(PDF) Polarization-Selective Optical Lithography

This article is based on a point made in the public discussion of the draft law on tertiary education. It was pointed out that the law permits profit-making organizations to establish non-profit "universities" - subsidiaries in Greece. This is a tailor-made provision for the venture between the fund CVC and the University of Nicosia, both profit-making entities. The critique to the government legislation has left the level of neoliberal and is now entering the level of ridiculous (PARON 18/2/2024)

Polarization-selective optical lithography Citation for published version (APA): Van, M. P. (2015). Polarization-selective optical lithography. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR783894 DOI: 10.6100/IR783894 Document status and date: Published: 22/01/2015 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: openaccess@tue.nl providing details and we will investigate your claim. Download date: 04. Dec. 2021 Polarization-Selective Optical Lithography PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op donderdag 22 januari 2015 om 16:00 uur door My Phung Van geboren te Venlo Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt: voorzitter: 1e promotor: 2e promotor: copromotor(en): leden: adviseur(s): prof.dr.ir. J.C. Schouten prof.dr. D.J. Broer prof.dr. H.P. Urbach (TUDelft) dr. C.W.M. Bastiaansen prof.dr. J.P.H. Benschop (UT) prof.dr.ir. J. Huskens (UT) prof.dr. R.P. Sijbesma Ing. J.M. Wijn (VDL ETG Technology & development) A catalogue record is available from the Eindhoven University of Technology library ISBN: 978-90-386-3760-0 Copyright © 2014 by My Phung Van The research described in this thesis has been financially supported by the Stichting voor Technische Wetenschappen (STW), project 10727 Contents Summary 1 1 1.1 1.1.1 1.1.2 1.1.3 1.2 1.2.1 1.2.2 1.3 1.3.1 1.3.2 1.4 1.5 Introduction Optical (mask) lithography The process Critical dimension Development of optical lithography Light Transversal polarization Longitudinal polarization Photoresists Conventional photoresists Polarization-selective photoresists Aim Outline 5 6 6 8 8 10 10 12 15 15 18 23 24 2 2.1 2.2 2.3 2.3.1 2.3.2 2.4 The smectic B photo-reactive material Introduction Experimental Results and discussion Liquid crystalline phase behavior The alignment of the dichroic photoinitiator in the smectic B host Conclusion Appendix A1 Appendix A2 31 32 33 35 35 40 42 45 47 3 3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.3.3.1 Polarization-selective photopolymerization 49 Introduction 50 Experimental 53 Results and discussion 56 Polymerization of an ordered smectic B host 56 Polarization-dependent polymerization kinetics 59 Dose-response behavior 63 Observations during characterization of the dose-response 64 behavior Conclusion 65 3.4 4 Patterning of the smectic B photo-reactive material with 69 linearly polarized light 4.1 Introduction 70 4.2 Experimental 72 4.3 Results and discussion 73 4.3.1 Planar alignment of the polarization-selective material 73 4.3.2 Patterning with polarization holography 74 4.4 Conclusion 77 5 5.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.4 Selectivity of the smectic B photo-reactive material towards longitudinal polarization Introduction Experimental Results and discussion Homeotropic alignment of the polarization-selective material Considering effects for oblique illumination The isotropic photo-reactive material Polarization-selectivity of the homeotropically aligned photoreactive material Conclusion 81 82 84 85 85 86 87 88 90 6 Development of a cholesteric filter for blocking transversal 93 polarizations 6.1 Introduction 94 6.2 Experimental 95 6.3 Results and discussion 97 6.3.1 Fabrication of the cholesteric filter 97 6.3.2 Cholesteric filter applied on a photoresist 100 6.4 Conclusion 102 7 Cholesteric filter for enhancement of the longitudinal polarization 7.1 Introduction 7.2 Experimental 7.3 Results and discussion 7.3.1 Radially polarized light 7.3.2 Effect of the cholesteric filter on radially polarized light 7.3.3 Theoretical study of the working-principle of the cholesteric filter 105 106 107 107 107 110 114 7.4 Conclusion 116 8 8.1 8.2 8.3 8.4 8.5 Technology assessment and prospects Assessment From negative to positive photoresists Challenges going from scanning to mask lithography Alternative lithography High polarization-contrast patterning in the third dimension 119 120 122 122 123 124 List of abbreviations 129 List of publications 131 Acknowledgements 133 Curriculum Vitae 135 Summary Summary Polarization-selective optical lithography Optical lithography is indispensable for many applications that require patterning on micrometer length scales. This technique utilizes light to image a pattern into a light-sensitive material called photoresist. The illuminated areas undergo a chemical change which in turn causes a change in the physical properties of the area. A pattern in the photoresist is created by choosing an appropriate developer to dissolve either the unilluminated or the illuminated regions. In commonly used photoresists the chemical changes depend on the intensity of the illumination source. In this thesis, photo-reactive materials that chemically respond to only one polarization state of the illumination source are studied, i.e. the material responds to one polarization state of the light and remains unchanged when exposed to other polarizations. The development of such a polarization-selective material adds an additional parameter besides intensity to control pattern formation in optical lithography. Depending on the type of lithography and the related optical set-up, the newly-developed photolithographic material should respond either to the transversal electric-field component of light or to the longitudinal electric-field component of light. In the first case the polarization is orthogonal to the propagation direction of the light. In the latter case the longitudinal component is a resultant of a radially polarized laser beam focused through a high numerical aperture lens. The longitudinal polarization component is parallel to the propagation direction. This is unique in the field of photolithography and requires special optics to generate light with such a polarization. The polarization-selective photolithographic material developed in this thesis is composed of a dichroic photoinitiator (DP) and a liquid crystalline (LC) host. Dichroic photoinitiators have their transition moment oriented along a single axis and are, therefore, polarization-selective. Polarizations of light which are parallel to this moment are absorbed while the orthogonal polarizations are transmitted. In the study presented here a DP was selected with the transition moment parallel to the long axis of the molecule. When the DP is dissolved as a guest in a reactive LC monomer, the DP closely follows the alignment of its LC host material. The coalignment of the LC host and the DP is controlled by alignment layers applied on the substrate, and the extent of alignment determines the sensitivity of the DP towards the different polarization states of light. Preferential absorption occurs of light polarized parallel to the alignment (director) which initiates radical 1 Summary polymerization to form an insoluble network. The polarization-selectivity is characterized by the dichroic ratio (DR), which is the ratio of intensities of absorbed light parallel and perpendicular to the molecular director. The DR can adopt values ranging from unity for an isotropic system to infinity for a perfectly aligned system. Selectivity towards longitudinal polarization requires a homeotropic alignment where the molecular director is orthogonal to the plane of the film while selectivity towards the transversal polarization requires planar alignment where the molecular director is parallel to the plane of the film. The case of planarly aligned photo-reactive material in combination with linearly polarized light was firstly studied. The DP alignment depends on the alignment of the LC host, so the order of the host is an important factor in determining the degree of polarization-selectivity. A reactive acrylate-based monomer mixture which exhibits the highly ordered smectic B (SmB) phase was selected as the LC host. A DR of 32 was obtained by means of absorption spectroscopy. This value is considerably higher than those found for lithographic materials that are processed in the more conventional nematic phase. Following a steady-state radical polymerization model, the polymerization contrast (the ratio of the polymerization rates initiated by light with the polarization of illumination parallel and perpendicular to the director) scales with the square-root of the DR in the initial stage of the polymerization. The polymerization contrast is further increased by the presence of an inhibitor molecule in the monomeric mixture. The contrast becomes infinite in the initial phase of the exposure where the photo-reactive material is sensitive to only one polarization along the director. The dose-response behavior, which gives the relative thickness of the film after development as a function of the exposure dose, shows an identical trend. Polarization holography experiments performed on these polarization-selective materials lead to defined pattern formation despite the polarization pattern of the illumination being sinusoidal. The nonlinear response of the photo-reactive material with respect to the dose enables well-defined pattern formation. For the photo-reactive material developed in this thesis to be sensitive to longitudinally polarized light, an alignment perpendicular to the substrate (homeotropic alignment) is needed to absorb this light. The homeotropic alignment of the photo-reactive film layer is confirmed by x-ray diffraction and atomic force microscopy characterizations. The polarization-selectivity of this material towards the longitudinal polarization was confirmed through illumination under an angle by plane waves. 2 Summary The aforementioned study is based on the selective absorption of the desired longitudinal polarization by the smectic B polarization-selective material. Another approach to record the longitudinal component is by applying a filter on top of a conventional photoresist which absorbs the unwanted broader transversal components while transmitting the narrower longitudinal polarization state. This transmitted component can subsequently be recorded in the underlying photoresist. Here it is shown that such a filter, consisting of a chiral nematic material containing a dichroic dye that follows the alignment of the nematic host, can be fabricated. However, a theoretical study demonstrates that this filter cannot function since elimination of the transversal components will also lead to the elimination of the longitudinal component. In conclusion, this thesis describes a basis for the fabrication of polarizationselective photo-reactive materials based on a dichroic photoinitiator and an aligned SmB liquid crystalline host. There are possible applications for these polarizationselective materials, e.g. for high-contrast three-dimensional patterning without masks. This can be achieved by using a twist alignment of the liquid crystalline photo-reactive material. The twist alignment shows molecular director rotation in the plane of the film. The polarization-selectivity varies throughout the thickness of these films due to the change of the orientation of the director through the depth of the material and, thus, results in a change in polarization-selectivity through the depth of the film which can be used for patterning in the third dimension. Another application is laser direct-write lithography with enhanced resolution. It is known that the spot size of the longitudinal component is 15% to 30% narrower than the Airy spot formed with linearly polarized light. Therefore, a photo-reactive material recording only the longitudinal component can potentially lead to higher resolution optical lithography. 3 Chapter 1 1 Introduction 5 Introduction Optical lithography is a patterning technique important for, e.g. for optical disk mastering, fabrication of microfluidic devices and the semiconductor industry where it is indispensable for the fabrication of integrated circuits. This technique utilizes light to fabricate patterns in light-sensitive materials called photoresists. The light induces a chemical change which results in a change in the physical property that is exploited to fabricate patterns. A more detailed description of the process of optical lithography will be given in the section below. Thus far, the chemical changes in the majority of the photoresists are induced by the intensity of light. In this study, a basis is proposed for the fabrication of photo-reactive materials that respond to the polarization state of light to induce chemical changes. It is anticipated that the polarization-selectivity can be utilized as an additional parameter next to intensity, e.g. to fabricate patterns in the third dimension next to the more widely applied inplane patterning method. Patterns created by direct laser writing might also benefit through resolution enhancement as will be explained in this chapter. Additionally, in this chapter an overview is given of the developments in optical lithography and photoresists. Light is an important factor in this technique and, therefore, the basic properties of light will also be described below. This chapter concludes with the aim of this study and the outline of the thesis. 1.1 1.1.1 Optical (mask) lithography The process Often optical lithography employs a mask, which contains a locally transmissive pattern, to produce patterns in a photoresist. This technique is referred to as optical mask lithography. In the semiconductor industry, the resist is often coated on a silicon wafer. In Figure 1.1, the basic process steps are shown. Firstly, the light emitted by the source passes a condenser lens which focuses the light onto the mask. The image of this pattern is then projected onto the photoresist by a lens. Illumination through a mask results in exposed and unexposed areas. The light induces a chemical change in the photoresist while no changes occur in the nonilluminated areas. This chemical change results in differences in the physical properties, such as polarity in the respective areas which can be utilized in the subsequent development of the photoresist. In this step either the illuminated or non-illuminated areas of the photoresist are removed by a solvent which can be organic or water-based (Figure 1.1b). The resist is referred to as positive when the illuminated areas are removed resulting in the formation of a pattern which is an exact replica of the mask. The resist is termed negative when the non-illuminated 6 Chapter 1 areas are removed, creating a pattern which is an inverse of the mask. The patterned resist protects the silicon wafer during the subsequent etching step where the pattern is transferred to the wafer. The resist can also be used as a means to dope or deposit materials at designated areas. In all cases, the resist acts as a protective barrier to prevent chemical or physical changes in parts of the surface of the silicon wafer. Lastly the resist is stripped. A three dimensional structure is formed by repeating the lithographic steps with different masks to build the desired pattern layer by layer.1 After illumination resist is developed Light source Condenser Positive Negative Mask Objective Lens Etch Strip resist Photoresist Silicon wafer (a) (b) Figure 1.1 a) A simplified schematic of the optical lithography process. b) After illumination, the negative or positive photoresist is developed and the silicon wafer is etched to transfer the desired pattern into the wafer; the resist act as a protective layer on the silicon wafer. After etching, the resist is stripped from the wafer. In the event that no mask is employed, as is done in optical disk mastering, a pattern in the photoresist is fabricated in a scanning fashion. The laser beam is focused into the photoresist and a pattern is formed by either moving the resist or sample stage and keeping the laser stationary or vice versa. This method, often referred to as laser direct-write, is more time-consuming to fabricate a full pattern, but offers the possibility to form complex structures which are very difficult or impossible to produce through optical mask lithography.2-13 7 Introduction 1.1.2 Critical dimension Improvements to the optical lithographic process have been made to facilitate the formation of smaller features. The resolution of any optical imaging process is defined by Rayleigh’s equation (Equation 1.1). Here, the critical dimension (CD) is defined as the minimum pitch divided by two, also referred to as the half-pitch. The pitch is defined as the sum of the line-width and the space-width. As shown in Equation 1.1, the CD is a function of the wavelength λ, numerical aperture NA and the process dependent factor k1 which is a factor that depends on the resist material, process technology and image-formation technique used.14 k1 ≥ 0.25 for single exposure optical lithography.15 NA is defined as shown in Equation 1.2 where n is the refractive index through which the image of the mask is projected and β is the angular aperture of the lens. Based on Equation 1.1, the CD can become smaller by lowering the wavelength or by increasing the NA. By changing these two factors, the depth of focus (DOF) is also lowered as shown by Equation 1.3. A DOF which is too small is unfavorable, resulting in an illumination process which is very sensitive to the alignment of optical set-up and the thickness and roughness of the photoresist since the light diverges very strongly moving away from the focus point.16 For the past decades, lowering of the CD has been achieved by lowering the wavelength, increasing the numerical aperture and by applying various resolution enhancement techniques. These enhancement techniques fall outside the scope of this study but for more information the reader is referred to the referenced literature. 14, 15, 17-28 CD  k1  Equation 1.1 NA NA  n sin  DOF  k 2 1.1.3  NA2 Equation 1.2 Equation 1.3 Development of optical lithography A general overview is shown in Figure 1.2 of the illumination sources and photoresist types used in optical lithography through the decades. In early 1960s, patterns were produced by optical lithography by bringing the mask into direct 8 Chapter 1 contact with the photoresist.15, 18 The mask was easily contaminated during the process which led to defect formation. Therefore, an air gap was introduced in proximity lithography which reduced damage to the mask, but the resolution was compromised by the air gap. Projection optical lithography was introduced in 1970s which showed a high resolution and the mask was not contaminated during the process. Negative cyclized rubber photoresist were used for contact lithography. As smaller features were produced by, e.g. reducing the wavelength of the illumination source, this negative photoresist showed poor resolution due to swelling of the formed polymer caused by developing the resist in an organic solvent. A positive photoresist was employed which did not exhibit swelling due to the use of an aqueous alkaline solution as developer. As the wavelength of the illumination source reduced to 248 nm, so-called chemically amplified resists were used to obtain sufficient sensitivity. Amplified resists are also used in extreme ultraviolet (EUV) 13.5 nm lithography. Additional information about these resist can be found in Section 1.3.1. EUV lithography is considered to be the next generation lithography. EUV lithography machines are available. However, several bottlenecks, such as low power output of the illumination source and low photoresist sensitivity impede high volume production.26, 29-33 Recently in literature new non-optical techniques are published based on selforganizing molecular systems such as block-copolymers.28, 34-36 They opt to bring the resolution down to tens of nanometers. Often these systems are being used in combination with optical techniques providing the offset for the molecular alignment within these systems. 9 Introduction Figure 1.2 The development of optical lithography. [Reprinted with permission.17 Copyright (2007) by Taylor & Francis Group, LLC] 1.2 Light As mentioned at the beginning of this chapter, in this study the aim is to form a basis for the development of polarization-selective materials to have an additional parameter next to intensity that can be used for patterning. Different forms of polarization states exist. Here, a distinction is made between transversal and longitudinal based on the direction of the polarization relative to the propagation direction. Transversal polarization refers to polarization states where the polarization is orthogonal to the propagation direction (Section 1.2.1) and longitudinal refers to a polarization state where the polarization is parallel to the propagation direction (Section 1.2.2). 1.2.1 Transversal polarization Light can be considered as an electromagnetic wave. As light propagates, the magnetic and electric field (E-field) oscillate orthogonal to the propagation direction as well as to each other (Figure 1.3). Due to the orthogonal direction relative to the propagation, these fields are considered as transverse fields. The polarization refers to the direction in which the E-field oscillates. This field is described by 10 Chapter 1 Equation 1.4 where E0 is the amplitude, k is the propagation number, ω is the angular temporal frequency, t is time and ε is the initial phase of the wave. E( z, t )  E0 sin(kz  t   ) Equation 1.4 z Electric field x Magnetic field y Figure 1.3 Light represented as an electromagnetic wave propagating in the z-direction. Light sources can be polarized or unpolarized, i.e. the polarization can exhibit directionality or is equal in all directions, respectively. The common types of polarization states are shown in Figure 1.4. Linear polarized or plane polarized refers to a state where the electric field oscillates in a single direction. Circularly polarized light is a superposition of two linearly polarized waves with the same amplitude E0 but with a π/2 phase shift. As the wave propagates, the E-field vector of these two waves rotates along the optical axis. When the phase shift is -π/2, the polarization rotates clockwise and it is referred to as right circularly polarized light. A +π/2 phase shift results in a counterclockwise rotation which is referred to as left circularly polarized. Linearly and circularly polarized light can be considered as two extreme cases of elliptically polarized light (Figure 1.4c). The characteristic of this polarization state is that the amplitude of the E-field vector changes during propagation. This type of polarization is created in two cases. The first case of equal amplitudes E0 of the interfering waves, elliptically polarized light is formed when the phase-shift is not ± π/2. In the second case, when E0 is different for the interfering waves elliptically polarized light is formed for all values of phase-shifts. 11 Introduction x z z z y z π/2 (a) (b) z π/2 z (c) Figure 1.4 In these images only the E-field is shown. a) Linearly polarized light and b) righthanded circularly polarized light which is the superposition of two linearly polarized waves orthogonal to each other and with a π/2 phase-shift. The E-field vector revolves around the optical axis with constant amplitude. c) Elliptically polarized light composed of two waves with different amplitude; the E-field vector revolves around the optical axis with varying amplitude. 1.2.2 Longitudinal polarization When the polarization direction of light is parallel to the propagation direction, the state is referred to as longitudinal polarized light. Longitudinal polarized light can be produced by focusing radially polarized light (Figure 1.5).37-40 Radial polarization refers to a transversal polarization state in a discrete light beam where 12 Chapter 1 the electric field is directed in a radial manner relative to the optical axis and where the electric-field in all points of the beam oscillates in phase (Figure 1.5a). x z x (a) z y (b) (c) Figure 1.5 a) A representation of radially polarized light when the light propagates in the zdirection. A side view of radially polarized light focused with b) a low numerical aperture lens and c) a high numerical aperture lens. The radially polarizations are depicted by grey arrows while the decomposed transversal (dashed) and longitudinal (continuous) component are indicated in black. In b) and c) the propagation is again in the z-direction. Different methods exist for creating radially polarized light. One method is based on converting linear polarized light through segmented half-wave plates into the radial polarization state41-43 or through the use of liquid crystal polarization converters.43-46 These methods are based on the local rotation of the polarization to form the desired radial polarization. Due to the limitation of segmentation of the wave plates due to losses at the boundaries between the segments, the quality of the radial polarization is low. Converting circularly polarized light to radially polarized light is also possible through circular wire grid polarizers (CWGPs).41, 47-49 These optical elements do not consist of segments and, therefore, are a more attractive method to form high quality radially polarized light. This polarizer consists of concentric metal rings on a substrate that reflects the polarizations which are parallel to the metal rings, i.e. it reflects azimuthal polarizations while transmitting the radial component (Figure 1.6). 13 Introduction Azimuthal component is reflected Circular wire grid polarizer Radial component is transmitted Figure 1.6 Radially polarized light is transmitted through the circular wire grid polarizer while azimuthal polarizations are reflected. The intensity of the resultant longitudinal component depends on the numerical aperture of the lens. The longitudinal component becomes stronger with high NA (compare Figure 1.5b and c). The longitudinal polarization only exists in the center of the focal point which is 15-30% narrower compared to the conventional Airy spot as demonstrated in Figure 1.7a where the profile of the intensity distribution in the focus point is depicted.50, 51 The extent of the reduction depends on the NA of the lens. The conventional Airy spot is the intensity distribution of a focused linearly polarized beam. It should be noted that only in the center of the focus spot, the electric-field is purely longitudinally polarized; outside the center, the transversal component also exists (Figure 1.7b).52 If a photoresist is used which is not polarization-selective, the recorded light is proportional to the sum of the longitudinal and transversal intensities (Figure 1.7b, green curve) which leads to a spot size that is actually broader than the Airy spot. Therefore, a longitudinal spot in combination with a conventional photoresist does not give higher resolution. However, when a photoresist is used which selectively responds only to the longitudinal component, a spot can be created with smaller dimensions as represented by the red curve in Figure 1.7b. Thus, the use of a polarization-selective material could lead to resolution enhancement. 14 Chapter 1 (a) (b) Figure 1.7 a) A comparison of the intensity distribution in the focal point of focused radially polarized light and focused linearly polarized light with its polarization parallel to the x-axis. [Reprinted with permission.50 Copyright (2008) by the American Physical Society] b) The intensity distribution of focused radially polarized light where both the longitudinal (Ez2) and transversal (Er2) components are shown separately and combined (Ez2 + Er2). [Reprinted with permission.52 Copyright (2008) by Macmillan Publishers Limited] 1.3 1.3.1 Photoresists Conventional photoresists Photoresists are vital for the optical lithographic process. It is this material that enables patterning. When a mask is used for patterning, light is partially blocked or transmitted by the mask. For this process a photoresist is used that is not polarization-selective. The transmitted light is absorbed by the photoresist which induces a chemical change. For well-defined patterning it is vital that the contrast between exposed and unexposed areas is large. Below, an overview is given of photoresists which have been used in the past or are still being used. In early optical lithography, negative photoresists were used based on crosslinking.53, 54 The formation of a polymer network makes the exposed areas insoluble. After illumination, the material was developed with an organic solvent. This step proved to be problematic for features smaller than 2 µm due to the swelling of the exposed regions of the resist in the solvent. A switch was made from negative resists to the positive Novolac-based resists. These phenol-formaldehyde systems can be developed in an aqueous alkaline solution which does not cause swelling of the resist. The Novolac is sensitized by photoactive compound (PAC) diazonaphtoquinone (DNQ) sulfonate esters. Through the photochemical Wolff rearrangement (Figure 1.8), DNQ is converted to the polar indene-3-carboxylic acid which promotes the solubility of the Novolac resin in the alkaline solution 15 Introduction (Figure 1.8). This photoresist was used for g-, h- and i-line (436 nm, 405 nm and 365 nm, respectively) illumination. These Novolac-based photoresists are often used for optical disk mastering of CDs and DVDs. Figure 1.8 The photochemical reaction of DNQ upon illumination. [Reprinted with permission.53 Copyright (2008) by SPIE] The Novolac based photoresists proved unsuitable for 248 nm (KrF) illumination due to extreme absorption by the Novolac resin and DNQ at this wavelength.16, 17 At this wavelength, chemically amplified resists (CARs) are used. In these resists a strong acid forms upon absorption by a photo acid generator (PAG). Through a post-exposure bake (PEB) this acid acts as a catalyst causing a cascade of a chemical reaction that changes the polarity and, thus the solubility, of the matrix of the resist. Chemical amplification refers to the catalytic behavior of the acid. Through this process, the chemical change is de-coupled from the absorption process and, therefore, does not require a high dose. In Figure 1.9a, an example of a CAR is shown where the t-butyloxycarbonyl (tBOC) protecting group is removed in the presence of acid to form the hydrophilic polyhydroxystyrene (PHOST. In the second example (Figure 1.9b), the resist is a copolymer of HOST and t-butyl acrylate (TBA). Again deprotection occurs in the presence of an acid to form a carboxylic acid which is base soluble. 16 Chapter 1 (a) (b) Figure 1.9 Two examples of CARs. a) Due to de-protection by tBOC, PHOST is formed which is hydrophilic. [Reprinted with permission.21 Copyright (2010) by American Chemical Society] b) A resist consisting of a copolymer of HOST and TBA also becomes hydrophilic upon deprotection by the tBOC forming a carboxylic acid. Both processes occur in the presence of an acid which acts as a catalyst. In 193 nm lithography (ArF) the HOST resin, commonly used in CARs, was not suitable due to intense absorption by the aromatic resin. PHOST resins were replaced by polymethacrylates with alicyclic side groups. The alicyclic side groups were introduced to improve the etching resistance of the polymethacrylates.55 Resist materials where carboxylic acids form, exhibit considerable swelling in aqueous alkaline solutions which compromises the resolution of the features. During research for 157 nm lithography, hexafluoroalcohol (HFA) resists were developed; these resist did not exhibit excessive swelling. Therefore, HFA resist were applied in 193 nm lithography.55 157 nm lithography was abandoned due to the lack of a suitable immersion fluid for this wavelength. Resolution enhancement techniques pushed the CD of 193 nm lithography to features as small as 22 nm.33, 56, 57 157 nm lithography was also abandoned because materials transparent at this wavelength are anisotropic. A large jump to 13.5 nm (EUV) was made; at this wavelength CARs are also used. However, the acid generation follows another mechanism compared to CARs for 248 nm and 193 nm lithography. In the latter two cases the acid is generated by the PAG through the absorption of photons. In the case of EUV lithography (EUVL), the acid is generated through polymer sensitization (the energy or electron transfer from excited polymers to PAGs).58 Therefore, an absorbing polymer resin is desired in case of EUVL. The development of EUV resists focuses mainly on derivatives of 248 and 193 nm resists and is still ongoing. 17 Introduction 1.3.2 Polarization-selective photoresists Polarization-selective photoresists are a new class of materials where the chemical changes leading to solubility differences are induced depending on the direction of the polarization of light. As already mentioned in Section 1.2.2, a polarization-selective material could possibly be useful for resolution enhancement by recording the narrow longitudinal polarization component. Another benefit of a polarization-selective material is for patterning in the third dimension as will be explained here. These studies have been performed on guest-host systems.59-63 A dichroic photoinitiator was dissolved as a guest in a liquid crystalline (LC) reactive monomer host. The dichroic photoinitiator exhibits polarization-selectivity due to the directionality of the transition moment along the long molecular axis of the rodshaped molecule. Liquid crystals are a class of materials that exhibit one or more intermediate phases between the isotropic liquid phase and the highly ordered crystalline state.64, 65 The molecules in these LC phases are less ordered than the crystalline state. These phases appear by a change in temperature or by a change in concentration of the LCs within a solvent. In the latter case the LC is classified as lyotropic while in the former case it is classified as thermotropic. The materials used in the research reported in this thesis are thermotropic. Within this category, a distinction is made based on the shape of the molecules, namely rod-shaped or disc-shaped. The rodshaped LCs are referred to as calamitic and the disc-shaped molecules are referred to as discotic. In this research calamitic LCs are used (Figure 1.10). These molecules have a rigid cylindrical shaped core with flexible spacers that can have various functions, e.g. to induce a preferred alignment or improve solubility in solvents. For the calamitic LCs the two most important LC phases are the nematic (N) phase, the chiral nematic phase (N*) and the smectic (Sm) phase. In the nematic phase the molecules exhibit orientational order in which the average orientation of the molecules is along a common direction. In the chiral nematic phase which is also known as the cholesteric phase (Ch) the molecules align in a helical fashion induced by a chiral center in the rod-like LC molecule itself or in additives to that. The pitch p is the required distance for a 2π rotation of the molecules. In the smectic phase the molecules also show directionality as in the nematic phase, but there is an additional positional order where the molecules align in layers. There are various kinds of smectic phases depending on type and degree of order. The most common smectic phases are smectic A, B and C which are depicted in Figure 1.10. In the smectic A phase the molecules are oriented similar to the nematic phase, but the molecules 18 Chapter 1 also organize in layers. In the smectic B phase the molecules are hexagonally packed within the layers. The smectic C phase is similar to smectic A, but the molecules have an additional tilt. Flexible spacer Rigid core p Chiral nematic Nematic Smectic A Smectic B Smectic C Figure 1.10 An example of a calamitic liquid crystalline molecule and a schematic of the molecule (top) and five commonly occurring liquid crystalline phases (bottom). In the chiral nematic phase the pitch p is defined as the distance required for a 2π rotation of the molecules. The alignment of the LC host can be controlled through the application of shear, alignment layers, a magnetic field or an electric field.66 The different alignments are shown in Figure 1.11. The planar and homeotropic alignment are two extreme cases where the average orientation of the molecules is towards a single direction indicated by the molecular director 𝑛⃑. In the former case the director is parallel to the plane of the film while in the latter the director is orthogonal to this plane. In a twisted alignment the director gradually rotates in the plane of the film, similar to the organization observed in the chiral nematic phase. However, in a twisted alignment the rotation can be smaller than 2π. In a splay alignment the director gradually rotates outside the plane of the film in a transition from planar to homeotropic.  n Planar  n Twist Splay Homeotropic Figure 1.11 The different kinds of alignments that can be adopted by liquid crystals (grey). The arrow indicates the molecular director 𝑛⃑. 19 Introduction The liquid crystal order can be frozen in a polymer network by modifying the liquid crystal molecules. An example is already given in Figure 1.10 where at both sides an acrylate moiety is attached that can undergo free-radical chain addition polymerization. In current literature polymerization is often initiated by UV irradiation.67-71 A small amount of an added photoinitiator absorbs the UV light and, after being brought to the excited state, defragments into free radicals which start the polymerization process. In the usual reactive LC systems the photoinitiator is not selective for the state of polarization of the UV light. But by co-aligning a dichroic photoinitiator with a LC host polarization-selectivity can be achieved. In uniaxially planar and homeotropic alignment polarization-selectivity is achieved for a single polarization of light which is parallel to the molecular director. In the splay and twist alignment and the alignment in the chiral nematic phase the polarizationselectivity varies throughout the thickness of the film. Where the polarization is parallel to the director the dichroic photoinitiator exhibits a large absorption of the light. When there is an increasing angle between the director and polarization the absorption is low. The difference in sensitivity enables patterning in the third dimension (Figure 1.12) which has been previously shown for the splay alignment60 and the chiral nematic phase.59-63 In these studies a nematic mixture of reactive acrylate LC molecules and non-reactive LC molecules was used. Upon exposing the mixture with polarized light a polymer was formed, i.e. it is a negative photo-reactive material. 20 Chapter 1 Twist Splay Chiral nematic E-field Low x Low Low Low z High High High High Figure 1.12 (top) A dichroic photoinitiator (white) co-aligned in a liquid crystal host (grey). When the light propagates in the z-direction and the polarization is in the x-direction there will be areas of high and low absorption throughout the thickness of the film. (Bottom) A model of the expected pattern formed due to the difference sensitivity of the film. After exposure and subsequent development to remove the non-reactive material, a triangular shaped polymeric pattern was the result in case of the splay alignment (Figure 1.13). The same shape is expected for the twist alignment. In the chiral nematic phase a stacked diamond pattern was expected as shown in Figure 1.12. However, separated polymeric layers throughout the thickness were formed with a periodicity corresponding to half of the pitch due to inhibition by oxygen (Figure 1.13). By utilizing the alignment a three-dimensional pattern can be formed through a single illumination instead of constructing it layer-by-layer as is done in optical mask lithography. 21 Introduction Splay Chiral nematic 20 µm 0.3 µm Figure 1.13 A SEM image of the fabricated polymeric patterns after development for splay alignment with the corresponding height-profile shown on the bottom of this figure.60 On the right a SEM image of the polymeric pattern in the chiral nematic phase after development.59, 60, 63 The polarization-selectivity of the dichroic photoinitiator in the LC host was shown to largely depend on the order of the LC host when the dichroic photoinitiator aligns with the host.60-63, 72 As already stated above, the average orientation of the LC molecules is represented by the director 𝑛⃑. The molecular order can be characterized by the order parameter S according to Equation 1.5, where θ is the tilt angle of each individual molecule with respect to the director.65 The brackets in Equation 1.5 represent the average tilt angle for the total ensemble of molecules. This parameter S ranges from zero for an isotropic system to unity for a perfectly uniaxially aligned system. A measure for the polarization-selectivity is the dichroic ratio (Equation 1.6) which is the quotient of the absorbance by the initiator parallel to the director A// and perpendicular to the director A⊥.73 The dichroic ratio (DR) ranges from unity for an isotropic system to infinity for a perfectly ordered system. The DR and S are related via Equation 1.7, assuming the dichroic initiator aligns perfectly with the host and assuming there is no additional angle between the transition moment and the long molecular axis of the initiator. S  3 cos 2   1  2 22 Equation 1.5 Chapter 1 DR  S A// A DR  1 DR  2 Equation 1.6 Equation 1.7 The studies above were performed in nematic phase which generally has an order parameter of 0.6-0.7 which is relatively low compared to the smectic phases which exhibit order parameters of larger than 0.7. Therefore, the expectation is that an anisotropic photoresist with higher polarization-selectivity can be achieved in these smectic phases. Smectic B monomers provided with a dichroic dye developed for display polarizers have shown to reach dichroic ratio values of 40 or higher. 74 It is anticipated that when the dye is exchanged with a suitable dichroic photoinitiator similar contrast ratios should be possible. The SmB phase is a phase that exhibits this high degree of molecular order due to a dense hexagonal packing.64, 75, 76 Consequently it is this phase that is anticipated to provide high polarizationselectivity to the photo-sensitive material. 1.4 Aim The objective of the work presented in this thesis is to develop a photopolymerizable material that has a high polarization-contrast for the state of polarization of the UV light that is applied to initiate the polymerization process. It is also an objective to provide insight in the important parameters that are needed to develop a polarization-selective photoresist material. Having control over the polarization-selectivity is anticipated to improve the resolution of direct laser writing in photoresist material as well opening pathways to create complex threedimensional resist patterns in a single UV exposure. The concept of the polarization-selective material is shown in Figure 1.14. This material is fabricated by co-aligning a dichroic photoinitiator in the smectic B LC host and adopting planar and homeotropic alignments. The alignments are to promote favorable absorption of the transversal and the longitudinal polarization component of light. Smectic B is chosen as the preferred phase as it exhibits a high degree of molecular order which should improve the contrast of the dichroic initiator. The proof of concept will be demonstrated on photo-crosslinking molecules which mean that they can form the basis of new negative resist materials, i.e. the crosslinking reaction creates areas with 23 Introduction low solubility in developer materials. This choice was based on the availability of some critical components for the photosensitive material being dichroic free radical photoinitiators and smectic acrylate monomers. Longitudinal E-field hν hν (a) (b) Figure 1.14 The polarization-selective material consisting of a smectic B LC host (grey) and dichroic photoinitiator (white). a) A planar alignment promotes the absorption of one of the transversal polarization states. b) A homeotropic alignment predominantly absorbs the longitudinal polarization component of light. In the latter case, both the polarization and propagation direction are along the grey arrow. 1.5 Outline The properties of the smectic B photo-reactive material are characterized for two cases. Firstly, for planar alignment for selectivity towards linearly polarized light and secondly for homeotropic alignment for selectivity towards the longitudinal polarization component of light. In chapter 2, the components are selected to create a negative polarizationselective photo-reactive material. In this chapter the phases are identified of the reactive, liquid crystalline and crosslinkable, monomers. Subsequently the alignment of the dichroic photoinitiator is characterized when embedded in the liquid crystalline host. The combination of a mixture of liquid crystal monomers, forming a smectic phase at the exposure conditions, and the photoinitiator forms the polarization-selective material. In chapter 3, the polarization-selectivity of this material is characterized based on measurements of the polymerization and crosslinking kinetics and the dose-response behavior. The latter behavior relates the resistance of the formed polymer to the development solvent as a function of the light exposure dose. Additionally, in this chapter the experimental conditions to achieve the highest polarization-selectivity are determined. Chapter 4 shows that the 24 Chapter 1 polarization-selective material can be used for patterning based on polarized light. For this first proof of principle polarization holography was used. In Chapter 5 the polarization-selective material was converted to a material that is sensitive for illumination with longitudinal polarization. To convert the polarization-selective material to a material with longitudinal sensitivity a homeotropic alignment of the smectic material was introduced. The polarizationselectivity of these photo-reactive materials towards the longitudinal component was characterized through angular illumination. In Chapter 6, a new optical component is developed that absorbs unwanted transversal polarizations meant as a potential tool for further enhancement of polarization lithography as discussed in this thesis. In Chapter 7, a theoretic study is presented to show whether the optical component developed in Chapter 6 can be applied in actual application. In chapter 8, a technological assessment and prospects are given concerning the use of the polarization-selective material in the lithographic process. 25 Introduction References 1. S. Nonogaki, T. 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Chem. Soc., 2002, 224, U404-U404. 73. R. D. B. Fraser, J. Chem. Phys., 1953, 21, 1511-1515. 74. E. Peeters, J. Lub, J. A. M. Steenbakkers and D. J. Broer, Adv. Mater., 2006, 18, 2412-2417. 75. R. M. Richardson, A. J. Leadbetter and J. C. Frost, Ann. Phys., 1978, 3, 177186. 76. A. J. Leadbetter, J. C. Frost and M. A. Mazid, J. Physique Lett., 1979, 40, L325-L329. 30 Chapter 2 2 The smectic B photo-reactive material* * This chapter is partially reproduced from: M.P. Van, C.C.L. Schuurmans, C.W.M. Bastiaansen, D.J. Broer, Polarization-selective polymerization in a photocrosslinking monomer film, RSC Advances, 2014, 4, 62499 31 The smectic B photo-reactive material 2.1 Introduction In Chapter 1 it has been argued that polarization-selectivity in lithographic materials gives an additional parameter for patterning next to intensity which could possibly be useful for patterning in the third dimension or to increase the resolution of optical lithography. The conditions for polarization-selectivity are twofold. First the photosensitive molecule in the mixture must have a distinct transition moment along one of its axes to enable excitation by one polarization state and remain relatively unaffected by the other(s). And second, to benefit from this so-called dichroic effect this photosensitive component must become macroscopically aligned. In the literature it was shown that a dichroic photoinitiator embedded in a nematic host fulfill these two conditions.1-4 The photoinitiator and its assumed transition moment are shown in Figure 2.1. Figure 2.1 An example of a dichroic photoinitiator. The initiator mainly absorbs UV light with the polarization parallel to the transition moment. The direction of the transition moment is indicated by the arrow. Upon absorption the initiator decomposes into radical species that initiate the polymerization process. This photoinitiator dissociates after excitation into two fragments with unpaired electrons. These so-called free-radicals are both capable to initiate a free-radical chain-addition polymerization in vinyl based monomers.5 For this reason a liquid crystal host material was chosen in which the molecules are provided with an acrylate endgroup allowing polymerization in their oriented state. The process described previously in the literature was based on a nematic liquid crystal host material.1-4 Nematic liquid crystals are known to have a limited order parameter of typically 0.6 to 0.7. For the description of the order parameter the reader is referred to Section 1.3.2. In the same chapter it was argued that, assuming that the photoinitiator adapts the degree of order within its liquid crystal host, this value of the order parameter would lead to only a limited contrast for polarized light. This contrast, expressed as the dichroic ratio (DR) being the absorbance for light with its polarization parallel to the molecular director divided by the absorbance of light with its polarization orthogonal to the director, is typically of the order of 5 to 7. As in common free-radical polymerization reactions, the initial polymerization rates scales with the root of the absorbed intensity6 which means 32 Chapter 2 that the contrast for polymerization will be intrinsically low. This was also found in literature with typical polymerization contrast ratio of 2 to 3. In this chapter, a photopolymerizable acrylate system with an improved order parameter is developed based on a smectic B liquid crystal monomer mixture containing a dichroic photoinitiator. The photo-reactive monomer formulation is characterized on its phase behavior, the degree of molecular order and its anisotropic properties. The latter forms the basis for the polarization-selective polymerization studies under various alignment conditions in following chapters. 2.2 Experimental The polarization-selective monomer mixture consists of a mono-acrylate liquid crystalline SmB host,7 a di-acrylate LC crosslinker,8 a dichroic photoinitiator and an inhibitor. The molecular structures are shown in Figure 2.2. The weight to weight ratio of host 1 and crosslinker 2 is 20/1. The dichroic photoinitiator 3 (1 wt%) was supplied by Merck. The inhibitor 4 (0.5 wt%) was supplied by Sigma-Aldrich. The thermal behavior of the reactive LC mixture was characterized with differential scanning calorimetry (DSC) TA Instrument Q1000 calorimeter. 1-2 mg of material was sealed in hermetically sealed aluminum pans. The LC mixture was heated from 20 °C to 160 °C and subsequently cooled back to 20 °C at a heating and cooling rate of 10 °C min-1. Polarized optical microscopy (POM) was performed on a Leica CTR 6000 microscope with crossed polarizers to examine the phase behavior. For this analysis the LC mixture was sandwiched between two glass slides coated with polyimide alignment layers JSR Micro AL-24101. The polyimide was spin coated on glass substrates with a two-step program: 1000 rpm for 5 s and subsequently 5000 rpm for 30 s. In both steps the acceleration was 2000 rpm s-1. After spin coating, the substrates were placed on a hotplate at 85 °C for 1 min to evaporate the solvent before being baked for 90 min in an oven set at 180 °C. In order to observe the microscopic textures to characterize the LC phases, occasionally defects were introduced to the polyimide surfaces by means of a razorblade. The material was heated and cooled down in a Linkam hot-stage THMS600; images were recorded with a Leica DFC420 C camera. 33 The smectic B photo-reactive material Liquid crystal host 1 Cr 45 °C SmB 56 °C SmA 112 °C N 147 °C Crosslinker 2 (SmC 89 °C ) Cr 108 °C N 153 °C I Dichroic photoinitiator 3 Inhibitor 4 Figure 2.2 The components of the photo-reactive mixture. Cr = Crystalline, SmB = Smectic B, SmA = Smectic A, N = Nematic and I = Isotropic. The temperatures refer to the transition temperatures between the various phases. Small and wide angle x-ray scattering (SAXS/WAXS) were performed on samples sealed in a 1 mm glass capillary inside a custom-built heating stage equipped with a 1 Tesla permanent magnetic field to stimulate molecular alignment in the liquid crystal phases. The monomer mixture was heated with a rate of 30 °C min-1 to the isotropic phase first before being cooled down with 10 °C min-1. The monomer mixtures were analyzed upon cooling. The samples were irradiated with a 1.54 Å GeniX-Cu ultra-low divergence source. The diffraction patterns were recorded on a Pilatus 300K silicon pixel detector. Silver behenate was used for calibration.9 The absorbance spectra of the dichroic photoinitiators in the aligned LC host were recorded on a Shimadzu UV-3102 spectrophotometer equipped with a wire grid polarizer (PPL05A Moxtek) to generate linearly polarized light. 2-4 µm thick films were prepared in a cell through the use of glass spacers. Uniaxial alignment was achieved by coating the glass plates with anti-parallel rubbed polyimide (JSR Micro AL-2410) as described here above. Rubbing was done manually with a cloth. The cells were filled through capillary action at 140 °C and cooled with an undefined rate to room temperature for the measurement. 34 Chapter 2 2.3 2.3.1 Results and discussion Liquid crystalline phase behavior Monomer 1 is the main component of the LC host material used to align the dichroic photoinitiator. As a reference, the phase behavior of monomer 1 was studied by DSC and POM (Figure 2.3a and Figure 2.4). DSC revealed that when heated above 147 °C this monomer is isotropic. But when cooled from the isotropic state to the liquid crystalline state first a nematic phase between 112 °C and 147 °C was observed. When cooling down further two smectic phases were found, namely a smectic A phase between 56 °C and 112 °C and a smectic B phase below 56 °C which can be preserved for approximately two hours in a supercooled state10 at room temperature which is below the equilibrium thermodynamic melting temperature. The crystal to smectic transition is, therefore, not seen in the DSC as this was measured in its second run starting from the supercooled monomer. In the first run, starting with the crystalline material, the crystal to smectic B transition was found at 45 °C. Monomer 1 was selected as the base component of the photoreactive system due to the broad smectic B phase allowing the material to be processed at room temperature. However, 1 is a monoacrylate which forms a linear polymer upon polymerization. Linear polymers are known to be less resistive against the solvent treatments during pattern development later in this thesis. In order to form a better solvent-stable polymer network a di-functional crosslinker 2 is mixed with 1 (20/1 wt/wt ratio of 1 and 2). Crosslinker 2 was selected based on the similarity of the molecular structure with host 1 to prevent disruption of the desired highly ordered smectic phases of 1. This crosslinker exhibits a nematic phase and a smectic C state when supercooled.8 The mixture was further enhanced with photoinitiator 3 (1 wt%) and inhibitor 4 (0.5 wt%). This hydroquinone based material is commonly used as inhibitor to prevent premature polymerization in the monomeric state but is also known to enhance the contrast in acrylate based lithographic materials.11-13 The polarization-selectivity of the photopolymerization process of the liquid crystalline photo-reactive mixture depends strongly on the type of the LC phase, the degree of order therein and the intrinsic dichroism of the dichroic photoinitiator. To get insight in these parameters first the phase transition temperatures of the LC monomer mixture was determined by DSC and compared with LC host 1 (Figure 2.3b). Upon heating, the mixture shows mesomorphic phase transitions at 54 °C and 108 °C; and above 145 °C the material becomes isotropic (Figure 2.4, P1). 35 The smectic B photo-reactive material Upon cooling, the material shows the same transition temperatures and remains liquid crystalline at room temperature (RT) for approximately two hours. The mesomorphic phase transitions temperatures are similar to host 1 which is a first indication that the LC phases of the host are unaffected by the addition of compounds 2, 3 and 4. However, polarized optical microscopy (POM) and x-ray characterization is necessary to assure the right identification of the phases of the photo-reactive mixture. 56 °C 1 Heat flow / W g-1 Heat flow / W g-1 54 °C 147 °C 113 °C Heat Cool 0 SmB SmA 112 °C N I 147 °C -1 145 °C 1 108 °C Heat 0 P4 56 °C P3 P2 40 80 120 160 0 200 Temperature / °C 40 P1 145 °C 54 °C 0 Cool 108 °C -1 80 120 160 200 Temperature / °C (a) (b) Figure 2.3 The DSC curves of a) liquid crystal host 1 and b) photo-reactive mixture where P1P4 are labels for the various phases. The POM results in Figure 2.4 for LC monomer 1 and the mixture show similar textures between crossed polarizers. Upon cooling down from the isotropic phase (Figure 2.4, P1), first thread-like textures appear which indicates the formation of the so-called Schlieren texture which is typical for the nematic phase.14 Upon further cooling to lower temperatures, fan-shaped structures appear which indicate the existence of two types of smectic phases.14 Based on these POM images, it is not possible to determine the exact type of smectic phase of the monomeric mixture and, therefore, further x-ray characterization is necessary. 36 Chapter 2 I N SmA SmB P1 P2 P3 P4 Figure 2.4 POM images of the LC phases P1-P4 of monomer 1 (top) and the photo-reactive mixture (bottom); from left to right going from high to lower temperatures. The images were captured upon cooling. The diffraction patterns for the three phases of molecularly aligned host 1 and the photo-reactive mixture are shown in Figure 2.5. The one-dimensional plots are shown in Appendix A.1 and the diffraction patterns of the photo-reactive mixture without crosslinker are shown in Appendix A.2. The diffraction patterns of monomer 1 and the mixture are similar. In every phase two sets of diffraction signals appear, namely a SAXS signal corresponding to the periodicity of the smectic layers (dL) and a WAXS signal corresponding to the intermolecular distance (di).10 The unexpected appearance of a SAXS signal in P2 (Figure 2.5, P2), the presumed simple nematic phase, indicates the formation of local layer-like domains. One can visualize this as some dynamic smectic pre-ordering without any long-range smectic order. This phase is known as the cybotactic nematic phase (Nc); thus, identifying P2 as the cybotactic nematic phase. The appearance of the dL signal in the smectic phases P3 and P4 are expected (Figure 2.5, P3 and P4) due to the layer organization of the molecules. More diffraction orders are visible for P4 compared to P3, explained by the higher degree of order of P4. The di signal of P4 is also more intense compared to P3. Based on this result, P3 and P4 are the LC phases smectic A and B, respectively. In the SmB phase, the molecules are hexagonally packed and, therefore, are more ordered compared to SmA. Based on the DSC, POM and SAXS/WAXS analyses, the liquid crystalline phases P2, P3 and P4 are Nc, SmA and SmB, respectively. Aside from minor changes in transition temperatures, the addition of crosslinker 2, dichroic photoinitiator 3 and inhibitor 4 does not affect the LC phase behavior of the pure monomer 1. It is argued that the transition from Nc, SmA to SmB, i.e. towards 37 The smectic B photo-reactive material increasingly ordered LC phases upon cooling is possibly beneficial for the gradual organization of the molecules, minimizing the defect formation that could lead to lower dichroic ratios and ultimately to lower polarization-selectivity. P2-Nc P3-SmA θθ dL (a) P4-SmB di (b) Figure 2.5 Diffraction patterns of a) monomer 1 and b) the photo-reactive mixture. Based on the WAXS signal an order parameter can be calculated according to Equation 2.1,15 where I(θ) is the intensity distribution as a function of the azimuthal angle θ as defined in Figure 2.5a. Assuming that the dichroic photoinitiator adopts the order of its LC host and that the transition moment is perfectly parallel to the long axis of initiator an apparent dichroic ratio can be calculated based on the order parameter (Equation 1.7). Both of these values are given in Table 2.1. 1 / 2 S   I ( ) 1 / 2 3 cos 2   1 sin d 2 1 / 2  1 I ( ) sin d 2 1/ 2 Equation 2.1 For both monomer 1 and the mixture the values are similar, again indicating that the addition of the crosslinker, initiator and inhibitor does not affect the LC phases. 38 Chapter 2 The order parameter (S) and dichroic ratio (DR) depend on the tilt of the molecules relative to the director. For the Nc phase the average tilt is the highest, resulting in a poor organization of the molecules and, thus, a relatively low S and DR value. The difference between SmA and SmB is due to the hexagonal packing in the latter.10, 16, 17 This packing prevents the molecules from exhibiting a large tilt and, therefore, resulting in a high S and DR for SmB. In the SmB phase the deviation of the DR between monomer 1 and the photo-reactive mixture is the largest. A probable cause is a large error in the calculation due to the baseline correction. This error becomes larger at high DR values due to division by a small number. Table 2.1 The order parameter (S) and dichroic ratio (DR) for the different LC phases calculated based on the WAXS data. Host 1 SWAXS DRWAXS 0.72 8.8 0.81 14 0.95 58 Nc SmA SmB Photo-reactive mixture SWAXS DRWAXS 0.70 8.0 0.82 15 0.94 48 The values for the layer spacing dL and intermolecular distance di are shown in Table 2.2. dL increases, going from Nc to SmA and to SmB. With increasing order, the molecules have a lower average tilt relatively to the director. Therefore, in the SmB phase the layer-spacing corresponds closest to the length monomer 1 which is approximately 40 Å derived in the elongated state. di values decrease for increasingly ordered phases, indicating a more densely packed LC phase, again due to the lower average tilt relative to the director. Table 2.2 The periodicity of the smectic layers (dL) and the intermolecular distance (di). Host 1 Nc SmA SmB dL (Å) di (Å) 36.5 37.8 39.3 4.65 4.55 4.47 Photo-reactive mixture dL (Å) di (Å) 35.2 4.62 37.8 4.58 39.7 4.47 The x-ray data suggest that a photo-reactive mixture with highest polarizationselectivity can be achieved by utilizing the SmB phase which exhibits an apparent dichroic ratio of 48. It should be noted that this value is based on the diffraction pattern and, thus, the average tilt angle of all the molecules in the photo-reactive mixture with respect to the director and not based on the absorption of the dichroic photoinitiator which is ultimately the relevant value. However, DRWAXS, is still interesting to examine the extent to which the dichroic photoinitiator aligns with the 39 The smectic B photo-reactive material LC host or to determine whether there is an angular deviation of the transition moment of the initiator relative to its long molecular axis. 2.3.2 The alignment of the dichroic photoinitiator in the Smectic B host The polarization-selectivity of the photo-reactive mixture arises from the absorption by the dichroic photoinitiator 3. It is important to verify that this initiator aligns with LC host 1. For this purpose the absorbance spectra, shown in Figure 2.6, are recorded for the uniaxially planarly aligned photo-reactive mixture. The absorbance maximum shifts to below 325 nm going from the SmB, SmA to Nc phase. A possible explanation is the change of molecular packing of the host molecules in the different LC phases that affect the electronic transition of the dichroic photoinitiator. An additional comment should be made regarding the shape of the absorbance spectra of the SmB phase parallel and perpendicular to the director which is not identical. This asymmetry could be caused by an intensification of higher energy electronic vibrations perpendicular to the long axis of the photoinitiator molecule. A definitive explanation requires further investigation which falls outside the scope of this study. Normalized absorbance / - 1.0 Cybotactic nematic // Cybotactic nematic _|_ Smectic A // Smectic A _|_ Smectic B // Smectic B _|_ 0.8 0.6 0.4 0.2 0.0 325 350 375 400 425 Wavelength / nm Figure 2.6 The absorbance spectra of the dichroic photoinitiator 3 in the planarly aligned photoreactive mixture. Only a partial absorbance spectrum is shown due to the absorption of host 1. An important conclusion is that all phases show polarization-selectivity due to the added photoinitiator. A dichroic ratio of 4.7, 9.5 and 32 at 355 nm is calculated (Equation 1.6) for the Nc, SmA and SmB phase, respectively. This wavelength was chosen for reference as it corresponds to the wavelength of later patterning experiments. The numbers correspond to order parameters of 0.55, 0.74 and 0.91 40 Chapter 2 for the respective phases (Equation 1.7). As with WAXS, the higher DR and S values calculated based on absorbance are also prone to errors due to baseline corrections. The DR of the photo-reactive mixture calculated based on the absorbance spectra is lower than the value calculated based on the WAXS data (compare DRabs and Sabs with Table 2.1). The lower values indicate an additional tilt angle. The average tilt of the molecules calculated according to Equation 1.5 based on the WAXS and absorbance data are compared in Table 2.3. Table 2.3 The average tilt angle of the molecules relative to the director calculated based on the WAXS and absorbance data. Nc SmA SmB WAXS 26.5° 20.1° 11.5° Absorbance 33.0° 24.6° 14.0° The difference can be caused by a misalignment between the dichroic initiator and the host molecules, i.e. the angle with the director for the host (θH) and dichroic photoinitiator (θi) are not equal (Figure 2.7). Based on the lower dichroic ratio via the absorbance measurements θi is larger than θH. Another explanation for the lower DR and S is an angle α between the transition moment and long axis of the dichroic photoinitiator as shown in Figure 2.7b. A combination of these two factors could also explain the deviation between the WAXS and absorbance measurements. Due to the slight asymmetry of the absorbance spectra shown in Figure 2.6, the angular deviation is likely caused by an angle α of the transition moment arising due to the intensification of higher energy electronic vibrations perpendicular to the long axis of the photoinitiator molecule. The angular deviation is 6.5°, 4.5° and 2.5° for the Nc, SmA and SmB phase, respectively. The difference in angles in the different LC phases is possibly due to minor changes of the electronic transition moment or minor changes in the molecular conformation affected by the molecular packing. Despite the small deviation, an important conclusion drawn from these results is that the host largely determines the alignment of the dichroic photoinitiator and that high polarization-contrast ratios are possible. 41 The smectic B photo-reactive material   n mi   n mH θH (a)  t θi α (b) Figure 2.7 The tilt angle θ between the director 𝑛⃑ and the long axis 𝑚 ⃑⃑ of the molecule where the subscript H and i is a) the host molecule and b) dichroic initiator, respectively. The angle α represents the angle between the long axis of the dichroic photoinitiator and the transition moment 𝑡 of the initiator. 2.4 Conclusion In this chapter we have shown that a mono-acrylate smectic B liquid crystalline host can be used as the base monomer for a polarization-selective lithographic material. The addition of a diacrylate functionalized crosslinker, dichroic photoinitiator and inhibitor slightly affects the transition temperatures, but does not change the type of liquid crystalline phases. Upon cooling from the isotropic phase, the cybotactic nematic, smectic A and smectic B phases appear in this order. At room temperature there is a stable supercooled state where the smectic B phase exists despite the fact that this is below the crystal melting temperature. This supercooled state enables processing of the photo-reactive mixture at room temperature. A comparison of the dichroic ratio and the order parameter obtained through x-ray and absorbance data show that the liquid crystalline host determines the alignment of the dichroic photoinitiator. However the values determined with the absorbance spectra are lower compared to the values calculated with the x-ray data. This deviation is likely attributed to an additional tilt angle between the long axis of the dichroic initiator molecule and the transition moment. The absorbance spectra show that highest polarization-selectivity can be achieved in the smectic B phase which exhibits a dichroic ratio of 32 that corresponds to an order parameter of the photoinitiator of 0.91. Therefore, this phase should be used to develop highly polarization-selective photopolymerization mixtures and are ultimately suited as the basis for new classes of lithographic materials. 42 Chapter 2 References 1. D. J. Broer, G. N. Mol, J. A. M. M. v. Haaren and J. Lub, Adv. Mater., 1999, 11, 573-578. 2. D. J. Broer, Curr. Opin. Solid State Mater. Sci., 2002, 6, 553-561. 3. D. J. Broer, G. N. Mol, J. van Haaren, J. Lub and N. Huck, Abstr. Pap. Am. Chem. Soc., 2002, 224, U404-U404. 4. B. Serrano-Ramon, C. Kjellander, S. Zakerhamidi, C. W. M. Bastiaansen and D. J. Broer, in Emerging Liquid Crystal Technologies III, Spie-Int Soc Optical Engineering, Bellingham, 2008, pp. 1-13. 5. M. P. Stevens, Polymer chemistry: an introduction, Oxford university press, Oxfrd, New York, 1999. 6. G. Odian, Principles of polymerization, Wiley-interscience, Hoboken, New Jersey, 2004. 7. E. Peeters, J. Lub, J. A. M. Steenbakkers and D. J. Broer, Adv. Mater., 2006, 18, 2412-2417. 8. D. J. Broer, in Radiation curing in polymer science and technology - volume III_Polymer mechanisms, eds. J. P. Fouassier and J. F. Rabek, Elsevier applied science, London, 1993. 9. K. Binnemans, R. Van Deun, B. Thijs, I. Vanwelkenhuysen and I. Geuens, Chem. Mat., 2004, 16, 2021-2027. 10. D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and W. Vill, Volume 1: Fundamentals, Wiley-VCH verlag GmbH, Weinheim, 1998. 11. S. Kawata, H. B. Sun, T. Tanaka and K. Takada, Nature, 2001, 412, 697-698. 12. T. F. Scott, B. A. Kowalski, A. C. Sullivan, C. N. Bowman and R. R. McLeod, Science, 2009, 324, 913-917. 13. Y. Cao, Z. Gan, B. Jia, R. A. Evans and M. Gu, Opt. Express, 2011, 19, 19486-19494. 14. I. Dierking, Textures of liquid crystals, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003. 43 The smectic B photo-reactive material 15. G. R. Mitchell and R. Lovell, Acta Crystallogr. Sect. A, 1981, 37, 189-196. 16. A. J. Leadbetter, J. C. Frost and M. A. Mazid, J. Physique Lett., 1979, 40, L325-L329. 17. R. M. Richardson, A. J. Leadbetter and J. C. Frost, Ann. Phys., 1978, 3, 177186. 44 Appendix A1 Appendix A1. q-plots of monomer 1 and the photo-reactive mixture 1.0 1.0 0.6 SmB SmA Nc 0.8 SmB SmA Nc Intensity / a.u. Intensity / a.u. 0.8 0.4 0.2 0.6 0.4 0.2 0.0 0.1 0.2 0.3 0.4 0.0 0.5 0.5 1.0 -1 q / Å-1 (a) (b) 1.0 2.0 2.5 1.0 0.8 0.8 SmB, polymerized SmB SmA Nc 0.6 Intensity / a.u. Intensity / a.u. 1.5 q/Å 0.4 0.2 SmB, polymerized SmB SmA Nc 0.6 0.4 0.2 0.0 0.1 0.2 0.3 0.4 0.0 0.5 0.5 -1 1.0 1.5 q/Å q / Å-1 (c) (d) 2.0 2.5 Figure A.1 q-plots of the a), c) SAXS signal corresponding to the periodicity of the smectic layers and the b), d) WAXS signal corresponding to the intermolecular distance of a), b) monomer 1 and c), d) the photo-reactive mixture. 45 46 Appendix A2 Appendix A2. Diffraction patterns of the photo-reactive mixture without crosslinker Nc SmA 35.3 Å 4.65 Å dL di SmB 37.8 Å 4.59 Å 40.7 Å 4.49 Å Figure A.2 Diffraction patterns of the liquid crystalline phases of the photo-reactive mixture without crosslinker; and the values for the periodicity of the smectic layers dL and intermolecular distances di. 1.0 1.0 0.8 SmB, polymerized SmB SmA Nc 0.6 Intensity / a.u. Intensity / a.u. 0.8 0.4 0.2 SmB, polymerized SmB SmA Nc 0.6 0.4 0.2 0.0 0.1 0.2 0.3 0.4 0.0 0.5 0.5 -1 1.0 1.5 q/Å q / Å-1 (a) (b) 2.0 2.5 Figure A.3 q-plots of the SAXS signal corresponding to the periodicity of the smectic layers and the WAXS signal corresponding to the intermolecular distance of the photo-reactive mixture without crosslinker. 47 Diffraction patterns of the photo-reactive mixture without crosslinker Table A.1 The order parameter (S) and dichroic ratio (DR) for the different LC phases calculated based on the WAXS data. Nc SmA SmB Photo-reactive mixture without crosslinker S DR 0.73 9.2 0.76 11 0.94 48 48 Chapter 3 3 Polarization-selective photopolymerization* * This chapter is partially reproduced from: M.P. Van, C.C.L. Schuurmans, C.W.M. Bastiaansen, D.J. Broer, Polarization-selective polymerization in a photocrosslinking monomer film, RSC Advances, 2014, 4, 62499 49 Polarization-selective photopolymerization 3.1 Introduction As introduced in Chapter 1, the objective is to develop lithographic materials which are polarization-selective. Specifically, here polarization-selectivity is a property referring to the formation of polymeric structures through a photopolymerization process which is initiated by one polarization direction of light while remaining relatively unaffected by other orthogonal polarizations. Chapter 2 showed the development of a liquid crystalline monomer mixture which is anticipated to exhibit this property. Especially a monomer mixture with a smectic B phase exhibits a high degree of order capable to orient a dissolved dichroic photoinitiator. This uniaxially aligned smectic B monomer host with photoinitiator exhibits a dichroic ratio (DR) of 32. This ratio means that the amount of light intensity absorbed by the initiator along the alignment direction (director) is 32 times higher compared to polarizations orthogonal to this direction. The physical reason behind the high DR is a well-defined transition moment of the initiator closely along the long molecular axis (Figure 3.1a). When the dichroic photoinitiator absorbs light, it decomposes into free radical species. A DR of 32 for this dichroic photoinitiator results in an amount of free-radicals 32 times higher for polarizations parallel to the director, assuming that the dissociation efficiency is independent of the absorbed intensity. The generated free radicals initiate the polymerization process, which proceeds by a chain addition process of the acrylate monomers to form a polymer. Radical polymerization of acrylates is a process that occurs in three steps, namely initiation, propagation and termination.1, 2 In the first step the initiator decomposes into radical species through the absorption of light (Figure 3.1a). This radical species reacts with the C=C acrylate bond of the monomer to form a radical monomer that reacts with other monomers during the propagation to form a polymer (Figure 3.1b). 50 Chapter 3 (a) (b) Figure 3.1 a) The dichroic photoinitiator absorbs mainly polarization states parallel to the transition moment. Upon absorption the initiator forms free radicals. b) The initiator free radicals I· initiate the polymerization of the acrylate-functionalized LC monomers. In the final step, the polymerization is terminated by the loss of the reactive radical species through mainly coupling and disproportionation (Figure 3.2). In the former process, two reactive species combine to form a single unreactive species while in the latter case two reactive chains react to form a saturated and an unsaturated fragment. Radical polymerization is a process that is prone to oxygen inhibition in which the polymerization process is stopped. For this reason, the polymerization is generally performed in a nitrogen environment. (a) (b) Figure 3.2 Termination through a) coupling and b) disproportionation. R represents the remainder of the polymer fragment. For radical polymerization, Equation 3.11, 3 describes the propagation rate or polymerization rate R. kp and kt are the propagation and termination constants, respectively. Mc, η and A are the monomer concentration, quantum yield for initiation and intensity of absorbed light by the initiator, respectively. Equation 3.1 is derived by making several assumptions. Firstly, the termination and propagation 51 Polarization-selective photopolymerization constants are independent of the size of the radical, i.e. the reactivity of the propagating radicals remains unaffected as the conversion increases. This assumption is only valid in the initial polymerization stage before gelation of the liquid crystal mixture occurs. Secondly, the decrease of the monomer concentration is attributed to the propagation only. The fraction of monomers that is consumed during the initiation process is neglected. Thirdly, a steady-state condition is assumed where the rate at which radicals form (initiation) is equal to the rate at which the radicals are consumed (termination). And fourth, the termination process is assumed to be a bi-molecular process as described in the processes above. 1/ 2  A  R  k p M c    kt  Equation 3.1 R//  R Equation 3.2 A//  DR A The polarization direction only affects the intensity of absorbed light A and, therefore, the polymerization scales with the square-root of A. This result has been confirmed for acrylate based materials in the nematic phase.4, 5 Equation 3.2 shows the relation between the dichroic ratio (DR) and the polymerization contrast, which is the ratio between the polymerization rate with the polarization of the illumination parallel (R//) and perpendicular (R⊥) to the director. For the nematic phase a typical value for the DR is 7 which corresponds to a polymerization contrast of 2.6. This result was shown previously for acrylic nematic materials where a different host and dichroic photoinitiator was used compared to what is used in this study. In order to increase the polymerization contrast high DR values are needed which can be established by selecting the smectic B phase for the monomeric mixture. The polymerization leads to the formation of a polymer network that is insoluble in the subsequent development step, i.e. a film layer remains after development. The thickness of the remaining layer normalized with the initial film thickness is referred to as the response of the photo-reactive material. The thickness of the film depends on the dose of exposure. The dose is the product of exposure time and the intensity of the illumination source. A model curve is shown in Figure 3.3.6 Ideally, the sensitivity of the photo-reactive material is a step function as shown in Figure 3.3 by the black curve (high contrast). Below a threshold no film remains while above the 52 Chapter 3 threshold the full film remains. In reality, this transition is more gradual as shown by the red curve in Figure 3.3. 100 Response / % High contrast Low contrast 50 0 10 100 Dose / mJ cm-2 Figure 3.3 A schematic picture for a low and high contrast photoresist. The response is defined as the relative thickness of the photo-reactive film remaining after development compared to the initial thickness of the film. In this chapter, it is shown that the increased dichroic ratio of the smectic B phase, compared to the nematic phase as mentioned above, also results in an increased polymerization contrast. The effect of the presence of an inhibitor on the polarization-selectivity is also characterized as well as the dose-response behavior of the photo-reactive material. 3.2 Experimental The composition of the liquid crystalline photo-reactive material is described in Section 2.2. To examine whether the SmB phase is maintained upon polymerization, the planarly aligned photo-reactive material was analyzed with small-angle and wideangle x-ray scattering (SAXS and WAXS) as described in Section 2.2. Polymerization was performed by applying a flood exposure (EXFO Omnicure Series 2000 mercury lamp, Figure 3.4). Absorbance spectra were recorded as described in Section 2.2. 53 Polarization-selective photopolymerization Absorbance photoinitiator Emission UV-source Emission, absorbance / a.u. 1.0 0.8 0.6 0.4 0.2 0.0 325 350 375 400 425 Wavelength / nm Figure 3.4 The absorbance spectrum of the dichroic photoinitiator dissolved in the planarly aligned smectic B host measured at room temperature. The absorbance spectrum was measured parallel to the molecular director. The graph also shows the emission spectrum of the UV illumination source. The conversion of the C=C acrylate bonds was monitored using Fourier Transform Infrared spectroscopy (FTIR) on a Varian 610-IR spectrometer that was equipped with a microscope; the spectra were recorded in reflection mode. For this purpose thin 2-4 µm films were prepared in a cell. In order to be able to measure in reflection mode a cell was made comprising of a glass plate coated with gold and polyimide (JSR Micro AL-2410) rubbed with a cloth as described in Section 2.2, and a CaF2 infrared transparent window also coated with rubbed polyimide. A wire grid polarizer (PPL05A Moxtek) was applied to form linearly polarized light from the emission of the EXFO mercury lamp (2.1 mW cm-2). No wavelength bandpass filters were applied. The measurements of the photo-reactive material in a cell were done at room temperature in both N2 atmosphere and in air. The degree of conversion is defined as the percentage of moles of C=C acrylate bonds (1408 cm-1) converted. All spectra were normalized (height) with the aromatic C=C signal (1511 cm-1) as shown in Figure 3.5. 54 Chapter 3 Acrylic C=C 1408 cm-1 t0 1.0 Intensity / a.u. Aromatic C=C 1511 cm-1 0.5 tend 0.0 1400 1440 1480 1520 Wavenumber / cm-1 Figure 3.5 An example of the infrared spectra used for calculating the C=C acrylate bond conversion as a function of time. The intensity of the signal for the C=C acrylate bond decreases upon exposure with time t. Dose-response curves were obtained by illuminating the photo-reactive film with different exposure times at room temperature. The photo-reactive material was aligned uniaxially in a cell where one glass substrate was coated with polyimide (JSR Micro AL-24101) and the other glass substrate was coated with polyvinyl alcohol (87-89% hydrolyzed, Sigma-Aldrich). Two different alignment layers were used to have the polymer layer remaining on a single substrate after opening the cell. The polymerized film adheres better to polyvinyl alcohol (PVA) than PI. A 5 wt% aqueous PVA solution was spin coated on the glass at 2000 rpm for 30 s with an acceleration of 500 rpm s-1. Subsequently, the substrate was baked for 30 min at 90 °C. Both alignment layers were rubbed with a cloth after which the cell was fabricated with controlled thickness through the use of glass spacers. Before filling the cell, the exact cell gap was measured by wavelength-dependent interference7 using a spectrophotometer (Shimadzu UV-3102). After filling the cell at 140 °C through capillary action, the photo-reactive material was exposed with linearly polarized light, through the use of a wire grid polarizer (PPL05A Moxtek), with a specific dose and subsequently opened. The material was developed in acetone for 5-10 s at room temperature. The remaining film thickness was measured using a 3D interferometer (Fogal Nanotech Zoomsurf). The response R is defined as the remaining film thickness d normalized with the cell gap d0 as shown in Equation 3.3. The response is given as a function of the dose D (mJ cm-2) which is the product of 55 Polarization-selective photopolymerization the intensity I of the UV-illumination (2.1 mW cm-2) and the time t of exposure (Equation 3.4). d d0 Equation 3.3 D  I t Equation 3.4 R 3.3 3.3.1 Results and discussion Polymerization of an ordered smectic B host Although the initial state of the molecular order is most relevant for the polarization-selectivity it is of interest to what extent this order is maintained after polymerization. A loss of order might indicate that the polarization contrast might decrease during the course of polymerization. X-ray characterization was performed to compare the initial state of the monomer with that of a fully cured polymer network. Figure 3.6 shows the diffraction patterns of planarly aligned photo-reactive material before and after polymerization within the SmB phase. The corresponding values for the periodicity of the smectic layers dL and intermolecular distance di can also be found in Figure 3.6. The intermolecular distance before and after polymerization remains constant at 4.47 Å. This is remarkable as free radical polymerization of acrylates is often accompanied with polymerization shrinkage upon conversion of the Van der Waals interactions to covalent bonds.8-10 This study shows that if shrinkage occurs, this does not manifest in the plane of the smectic layers, most probably related to the dense packing of the liquid crystals within these layers. In contrast, after polymerization, the dL value decreases from 39.7 Å to 37.8 Å (5 % decrease). Apparently, the shrinkage predominantly occurs in the direction of the director. Additionally, another SAXS signal appears close to the beamstop and the corresponding higher diffraction order signal appears at 23.4 Å (Figure 3.6 and Figure 3.7). Assuming the signal at the beamstop is the first diffraction order than the signal at 23.4 Å corresponds to the third order which means dL is 70.2 Å. The second order diffraction signal is not well visible due to the overlap with the 37.8 Å signal. It is anticipated that this signal relates to the local formation of double layers where the nonpolar hexyl endgroups of LC monomer 1, the monomer which is most prominently present in the monomeric mixture, tend to agglomerate over longer distances. The formation of these double layers seems to be induced by the polymerization. Due to interdigitation of the alkyl tails the double layer can become 56 Chapter 3 smaller than the double distance of a single layer which is a phenomenon that has previously been observed for liquid crystalline materials.11-14 Importantly, based on the diffraction pattern, one can conclude that the molecular order typical for the SmB phase is maintained upon polymerization. This conclusion is drawn based on the appearance of several diffraction orders of the dL signal which indicates a highly ordered phase. An additional motivation to draw this conclusion is the defined di signal which indicates defined packing of the molecules due to the hexagonal packing in the SmB phase.15-17 Before polymerization After polymerization dL di dL di 39.7 Å 4.47 Å 70.2 Å and 37.8 Å 4.47 Å Figure 3.6 The diffraction pattern of the photo-reactive material in the SmB phase before and after polymerization and the corresponding dL and di values. Despite the fact that the type of phase is maintained during the polymerization reaction, the order parameter decreases from 0.94 to 0.91 (calculated based on Equation 2.1 from the broadening of the di signal). The lower value relates to a larger average tilt of the molecules with respect to the director. The reactive groups conform to participate in the polymerization process. Therefore, the larger tilt is attributed to the disruption of the order of the rigid LC core upon forming a polymer network.18 Assuming a similar loss of order of the photoinitiator it can be calculated that the dichroic ratio decreases from 48 to 31 (Equation 1.7). 57 Polarization-selective photopolymerization 1 SmB, polymerized SmB Intensity / a.u. 0.1 3rd order 23.4 Å 0.01 1E-3 1E-4 1E-5 0.1 1 q/Å -1 Figure 3.7 The q-plot of the diffraction pattern of the photo-reactive material. To verify the decrease in dichroic ratio during polymerization a reference measurement was performed. The dichroic ratio can be measured by polarizing UVvis spectroscopy as was shown in Chapter 2. To measure the dichroic ratio after polymerization a problem had to be overcome which is the change of absorbance of the initiator due to UV initiated decomposition. In the reference experiment this could be prevented by adding a second photoinitiator (1 wt% of Irgacure 819, Ciba Specialty Chemicals) in the photo-reactive mixture. This particular initiator has an absorption band which is located at higher wavelengths compared to the absorption band of the dichroic photoinitiator. Polymerization was performed by applying a flood exposure with a 470 nm bandpass filter to ensure absorption by Irgacure 819 alone, which is necessary for polymerization, while leaving the dichroic photoinitiator unaffected. The absorbance spectra are measured before and after polymerization (Figure 3.8). The DR at 355 nm decreases from 32 to 18 after polymerization which is a result of the loss of order upon polymerization as determined from the aforementioned x-ray data. The absorbance maxima for parallel and orthogonal illumination are not at the same wavelength as observed previously in Section 2.3.2. As discussed this observation could possibly be caused by an intensification of higher energy electronic vibrations perpendicular to the long axis of the photoinitiator molecule. 58 Chapter 3 // Before polymerization _|_ Before polymerization // After polymerization _|_ After polymerization Normalized absorbance / - 1.0 0.8 0.6 0.4 0.2 0.0 325 350 375 400 425 Wavelength / nm Figure 3.8 The absorbance spectra before and after polymerization performed in the smectic B phase. The lower DR means that the polarization-selectivity becomes less during the polymerization which is undesirable. This is an important observation with implications for the strategy to obtain highest contrast possible. Photo-exposure preferably should not proceed until the end-conversion but already stopped in an early phase of the reaction to maintain the high polarization-selectivity. The conversion can be increased by applying a post-exposure bake (PEB) to form a polymer network that can withstand the subsequent development procedure. The PEB is a commonly applied step in lithographic processes to increase the contrast between exposed and unexposed areas.19, 20 The kinetics studied in the following section as well as the measurements of dose-response behavior in the section thereafter will reveal the necessary information on the optimum dose where the highest possible contrast can be obtained. 3.3.2 Polarization-dependent polymerization kinetics The dichroic photoinitiator exhibits the highest dichroic ratio of 32 in the SmB phase. This ratio should translate to polarization-selective polymerization. Based on Equation 3.2 a polymerization contrast of 5.7 is expected. The C=C acrylate bond conversion was measured as a function of time of illumination for uniaxially planarly aligned photo-reactive films. The curves measured in N2 atmosphere and in air are shown in Figure 3.9. Equation 3.2 is only valid in the initial polymerization stage which is shown in Figure 3.9b. The initial polymerization rate is defined as the 59 Polarization-selective photopolymerization 100 100 80 80 Conversion / mol% Conversion / mol% tangent of the conversion-time curve at the onset of polymerization directly after the inhibition period. A clear difference in polymerization rate between the parallel and perpendicular configuration is observed in both nitrogen atmosphere and in air. The contrast in polymerization rate can be assigned to the difference in absorption by the dichroic photoinitiator for the two illumination configurations. In nitrogen atmosphere, the observed inhibition period is assumed to be caused by the presence of inhibitor 4 through the reaction shown in Figure 3.10a. This reaction produces a radical species with very low reactivity.1, 21 The inhibitor molecules largely inhibit polymerization until they are all consumed by the first free initiator radicals formed. After the inhibition period, the polymerization rates of the reaction are 12.1 mol% s-1 for parallel illumination and 2.3 mol% s-1 for perpendicular illumination, which results in a polymerization contrast of 5.3, which roughly corresponds to the expected value of 5.7 based on Equation 3.2. This value for the SmB phase is considerably higher compared to the kinetic contrast data for the nematic phase which is 2.6.4, 5 This confirms the assumption that the enhancement of the uniaxial alignment of the dichroic photoinitiator in the SmB host, results in a two-fold increase of the polymerization contrast and, therefore, the polarization-selectivity is improved. 60 N2 // 40 N2 _|_ 20 Air // Air _|_ 0 N2 // N2 _|_ Air // Air _|_ 60 40 20 0 0 1000 2000 3000 4000 5000 0 Time / s 20 40 60 80 Time / s (a) (b) Figure 3.9 a) Conversion as function of time and b) a magnification of the initial polymerization stage. The curves were measured at room temperature where the photo-reactive material is in the smectic B phase. For polymerization performed in air, the rates at the onset of the reaction are 3.8 mol% s-1 and 0.5 mol% s-1 for parallel and perpendicular illumination, respectively. This leads to a polymerization contrast of 7.6. The lower polymerization rates are explained by an enhanced inhibition due to the synergy between inhibitor 4 and the presence of oxygen. Oxygen itself is known to be an 60 Chapter 3 effective inhibitor by forming a peroxy radical 1, 3, 22 as shown in Figure 3.10b. This particular radical is relatively unreactive and reacts mainly in termination reactions. Oxygen is also known to increase the activity of phenol inhibitors.1, 23 The synergistic effect can be explained by the relatively fast reactions of the propagating radical with oxygen to form the peroxy radical (Figure 3.10b) and the subsequent reaction of the peroxy radical with inhibitor 4 (Figure 3.10c) compared to the slower reaction between inhibitor 4 and the propagating radical (Figure 3.10a). Slow (a) Fast (b) Fast (c) Figure 3.10 a) The relatively slow inhibition reaction of the acrylate monomer with inhibitor 4. The relatively fast processes of b) the inhibition reaction of the propagating monomer with oxygen and c) the subsequent reaction with inhibitor 4. Due to the presence of inhibiting species, an inhibition period appears where no polymerization occurs. Performing the polymerization in air prolongs this inhibition period due to the presence of oxygen. Normally oxygen inhibition is seen as a disadvantage of photo-curable coatings. However, here inhibition might have a positive effect on the contrast in pattern formation as is also observed for other patterning materials.24-26 The length of the inhibition period, both in the presence of inhibitor in both N2 atmosphere and air, clearly depends on the state of polarization of the UV illumination (Figure 3.9). Under nitrogen at the end of the perpendicular inhibition period, a condition can be reached where the conversion for parallel illumination is 40 mol% while the conversion for perpendicular illumination is below the sensitivity of our measuring set-up and apparently close to 0 mol%. In the presence of oxygen the conversion at the end of the inhibition period is 50 mol% and 0 mol%, respectively, i.e. the contrast under these conditions is practically infinite. The difference in conversion can be tuned by controlling the amount of inhibitor and/or the controlled addition of oxygen. It should be noted that within a cell it is difficult to control the amount of oxygen dissolved in the LC monomer mixture which has its effect on the reproducibility of the results. Control over the inhibition is more convenient when the photo-reactive material is planarly aligned 61 Polarization-selective photopolymerization on a single substrate. Attempts to achieve planar alignment were unsuccessful during this study due to the very strong tendency of smectic layers to align perpendicularly to free surface driven by the low surface tension of the endgroups. Figure 3.9 shows that illumination in nitrogen with the applied intensity of the UV source preferably should not exceed 15 s to maintain high polarization-selectivity of the photo-reactive material. A post-exposure bake (PEB) is applied to increase the difference in conversion between areas where the polarization is parallel to the director and areas where the polarization is perpendicular to the director. For lithographic purposes, the increased contrast will favor the formation of welldefined patterns. In Figure 3.11 the conversion is shown for 15 s illumination with polarization parallel and perpendicular to the director. The conversion is 22 mol% and 0 mol% for parallel and perpendicular illumination, respectively. 22 mol% is lower than the 40 mol% shown in Figure 3.9b. This difference is attributed to the amount of dissolved oxygen in the monomeric mixture in a cell which is difficult to control as mentioned above. By applying a post-exposure bake for 60 s at 80 °C the conversion is increased to 35 mol% for parallel exposure while for orthogonal illumination it remains 0 mol%, which means the conversion difference of the photo-reactive film is increased even further by applying a PEB. As a control experiment, a continuous flood exposure was applied over a longer period; and in both cases an end-conversion of about 80 mol% is reached. These results show that a PEB can be applied to enhance the conversion difference between parallel and orthogonal illumination. Conversion / mol% 1.0 // _|_ 0.8 0.6 0.4 0.2 0.0 15 s 15 s + PEB Flood exposure Figure 3.11 Increasing the conversion by applying a post-exposure bake (PEB) at 80 °C for 60 s after illuminating for 15 s. The illumination and PEB were performed in nitrogen atmosphere. 62 Chapter 3 3.3.3 Dose-response behavior In the previous section it was shown that the polymerization rate depends on the direction of the polarization. For lithographic purposes this polymerization should translate to a contrast in film thickness after solvent development using the induced change in solubility between illuminations performed parallel and perpendicular to the director. A dose-response curve provides information on the relative thickness of the polymerized pattern that is remaining after the development step as a function of the UV exposure dose. In other words, it demonstrates the sensitivity difference of the photo-reactive material towards the polarization of the illumination. Figure 3.12 shows the dose-response curve for two polarization directions of light with respect to the director of the photo-reactive material. The response in this case is the remaining film thickness after development in acetone divided by the initial thickness. These curves are obtained in nitrogen atmosphere. Based on the kinetic data, the same trend is expected when the polymerization is performed in air. Up to a dose of 30 mJ cm-2, films could be measured up to a value of 60% of the initial thickness, but only when the film was exposed with the polarization of the UV illumination parallel to the director. Under similar conditions, the substrate remains clean when exposed with orthogonal polarization. This is a favorable condition for lithographic purposes for defined pattern formation. This result shows that the illumination dose should not exceed 30 mJ cm-2 to maintain the high polarization-selectivity of the material. 0.8 1.0 0.6 Response / - Response / - 0.8 0.6 0.4 0.2 // _|_ 0.0 0.4 0.2 // _|_ 0.0 0 200 400 600 0 Dose / mJ cm-2 40 80 120 160 Dose / mJ cm-2 (a) (b) Figure 3.12 a) The full dose-response curve and b) the magnified initial stage. Illumination was performed in nitrogen atmosphere. 63 Polarization-selective photopolymerization 3.3.3.1 Observations during characterization of the dose-response behavior The dose-response behavior was obtained by applying a homogenous exposure to the photo-reactive material in a cell with linearly polarized light parallel or perpendicular to the director. Subsequently, the cell was opened for the development with acetone. However, upon opening the cells the film often formed cracks orthogonal to the alignment direction. In Figure 3.13 a few examples are shown of the developed film exposed parallel to the director with different illumination times. The same effect is observed for perpendicular illumination. The cracks only appear when the illumination time is short (Figure 3.13). The photoreactive film consists of a mixture of monoacrylate and di-acrylate crosslinker in a 20/1 weight ratio. With short illumination times the degree of conversion of acrylate bonds is low, therefore, no full polymer network is formed. The directionality of the cracks is caused by organization of the molecules in layers in the smectic B phase. Results from the x-ray data (Section 3.3.1) seem to be relevant for this observation: 1. The polymerization shrinkage manifests itself mainly in the direction along the director. This shrinkage induces stress in the polymer film perpendicular to the smectic layers. 2. The smectic organization tend to form spatially distributed double layers after polymerization where the organization of the non-reactive aliphatic endgroups locally reduce the concentration of polymerizable endgroups and, thus, inducing areas between the smectic layers. The combination of these two effects could be the reason that the film tend to break orthogonal to the molecular director. It should be noted that the thickness of the smectic layers is in the nanometer range while the cracks observed with POM occurs on the micrometer scale. Based on this information, the location of the cracks is most likely determined by statistics. Once a crack is formed the stresses release in its direct vicinity. These cracks prevent correct pattern formation. Therefore, this effect is undesirable and should be prevented by achieving a higher conversion in the exposed regions through the application of a post-exposure bake (Section 3.3.2). 64 Chapter 3 200 µm 6s 200 µm 15 s 200 µm 50 s Figure 3.13 Optical microscopy images between crossed polarizers (red arrows) of the developed photo-reactive material illuminated with different exposure times. The illumination was performed with the polarization parallel to the director (white arrow). 3.4 Conclusion In this chapter it is shown that the dichroic ratio of the SmB photo-reactive material decreases from 32 to 18 after polymerization. It is anticipated that this loss of polarization-selectivity is caused by a disruption of the molecular order of the liquid crystal host material upon polymerization. This result shows that the polarization-selectivity is high in the initial stage but decreases upon polymerization, which implies that the most defined lithographic structures can be formed when the UV dose is limited to remain in the initial polymerization stage. This assumption is further confirmed by kinetic measurements where the polarization-selectivity of the radical polymerization can be related to the dichroism in absorption of the initiator through a conventional kinetic model. This model proves to be valid only in the initial polymerization stage where the polymerization contrast scales with the squareroot of the dichroic ratio of the dichroic photoinitiator. The presence of inhibiting molecules in the monomeric mixture creates a condition where the photo-reactive material is only selective for polarizations parallel to the director. For polarizations orthogonal to the director, the conversion remains zero. Based on this information, the illumination step should only be performed during this period to maintain the high polarization-selectivity of the photo-reactive material. By applying a postexposure bake the conversion contrast can be increased even further. For lithographic purposes the dose-response behavior, which shows the relative film thickness after development as function of the exposure dose, is of importance. The dose-response behavior also shows an identical trend as observed with the kinetic measurements. Below a dose of 30 mJ cm-2 a condition is present where a film remains after development for parallel illumination while no film remains for perpendicular illumination. This condition is favorable for defined patterning. 65 Polarization-selective photopolymerization References 1. G. Odian, Principles of polymerization, Wiley-interscience, Hoboken, New Jersey, 2004. 2. M. P. Stevens, Polymer chemistry: an introduction, Oxford university press, Oxfrd, New York, 1999. 3. E. Andrzejewska, Prog. polym. Sci., 2001, 26, 605-665. 4. D. J. Broer, G. N. Mol, J. A. M. M. v. Haaren and J. Lub, Adv. Mater., 1999, 11, 573-578. 5. D. J. Broer, Curr. Opin. Solid State Mater. Sci., 2002, 6, 553-561. 6. U. Okoroanyanwu, Chemistry and Lithography, SPIE and John Wiley & Sons Inc., Bellingham, Washington, 2011. 7. K. H. Yang, J. Appl. Phys., 1988, 64, 4780-4781. 8. W. K. Neo and M. B. Chan-Park, Macromol. Rapid Commun., 2005, 26, 10081013. 9. Y. Jian, Y. He, T. Jiang, C. Li, W. Yang and J. Nie, J. Polym. Sci. B, 2012, 50, 923-928. 10. D. J. Broer, G. P. Crawford and S. Zumer, in The liquid crystals book series, ed. V. percec, CRC press Taylor & Francis group, Boco Raton, 2011. 11. A. A. Craig and C. T. Imrie, Macromol., 1999, 32, 6215-6220. 12. K. Satoh, S. Mita and S. Kondo, Chem. Phys. Lett., 1996, 255, 99-104. 13. J. S. van Duijneveldt, A. Gil-Villegas, G. Jackson and M. P. Allen, J. Chem. phys., 2000, 112, 9092-9104. 14. O. Francescangeli, D. Rinaldi, M. Laus, G. Galli and B. Gallot, J. Phys. II, 1996, 6, 77-89. 15. D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and W. Vill, Volume 1: Fundamentals, Wiley-VCH verlag GmbH, Weinheim, 1998. 16. A. J. Leadbetter, J. C. Frost and M. A. Mazid, J. Physique Lett., 1979, 40, L325-L329. 66 Chapter 3 17. R. M. Richardson, A. J. Leadbetter and J. C. Frost, Ann. Phys., 1978, 3, 177186. 18. E. Peeters, J. Lub, J. A. M. Steenbakkers and D. J. Broer, Adv. Mater., 2006, 18, 2412-2417. 19. K. Suzuki and B. W. Smith, Microlithography, Taylor & Francis Group, LLC, Boca Raton, 2007. 20. H. Ito, J. Photopolym. Sci. Technol., 2008, 21, 475-491. 21. L. B. Levy, 1985, 23, 1505-1515. 22. F. R. Wight, J. Polym. Sci. Polym. Lett. Ed., 1978, 16, 121-127. 23. J. J. Kurland, Polym. Sci. Polym. Chem. Ed., 1980, 18, 1139-1145. 24. T. F. Scott, B. A. Kowalski, A. C. Sullivan, C. N. Bowman and R. R. McLeod, Science, 2009, 324, 913-917. 25. Y. Cao, Z. Gan, B. Jia, R. A. Evans and M. Gu, Opt. Express, 2011, 19, 19486-19494. 26. S. Kawata, H. B. Sun, T. Tanaka and K. Takada, Nature, 2001, 412, 697-698. 67 Chapter 4 4 Patterning of the smectic B photoreactive material with linearly polarized light 69 Patterning of the smectic B photo-reactive material with linearly polarized light 4.1 Introduction In Chapter 3, it was shown that high polarization-selectivity for polymerization can be obtained through the use of the reactive smectic B liquid crystal with a dichroic photoinitiator. The initiator adapts the order of the high-order parameter liquid crystal host which makes the mixture especially sensitive for the polarization of UV light parallel to the liquid crystal director. The presence of an inhibitor prevents polymerization at low free-radical concentration which enhances the contrast. A favorable condition for new types of lithography would be reached if the monomer mixture is only sensitive to one state of polarization. In this chapter, the focus is on utilizing the polarization-selectivity to create patterns. To demonstrate patterning polarization holography was selected. This technique has been often applied to produce polarization gratings.1-4 Holography or interference lithography is a technique to fabricate periodic patterns based on the interference of two or more coherent laser beams.5, 6 The interference results in the modulation of the intensity and/or polarization which can be recorded in a photoresist. In two beam holography the interference results in a one dimensional pattern with pitch Ʌ which depends on the half-angle ϑ between the two interfering beams and the wavelength λ (Figure 4.1). Different kinds of intensity or polarization patterns can be produced depending on the polarization and intensity of the interfering beams.6, 7 Commonly, two extreme cases of intensity and polarization holography are applied to create regular periodic patterns. In the case of intensity holography often both beams are linearly and parallel polarized with equal intensities. The interference results in an intensity pattern while the polarization remains constant (Figure 4.2a). In the case of polarization holography the two beams are orthogonally circularly polarized (leftand right-handed), also with equal intensities. A linear polarization pattern forms while the intensity remains constant as a result of the interference (Figure 4.2b). 70 Chapter 4 ˄ Photoresist  -ϑ +ϑ  2 sin  Figure 4.1 Two beam holography where two coherent beams with wavelength λ interfere at an angle of +ϑ and -ϑ to create a periodic pattern with pitch Ʌ. Different resist materials are required to record the interference pattern depending on the type of holography. The focus has been primarily on intensity holography due to the availability of photoresists developed for optical mask lithography. This type of holography requires an isotropic photoresist where chemical changes occur depending on the intensity profile. These materials have been explored extensively in the past.8-10 Polarization holography requires an anisotropic or polarization-selective material such as the material developed and characterized in previous chapters. This technique is useful to determine whether the polarization-selective material is suitable for lithographic purposes. Polarization Intensity Intensity Polarization 0 0 Position Position (a) (b) Figure 4.2 The resultant polarization and intensity profiles of a) two linearly polarized beams and b) of two orthogonally circularly polarized beams. In both cases, the intensities of both interfering beams are equal to each other. In previous chapters the polarization-selectivity of the photo-reactive material has been characterized for two extreme cases, namely polarizations perfectly parallel to the director or orthogonal to it. Polarization holography offers the opportunity to characterize the photo-reactive material with a polarization pattern that varies 71 Patterning of the smectic B photo-reactive material with linearly polarized light sinusoidally, i.e. for all angles relative to the director. For this purpose the photoreactive material needs to be aligned planarly as shown in Figure 4.3. In this chapter it is verified that the high polarization-selectivity of the photo-reactive material developed in previous chapters can be used for well-defined patterning. Figure 4.3 With polarization holography a spatially rotating polarization pattern is formed as indicated by the arrows while the intensity remains constant. When the uniaxially planarly aligned photo-reactive material with liquid crystals (grey) and dichroic photoinitiator (white) is exposed with this polarization pattern, polymerization will occur mainly in areas where the polarization is parallel to the director (marked areas). 4.2 Experimental The composition of the photo-reactive material is described in Section 2.2. Polarization holography was performed on planarly aligned photo-reactive material. Uniaxial planar alignment was achieved by using a glass cell; one glass plate was coated with rubbed polyvinyl alcohol and the other glass plate was coated with rubbed polyimide (JSR Micro AL-24101, described in Section 2.2). The procedure for preparing rubbed PVA is described in Section 3.2. The 2-4 µm thick cells were filled at 140 °C through capillary action and subsequently cooled to room temperature for illumination. After exposing the photo-reactive material a postexposure bake (PEB) was applied for 1 min at 80 °C. Subsequently, the cell was opened and developed by dipping into acetone for 5-10 s. The film was dried with a nitrogen flow. The polarization holography set-up is shown in Figure 4.4. The intensity of the laser (Quanta-Ray Nd:Yag Lab-150 355 nm pulsed laser, 10 Hz, seeded) is regulated by a half-wave plate (λ/2) and a polarizing beamsplitter (PBS). The reflected s-polarized beam by the PBS passes a shutter before it is reflected by a mirror to a beamsplitter after which the two beams pass through a quarter-wave plate (λ/4) where the s-polarized light is converted to left (LCP) or right (RCP) circularly polarized light. The sample is placed at a position where the two beams interfere and this location depends on the desired pitch determined by the equation 72 Chapter 4 in Figure 4.1. Here, a pitch of 6 µm was created with a half angle ϑ of 1.7° between the two beams. Beamstop λ/2 PBS 355 nm Nd:YAG p S Shutter λ/4 Mirror Mirror Photoresist LCP ϑ BS RCP Beamstop λ/4 Mirror Figure 4.4 The polarization holography set-up. Polarized optical microscopy (POM) and scanning electron microscopy (SEM) were used to analyze the fabricated structures. POM was performed on a Leica CTR 6000 microscope equipped with crossed polarizers. The images were recorded with a Leica DFC420 C camera. For SEM (JEOL JSM 5600) analysis a 40 nm thick gold layer was deposited on the grating. 4.3 4.3.1 Results and discussion Planar alignment of the polarization-selective material To achieve polarization-selective polymerization, the photo-reactive material needs to be aligned planarly. It is well-known that LCs exhibit birefringence due to the anisotropic shape of the molecule.11, 12 Figure 4.5 shows polarized optical microscopy (POM) images of the photo-reactive material in a cell between crossed polarizers. A dark and a bright state appears when the director is 0° or 45° rotated relative to the polarizers, respectively. At an angle of 45° the linear polarization state produced by the first polarizer changes to elliptically polarized light as the light passes through the material as a result of the birefringence of the LC molecule. The second polarizer placed orthogonal to the first polarizer only blocks part of the light and, therefore, a bright-state appears. At an angle of 0° the polarization does not change passing through the material. Therefore, the light is fully blocked by the 73 Patterning of the smectic B photo-reactive material with linearly polarized light second polarizer resulting in a dark state. This result confirms the uniaxial planar alignment of the photo-reactive material. 0° 45° (a) (b) Figure 4.5 Optical images of the uniaxially and planarly aligned photo-reactive material between crossed polarizers with the director a) 0° or b) 45° degrees rotated from one of the polarizers. The arrows indicate the alignment direction. 4.3.2 Patterning with polarization holography In this section the photo-reactive material is exposed with the polarization pattern. In Figure 4.6a, a POM image is shown of the photo-reactive material before illumination. The image is recorded between crossed polarizers where there is a small angle between the director and the polarizer/analyzer. This angle was introduced to observe the birefringence of the photo-reactive material. Before exposure, there are slight color variations, which indicate differences in retardation. This effect is likely caused by small variation in the thickness of the film. The photoreactive material was illuminated with polarization holography using different doses. After exposure an additional post-exposure bake (PEB) was applied. An example of a POM image of the photo-reactive material after illumination and the application of a PEB is shown in Figure 4.6b. Straight lines appear parallel to the director and these lines are visible due to a change of the birefringence upon polymerization.13, 14 The birefringence is the difference between the extraordinary and ordinary refractive index. The change in birefringence is caused by the decrease in order of the LC host upon polymerization. The decrease of the order parameter upon polymerization was shown in Section 3.3.1. The change of birefringence leads to changes in the optical retardation colors as observed in Figure 4.6b. The pitch of this grating is 6 µm which is equal to the pitch set by the angle between the two interfering laser beams. This result again confirms the polarization-selectivity of the LC photo-reactive material. The POM image shows more than the two colors corresponding to 74 Chapter 4 polymerized and unpolymerized areas. As the film polymerizes the birefringence changes which might affect the polarization of the illumination and, thus, possibly the inhomogeneity observed in the image. After opening the cell cracks appear perpendicular to the director as shown in Figure 4.6c and Figure 4.7 as previously observed in Section 3.3.3.1. 50 µm (a) 50 µm (b) 50 µm (c) Figure 4.6 Optical images between crossed polarizers a) before illumination and b) after illumination (120 mJ cm-2) and a post-exposure bake at 80 °C for 1 min and c) after opening the cell. The orientation of the polarizers is indicated by the red crossed arrows and the white arrow indicates the alignment direction. After the post-exposure bake, the photo-reactive material was developed with acetone to remove the unpolymerized areas. The SEM images of the resulting gratings after development are shown in Figure 4.7. The LC polymer detaches from the underlying alignment layer. This is partially a result of opening the cell and it is possibly also aggravated by the solvent used for development. Despite the detachment and the cracks, an important observation is the formation of freestanding polymer lines, which have straight walls (Figure 4.7 bottom), parallel to the director. 75 Patterning of the smectic B photo-reactive material with linearly polarized light 50µm 50µm 40 mJ cm-2 60 mJ cm-2 50µm 50µm 80 mJ cm-2 120 mJ cm-2 5µm 120 mJ cm-2, magnified Figure 4.7 SEM images of the fabricated gratings which were exposed with different dose. Each photo-reactive film received the same post-exposure bake of 1 min at 80 °C. A magnified area where the polymer lines detached from the substrate, showing the block-shape of the lines. The arrow indicates the director. A magnification of the gratings in Figure 4.7 is shown in Figure 4.8. One can see that the lines become broader with increasing exposure-dose. This result can be explained by increasing absorption of polarizations that have an increasing angle with the director when a higher dose is applied. Additionally, one can see that the space between the formed polymer lines becomes cleaner with higher dose. A possible explanation is that at lower conversion the network is not strong enough to fully withstand the development procedure where the material is swollen in acetone. Since, the network is too weak, this leads to failure of the network resulting in polymeric residue fragments appearing between the lines. This explanation also supports the observation of more porous polymer lines fabricated at low exposure dose (Figure 4.8a and b). Another observation is that after opening of the cell fewer cracks are formed when higher exposure dose are applied. A higher dose results in a 76 Chapter 4 higher conversion and, thus, a stronger polymer network which can withstand the development procedure. 20 µm 20 µm 40 mJ cm-2 60 mJ cm-2 20 µm 20 µm 80 mJ cm-2 120 mJ cm-2 Figure 4.8 SEM images of the fabricated gratings which were exposed with different dose. Every photo-reactive film received the same post-exposure bake of 1 min at 80 °C. The arrow indicates the director. The SEM images in Figure 4.8 show that polarization-selectivity of photopolymerization can be utilized for patterning. Block-shaped lines (Figure 4.7 bottom) are created due to the nonlinear response of the photo-reactive material despite the sinusoidal polarization pattern as the illumination input (Figure 4.3). 4.4 Conclusion The high polarization-selectivity of the smectic B photo-reactive material characterized in Chapter 3 is utilized for structure formation with polarization holography. The required planar alignment of the photo-reactive material can be achieved through the application of alignment layers. After illuminating the photoreactive material, lines parallel to the director are observed under crossed polarizers due to a change of the birefringence upon polymerization. The pitch of the lines is 6 µm which corresponds to the pitch set by the angle between the two interfering 77 Patterning of the smectic B photo-reactive material with linearly polarized light laser beams. After applying a post-exposure bake, the cell is opened and the photoreactive material is subsequently developed. The polymer lines that remain are block-shaped despite the sinusoidally varying polarization pattern with which the photo-reactive material is illuminated. Well-defined gratings can be fabricated due to the nonlinear response of the photo-reactive material with respect to the dose. 78 Chapter 4 References 1. L. Nikolova, T. Todorov, M. Ivanov, F. Andruzzi, S. Hvilsted and P. S. Ramanujam, Appl. Opt., 1996, 35. 2. T. Todorov, L. Nikolova and N. Tomova, Appl. Opt., 1984, 23. 3. C. Oh and M. J. Escuti, Opt. Lett., 2008, 33, 2287-2289. 4. J. Kim, R. K. Komanduri, K. F. Lawler, D. J. Kekas and M. J. Escuti, Appl. Opt., 2012, 51, 4852-4857. 5. M. Maldovan and E. L. Thomas, Periodic materials and interference lithography for photonics, phononics and mechanics, Wiley-VCH, Weinheim, 2009. 6. G. Saxby, Practical holography, IoP, Bristol, 2004. 7. L. Nikolova and P. S. Ramanujam, Polarization holography, Cambridge University Press, Cambridge, 2009. 8. W. S. Colburn, J. Imaging Sci. Techn., 1997, 41, 443-456. 9. K. Suzuki and B. W. Smith, Microlithography, Taylor & Francis Group, LLC, Boca Raton, 2007. 10. H. Ito, J. Photopolym. Sci. Technol., 2008, 21, 475-491. 11. I. Dierking, Textures of liquid crystals, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003. 12. D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and W. Vill, Volume 1: Fundamentals, Wiley-VCH verlag GmbH, Weinheim, 1998. 13. D. J. Broer, H. Finkelmann and K. Kondo, Makromol. chem., 1988, 189, 185194. 14. D. J. Broer, G. P. Crawford and S. Zumer, in The liquid crystals book series, ed. V. percec, CRC press Taylor & Francis group, Boco Raton, 2011. 79 Chapter 5 5 Selectivity of the smectic B photoreactive material towards longitudinal polarization 81 Selectivity of the smectic B photo-reactive material towards longitudinal polarization 5.1 Introduction In Chapters 2, 3 and 4, the polarization-selectivity of the smectic B material was shown based on experiments where the molecules were aligned planarly and illuminated with the transversal linearly polarized light. However, there are situations where it is beneficial to illuminate the polarization-selective photo-crosslinking material with longitudinally polarized light. Longitudinal polarization refers to a polarization state where the electric-field vector is parallel to the main direction of propagation. As discussed in Section 1.2.2, the longitudinal spot is created by focusing radially polarized light which results in an intensity distribution of the longitudinal component which is narrower than the Airy spot obtained by focusing a linearly polarized plane wave.1-4 Outside the center of the spot obtained by focusing radially polarized light the transversal polarization component is substantial. The combination of the transversal and longitudinal component in focused radially polarized light causes the total intensity distribution of the pattern to be actually broader than that of focused linearly polarized plane wave. Therefore, a polarization-selective material is required that solely records the narrow longitudinal polarization component. To achieve this selectivity, the photo-reactive material needs to be aligned homeotropically (perpendicular to the substrate) for preferential absorption of this longitudinal component by the dichroic photoinitiator (Figure 5.1). 82 Chapter 5 Longitudinal hν Figure 5.1 The homeotropically aligned photo-reactive material consisting of a liquid crystalline smectic B host (grey) and a dichroic photoinitiator (white) that aligns with the liquid crystalline host. In order to investigate whether the homeotropically aligned photo-reactive material selectively reacts to the longitudinal polarization, illuminations are performed with s- and p-polarized light under an angle of incidence of 45° (Figure 5.2). In the case of illumination with p-polarized light, the angle introduces an electric-field component normal to the surface, i.e. parallel to the director 𝑛⃑ of the photo-reactive material. In the case of s-polarization, the electric-field is perpendicular to the incident wave vector and, hence, in this case the electric-field component parallel to the director vanishes. Therefore, the expectation is much higher sensitivity of the photo-reactive material towards the p-polarization than for s-polarization if the material is polarization-selective towards the longitudinal component. Liquid crystal p-polarization s-polarization 45° y 45° x z  n Figure 5.2 The set-up used to characterize the sensitivity of the homeotropically aligned liquid crystalline photo-reactive material towards the polarization component parallel to the alignment director 𝑛⃑ through illumination by p- and s-polarized light. The angle of incidence is 45°. In this chapter, the required alignment of the photo-reactive material is characterized for preferential absorption of the longitudinal polarization. The 83 Selectivity of the smectic B photo-reactive material towards longitudinal polarization sensitivity of the polarization-selective material is characterized through exposures under an angle of 45° with s- and p-polarized light to obtain the dose-response behavior. 5.2 Experimental The composition of the polarization-selective material is described in Section 2.2. To promote the adhesion of this material, the glass was functionalized with 3(trimethoxysilyl)propyl methacrylate (Sigma-Aldrich). This was done by dip coating the glass substrate for 5-10 s in a solution of 2 vol% of this compound dissolved in isopropanol. Subsequently the substrate was placed on a hotplate at 120 °C for 90 min. After cooling back to room temperature the substrates were rinsed with isopropanol to obtain a monolayer. These films were prepared by spin coating a 5.3 wt% solution of the monomer mixture in cyclopentanone on silanefunctionalized substrates in two steps: 100 rpm for 5 s and 3000 rpm for 60 s. For both steps the acceleration was 100 rpm s-1. A 110 nm thick layer was obtained which was measured using a 3D interferometer (Fogal Nanotech Zoomsurf). The films were polymerized through a UV flood exposure in nitrogen atmosphere to prevent crystallization during transport and characterization. The alignment of this thin film was characterized with grazing incidence x-ray diffraction (GIXD) and atomic force microscopy (AFM). GIXD was performed at the DUBBLE beam line at the ESRF in Grenoble (France).5 A Frelon CCD camera with pixel size of 46.9 x 46.9 µm was used to record the diffraction patterns. Calibration was carried out using silver behenate and alpha aluminum as standards. AFM measurements were performed using a Dimension ICON atomic force microscope (Bruker Nano Inc., Santa Barbara, CA). Silicon nitride tips (Bruker SNL-10) were used for characterization of samples. To examine the polarization-selectivity of the photo-reactive material for s- and p-polarized illuminations under oblique exposures, homeotropic films were prepared by spin coating (1000 rpm, 200 rpm/s, 30 s) of a 25 wt% solution in cyclopentanone on a silane-functionalized glass substrate. After spin coating, the 1.3 µm thick film was baked at 60 °C for 30 s to remove the solvent and subsequently placed in a nitrogen atmosphere for about 15 minutes to remove dissolved oxygen before illumination with s- or p-polarized light by either a QuantaRay Nd:Yag Lab-150 355 nm pulsed laser (10 Hz, seeded) or a 405 nm laser (Toptica, iPulse). Illumination was also performed in a nitrogen atmosphere to prevent inhibition by oxygen. After illumination a post-exposure bake was applied at 84 Chapter 5 80 °C for 60 s in nitrogen atmosphere. After cooling to room temperature the film was developed for 10 s in cyclopentanone and, finally, dried with a nitrogen air gun. 5.3 Results and discussion 5.3.1 Homeotropic alignment of the polarization-selective material The alignment of the photo-reactive film is crucial for the material to be selectively reactive to the longitudinal polarization component. The alignment of a polymerized film was characterized with grazing incidence x-ray diffraction (GIXD). In Figure 5.3 the diffraction pattern and q-plots are shown corresponding to the smectic layer spacing (dL) and the intermolecular distance (di). The orientation of the diffraction pattern strongly indicates the needed homeotropic alignment of the photo-reactive film. The strong signal in Figure 5.3b corresponds to 40.8 Å which roughly corresponds to the length the smectic B monomer host. Below this signal a weak signal closer to the beam stop arises due to the local formation of double layers as observed before in Section 3.3.1. The intermolecular distance is 4.36 Å (Figure 5.3c). The orientation of the diffraction pattern combined with the value of the layer spacing confirm the homeotropic alignment on glass substrates which are functionalized with 3-(trimethoxysilyl)propyl methacrylate. dL di 1.0 1.0 0.8 0.8 Intensity/ a.u. Intensity / a.u. (a) 0.6 0.4 0.2 0.6 0.4 0.2 0.0 0.0 1 2 3 q / nm 4 5 6 13 -1 14 15 16 q / nm-1 (b) (c) Figure 5.3 a) The diffraction pattern of a homeotropically aligned polymerized photo-reactive material spin coated on a glass substrate and the q-plots of b) the smectic layer spacing dL and c) the intermolecular distance di. 85 Selectivity of the smectic B photo-reactive material towards longitudinal polarization The AFM topography image of a polymerized film of the photo-reactive material (Figure 5.4) shows steps of about 38 Å that corresponds to the length of the elongated smectic B LC host monomer which is approximately 40 Å. This result further confirms the homeotropic alignment of the molecules. A comment should be made that AFM is a technique that probes the surface of the specimen and not the bulk. The properties can be different for the bulk. However, in combination with the x-ray data it is a strong indication that the bulk properties are similar to the properties at the sample/air interface. 4 Height / nm 3 2 1 0 1 µm 0.0 0.5 1.0 1.5 2.0 Position / m (a) (b) Figure 5.4 a) A topography image of the top surface of a homeotropically aligned polymerized smectic B photo-reactive material and b) an averaged profile image of the steps. 5.3.2 Considering effects for oblique illumination In the previous section, the required homeotropic alignment is confirmed through x-ray and AFM characterization. Prior to performing angular illuminations, the reflections of s- and p-polarized light should be taken into account when the angle of incidence is 45° (Figure 5.2). Using Fresnel equations the reflectance R, which is the ratio of the intensity of reflected light and the intensity of the incident light, of p- and s-polarized light are described by Equation 5.1 and Equation 5.2, respectively.6 θi is the angle of incidence and θt is the angle of transmission. Using Snell’s law (Equation 5.3) where ni and nt are the refractive indices of the medium of incidence and transmission, respectively, the fraction of s and p light reflected can be calculated. The photo-reactive material used here is anisotropic due to the homeotropic alignment of the liquid crystals. For this purpose an effective refractive nt,eff is used instead of nt and calculated based on Equation 5.47 where ne and no are the extraordinary and ordinary refractive index of the liquid crystal, respectively. A nt,eff of 1.6 is calculated under the assumption of a ne of 1.7 and no of 1.5. The 86 Chapter 5 material is illuminated in a nitrogen atmosphere where ni is assumed to be 1 and θi is set at 45°. The calculated reflections are 1.3 % and 12 % for p and s-polarized light, respectively. Based on this result, when the same dose is applied for both polarizations, the power of the light passing through the photo-reactive material in the case of s-polarization is 11 % lower due to the higher reflection. Therefore, the difference of the response, which is the remaining thickness after exposure and development, normalized with the initial film thickness should be larger than 11 % to be considered as polarization-selective towards the longitudinal component. RP  tan 2 ( i   t ) tan 2 ( i   t ) Equation 5.1 RS  sin 2 ( i   t ) sin 2 ( i   t ) Equation 5.2 ni sini  nt sint Equation 5.3 nt ,eff  5.3.3 ne no n cos  i  no2 sin 2  i 2 e 2 Equation 5.4 The isotropic photo-reactive material As an extra control to confirm the polarization-selectivity of the homeotropically aligned photo-reactive material with dichroic photoinitiator, also a non polarizationselective material is developed. This material which is not polarization-selective is referred to as the ‘isotropic’ photo-reactive material. For this purpose, the dichroic photoinitiator is substituted by initiator 5 (Irgacure 819, Ciba specialty chemicals) shown in Figure 5.5. The remainder of the composition the photo-reactive material and the coating procedure remain unaltered. This material was aligned planarly (parallel to the substrate) to measure the absorbance spectrum parallel and perpendicular to the director (Figure 5.5). The dichroic ratio at 355 nm and 405 nm is 1.7 and 1.4, respectively which can be considered as a minor degree of polarization-selectivity. The small difference between the two wavelengths falls within the error of the measurement. 405 nm illumination is interesting for optical disk mastering and, therefore, the polarization-selectivity at this wavelength is also studied. The reader is referred to Section 1.3.2 for additional information about the 87 Selectivity of the smectic B photo-reactive material towards longitudinal polarization dichroic ratio. This isotropic photo-reactive material was chosen as a reference instead of a commercially available isotropic resist to retain the same material properties such as the refractive indices and the chemically induced reaction upon illumination and, additionally, the processing and development of the photo-reactive material. These factors determine the process-dependent factor in Rayleigh’s equation (Section 1.1.2) and, therefore, should be kept constant for a proper comparison.8-10 0.3 Absorbance / - Parallel Perpendicular 0.2 0.1 0.0 360 400 440 Wavelength / nm Photoinitiator 5 Figure 5.5 (left) The molecular structure of the photoinitiator which is not polarization-selective that is planarly aligned in the smectic B host. (right) The absorbance spectra of this initiator measured parallel and perpendicular to the molecular director. 5.3.4 Polarization-selectivity of the homeotropically aligned photo-reactive material As shown by the kinetic measurements in Section 3.3.2, the high polarizationselectivity is only present at the onset of the polymerization. Therefore, Figure 5.6 contains the initial stage of the dose-response behavior of the homeotropically aligned photo-reactive material to show the selectivity towards s- and p-polarized light. Figure 5.6a shows the dose-response behavior of the polarization-selective material with 355 nm illumination. The response is the thickness of the remaining film layer after development with cyclopentanone normalized with the initial film thickness. For illumination with s-polarization, above a dose of 10 mJ cm-2 film formation starts to occur. This threshold is attributed to the presence of the inhibitor that prevents polymerization in the initial stage as explained in Section 3.3.2. The response for p-polarized light is about 20% higher compared to spolarized light. The higher response is a strong indication of the polarizationselectivity of the material towards the longitudinal polarization component of light. 88 Chapter 5 The observed difference between s- and p-polarization in Figure 5.6a is not solely a result of the 11% higher reflection of s-polarized light (Section 5.3.2). The higher response for p-polarized light is a result of the preferential absorption of p-polarized light by the dichroic photoinitiator and confirms the polarization-selectivity of the homeotropically aligned photo-reactive material towards the longitudinal polarization component. 0.6 p-polarization s-polarization 0.4 Response / - Response / - 0.6 0.2 0.0 0.4 p-polarization s-polarization 0.2 0.0 0 20 40 60 80 0 Dose / mJ cm-2 20 40 60 80 Dose / mJ cm-2 (a) (b) Figure 5.6 The initial stage of the dose-response curves obtained for 355 nm illumination of a) the polarization-selective material and b) the isotropic photo-reactive material. To confirm the afore-mentioned polarization-selective material and also to confirm the absence of polarization-selectivity of the isotropic photo-reactive material developed in the previous section, the same s- and p-illuminations were performed on the latter material. Again, only the initial dose-response behavior is shown in Figure 5.6b. The response for p-polarization is about 12% higher compared to the s-polarization which can be explained by the preferential reflection of the s-polarization. Based on this result the isotropic photo-reactive material, as expected, does not exhibit polarization-selectivity. Figure 5.7a shows the dose-response behavior of the polarization-selective material for 405 nm illumination which shows the same trend as for 355 nm illumination. This result again confirms the polarization-selectivity towards the longitudinal polarization component. There is a mismatch between the illumination wavelength and absorption band of the dichroic photoinitiator (Figure 5.7c). Due to this, the required dose for patterning is a factor 1000 higher compared to 355 nm illumination. However, the observation of the polarization-selectivity at both 355 nm and 405 nm confirms that the same transition moment along the long molecular axis of the initiator is responsible for the production of the initiator radical species to start the polymerization process and that the polarization-selective 89 Selectivity of the smectic B photo-reactive material towards longitudinal polarization behavior is present for a broad wavelength range. Figure 5.7b shows the doseresponse behavior of the isotropic photo-reactive material for 405 nm illumination. Only a minor difference in response is observed which can be attributed to the preferential reflection of the s-polarization as explained above. 0.6 0.5 P-polarization S-polarization Response / - Response / - 0.4 0.4 0.2 0.3 0.2 p-polarization s-polarization 0.1 0.0 0.0 0 50 100 150 3 Dose / 10 mJ cm 0 200 6 12 18 24 30 Dose / mJ cm-2 -2 (a) (b) Normalized absorbance / - 1.0 0.8 0.6 0.4 0.2 0.0 325 350 375 400 425 Wavelength / nm (c) Figure 5.7 a) The initial stage of the dose-response behavior obtained for 405 nm illumination of a) the polarization-selective material and b) the isotropic photo-reactive material. c) The absorbance of the dichroic photoinitiator measured at room temperature in the smectic B monomeric host. 5.4 Conclusions The homeotropic alignment of the polarization-selective material on a single glass substrate is confirmed through x-ray and AFM characterization. In order to confirm the polarization-selectivity of the photo-reactive material with dichroic photoinitiator, an isotropic (not polarization-selective) photo-reactive material has been developed by substituting the dichroic photoinitiator with a photoinitiator which is not polarization-selective while using the same liquid crystalline host. The isotropic photo-reactive material serves as a reference. The dichroic ratio of about 90 Chapter 5 1.6 which is calculated at 355 nm and 405 nm show that the material is not polarization-selective. The isotropy and the polarization-selectivity of the two homeotropically aligned photo-reactive materials are confirmed through 45° angular illuminations with sand p-polarized light. In the case of the isotropic material a higher response of roughly 12% is obtained for p-polarization compared to the s-polarization, but is caused by an 11% higher reflection of s-polarized light at a 45° incidence. Due to this reflection less light passes through the photo-reactive material in case of spolarization which results in a lower response. Therefore, the observed difference in response is almost fully attributed to the reflection which confirms the isotropic behavior of this material. In the case of the polarization-selective material a higher response of about 20% is obtained for p-polarized light compared to s-polarized light. The difference in response is higher than the preferential reflection of the spolarization which means that the difference is caused by the selective absorption of polarizations parallel to the director. This result confirms the selectivity of this material towards the longitudinal polarization component. 91 Selectivity of the smectic B photo-reactive material towards longitudinal polarization References 1. S. Quabis, R. Dorn, M. Eberler, O. Glöckl and G. Leuchs, Opt. Commun., 2000, 179. 2. R. Dorn, S. Quabis and G. Leuchs, Phys. Rev. Lett., 2003, 91. 3. Y. Ma and R. Wu, Opt. Rev., 2014, 21, 4-8. 4. T. Grosjean, D. Courjon and C. Bainier, Opt. Lett., 2007, 32, 976-978. 5. W. Bras, I. P. Dolbnya, D. Detollenaere, R. van Tol, M. Malfois, G. N. Greaves, A. J. Ryan and E. Heeley, J. Appl. Crystallogr., 2003, 36, 791-794. 6. E. Hecht, Optics, Addison Wesley, San Francisco, 2002. 7. T. Scharf, Polarized light in liquid crystals and polymers, John Wiley & Sons, Inc., New Jersey, 2007. 8. T. Ito and S. Okazaki, Nature, 2000, 406, 1027. 9. C. Mack, Fundamental principles of optical lithography - The science of microfabrication, John Wiley & Sons, Ltd, West Sussex, 2007. 10. G. M. Wallraff and W. D. Hinsberg, Chem. Rev., 1999, 99, 1801-1821. 92 Chapter 6 6 Development of a cholesteric filter for blocking transversal polarizations 93 Development of a cholesteric filter for blocking transversal polarizations 6.1 Introduction Previous chapters showed the development of a polarization-selective liquid crystalline material. In planar or homeotropic alignment this material is designed to be either responsive to the transversal or longitudinal polarization component of light, respectively. The concept is based on the direct absorption of the desired polarization by the polarization-selective material. Although this principle is new and promising for future lithography it also has a number of challenges that need to be resolved. For instance, the polarization-selective material is still in its liquid state and, thus, sensitive to dust uptake. Also, so far the principle has been demonstrated on a negative resist whereas microelectronic manufactures often use positive resist materials.1 During this research on polarization-selective materials an alternative approach was suggested. In this approach an optical element is applied on top of a photoresist that serves as a filter to absorb the unwanted polarizations and to transmit the desired polarization. The underlying photoresist does not need to be polarization-selective. Therefore, this method offers the advantage that widely applied commercial resists can be used which have proven track record in the lithographic process, such as those described in Chapter 1. Parallel to an analysis of the related optics, performed in a joint project with the Delft University of Technology,2, 3 it was studied whether it is possible to create such a filter by simple means. In this chapter, a polarization-selective filter is developed that absorbs transversal polarizations. This optical component could be useful in case of focused radially polarized light (Section 1.2.2). The filter would function by absorbing the transversal polarizations surrounding the narrow longitudinal polarization component and transmitting the latter component. The transversal polarization component broadens the Airy spot of a focused radially polarized laser beam and, therefore, is undesired. The filter is composed of a chiral nematic, also named cholesteric, host to which a dichroic dye is added as the absorbing component as shown in Figure 6.1. As mentioned in Chapter 1, the cholesteric phase is a type of liquid crystalline phase where the average orientation of the rod-like liquid crystal molecules describes a helix perpendicular to the long axis of the molecules.4-8 The helicoidal organization is induced by liquid crystal molecules with a chiral center in their molecular structure. Alternatively, a cholesteric phase can be induced through the addition of a chiral material (dopant) to a nematic host. The chirality of the dopant determines whether the helix is left- or right handed. The pitch p is defined as the distance 94 Chapter 6 required for a 2π rotation of the average molecular orientation. In the case this pitch is on the order of the wavelength of the light impinging parallel to the helical axis of the cholesteric layer, this light gets reflected. Only left or right circular polarized is reflected depending on the handedness of the helix. When the pitch of the helix is chosen to be smaller than the wavelength of light the film behaves as a negative-C plate with two high refractive indices in the plane of the film and a low refractive index perpendicular to that. In the case a dichroic dye is added to this small-pitch material it adapts the helicoidal organization with a large absorption in the plane of the film and a low absorption in the direction perpendicular to that. T Cholesteric layer L Photoresist Not polarization-selective (a) (b) Figure 6.1 a) A cholesteric layer consisting of a nematic host (grey) and a dichroic dye (black) coated on top of a photoresist which is not polarization-selective. The dichroic dye absorbs the transversal (T) polarized light while transmitting the longitudinal (L) component that is recorded by the photoresist. b) A top view of the same for a single cholesteric helix showing favorable absorption of the transversal component by the dichroic dye due to the alignment with the nematic host. In this chapter, the above-described cholesteric filter is developed and characterized. It is also shown that this cholesteric can be fabricated on top of a known Novolac-based photoresist (see Section 1.3.1 for more information about this type of resist). 6.2 Experimental The cholesteric consists of a nematic host (BASF), a chiral dopant and dichroic dye9 (0.5 wt%) and a planarization agent (1 wt%) which are shown in Figure 6.2. The pitch p of the material is determined by the concentration of the chiral dopant Cw and the helical twisting power HTP of this component (Equation 6.1). The righthanded chiral dopant (BASF) has a HTP of 37 µm-1 wt%-1 and 5 wt% was mixed with the nematic host. 95 Development of a cholesteric filter for blocking transversal polarizations p 1 C w HTP Equation 6.1 The LC mixture was dissolved into xylene for spin coating. The amount of the solvent xylene was added in different concentrations. The cholesteric was either spin coated on a rubbed polyvinyl alcohol (PVA) alignment layer or a negative Novolac photoresist ma-N 141010 (Micro resist technology GmbH). The preparation of the PVA layer is described in Section 3.2. In the case where the cholesteric was coated on PVA, a 50 wt% solution in xylene was used. In the case where the cholesteric was coated on top of the photoresist, a 33 wt% solution in xylene was used for spin coating. For both solutions the following program was used: 1000 rpm for 30 s with acceleration of 500 rpm s-1. After having fulfilled its function in the process, the cholesteric layer is removed by a 10 s immersion in xylene and subsequently dried with a nitrogen flow. A thin film of the ma-N 1410 resist is formed on a glass substrate by spin coating (4000 rpm for 30 s, 500 rpm s-1) to create a layer of approximately 0.8 µm on a glass substrate. After spin coating the resist was pre-baked at 120 °C for 90 s before illumination. In case of an exposure, a flood exposure was performed with an EXFO Omnicure Series 2000 mercury lamp. The resist was developed in a tetramethylammoniumhydroxide solution (ma-D 533/S, Micro resist technology GmbH) for 30 s and subsequently rinsed with deionized water for 10 s. As a final step the resist was dried with a nitrogen air gun. The transmission spectra were recorded on a Shimadzu UV-3102 spectrophotometer equipped with a polarizer to create linearly polarized light and an additional quarter wave plate to form circularly polarized light. The topography of the cholesterics was measured using a 3D interferometer (Fogal Nanotech Zoomsurf). 96 Chapter 6 Nematic host Cr 67 °C N 123 °C I Chiral dopant Cr 100 °C I Dichroic dye Surfactant Figure 6.2 The components of the cholesteric layer. Where Cr, N and I are the crystalline, nematic and isotropic phases, respectively. 6.3 6.3.1 Results and discussion Fabrication of the cholesteric filter A reactive nematic host was selected in order to form a polymerized cholesteric film for analyses performed in this section. Polymerization of this material prevents crystallization of the film over time. In this section the cholesteric film is polymerized by adding a photoinitiator (Irgacure 651, Ciba specialty chemicals) to the cholesteric mixture. The film was polymerized in nitrogen atmosphere through a flood exposure. The pitch of the cholesteric filter here was tuned to reflect at 550 nm to easily observe the formation of the desired cholesteric planar alignment and to show that a cholesteric film can be formed on top of the Novolac-based negative photoresist. It should also be noted that the pitch should not be set such that the reflection band 97 Development of a cholesteric filter for blocking transversal polarizations overlaps with the wavelength of illumination, i.e. n p   where n is the average index of refraction, p the pitch and λ the wavelength needed for the underlying photoresist. For illumination with wavelengths of 405 nm or lower, a filter with 550 nm pitch is too large as will be explained here. The degree of absorption by the filter depends on the concentration of the co-aligning dye in the cholesteric host and the thickness of the prepared film. The thickness of the filter is limited to a value smaller than the depth of focus (DOF) since the DOF needs to encompass the filter as well as the underlying photoresist. The DOF for an optical system follows Equation 1.3. For an illumination wavelength of 405 nm, a numerical aperture of the focusing lens of 0.9 and assuming the process-dependent factor is 1, the DOF is 500 nm and, therefore, the thickness of the cholesteric is limited to several 100 nanometers which prevents high absorption especially for a cholesteric with a large pitch. The pitch should be tuned as small as possible to have many 2π rotations to achieve enough absorption of the transversal polarizations. A 550 nm pitch is chosen here to show the proof of concept for the fabrication of this filter. The same behavior is expected for cholesterics with smaller pitch. For the cholesteric film to function as an absorbing filter a dye needs to be added that aligns with the liquid crystalline nematic host. The dichroic dye shown in (Figure 6.2) is selected based on the ability to absorb at the wavelength of illumination of 405 nm. These dye containing cholesterics were prepared on PVA alignment layers. The surface of the cholesteric is irregular which is attributed to the coating procedure (Figure 6.3a). The absorbance spectra of this cholesteric film show that the central wavelength of the absorption band of the dichroic dye is located at 395 nm while the reflection band of the cholesteric is centered at 550 nm (Figure 6.3b). The absorbance of linearly polarized light was analyzed at different angles along the azimuth where the cholesteric film was rotated while keeping the transmission axis of the linear polarizer stationary as shown in Figure 6.3c. The reflection band changes slightly upon rotation which is likely caused by small inhomogeneities in the cholesteric film as can be observed in the optical image shown in Figure 6.3b. The intensity of the absorbance band of the dichroic dye remains constant upon rotation which means that the dye is homogeneously distributed as is desired for filtering the unwanted radial components upon illumination. However, based on this result, it is not possible to determine whether this homogeneity arises due to the co-alignment of the dichroic dye with the cholesteric host or due to a random orientation of the dye within the host. The latter case is undesirable because the random orientation will also lead to absorption of the longitudinal polarization component of light by the dye. 98 Chapter 6 0.5 1 1.5 2 2.5 4 0.5 3 1 2 µm 1 1.5 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330° 360° 0.4 Absorbance / a.u. 0 0 0 0.3 0.2 0.1 0.0 mm 400 500 600 700 800 Wavelength / nm (a) (b) Cholesteric Light source polarizer (c) Figure 6.3 a) The topography of the dichroic dye containing cholesteric film and b) the corresponding absorbance spectra and picture of the cholesteric film. c) The absorbance spectra were measured with linearly polarized light at different rotation angles as indicated by the arrow. To investigate whether the dichroic dye aligns with the host, the dye is uniaxially planarly aligned in the same nematic host (Figure 6.2) but in the absence of the chiral dopant. The layer was fabricated through spin coating on a PVA layer. The absorbance spectra parallel and perpendicular to the director are shown in Figure 6.4. The dichroic ratio of the dye, which is a measure for the polarizationselectivity, is 8 at 405 nm which is a conventional ratio for the nematic phase.11, 12 This result indicates co-alignment of the dichroic dye with the nematic host and, therefore, it is probable that the co-alignment also occurs in the cholesteric phase which leads to favorable absorption of the transversal polarizations of light. 99 Development of a cholesteric filter for blocking transversal polarizations 0.5 Absorbance / a.u. 0.4 Parallel Perpendicular 0.3 0.2 0.1 0.0 350 400 450 500 550 600 Wavelength / nm Figure 6.4 The absorbance spectra of the dichroic dye in the planarly aligned nematic host measured parallel and perpendicular to the director; and analyzed with linearly polarized light. 6.3.2 Cholesteric filter applied on a photoresist Due to the limitation of the DOF mentioned in the previous section the cholesteric film needs to be coated directly on top of the negative photoresist. For this purpose a cholesteric mixture was spin coated on top of the photoresist. The transmittance, measured with right-handed circularly polarized (RCP) light, of the bare photoresist and of the resist with a cholesteric coated on top is shown in Figure 6.5. The transmittance data show that the photoresist absorbs light below 420 nm. The transmittance also shows a dip around 550 nm which corresponds to the reflection band of this cholesteric film and, therefore, the film is green colored. This film is about 1 µm thick which is too thin to obtain 100 % reflection of the RCP light. The appearance of the reflection band demonstrates that a cholesteric can be fabricated directly on the photoresist. The absorption band of the photoresist remains the same after the coating procedure where xylene is used as a solvent which means the photoresist is unaffected by the application of the cholesteric film. 100 Chapter 6 100 Transmittance / % 80 Resist Resist + cholesteric 60 40 20 0 400 500 600 700 800 900 Wavelength / nm Figure 6.5 The transmittance spectra of the photoresist with and without a cholesteric film coated on top measured with right-handed circularly polarized light. The inset in the graph shows an optical image of the cholesteric film. The cholesteric film should only serve as a filter and needs to be removed after exposure of the photoresist while leaving the photoresist unaffected. Xylene is used to remove the cholesteric and as shown in Figure 6.6a, the transmission band of the photoresist remains the same after the cholesteric is stripped. As a control experiment the photoresist is developed without being exposed. The transmittance data show that the negative photoresist can be removed completely. In the lithographic process the photo-induced conversion enhances the solubility of the photoresist in an alkaline solution. For correct pattern transfer, it is undesirable for the cholesteric to influence this chemical process. Therefore, a flood exposure was applied to the photoresist with the cholesteric top coating. The transmittance spectrum is shown in Figure 6.6b. The transmittance becomes lower between 450 nm and 550 nm which can be a result of an interaction of the resist with the cholesteric or a change of the absorption band of the photoresist upon illumination. For this purpose, spectra are recorded of the resist before and after illumination without applying a cholesteric film (Figure 6.6c). Based on this observation, it is concluded that the lower transmittance between 450 nm and 550 nm is the result of a shift of the absorption by the photoresist. Based on the aforementioned results, it is concluded that a cholesteric can be fabricated on top of the photoresist and be removed while leaving the photoresist completely unaffected. 101 Development of a cholesteric filter for blocking transversal polarizations 100 80 60 40 Resist Resist + cholesteric Unexposed resist + cholesteric stripped Unexposed resist + developed 20 0 400 500 600 700 800 Transmittance / % Transmittance / % 100 80 60 40 Resist Resist + cholesteric Exposed resist + cholesteric Exposed resist + cholesteric stripped 20 0 900 400 500 600 700 800 900 Wavelength / nm Wavelength / nm (a) (b) Transmittance / % 100 80 Before exposure After exposure 60 40 20 0 400 500 600 700 800 900 Wavelength / nm (c) Figure 6.6 The transmittance spectra of a cholesteric coated on top of a commercial negative photoresist a) without a subsequent flood exposure and b) with the application of a flood exposure. c) The transmittance spectra of the photoresist before and after exposure without the application of a cholesteric filter. The spectra are recorded with right-handed circularly polarized light. 6.4 Conclusions This chapter shows that a filter, which absorbs transversal polarizations, consisting of a cholesteric host and a dichroic dye can be fabricated. Through absorbance measurements it is shown that the dichroic dye aligns with the host and that this occurs in a homogeneous fashion resulting in equal absorbance of the transversal polarizations of light at every angle. It is shown that this cholesteric filter can be applied on a commercially available Novolac-based negative photoresist while leaving the photoresist unaffected. After illumination the cholesteric film can be removed from the photoresist without affecting the resist. 102 Chapter 6 References 1. K. Suzuki and B. W. Smith, Microlithography, Taylor & Francis Group, LLC, Boca Raton, 2007. 2. K. Ushakova, A. Assafrao, S. Pereira and P. Urbach, Near UV- VIS radial wire grid polarizer, To be published in Opt. Express. 3. K. Ushakova, S. Pereira and H. P. Urbach, Thesis in preparation, Delft University of Technology, 2015. 4. D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and W. Vill, Volume 1: Fundamentals, Wiley-VCH verlag GmbH, Weinheim, 1998. 5. S. Chandrasekhar, Rep. Prog. Phys., 1976, 39. 6. I. Dierking, Textures of liquid crystals, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003. 7. I. Dierking, Symmetry, 2014, 6, 444-472. 8. I. Nishiyama, The Chemical Record, 2009, 9, 340-355. 9. E. Peeters and D. J. Broer, Dichroic guest-host polarizer, United States patent application publication, Koninklijke Philips electronics N.V., 2010. 10. A. Voigt, M. Heinrich, K. Hauck, R. Mientus, G. Gruetzner, M. Töpper and O. Ehrmann, Microelectron. Eng., 2005, 78–79, 503-508. 11. C. Sanchez, B. Villacampa, R. Cases, R. Alcala, C. Martinez, L. Oriol and M. Pinol, Journal of applied physics, 2000, 87. 12. E. Peeters, J. Lub, J. A. M. Steenbakkers and D. J. Broer, Adv. Mater., 2006, 18, 2412-2417. 103 Chapter 7 7 Cholesteric filter for enhancement of the longitudinal polarization 105 Cholesteric filter for enhancement of the longitudinal polarization 7.1 Introduction The preceding chapter showed the development of a cholesteric filter with a coaligning dichroic dye to remove the transversal polarization components of light. It was proposed that this optical element is possibly useful for focused radially polarized light. The proposed mechanism of the filter is that it transmits the narrow longitudinal polarization component while absorbing the unwanted broadening transversal polarization component. As mentioned in Chapter 1, there are several methods to form radially polarized light, namely through the application of segmented wave plates,1-4 liquid crystal polarization converters4-7 or through the use of a circular wire grid polarizer. 2, 8-10 The first two options are less desirable due to losses at the boundaries due to segmentation of the wave plates or change in alignment of the liquid crystal director that compromise the quality of the radially polarized light. Circular wire grid polarizers consist of concentric metal rings that reflect the azimuthal polarization and transmit the radial polarization (Figure 7.1a). These polarizers are not segmented and, thus, no losses occur at boundaries. Therefore, this method is the preferred option for creating radially polarized light. A simplified schematic of the laser set-up is shown in Figure 7.1b. A quarter wave plate (λ/4) converts the linear polarization of a laser beam to a circular polarization state. Upon passing the circular wire grid polarizer (CWGP), radially polarized light is created. Due to the circular polarization state before passing the CWGP the phase needs to be corrected by a spiral phase plate (SPP). Forming a coherent radially polarized beam is important for the formation of a strong resultant longitudinal electric-field vector in focus. It is important for the cholesteric filter not to change the radial polarization state of the incoming laser beam. Changes in this polarization state will prevent the formation of a strong longitudinal polarization component in focus. In this chapter the effect of the cholesteric filter on the radial polarization is examined; and in addition a theoretic study is given that shows whether the cholesteric filter can properly function in the manner as described above. 106 Chapter 7 Azimuthal component is reflected λ/4 Circular wire grid polarizer CWGP SPP Radial component is transmitted (a) (b) Figure 7.1 a) Radially polarized light is transmitted through the CWGP while azimuthal polarizations are reflected. b) A simplified schematic of the set-up to create radially polarized light containing a quarter wave plate (λ/4), a circular wire grid polarizer (CWGP) and a spiral phase plate (SPP). 7.2 Experimental The composition of the cholesteric mixture is described in Section 6.2. A polymerized cholesteric film without and with dichroic dye is fabricated by adding a photoinitiator (Irgacure 651, Ciba specialty chemicals) to the cholesteric mixture. The film was polymerized in nitrogen atmosphere through a flood exposure. This was done to prevent crystallization during study of the effect of the cholesteric on the radial polarization state. The LC mixture was dissolved in xylene (50 wt%) for spin coating (1000 rpm, 30 s, 500 rpm s-1) on a rubbed polyvinyl alcohol (PVA) alignment layer. The preparation of the PVA layer is described in Section 3.2. The transmission spectra of the cholesteric films were recorded on a Shimadzu UV-3102 spectrophotometer equipped with a polarizer and a quarter wave plate to form circularly polarized light. The topography of the cholesteric film was measured using a 3D interferometer (Fogal Nanotech Zoomsurf). 7.3 Results and discussion 7.3.1 Radially polarized light Prior to placing a cholesteric film in the path of the radially polarized laser beam, it is important to confirm that radially polarized light is indeed created by the set-up shown in Figure 7.1b. This characterization is performed by placing a linear 107 Cholesteric filter for enhancement of the longitudinal polarization polarizer after the SPP and determining the total intensity and intensity distribution of the laser beam passing the polarizer (Figure 7.2). λ/4 CWGP SPP Power meter or beam profiler Linear polarizer Figure 7.2 A simplified schematic of the 405 nm laser set-up where a quarter wave plate (λ/4), circular wire grid polarizer (CWGP) and spiral phase plate (SPP) is used to form radially polarized light. The polarization state of the laser light that passes the SPP is analyzed with a polarizer, power meter and beam profiler. Figure 7.3 shows the intensity distribution of the linearly polarized 405 nm laser beam without polarizer.11 The diameter of the beam is 3.8 mm. The quality of this beam is not perfect which reflects in the inhomogeneous intensity distribution. This same inhomogeneity is seen in the intensity distribution of the radially polarized beam. The intensity of the beam reduces due to losses upon passing several optical elements. The intensity distribution analyzed with a linear polarizer shows a twolobe pattern that follows the transmission axis of the polarizer. This is as expected because radially polarized light can be considered as locally linearly polarized.3, 12, 13 This result confirms the radial polarization state of the beam. 108 Chapter 7 Linear Radial (a) (b) (c) 0 Intensity 100 Figure 7.3 The intensity distribution of a) the full linearly and b) the full radially polarized laser beam; the diameter of the beam is 3.8 mm. c) The intensity distribution with a polarizer indicated by the dashed line set at rotation angles of 0° to 180° with 30° increments. The total intensity of the beam after passing the linear polarizer set at different rotation angles is shown in Figure 7.4. The intensity is 0.21 µW on average but does periodically vary which could be caused by the inhomogeneous intensity distribution of the laser beam as mentioned above. Despite these small deviations from the ideal laser spot, the quality of the state of radial polarization is considered to be sufficient for the analyses performed in this chapter. 109 Cholesteric filter for enhancement of the longitudinal polarization Intensity / W 0.3 0.2 0.1 0.0 0 60 120 180 240 300 360 Angle / ° Figure 7.4 The measured intensity of the radially polarized laser beam upon passing a polarizer set at different angles. 7.3.2 Effect of the cholesteric filter on radially polarized light The birefringent LC molecules in the cholesteric filter rotate to form helices which may change the radial polarization state. A change in this polarization state prevents the formation of a strong longitudinal component upon focusing and, therefore, is undesirable. By placing a cholesteric between the SPP and linear polarizer and analyzing the laser light which has passed through the cholesteric filter (Figure 7.2), a potential effect of the cholesteric film on the radial polarization state can be investigated. Figure 7.5a shows the spectra of a cholesteric film with and without dichroic dye. Figure 7.5b shows an example of a topography of a cholesteric film which shows that the cholesteric film is not perfectly flat. Firstly, the cholesteric without dye is placed in the laser set-up. 110 Chapter 7 100 0 Transmittance / % 80 0 Cholesteric Cholesteric with dichroic dye 60 0.5 1 1.5 2 2.5 mm 4 0.5 3 1 2 µm 40 1 1.5 20 0 mm 0 400 500 600 700 800 900 Wavelength / nm (a) (b) Figure 7.5 a) The transmittance spectra of a cholesteric without and with dichroic dye measured with right-handed circularly polarized light. b) An example of the topography of a cholesteric film. The intensity profiles of the beam without polarizer in Figure 7.6 shows that the intensity distribution changes by placing a cholesteric film, which does not contain a dichroic dye, in the radially polarized beam. This change is attributed to the roughness of the surface of the cholesteric which is shown in Figure 7.5b. This roughness means there is a variation in thickness which changes the phases inhomogeneously. This effect is undesirable because it prevents the formation of a strong longitudinal component upon focusing. This result shows that it is important to form a cholesteric film with a flat surface. 111 Cholesteric filter for enhancement of the longitudinal polarization (a) (b) 0 Intensity 100 Figure 7.6 a) The intensity distribution of the full radially polarized laser beam after passing the cholesteric film. b) The intensity distribution analyzed with a polarizer indicated by the dashed line set at rotation angles of 0° to 180° with 30° increments. The continuous line represents the symmetry axis of the two-lobe pattern. Intensity distributions imaged after the light has passed the polarizer show the expected two-lobe pattern3, 12, 13 (Figure 7.6) as observed in Section 7.3.1 which indicates the radial polarization state. However, there is an off-set angle of about 10° between the transmission axis of the polarizer and the symmetry axis of the twolobe pattern. This is attributed to the total thickness of the cholesteric film. In a cholesteric the molecular director changes continuously. When the thickness of the cholesteric film is not equal to nλ where n is an integer, the polarization state is altered upon passing the cholesteric which results in an off-set angle. The thickness of this cholesteric is 2.3 µm and, therefore, not a multiple of the 405 nm illumination. The two-lobe intensity distribution observed after the polarizer does indicate that the radial polarization is still present. However, the phases are altered. 112 Chapter 7 The previous results are obtained by using a cholesteric without dichroic dye. Figure 7.7 shows the intensity distribution when the cholesteric contains dichroic dye. The same effects are observed as above without dye. The intensity distribution changes and, additionally, there is an off-set angle between the polarizer and the symmetry axis of the lobes profile. The same explanations apply here as above. The only difference is the reduction in intensity due to the absorption by the dichroic dye. The result shows that the cholesteric filter can be used to absorb the transversal polarization components. (a) (b) 0 Intensity 100 Figure 7.7 a) The intensity distribution of the full radially polarized laser beam after passing a cholesteric that contains dichroic dye. b) The intensity distribution analyzed with a polarizer indicated by the dashed line set at rotation angles of 0° to 180° with 30° increments. The continuous line represents the symmetry axis of the two-lobe pattern. The total intensity measured after the polarizer which is set at different rotation angles is shown in Figure 7.8. The intensities are shown for the beam with or 113 Cholesteric filter for enhancement of the longitudinal polarization without the cholesteric film and also for a cholesteric film with or without dichroic dye. For a perfectly radially polarized beam, the intensity is expected to remain constant as a function of the angle of the polarizer.13 However, it varies periodically which is attributed to the quality of the initial linear polarized laser beam as discussed in the previous section. Placing a cholesteric in the set-up does not change the intensity of the polarizer which indicates there are no optical losses. The addition of the dichroic dye in the cholesteric reduces the intensity due to the absorption by the dye. The intensity decreases about 50% which is supported by the transmittance spectrum in Figure 7.5. Intensity / W 0.3 0.2 0.1 Without cholesteric With cholesteric With dichroic dye containing cholesteric 0.0 0 60 120 180 240 300 360 Angle / ° Figure 7.8 The measured intensity of the radially polarized laser beam upon passing a polarizer set at different angles. 7.3.3 Theoretical study of the working-principle of the cholesteric filter Consider an anisotropic medium of which the principal axes are xˆ , yˆ and ẑ , then on the basis of the eigenvectors xˆ , yˆ and ẑ the relative electric permittivity tensor  is given by Equation 7.1. The energy absorbed per unit time and per unit volume W is given by Equation 7.2.  x    0 0  0 y 0 0  0  z  114 Equation 7.1 Chapter 7  2 1 2 W   0 Im( x ) E x  Im( y ) E y  Im( z ) E z 2 2  Equation 7.2 Where  is the frequency of the light,  0 is the permittivity of vacuum, ‘Im’ refers to the imaginary part and E j is the amplitude of the electric field where j is x, y or z. When only the z-component is absorbed, then Im(  x )  Im(  y )  0 and W becomes: 1 W   0 Im( z ) E z 2 2 Equation 7.3 Which means the absorbed energy is proportional to the squared amplitude of the z-component of the electric field. For simplicity it is assumed that the anisotropic medium is uniaxial and that the z-axis is the optical axis, then  x   y . Also assume that there is an interface of which the z-axis is the normal, than any field propagating in the medium can be expanded in plane waves of which the tangential wave vector components are real, i.e. k x and k y are real. Here, only one such plane wave is considered and it is assumed that k y  0 . There exist two polarizations of such a wave, namely the ordinary and the extraordinary wave. The ordinary wave is first considered. In this case, Equation 7.4 and Equation 7.5 are valid where k 0 is the wave number in vacuum and A is the amplitude. k z( o )  k02 x  k x2 Equation 7.4  (o) E  Ayˆe i ( k x xk z z ) Equation 7.5 It should be noted that k z(o ) is real if k x  k0  x which is supported by the fact that this wave is not absorbed by the anisotropic medium since it has the electric field along the y-direction. As for the extraordinary wave ‘(e)’, the k z(e) can be determined from Equation 7.6 and Equation 7.7. k x is real,  z is complex and it is assumed that k y ,  y  0 . 115 Cholesteric filter for enhancement of the longitudinal polarization Substituting k z(e) (Equation 7.8) in Equation 7.7 yields Equation 7.9. Note that both the z- and x-component of the electric-field decrease exponentially fast with the propagation distance z, i.e. elimination of transversal electric-field component x will also result in the elimination of the longitudinal electric-field component z. Therefore, the cholesteric filter cannot transmit only the longitudinal component when the transversal components are absorbed.      k02E      E  0 Equation 7.6    (e) E ( x , z )  Ae i ( k x xkz z ) Equation 7.7 k z( e )   k 02 x  k x2  i  k x x     E ( x , z )  Ae  7.4 x z k02 x  k x2  x  z  z  Equation 7.8 Equation 7.9 Conclusions Here, the effect of a transversal filter, consisting of a cholesteric host and coaligning dichroic dye, on radially polarized light is studied. A two-lobe pattern is observed by analyzing the radially polarized light passing the cholesteric. The intensity distribution analyzed through a polarizer shows a two-lobe pattern which indicates that the beam remains radially polarized. This statement is also supported by the relative constant total intensity. However, the phases of the radial polarization do seem to change upon passing the cholesteric due to an angular offset between the transmission axis of the polarizer and the symmetry axis of the twolobe intensity distribution pattern. The phase-change is attributed to the surface roughness and also to the total thickness of the cholesteric film. The study performed here was made under the assumption that the longitudinal component can propagate and be transmitted by the cholesteric filter without the transversal component. Further study shows that the longitudinal component cannot exist without the transversal component and, therefore, this approach where 116 Chapter 7 a cholesteric filter is applied to decrease the dimensions of illuminated spots in an underlying commercial photoresist cannot function. 117 Cholesteric filter for enhancement of the longitudinal polarization References 1. R. Dorn, S. Quabis and G. Leuchs, Phys. Rev. Lett., 2003, 91. 2. T. Wakayama, O. G. Rodríguez-Herrera, J. S. Tyo, Y. Otani, M. Yonemura and T. Yoshizawa, Opt. Express, 2014, 22, 3306-3315. 3. H. Kawauchi, Y. Kozawa, S. Sato, T. Sato and S. Kawakami, Opt. Lett., 2008, 33, 399-401. 4. Q. Zhan, Adv. Opt. Photon., 2009, 1, 1-57. 5. R. Yamaguchi, T. Nose and S. Sato, Jpn. J. Appl. Phys., 1989, 28, 1730-1731. 6. M. Stalder and M. Schadt, Opt. Lett., 1996, 21. 7. G. Miyaji, N. Miyanaga, K. Tsubakimoto, K. Sueda and K. Ohbayashi, Appl. Phys. Lett., 2004, 84, 3855-3857. 8. Z. Ghadyani, I. Vartiainen, I. Harder, W. Iff, A. Berger, N. Lindlein and M. Kuittinen, Appl. Opt., 2011, 50, 2451-2457. 9. Z. Bomzon, V. Kleiner and E. Hasman, Appl. phys. lett., 2001, 79, 1587. 10. Z. e. Bomzon, G. Biener, V. Kleiner and E. Hasman, Opt. Letters, 2002, 27, 285-287. 11. K. Ushakova, A. Assafrao, S. Pereira and P. Urbach, Near UV- VIS radial wire grid polarizer, To be published in Opt. Express. 12. G. M. Lerman, L. Stern and U. Levy, Opt. Express, 2010, 18, 27650-27657. 13. Y. Ma and R. Wu, Opt. Rev., 2014, 21, 4-8. 118 Chapter 8 8 Technology assessment and prospects 119 Technology assessment and prospects 8.1 Assessment The aim of this thesis is to form a basis for the fabrication of photopolymerizable materials with a high sensitivity difference for the state of polarization of light that is used to initiate the polymerization process. The liquid crystalline (LC) photo-reactive material developed in this thesis forms an insoluble polymer network upon illumination, i.e. it acts as a negative resist material. This choice was made based on the availability of important compounds such as the dichroic photoinitiator which is capable to select the desired polarization for initiating a chain-addition polymerization. Another important compound is the liquid crystalline monomer host with the highly ordered smectic B phase that directs the orientation of the initiator with a small distribution around a preferred axis. In this thesis it was shown that high polarization-selectivity was achieved by using this smectic B phase. Polarization-selectivity towards linearly polarized light was achieved by aligning the material planarly while selectivity towards longitudinal polarization was achieved through a homeotropic alignment. The presence of inhibiting species in the monomer mixture resulted in a nonlinear response of this material with respect to dose which enabled well-defined patterning by polarization holography in the case of planar alignment. For actual application in optical lithography of the SmB photo-reactive material as developed in this thesis it is recommended to prepare the films shortly prior to illumination due to absence of a glassy state at room temperature. The smectic B phase is preserved for approximately two hours at room temperature before crystallization occurs. A solution of the LC material in cyclopentanone can be stored over a time-span of at least several months. During this research layers of about 100 nm to several micrometers could be prepared on a single substrate through spin coating. The roughness of the layers that are several hundred nanometers thick needs to be diminished by working in a cleanroom environment and by using a different solvent. Upon illumination of the photo-reactive material, a polymer network forms through radical polymerization. This process is inhibited when it is performed in an environment with high oxygen content. Therefore, an inert environment such as a nitrogen atmosphere is needed. However, the presence of inhibitor/oxygen in low concentrations is desirable as shown in Chapter 3 where polarization-selectivity towards a single polarization was achieved by the presence of these materials. Optimizing the inhibitor/oxygen concentration to achieve the highest possible polarization-selectivity is advised. 120 Chapter 8 Negative resist materials are often used for the formation of permanent micrometer-sized structures in devices. Examples are optical grating in diffraction optics,1, 2 microchannels in microfluidic devices3, 4 and membranes.5, 6 Well-known examples are SU8, an epoxide monomer which can be cured by a cationic mechanism and leads to a densely crosslinked film, and acrylate monomers containing 3, 4 or even 5 functional acrylate groups also to enhance the crosslink density. In order to compare the status of our material with the properties of existing photolithographic materials there are still two dominant factors which needs to be improved: 1. The crosslink density. High crosslink densities are necessary to reduce swelling in organic solvents during the develop step. The best performing photolithographic materials have a large number of functional groups per monomeric unit. In the case of SU8 even eight. In the case of the acrylates 3 or more. In our case we have an average number of slightly higher than 1, just sufficient to make the cured films insoluble but not high enough to prevent swelling. In this proof of principle research this was chosen to optimize on the formation of the smectic phases using already existing materials. A large improvement of the resolution is anticipated if new molecules are designed that combine smectic phases with a higher functionality. 2. The formation of a glassy phase at room temperature. A glassy phase of the photolithographic material at room temperature during the lithographic exposure step has three important advantages. Firstly, it allows contact mask exposures. Secondly, the materials are not sensitive for dust uptake during processing and eventual storage. And thirdly, during the mask exposure no polymerization-induced diffusion can take place that reduces the resolution. Our materials, although smectic B at room temperature, are viscous liquids. A liquid crystalline material is desired that exhibits the smectic B phase above room temperature, and where the phase and the desired molecular order is frozen-in when cooled to room temperature when the material goes through its glass transition temperature. It is obvious that, in order to improve on the shortcoming, new liquid crystalline molecules need to be designed and studied. The functionality can be improved by composing the smectic B formulation using higher concentrations of bi-functional acrylate monomers. But also higher functionalities are possible by functionalizing 121 Technology assessment and prospects the aromatic rod-like cores not only at para position but also at the meta positions of the outer rings to functionalize them with spacered acrylate groups. This will also suppress the crystallization temperature and is anticipated to introduce a glass transition. A glass transition can be further introduced by considering so-called twin-molecules as liquid crystal molecules.7, 8 In literature it is shown that by extending the length of the molecules the glass temperatures shifts to higher values. 8.2 From negative to positive photoresists The lithographic material developed in this thesis forms the basis for negative photoresist materials. However, in present semiconductor industry positive photoresist are predominantly used. These materials, often based on polyvinylphenol activated by a photo-acid generator (PAG), are preferred as they have a better resistance against swelling during the development. Swelling was not observed for positive photoresists because aqueous alkaline solutions could be used as developer. These water based developers have a smaller physical interaction with the remaining polymer structures. As the acrylate-based polymers that were investigated in this thesis need organic solvents for the development step they are also susceptible to swell when the structures are being formed.9, 10 This leads to the conclusion that it would be beneficial to invent polarization-selective positive photoresists. The previous chapters show high polarization-selectivity of the acrylic photo-reactive material due to the preferential absorption axis of the initiator. This principle of polarization-selectivity or dichroism is not restricted to negative photoresist materials. The dichroism arises due to the directionality of the conjugation system/transition moment of the molecule. The same dichroic behavior can be applied to photo acid generators used in positive photoresist. This can be achieved by synthesizing a PAG with a conjugated system that follows mainly one direction. The dichroic PAG requires a polymeric liquid crystalline host, with the right solubility parameters, to align the PAG and to become soluble after exposure. 8.3 Challenges going from scanning to mask lithography In this thesis, it was proposed that a polarization-selective material could be used for resolution enhancement by recording the narrow longitudinal electric-field vector that is created upon focusing radially polarized light. High-resolution patterning with a high throughput is the key-factor to obeying Moore’s law which states that the number transistors doubles roughly every two years.11-13 Optical mask lithography has enabled this trend in which three-dimensional structures are 122 Chapter 8 constructed layer-by-layer by projecting two-dimensional images of various masks. The longitudinal component is a resultant of a radially polarized laser beam focused through a high numerical aperture lens, i.e. a single spot is formed. To form a full pattern through point-illumination requires scanning which is time-consuming and, therefore, impedes high throughput. Converting this point-illumination system to mask lithography will be challenging. A pattern containing mask can be illuminated by the radial polarized light, however, diffraction occurs as this light passes through the mask. The diffraction will lead to a loss of coherency of the radial polarization state and consequently the formation of a weak longitudinal polarization component and a stronger transversal component in focus. The stronger transversal component possibly broadens the spot size. The broadening depends on the contrast of the intensity between the longitudinal and the transversal polarization component and the contrast of the polarization-selective photoresist. 8.4 Alternative lithography Interference lithography or holography and laser direct-write lithography are other types of lithography that are interesting for polarization-selective patterning. Polarization holography was used in Chapter 4 to show the high polarizationselectivity of the photo-reactive material. In that polarization holography study two orthogonally circularly polarized laser beams were employed. The interference of these two beams produces a polarization pattern while the intensity remains constant. However, holography is not restricted to two laser beams nor is there a restriction on keeping either the polarization or intensity constant. Both parameters can be tuned simultaneously depending on the polarization of the interfering beams. The polarization-selectivity of the smectic B photo-reactive material enables patterning based on polarization as well as intensity. Conventional photoresists do not possess this property since they only respond to intensity modulation. This could be useful for three dimensional patterning without masks. Laser direct-write refers to any technique that is able to create patterns in a surface or volume through spot-by-spot illumination by either moving the laser with a fixed sample stage or with a fixed laser and a moving sample stage.14 Based on this definition, the term ‘direct-write’ does not only refer to optical techniques but also encompasses techniques such as electron-beam, ion-beam and atomic force lithography. Here, the focus is on optical or laser direct-write lithography. Despite the slow nature of these types of techniques it offers one main advantage which is high-resolution patterning of complex two- and three-dimensional structures15-26 123 Technology assessment and prospects which are extremely difficult or impossible to produce with mask lithography. Like in optical mask lithography, in optical direct-write techniques there is also a drive to increase the resolution. There have been different approaches to produce subdiffraction-limited spots such as two-photon illumination15-17, 19, 21, 22 and two-color single-photon illumination.20, 21, 23, 24 It is anticipated that the use of focused radially polarized light will lead to further enhancement of the resolution in direct-write lithography. This could lead to e.g. a higher information density in optical data storage. 8.5 High polarization-contrast patterning in the third dimension As already mentioned in Chapter 1, the control over the alignment of the liquid crystalline host enables patterning in the third dimension depending on the polarization of the illumination source. This was previously demonstrated for the nematic phase in a splay27 or helicoidal alignment.27-31 In this thesis the smectic B phase was utilized due to the high directional order of the molecules in this phase. This phase was selected to show the proof of concept for fabricating a highly polarization-selective material. In smectic phases the molecules align in a layered structure. Due to this organization, a continuous change of the molecular director as for the nematic phase cannot be achieved. The change in the direction of the director in smectic materials is discontinuous or discrete. The helicoidal alignment can be achieved in smectics which exhibit strong chirality. This phase is also referred to as the twisted grain boundary phase (TGB, Figure 8.1).32-35 In the TGB phase smectic domains are present with a local molecular director 𝑛⃑ which rotates gradually along the helical axis; and the domains are separated by grain boundaries. It is anticipated that this type of smectic phase can be used for fast high-contrast patterning in the third dimension without masks.  n Grain boundaries Figure 8.1 A schematic view of the twisted grain boundary phase for chiral smectic materials.32 The local orientation is represented by the director 𝑛⃑. 124 Chapter 8 References 1. D. Chambers, G. Nordin and S. Kim, Opt. Express, 2003, 11, 27-38. 2. T. M. de Jong, D. K. G. de Boer and C. W. M. Bastiaansen, Opt. Express, 2011, 19, 15127-15142. 3. J. Zhang, M. B. Chan-Park and S. R. Conner, Lab on a Chip, 2004, 4, 646653. 4. M. B. Chan-Park, J. Zhang, Y. Yan and C. Y. Yue, Sens. Actuators, B, 2004, 101, 175-182. 5. C. Junseo, R. Anish, K. Namwon, E. N. Dimitris, L. Wonbong, H. Haksoo and P. Sunggook, J. Micromech. Microeng., 2010, 20, 085011. 6. A. M. Prenen, in Chemical engineering and chemistry, Eindhoven university of technology, Eindhoven, 2009. 7. K. Kürschner and P. Strohriegl, Liq. Cryst., 2000, 27, 1595-1611. 8. K. Kürschner, P. Strohriegl, P. V. D. Witte and J. Lub, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 2000, 352, 301-310. 9. B. A. Miller-Chou and J. L. Koenig, Prog. Polym. Sci., 2003, 28, 1223-1270. 10. D. Braun, H. Cherdron and H.Ritter, Polymer Synthesis: Theory and Practice, Springer-Verlag, Berlin Heildelberg, 2001. 11. G. E. Moore, Electronics, 1965, 8, 114-117. 12. S. E. Thompson and S. Parthasarathy, Mater. Today, 2006, 9, 20-25. 13. C. A. Mack, IEEE Transactions on semiconductor manufacturing, 2011, 24, 202207. 14. C. B. Arnold and A. Piqué, MRS Bulletin, 2007, 32, 9-15. 15. H. Jeon, R. Schmidt, J. E. Barton, D. J. Hwang, L. J. Gamble, D. G. Castner, C. P. Grigoropoulos and K. E. Healy, J. Am. Chem. Soc., 2011, 133, 6138-6141. 16. S. Maruo and J. T. Fourkas, Laser & Photonics Reviews, 2008, 2, 100-111. 125 Technology assessment and prospects 17. T. W. Lim, Y. Son, D. Y. Yang, H. J. Kong and K. S. Lee, Appl. Phys. A, 2011, 103, 1111-1116. 18. A. Goswami, A. Phani, A. M. Umarji and G. Madras, Rev. Sci. Instrum., 2012, 83, 095003-095003. 19. S. Kawata, H. B. Sun, T. Tanaka and K. Takada, Nature, 2001, 412, 697-698. 20. Y. Cao, Z. Gan, B. Jia, R. A. Evans and M. Gu, Opt. Express, 2011, 19, 19486-19494. 21. Z. Sekkat and S. Kawata, Laser Photonics Rev., 2014, 8, 1-26. 22. M. Malinauskas, M. Farsari, A. Piskarskas and S. Juodkazis, Phys. Rep., 2013, 533, 1-31. 23. J. T. Fourkas and J. S. Petersen, PCCP, 2014, 16, 8731-8750. 24. T. F. Scott, B. A. Kowalski, A. C. Sullivan, C. N. Bowman and R. R. McLeod, Science, 2009, 324, 913-917. 25. A. C. Sullivan, M. W. Grabowski and R. R. McLeod, Appl. Opt., 2007, 46, 295-301. 26. J.-H. Lee, C. Y. Koh, J. P. Singer, S.-J. Jeon, M. Maldovan, O. Stein and E. L. Thomas, Adv. Mater., 2014, 26, 532-569. 27. B. S. Ramón, in Chemical engineering and chemistry, Eindhoven university of technology, Eindhoven, 2008, p. 117. 28. B. K. C. Kjellander, in Chemical engineering and chemistry, Eindhoven university of technology, Eindhoven, 2006, p. 209. 29. D. J. Broer, G. N. Mol, J. A. M. M. v. Haaren and J. Lub, Adv. Mater., 1999, 11, 573-578. 30. D. J. Broer, Curr. Opin. Solid State Mater. Sci., 2002, 6, 553-561. 31. B. Serrano-Ramon, C. Kjellander, S. Zakerhamidi, C. W. M. Bastiaansen and D. J. Broer, in Emerging Liquid Crystal Technologies III, Spie-Int Soc Optical Engineering, Bellingham, 2008, pp. 1-13. 32. I. Dierking, Symmetry, 2014, 6, 444-472. 33. I. Nishiyama, The Chemical Record, 2009, 9, 340-355. 126 Chapter 8 34. D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and W. Vill, Volume 1: Fundamentals, Wiley-VCH verlag GmbH, Weinheim, Germany, 1998. 35. I. Dierking, Textures of liquid crystals, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2003. 127 List of abbreviations List of abbreviations AFM CAR CD Ch Cr CWGP DNQ DOF DP DR DSC E-field EUV EUVL FTIR GIXD HFA I LC LCP N NA PAC PBS PEB PHOST PI POM PVA RCP RT SAXS SEM SmA Atomic force microscopy Chemically amplified resist Critical dimension Cholesteric Crystalline Circular wire grid polarizer Diazonaphtoquinone Depth of focus Dichroic photoinitiator Dichroic ratio Differential scanning calorimetry Electric field Extreme ultraviolet Extreme ultraviolet lithography Fourier transform infrared spectroscopy Grazing incidence x-ray diffraction Hexafluoroalcohol Isotropic Liquid crystal Left-handed circularly polarized Nematic Numerical aperture Photoactive compound Polarizing beamsplitter Post-exposure bake Polyhydroxystyrene Polyimide Polarized optical microscopy Polyvinyl alcohol Right-handed circularly polarized Room temperature Small angle x-ray scattering Scanning electron microscopy Smectic A 129 List of abbreviations SmB SmC SPP TBA tBOC TGB UV WAXS Smectic B Smectic C Spiral phase plate t-Butyl acrylate t-Butyloxycarbonyl Twisted grain boundary Ultraviolet Wide angle x-ray scattering 130 List of publications List of publications M.P. Van, C.W.M. Bastiaansen, D.J. Broer, Polarization holography using a highly polarization-selective photopolymerizable film, in preparation M.P. Van, C.C.L. Schuurmans, C.W.M. Bastiaansen, D.J. Broer, Polarizationselective polymerization in a photo-crosslinking monomer film, RSC Advances, 2014, 4, 62499 M.P. Van, C.W.M. Bastiaansen, D.J. Broer, Polarization-selective photoresist based on reactive liquid crystals doped with a dichroic photoinitiator, Proceedings of SPIE, 2013, 8682, 8682-65 Awarded the 2013 Hiroshi Ito Memorial Best Student Paper Award at the SPIE Advanced Lithography conference Publications not related to this thesis J. Bauer, P.P.C. Verbunt, W. Lin, Y. Han, M.P. Van, H.J. Cornelissen, J.J.H. Yu, C.W.M. Bastiaansen, D.J. Broer, Thermoresponsive scattering coating for smart white LEDs, Optics Express, 2014, 22, A1868 M.G. Debije, M.P. Van, P.P.C. Verbunt, M.J. Kastelijn, R.H.L. van der Blom, D.J. Broer and C.W.M. Bastiaansen, The effect on the output of a luminescent solar concentrator on application of organic wavelength-selective mirrors, Applied Optics, 2010, 49, 745 A.C.C. Esteves, J. Brokken-Zijp, J. Lavèn, H.P. Huinink, N.J.W. Reuvers, M.P. Van and G. de With, Garnet particles effect on the cross-linking of PDMS and the network structures formed, Polymer, 2010, 51, 136 A.C.C. Esteves, J. Brokken-Zijp, J. Lavèn, H.P. Huinink, N.J.W. Reuvers, M.P. Van and G. de With, Influence of cross-linker concentration on the cross-linking of PDMS and the network structures formed, Polymer, 2009, 50, 3955 131 132 Acknowledgements Acknowledgements The work presented in this thesis could not have been accomplished without the help of other people. First of all, I want to thank Dick Broer and Cees Bastiaansen for giving me the opportunity to work on this project in the Functional Organic Materials & Devices group. Dick, your pragmatic approach, advice, ideas and your patience have been invaluable to this work. Cees, during our discussions in the last few months frustrations from both sides rose considerably but I believe those discussions have led to a better thesis. I would like to thank prof.dr. H.P. Urbach, prof.dr. J.P.H. Benschop, prof.dr.ir. J. Huskens, prof.dr. R.P. Sijbesma and ing. J.M. Wijn for their advice and comments on my thesis and for being a part of my defense committee. Joep, I am very grateful to you for your hands-on input during difficult times of my research. I would also like to acknowledge the people in Delft: Kate, Quincy, Silvania, Paul, Thim and Roland, Ewan and Roel. Thank you for all your input and advice. I would also like to acknowledge Edgar for helping me with the derivations of equations. Of course, I am also grateful for the people in Eindhoven who helped me to perform my research. Carl, thank you for working on my project. Luc and Jeroen, thank you for your help with the synthesis and purifications. Berry and Marco, thank you for your help and patience. I also want to thank Huub, Dirk-Jan and Giuseppe Portale for measuring my samples in Grenoble even though you were in a sleepdeprived state. In addition, thank you to Günter Hoffmann and Philippe Leclère for helping me with the AFM analyses. A thank you to Ties for familiarizing me with polarization holography and also thank you for a fun time in the office; this also goes for Huub and Kamlesh. Michael, it has been already 7 years ago since we met when I worked for you as a student assistant. I valued your creativity and infectious enthusiasm then and also did during my PhD. I am sure you will inspire many more (PhD) students. Together with Paul you also introduced me to some really nice board and card games which I enjoyed (‘Wie is de ezel?’ forever! : ) ). Paul, I enjoyed sharing a beer with you and also appreciated our talks and your advice. In the future, I will try to vent my emotions more instead of holding it in. Ivi, because of your always ready party-spirit I really enjoyed our time at and outside of work. Also, thank you for your comforting and encouraging words when I needed it. May Gabi, Nico and you find the life you desire in Australia. Jelle and Natalia, thank you for a really nice time outside work and making my tolerance to alcohol better. It is still not that high yet, but something to strive for in 133 Acknowledgements the future. Jelle, thank you for introducing me to interesting Dutch singers and classic songs such as ‘de fles’ and I will always remember our special night with Django Wagner. Natalia, I have enjoyed your fun and optimistic company and going to concerts with you. May more follow. Down the line you also picked up a German package called Fabian who thoroughly enjoys making ‘My’-jokes. Thank you for that too…….sigh…… ; ) Fabian, thank you for the good drinking times and I hope you manage to invent the best ‘My’-joke. Cees W., thank you for bringing me to the dirt race-car event in Denekamp. That was truly a nice experience. Your kind and cheerful personality made our nights out incredibly enjoyable. I know I have been absent for a while, but I am sure there will be more nights in the future. Ele, I would like to thank you for having the patience to play tennis/squash with me when I just started playing. Thanks to you I found a good way to relax. Of course I also enjoyed our time outside of sports. Good luck with your PhD in Loughborough. Jurica, thank you for keeping me sane during the final stretch of my PhD by helping me putting things into perspective and for letting me vent my frustrations with tennis. But most of all, thank you for consistently reminding me to enjoy life outside of work. Furthermore, I would like to thank all current and former SFD members for a great time in the group during work and outside of it. Also, a thank you to the people from PTG/e, Peer+ and my sport buddies for a good time. Lastly, I would like to thank my family for letting me pursue things in life that I enjoy. Mom, thank you for cooking enormous amounts of wonderful food when I came by. I am grateful to you all, My 134 Curriculum Vitae Curriculum Vitae My Phung Van was born on 19 May 1985 in Venlo. After finishing VWO in 2003 at the Den Hulster in Venlo, she studied Chemical Engineering and Chemistry at Eindhoven University of Technology. In 2009 she graduated within the Materials and Interface Chemistry group on ‘Thin polymer films containing garnet particles for lighting applications’. After receiving her master’s degree she started working at Océ where she studied the crystallization behavior of hotmelt ink. In 2010 she started her PhD project at the Eindhoven University of Technology of which the results are presented in this dissertation. 135 Printed by Uitgeverij BOXPress, ‘s-Hertogenbosch Cover design by My Phung Van 136

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