FI20235855A1 - An antireflection coating and method for manufacturing such - Google Patents

Abstract

An anti-reflective coating for a device surface (106), comprising: a first material layer (102) having a first and second surfaces, having a first continuous layer of material, wherein the first material layer provides a first refractive index; and a second material layer (104) having a first and second surfaces, comprising a second material that is a different material than the first material, wherein the second material layer is configured to provide a variable refractive index that decreases from a maximum value on a first surface of the second material layer to a minimum value on a second surface of the second material layer, wherein each refractive index provided by the second material layer is less than the first refractive index of the first material layer, wherein the first surface of the first material layer is configured to contact at least a portion of the device surface, and the second surface of the first material layer is configured to contact at least a portion of the second surface of the second material layer, and wherein the first refractive index and the variable refractive index are configured to provide refractive indices that are less than the refractive index of the device surface.

Description

AN ANTIREFLECTION COATING AND METHOD FOR

MANUFACTURING SUCH

TECHNICAL FIELD OF THE INVENTION

The invention relates to optics in general. More specifically, the invention relates to an antireflection coating for a device surface and a method of manufacturing an antireflection coating for a device surface, where the antireflection coating comprises a first material layer and a second material layer with selected refractive indices.

BACKGROUND OF THE INVENTION

Many applications require the use of antireflection (AR) coatings on optical surfaces to reduce the reflectance of the surface. In particular, materials such as silicon, germanium, and compound semiconductors, which have a high refractive index, suffer from reflection losses.

Single-layer and multilayer coatings are known for use as antireflection coatings. Single-layer coatings, also known as quarter-wavelength or lambda/4 AR coatings, are simple to manufacture but only provide anti- reflective properties in a limited narrow bandwidth. Furthermore, in some use cases a suitable material providing a feasible refractive index for a single-layer antireflection coating may not be found.

Multilayer antireflection coatings may provide antireflective properties in a broader bandwidth or wavelength range than single-layer antireflective coatings. Broadband anti-reflective (BBAR) coatings have multiple layers. In

Q the case of a three-layer BBAR the layer thicknesses are chosen to have

N optical thicknesses (refractive index times physical thickness) of guarter

Od 25 wavelength for top layer facing the environment, half wavelength for the = middle layer, and again quarter wavelength for the bottom layer, with the

I refractive indices having an order of low/high/medium from top to bottom. > The exact values for the low/high/medium indices depends on the substrate

O for which the three-layer BBAR is used. A two-layer AR coating where both 2 30 layers have a guarter wavelength optical thickness with the outer layer

S having lower index is also sometimes used, but generally has inferior performance to the three-layer BBAR. Yet, the fabrication of multilayer antireflection coatings is complex and may e.g. involve high temperatures, such as over 700 °C.

Specifically in relation to e.g. (thick) silicon on insulator (SOI) devices, such as silicon waveguides or photonic circuits, and quantum computers that are based on silicon photonics, the devices may have non-planar device surfaces that should be coated with an antireflection coating. Many known multilayer AR coatings are manufactured using line-of-sight methods, whereby the coating of nonplanar device surfaces that have areas that are to be coated that face different directions is cumbersome and cannot be done in one run. Providing of antireflection coatings on such surfaces may also not be scalable, and can require several different tools or complicated machinery, which may be costly.

SUMMARY OF THE INVENTION

An object of the invention is to alleviate at least some of the problems relating to the known prior art. In one aspect of the invention an antireflection coating for a device surface is provided wherein the antireflection coating comprises: a first material layer comprising a first surface and a second surface, wherein the first material layer comprises a first material, said first material layer being a uniform material layer, and wherein the first material layer provides a first refractive index; and a second material layer comprising a first surface and a second surface, wherein the second material layer comprises a second material, said second material being different from the first material, wherein the second material layer is configured to provide a varying refractive index that e decreases from a largest value at the first surface of the second material

S 25 layer to a smallest value at the second surface of the second material layer, & wherein any refractive index provided by the second material layer is = smaller than the first refractive index of the first material layer,

O

E wherein the first surface of the first material layer is configured to contact at

LO least a portion of the device surface and the second surface of the first 2 30 material layer is configured to contact at least a portion of the first surface of

S the second material layer, and wherein the first refractive index and the

N varying refractive index are configured to provide refractive indices that are smaller than a refractive index of the device surface.

In one further aspect of the invention, a method of manufacturing an antireflection coating is provided, the method comprising obtaining a device surface, - forming a first material layer comprising a first material as a uniform material layer onto at least a portion of the device surface, wherein the first material layer is formed to provide a first refractive index, and - forming a second material layer comprising a second material, said second material being different from the first material, onto at least a portion of a surface of the first material layer, such that second material layer provides a varying refractive index that decreases from a highest value at the interface between the first material layer and the second material layer to a lowest value at a surface of the second material layer which is furthest away from the first material layer, wherein any refractive index provided by the second material layer is smaller than the first refractive index of the first material layer, wherein the first refractive index and the varying refractive index are configured to provide refractive indices that are smaller than a refractive index of the device surface.

The present invention provides an antireflection coating where the desired antireflective behavior is advantageously obtained with only two material layers.

Broadband antireflective properties (which may refer to reflectance minimum that is broader than with lambda/4 AR coatings) may be exhibited by the AR = coating of the invention, and may thus provide antireflection in a broader

N 25 wavelength range than antireflective properties of traditional single-layer AR 3 coatings. Yet, the AR coating structure of the invention is simpler than the

S multilayer broadband antireflection coatings of the prior art, where three or

I more material layers are utilized. 2 A flat reflectance spectrum may also be provided, meaning that all 3 30 wavelengths may be transmitted essentially equally, a flat reflectance

O spectrum often being the goal of broadband antireflective coatings.

The AR coating of the invention may provide a solution with lower coupling losses by reduced optical reflection to and from e.g. silicon photonics devices and/or high refractive index semiconductor devices as compared with the prior art.

The second surface of the second material layer may be configured to face an environment of the antireflection coating, wherein the environment comprises an outer medium such as a gaseous medium, a liquid medium, or a vacuum.

The antireflection coating may be formed utilizing a conformal deposition method, such as chemical vapor deposition (CVD), dip coating, or atomic layer deposition (ALD). When a conformal deposition method is utilized, it may be possible to provide the AR coating onto selected surface area portions of a device surface by patterning the coating, for example by removing both layers of the coating simultaneously from selected areas. For instance, silicon waveguide facets and coupling mirrors comprising surfaces that should be AR coated can have said surfaces facing multiple directions and can be coated in one run with the present invention, which may not be possible with broadband antireflection coatings of the prior art. This may increase speed and flexibility of the coating process, while reducing costs.

Other embodiments of the AR coating as described above comprising a first material layer and a second material layer may be formed utilizing non- conformal deposition methods.

It may be noted that in addition to the already mentioned photonic applications, the present invention may also provide an antireflection coating for light emitting diodes (LEDs) and photodetectors. In the case of e LEDs, the AR coating may increase the light output by preventing reflection

S 25 when the light created in the LED reaches the LED surface.

S Further advantages of providing the AR coating with a conformal deposition

O method include scalability to batch processes. For example, a 25 wafer

E batch process, treating 25 device surfaces at the same time, may be carried

LO out instead of a 1 wafer non-batch process. The coating may be formed 2 30 using only one tool, making the fabrication machinery simpler than that in & the prior art, where several tools should be utilized.

Al

The coating may also be formed in a process where temperatures are maintained lower than in prior art processes, such as at a maximum temperature of 170 °C. Furthermore, the process may be CMOS compatible, as the coatings may be formed using materials suitable for gate oxides. Photonic components and CMOS circuits may then be fabricated in a single silicon die.

In one embodiment, the antireflection coating may be formed as a patterned 5 antireflection coating by depositing the first material layer, removing at least a portion of the first material layer, depositing the second material layer, and removing at least a portion of the second material layer or by depositing the first material layer, depositing the second material layer, and removing at least a portion of the second material layer and optionally removing at least a portion of the first material layer. A patterned AR coating may be formed by removing the AR coating from one or more areas to provide one or more shaped AR coated regions on the device surface. The layers of the AR coating may be removed simultaneously, or one at a time, or the first material layer may be deposited then patterned followed by the second layer deposition and patterning. Furthermore it may be favorable, e.g. for saving time, to deposit both material layers and only pattern the second material layer leaving the first material layer intact (at least in one area from which the second material layer is removed), thus having the described AR coating in some regions but leaving areas on the surface device where only the first material layer is present.

With the present invention, the AR coating can be fabricated onto a plurality of device surface areas or portions of the device surface in one run. The antireflection coating of the invention may thus be provided such that the device surface is a non-planar surface, further wherein the antireflection coating is configured to cover only a portion of said device surface. The AR & coating may be formed onto a non-planar device surface comprising second

N areas of the device surface simultaneously even if the second areas of the

S device surface are surfaces that face different directions. This may be

O impossible with non-conformal methods used in prior art AR coating

E 30 processes. With non-conformal methods, it may also be impossible to

LO fabricate an AR coating that would be essentially homogeneous across the 2 device surface regarding e.g. thickness of the first and second material

S layers.

The second material layer may comprise a nanoporous, microporous, nanourough, or microrough material. The second material layer may comprise a graded refractive index or gradient refractive index (GRIN)

profile, meaning that the refractive index of the material changes depth-wise from a largest value at the first surface of the second material layer to a smallest value at the second surface of the second material layer. The refractive index profile as a function of the material depth can change linearly or non-linearly from one surface to the other. A gradient/graded refractive index material (GRIM) can be made from a number of materials. .

The second material layer, the GRIM, may comprise porous or rough silica or other porous or rough oxides such as aluminum oxide (e.g. grass-like alumina) or titanium oxide, porous magnesium fluoride or other porous fluorides, or porous organic materials for example organic layers that have been plasma etched or treated with plasma, or parylene that has been made porous or roughened by plasma methods.

The first material layer may comprise a dielectric material. The material of the first material layer may comprise stoichiometric or non-stoichiometric materials such as oxides (e.g. aluminium oxide (alumina), silicon oxide (silica), hafnium oxide (hafnia), zirconium oxide (zirconia), or titanium oxide (titania), nitrides (e.g. silicon nitride or aluminium nitride), or other compound materials or elemental materials that exhibit dielectric characteristics at a selected wavelength or wavelength range of interest. The selected wavelength range may be a wavelength range where the AR coating is to exhibit antireflective properties. The first material layer may comprise a dielectric material that may also be a mixture of materials such as aluminium-titanium dioxide AITiOx or an oxynitride or other mixture of materials.

A thickness of the first material layer may be about between 20 nm and 300 & nm, such as between 50 nm and 250 nm, at least for visible and near infra- 3 red applications. 5 According to an embodiment of the invention, the refractive index of the first

I material layer, the first material, and/or a thickness of the first material layer > 30 may be selected based on predetermined refractive indices of the second

O material layer, the second material, and/or a thickness of the second 2 material layer and a predetermined wavelength range at which the

S antireflection coating is to provide an antireflective effect. Alternatively, the refractive indices of the second material layer, the second material, and/or a thickness of the second material layer may be selected based on a predetermined refractive index of the first material layer, the first material,

and/or a thickness of the first material layer and a predetermined wavelength range at which the antireflection coating is to provide an antireflective effect.

The predetermined refractive indices of the second material layer may constitute a refractive index profile of the second material layer. The refractive index profile could correspond to a linear gradient in the refractive index or could e.g. correspond to a quintic profile. The gradient could be selected as a continuous gradient.

A method of manufacturing an antireflection coating may then comprise - selecting a wavelength range for which the antireflection coating shall provide an antireflective effect, - selecting a refractive index of the first material layer or a refractive index range and/or profile of the second material layer, and selecting the first or second material, and/or a thickness of the first or second material layer, and - selecting a refractive index of the first material layer or a refractive index range and/or profile of the second material layer, and selecting the first or second material, and/or a thickness of the first or second material layer based on the above selections.

When a use case or application of the AR coating is known, the wavelength range for which the antireflection coating shall provide an antireflective effect may be selected. One or more of the parameters of refractive index,

Q material, and/or thickness of the first or second material layers may then be

R fixed, selected, or preknown. For example, one may select a refractive index

Od 25 or refractive indices for the second material layer, a second material that is = to be used, and a thickness of the second material layer. Based on these

I selections, properties of the first material layer may be selected/determined, > such that the AR coating will exhibit the antireflective properties (e.g. a

O reflectance under a preferred value) at the selected wavelength range.

LO

& 30 At least properties comprising the refractive index (or indices) of the first or

N second material layer (one of which is not preknown or preselected) may be determined or selected utilizing a computational electromagnetics method such as the transfer-matrix method based on the previously selected property or properties of the other material layer.

One or more simulations may be carried out, wherein the properties of the first and/or second material layers are varied and a resulting reflectance of the AR coating is determined. One or more properties of the first and/or second material layer may be selected/determined based on the simulations.

In embodiments of the invention, the AR coating may be configured to cover at least a portion of a device surface, wherein the device material comprises a structured surface, such as black silicon or silicon comprising an inverted pyramid structure commonly used in silicon solar cells. Therefore, in addition or as an alternative to being able to be configured for a non-planar device surface, where the device surface may be shaped to comprise a certain topography or form separate device structures, such as the aforementioned waveguide facets, the antireflection coating may be utilized in connection with device surface materials that exhibit a surface structure.

Materials with structured surfaces may be employed in e.g. photodetectors and photovoltaic devices, but also in for instance LEDs. The AR coating of the invention may enhance the operation of devices with structured surface materials. Using prior art methods, it may be cumbersome or impossible to fabricate an antireflection coating on such device surfaces.

In some embodiments, the antireflection coating may be configured for a device surface comprising at least one material comprising a refractive index of equal to or over 2, such as silicon, germanium, or a compound

Q 25 semiconductor. Known AR coating solutions known in the prior art

N specifically for use in connection with device surfaces comprising high (2 or

Od over) refractive indices usually comprise three or more material layers. = Traditional prior art manufacturing methods have not permitted the z manufacturing of feasible GRIN layer structures conformally, and the > 30 combination of one GRIN layer with only one dielectric layer as an AR

O coating has not been applied, especially in relation to device surfaces 2 comprising high refractive indices. oo

N The exemplary embodiments presented in this text are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this text as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific example embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

Figure 1 schematically illustrates an antireflection coating on a device surface,

Figure 2 shows representative examples of an antireflection coatings on device surfaces,

Figure 3 illustrates a flow chart of a method of manufacturing an antireflection coating,

Figure4 shows a flow chart of a method of manufacturing an antireflection coating,

Figure 5 shows a flow chart of a method of manufacturing an

S antireflection coating,

Od Figure 6 shows at 6A a reflectance spectrum of a first example of an AR

S coating according to the invention and at 66 a reflectance spectrum of an

I 25 ideal prior art AR coating,

X Figure 7 shows at 7A a reflectance spectrum of a second example of an 3 AR coating according to the invention and at 7B a reflectance spectrum of

N an ideal prior art AR coating,

Al

Figure 8 shows at 8A a reflectance spectrum of a third example of an AR coating according to the invention and at 8B a reflectance spectrum of an ideal prior art AR coating,

Figure 9 illustrates a mean reflectance of an AR coating as a function of thickness of the first material layer,

Figure 10 shows a mean reflectance of an AR coating as a function of thickness of the first material layer, and

Figure 11 shows a mean reflectance of an AR coating as a function of thickness of the first material layer.

DETAILED DESCRIPTION

Figure 1 shows an antireflection coating on a device surface according to one embodiment of the invention. It should be noted that the figures presented herein depicting AR coatings and devices are not drawn to scale and embodiments of an antireflection coating and associated devices or device surfaces may differ greatly from those depicted.

The antireflection coating comprises a first material layer 102 and a second material layer 104. The AR coating may be disposed on at least a portion of a device surface 106, which is a surface of a device 108. The device surface 106 may be an essentially planar surface as shown in Fig. 1 or it may be a nonplanar surface.

The term “device” may refer to one or more components which may provide e some intended functionality or use in a certain application. A device 108

N . .

S may at least in some embodiments be a substrate. Other examples of a & device 108 may comprise silicon photonic devices comprising device = 25 components provided on a surface. A device may be a thick SOI photonics = device, in connection with which e.g. tapered waveguide to fiber approach = of photonics interconnects cannot be used because of the very nature of

O thick SOI photonics that inhibits light outcoupling from the thick silicon. 00

LO

& The first material layer 102 is a uniform material layer, meaning that the first

N 30 material layer 102 is substantially homogeneous and provides essentially the same refractive index at all locations of the first material layer 102. The first material layer 102 provides a first refractive index.

The first material layer may comprise a dielectric material. A dielectric material may be a material that is not metallic or otherwise heavily light- absorbing. Light that is absorbed within a metallic material is absorbed rapidly and does not traverse very far within the material. A material which is referred to as dielectric, on the other hand, comprises material within which an electromagnetic wave penetrates the material to a large extent and a light absorbance of such materials is essentially nonexistent or at least low compared to the metallic materials.

A first surface of the first material layer 102 is configured to contact at least a portion of the device surface 106.

The second material layer 104 comprises a first surface and a second surface, wherein the first surface is configured to contact at least a portion of the second surface of the first material layer 102. Thus, the first material layer is sandwiched between the device surface 106 (or device 108) and the second material layer 104.

The second surface of the second material layer 104 may be configured to face the environment of the device 108 and the AR coating. The antireflection coating may thus consist of the first material layer 102 and second material layer 104, with no further material layers being provided as part of the AR coating.

The second material layer 104 comprises a second material, where the second material is different from the first material of the first material layer 102. The second material may comprise a homogenous material or it may e comprise one or more materials, wherein the second material is arranged to

S 25 asanon-homogeneous material, comprising e.g. a nano- or microstructure, & such as a nanoporous material.

O The second material layer is configured to provide a varying refractive index

E that decreases from a largest value at the first surface of the second

LO material layer to a smallest value at the second surface of the second 2 30 material layer. Any refractive index provided by the second material layer is

N smaller than the first refractive index of the first material layer. The first

N refractive index of the first material layer 102 and the varying refractive index of the second material layer 104 are configured to provide refractive indices that are smaller than a refractive index of the device surface 106 or a refractive index of the material of the device 108 or at least of the portion of the device 108 that comprises the device surface/surfaces that are to be coated to with the AR coating.

The varying refractive index refers to the refractive index of the second material layer 104 providing a gradient along the transverse direction of the second material layer 104, denoted by a direction x in Fig. 1. The refractive index of the second material layer 104 may decrease from a first gradient refractive index value at the first surface of the second material layer to a final gradient refractive index value at the second surface of the second material layer 104, with the second surface of the second material layer being the outer surface of the AR coating. The refractive index value may change linearly or nonlinearlyalong the direction x of the second material layer 104. In a plane in the longitudinal direction perpendicular to the direction x, the second material layer 104 may comprise a refractive index that is essentially equivalent at all locations, or may vary only to a small degree, having a gradient that is significantly smaller than the gradient exhibited by the refractive index variation in the direction x.

Properties of the first material layer 102 and second material layer 104 may be selected based on an intended use. The intended use may entail obtaining knowledge regarding the device 108, comprising at least one or more known properties of the device surface 106 (such as at least material or refractive index), and a wavelength range where the antireflection coating shall provide an antireflective function. The AR coating providing an antireflective function at a selected wavelength may refer to the AR coating

N 25 exhibiting a reflectance that e.g. is below a selected value.

N

N The selected properties of the first material layer 102 and second material

Od layer 104 may comprise at least a thickness of the material layer, a material = of the material layer, and/or a refractive index of the material layer (or

I indices that constitute the refractive index profile, in the case of the second > 30 material layer 104).

LO

& Figure 2A shows an example of an antireflection coating on a device

N surface 106 of a device 108. The device surface106 in this example is a

N non-planar device surface 106. The antireflection coating may be configured to cover only a portion of the device surface 106. In the embodiment of Fig. 2A, the device surface comprises a plurality of device surface areas 106a,

106b, 106c that are to be provided with the AR coating. The device surface areas 106a, 106b, 106c may be for instance surfaces of components of the device, such as the aforementioned mirrors.

The different device surface areas 106a, 106b, 106c may in isolation comprise planar surfaces, such as the device surface areas 106a and 106c, or non-planar surfaces, such as the device surface area 106b.

In the embodiment of Fig. 2A, the antireflection coating with first material layer 102 and second material layer 104 may be considered as a discontinuous coating layer that covers only a portion of the device surface 108 orit may be considered that the device surface 106 is provided with a plurality of separate antireflection coating portions, which are however essentially eguivalent in their properties.

It should be noted that for the antireflection coating of Fig. 2A, the direction denoted as x in connection with Fig. 1 will vary along the different locations ofthe AR coating.

Figures 2B and 2C show an AR coating on a waveguide facet. Fig. 2B shows a side view and Fig. 2C shows a top view. The waveguide 108 may be considered as the device, comprising a device surface 106 that is coated, while a silicon substrate of the entire device is denoted in Figs. 2B and 2C as 202 with a silicon dioxide layer 204 being provided.

Figure 3 shows a flow chart of a method of manufacturing an antireflection coating. The method comprises obtaining 110 a device 108 comprising a

N device surface 106 that is to be antireflection coated.

N

N A first material layer 102 is then formed 112 to cover at least a portion of the

Od 25 device surface 106. The forming 112 of the first material layer 102 may be = carried out by a deposition method. Advantageously, a conformal deposition

I method is utilized.

X A second material layer 104 is subseguently formed 114 to cover at least a 3 portion of the first material layer 102. The second material may be deposited

N 30 onto the surface of the previously formed first material layer 102, preferably

N using a conformal deposition method. The first material layer 102 and second material layer 104 may be formed using the same deposition method.

The second material layer 104 may in some embodiments comprise grass- like alumina. A method for fabricating at least the second material layer 104 may then comprise atomic layer deposition and subsequent immersion into heated (e.g. at least 40-50 °C) deionized water to induce a morphology change from film to grass-like alumina. As an example, grass-like alumina may provide a second material layer 104 that provides a refractive index profile with a continuously varying refractive index profile that may be produced without having to rely on line-of-sight manufacturing methods.

Figure 4 shows an alternative method of manufacturing an AR coating. The method of Fig. 4 may otherwise correspond to the method of Fig. 3, but additionally comprises patterning of the AR coating. This may be realized through removing 002 at least a portion of the first material layer 102 and removing 004 at least a portion of the second material layer 104 after the material layers have been formed. The removing 002 of the first material layer may occur after forming 112 of the first material layer but before forming 114 of the second material layer 104 or the removing 002 of the first material layer 102 may occur after both forming 112 the first material layer 102 and forming 114 of the second material layer 104 have been carried out. The removing of the portions of the material layers may be realized e.g. through etching.

The steps 002 and 004 of removing are optional and may apply in cases where a device comprises a surface 106 of which only portions are to be antireflection coated.

In further alternative embodiments of a method, the AR coating may be

Q 25 patterned such that at least a portion of the first material layer 102 and at

N least a portion of the second material layer 104 are removed 3 simultaneously. = Figure 5 shows a flow chart of a further method of manufacturing an = antireflection coating. The steps 110, 112, and 114 may correspond to the 3 30 steps of Fig. 3. The additional steps of Fig. 4 may also be applied in the

O method of Fig. 5.

N

N The method of Fig. 5 comprises, before a fabrication phase of the AR coating, a design phase comprising at least selecting 006 a wavelength range for antireflection functionality of the AR coating (for a known device surface material), selecting 008 properties for one material layer, and determining 010 properties of the other material layer.

The fabrication phase of manufacturing an AR coating may be carried out using different equipment than the design phase. The different phases may also be carried substantially at different times. Yet, one piece of equipment may be configured to carry out all the method steps of Fig. 5.

For the fabrication phase, e.g. a deposition equipment/device such as an

ALD reactor, LPCVD reactor, PECVD reactor or other reactor may be utilized, while for the design phase a device comprising at least one processor, such as a personal computer device, may be employed.

At 008, all properties of one of the material layers may be selected. For example, all properties the second material layer 104 may be known or selected/set to some predetermined value. The second material layer 104 may generally be more complicated to manufacture than the first material layer 102, and there may be less options available regarding material to be used and refractive indices that are to be provided. It may then be advantageous to set the properties of the second material layer at 008.

At 010 the properties of the other material layer may be determined. If properties of the second material layer 104 are selected at 008, the properties of the first material layer 102 may be determined at 010.

At 010, at least one of the properties of the other material layer may be determined or selected without being based on any other property. At 010, at least one of the properties of the other material layer is determined based & on at least some of the previously selected parameters.

N

Od 25 Atleast some of the parameters selected in steps 006 and 008 are utilized

S in step 010 to determine at least one property of the other material layer

I utilizing a calculation method suitable for electromagnetics, such as the > transfer matrix method.

LO

LO

3 Step 010 may comprise carrying out a plurality of simulations of AR coatings

N 30 comprising one material layer with properties selected at 008 and the other

N material layer comprising varying properties.

Based on the simulations, properties of the other material layer that result in a desired antireflection functionality may be selected. For example, the simulations may involve determining reflectances of antireflection coatings comprising selected or varying properties of one and the other material layers. An antireflection coating yielding a selected reflectance may be selected as an antireflection coating that will be manufactured in the fabrication phase. A selected reflectance may e.g. be a selected mean reflectance that is provided at the selected wavelength range.

Next, three exemplary use case scenarios will be presented. The examples give three different examples of antireflection coatings that are advantageous for different use cases. Each case may involve forming an antireflection coating on a device surface. Each case may additionally involve a design phase.

A first example of an AR coating comprises a first material layer 102 comprising a dielectric material having a refractive index of 2.20 and a thickness of 98 nm. The material of the first material layer 102 may be any dielectric material that provides the selected refractive index of 2.20. The AR coating comprises a second material layer 104 having a thickness of 250 nm and providing a varying refractive index that varies from 1.00 to 1.85.

The material of the AR coating may be selected to provide the aforementioned refractive indices.

The first example of an AR coating may be advantageous for a device surface 106 where the device surface comprises silicon. An intended use may be an AR coating for a silicon optical waveguide facet. Silicon has a

Q 25 refractive index that is considered high, being 3.5 at 1550 nm wavelength.

N Si optical waveguides that shall be coupled through air to fiber optical cable

Od or other Si waveguide -air interfaces may reguire an antireflection coating to = minimize reflections. Broadband AR coatings are highly preferred on the

I waveguide facets, as these enable a broad spectrum of light to be coupled > 30 into and out of the waveguide efficiently.

LO

& The first example of an AR coating may be manufactured by first carrying

O out a design phase. The design phase may comprise selecting the device surface, here silicon, and selecting a wavelength range for AR performance, which in this example is 1200 nm — 1850 nm. The properties (material, refractive indices, and/or thickness) of the second material layer 104 may be selected and based on the selections thus far, a plurality of different properties for the first material layer 102 may be tested/simulated. The testing may result in reflectances of a corresponding AR coating being determined. A first material layer 102 providing a desired reflectance spectrum may be determined as a final first material layer 102. The corresponding AR coating may then be determined as a final AR coating that is given here as the first example of an AR coating.

The AR coating determined through the design phase may then be manufactured in a fabrication phase.

Figure 6A shows a reflectance spectrum of the first example of an AR coating that has been determined as the final AR coating. The figure depicts with a solid line the reflectance as a percentage as a function of wavelength of incident light and with a dashed line the reflectance in dB as a function of wavelength. The reflectance in dB corresponds to 10*l0g10(R), where R is reflectance.

Figure 6B shows a reflectance spectrum as a function of wavelength of incident light for a prior art AR coating comprising a traditional lambda/4 AR coating centered at 1550 nm. The properties of the lambda/4 AR coating have been selected to show an ideal AR performance of this type of coating for the use case of the first example. The material of the lambda/4 AR coating is silicon nitride (SiN) and thickness of the coating is 207 nm.

Figures 6A and 6B show that the AR coating according to the invention (the first example of an AR coating shown in Fig. 6A) has lower mean e reflectance in a broad wavelength band, giving better AR performance for

S 25 example to silicon optical waveguides, even compared to the conventional & coating with ideal refractive index (which is simulated in Fig. 6B, yet even = hard to achieve/manufacture in practice).

O

E A second example of an AR coating comprises a first material layer 102

LO comprising zirconium dioxide having a refractive index of 2.20 and a 2 30 thickness of 135 nm. The AR coating comprises a second material layer 104 & having a thickness of 200 nm and providing a varying refractive index that

N varies from 1.00 to 1.60.

The second example of an AR coating may be advantageous for a device surface comprising germanium (Ge), which has a refractive index of 4.25 at

1550 nm wavelength. Germanium may be used in a device 108 which is utilized as a photodetector for detecting infrared light, especially between 1.2 um and 2 um where silicon detectors are very insensitive or unable to detect the light. Due to the very high refractive index of Ge, a large fraction of light hitting the device surface would be reflected and would thus not be detected without an AR coating. A selected wavelength range where the AR coating shall provide AR properties is 1200 nm — 1850 nm.

The second example of an AR coating may be manufactured (designed and/or fabricated) as has been discussed previously.

Figure 7A shows a reflectance profile of the second example of an AR coating. The figure depicts with a solid line the reflectance as a percentage as a function of wavelength of incident light and with a dashed line the reflectance in dB as a function of wavelength.

Figure /B shows a reflectance profile as a function of wavelength of incident light for a prior art AR coating comprising a traditional lambda/4 AR coating centered at 1550 nm. The properties of the lambda/4 AR coating have been selected to show an ideal AR performance of this type of coating for the use case of the second example. The material of the lambda/4 AR coating is hafnium oxide (HfO>2) and thickness of the coating is 188 nm.

Fig. 7A shows that the AR coating according to the invention has lower mean reflectance in a broad wavelength band compared to the traditional lambda/4 AR coating on Ge shown in Fig. 7B.

A third example of an AR coating comprises a first material layer 102 & comprising titanium dioxide having a refractive index of 2.40 and a thickness

N 25 of 72 nm. The AR coating comprises a second material layer 104 having a ? thickness of 200 nm and providing a varying refractive index that varies from

O 1.00 to 1.60. , The third example of an AR coating may be advantageous for a device

O surface 106 comprising gallium arsenide (GaAs), which has a refractive 2 30 index of 3.55 at 870 nm. GaAs may be used e.g. in devices such as near

S infrared light emitting diodes (LEDs) for telecommunication or electronics products.

GaAs LED devices may be manufactured to emit light near their band gap energy, producing light centered roughly at 870 nm wavelength, at which the associated refractive index is very high. Without e.g. an AR coating, a large fraction of light that is generated inside a GaAs LED or similar device is trapped in the GaAs material due to reflection when light propagating towards the GaAs device surface is reflected at the GaAs-air interface.

Another strategy that can be used to increase the performance of the device by itself or together with an AR coating that can be used to prevent the undesired reflection is epoxy encapsulation, but there are cases where it is preferred that a GaAs LED emits light from inside the device material directly into air (without epoxy). Reasons for avoiding the use of epoxy encapsulation may be for example hygiene, epoxy coloration with age, thermal requirements, miniaturization need (epoxy domes are large), or use in a micro LED array or other specialty display application. Suitable AR coatings on GaAs LEDs can thus be beneficial, and may eliminate the need for epoxy encapsulation altogether.

Also the third example of an AR coating may be manufactured (designed and/or fabricated) as has been discussed previously. A wavelength range where the AR coating shall provide AR properties in the third example is 800 nm — 950 nm.

Figure 8A shows a reflectance spectrum of the third example of an AR coating. The figure depicts with a solid line the reflectance as a percentage as a function of wavelength of incident light and with a dashed line the reflectance in dB as a function of wavelength.

Q 25 Figure 8B shows a reflectance spectrum as a function of wavelength of

N incident light for a prior art AR coating comprising a traditional lambda/4 AR

Od coating centered at 870 nm. The properties of the lambda/4 AR coating = have been selected to show an ideal AR performance of this type of coating

I for the use case of the third example. The material of the lambda/4 AR > 30 coating is SiN and thickness of the coating is 116 nm.

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& Fig. 8A shows that the AR coating according to the invention has lower

N mean reflectance in a broad wavelength band compared to the traditional

N lambda/4 AR coating on GaAs shown in Fig. 8B.

The design phase of a method of manufacturing an AR coating may comprise determining a reflectance of an AR coating at one or more selected wavelengths of impinging light with at least some of the properties of the first and/or second material layer 102, 104 being fixed and varying at least one property of the first or second material layer.

Figures 9-11 show example visualizations of simulations/calculations that may be carried out using the transfer matrix method that may be used to determine properties of the “other” material layer (the one which is not fixed or selected beforehand) that yield a desired or selected AR functionality.

Figure 9 relates to the first example discussed above. Figure 9 shows a mean reflectance of an AR coating at a wavelength range of 1200 nm — 1850 nm as a function of thickness of the first material layer 102, where the other properties of the first and second materials have been fixed at selected values. Fig. 9 shows that with a thickness of 98 nm, which has been used in the first example of an AR coating gives a minimum value for the mean reflectance and is thus advantageous to select as the used thickness.

Figure 10 relates to the second example discussed above. Figure 10 shows a mean reflectance of an AR coating at a wavelength range of 1200 nm — 1850 nm as a function of thickness of the first material layer 102, where the other properties of the first and second materials have been fixed at selected values. Fig. 10 shows that with a thickness of 135 nm, which has been used in the second example of an AR coating gives a minimum value for the mean reflectance and is thus advantageous to select as the used = 25 thickness. & Figure 11 relates to the third example discussed above. Figure 11 shows a = mean reflectance of an AR coating at a wavelength range of 800 nm — 950 = nm as a function of thickness of the first material layer 102, where the other = properties of the first and second materials have been fixed at selected 1 30 values. Fig. 11 shows that with a thickness of 72 nm, which has been used 3 in the second example of an AR coating gives a minimum value for the

O mean reflectance and is thus advantageous to select as the used thickness.

In the cases of Figs. 9-11, only the thickness of the first material layer 102 has been varied. In other use cases, for example also a material and therefore a refractive index of the first material layer 102 may be a parameter that is varied to determine e.g. mean reflectance at a selected wavelength range to select an appropriate/optimum refractive index (or material) for the e.g. first material layer 102. As may be understood by the skilled person, the properties or parameters which are fixed and those which are varied may differ based on the use case.

The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims.

The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.

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Claims (14)

1. An antireflection coating for a device surface (106), wherein the antireflection coating comprises: - a first material layer (102) comprising a first surface and a second surface, wherein the first material layer comprises a first material, said first material layer being a uniform material layer, and wherein the first material layer provides a first refractive index; and - a second material layer (104) comprising a first surface and a second surface, wherein the second material layer comprises a second material, said second material being different from the first material, wherein the second material layer is configured to provide a varying refractive index that decreases from a largest value at the first surface of the second material layer to a smallest value at the second surface of the second material layer, wherein any refractive index provided by the second material layer is smaller than the first refractive index of the first material layer, wherein the first surface of the first material layer is configured to contact at least a portion of the device surface and the second surface of the first material layer is configured to contact at least a portion of the first surface of the second material layer, and wherein the first refractive index and the varying refractive index are configured to provide refractive indices that are smaller than a refractive index of the device surface.

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2. The antireflection coating of claim 1, wherein the antireflection coating N 25 is formed utilizing a conformal deposition method, such as chemical vapor I deposition (CVD), dip coating, or atomic layer deposition (ALD).

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3. The antireflection coating of any previous claim, wherein the second > material layer comprises a nanoporous, microporous, nanourough, or O microrough material. LO & 30

4. The antireflection coating of any previous claim, wherein the second N surface of the second material layer is configured to face an environment of the antireflection coating, wherein the environment comprises an outer medium such as a gaseous medium, a liquid medium, or a vacuum.

5. The antireflection coating of any previous claim, wherein the first material layer comprises a dielectric material.

6. The antireflection coating of any previous claim, wherein a thickness of the first material layer is between about 20 nm and 300 nm.

7. The antireflection coating of any previous claim, wherein properties comprising the refractive index of the first material layer, the first material, and/or a thickness of the first material layer is selected based on predetermined refractive indices of the second material layer, the second material, and/or a thickness of the second material layer and a predetermined wavelength range at which the antireflection coating is to provide an antireflective effect or wherein properties comprising the refractive indices of the second material layer, the second material, and/or a thickness of the second material layer is selected based on a predetermined refractive index of the first material layer, the first material, and/or a thickness of the first material layer and a predetermined wavelength range at which the antireflection coating is to provide an antireflective effect, optionally wherein the properties are selected utilizing a computational electromagnetics method such as the transfer-matrix method.

8. The antireflection coating of any previous claim, wherein the antireflection coating is configured to cover a non-planar device surface, further wherein the antireflection coating is optionally configured to cover only a portion of said device surface.

9. The antireflection coating of any previous claim, wherein the e antireflection coating is configured to cover at least a portion of a device S 25 surface, wherein the device material comprises a structured surface, such & as black silicon or silicon comprising an inverted pyramid structure. O

10. The antireflection coating of any previous claim, wherein the E antireflection coating is configured for a device surface comprising at least LO one material comprising a refractive index of over 2, such as silicon, 2 30 germanium, or a compound semiconductor. O O

11. A method of manufacturing an antireflection coating for a device surface, the method comprising at least: - obtaining (110) a device comprising a device surface,

- forming (112) a first material layer comprising a first material as a uniform material layer onto at least a portion of the device surface, wherein the first material layer is formed to provide a first refractive index, and - forming (114) a second material layer comprising a second material, said second material being different from the first material, onto at least a portion of a surface of the first material layer, such that second material layer provides a varying refractive index that decreases from a highest value at the interface between the first material layer and the second material layer to a lowest value at a surface of the second material layer which is furthest away from the first material layer, wherein any refractive index provided by the second material layer is smaller than the first refractive index of the first material layer, wherein the first refractive index and the varying refractive index are configured to provide refractive indices that are smaller than a refractive index of the device surface.

12. The method of claim 11, wherein the first material layer and the second material layer are formed utilizing a conformal deposition method.

13. The method of claim 11 or claim 12, wherein the method comprises - selecting (006) a wavelength range for which the antireflection coating shall provide an antireflective effect, - selecting (008) a refractive index of the first material layer or selecting a refractive index range and/or profile of the second material layer, e selecting the first or second material, and/or a thickness of the first or S 25 second material layer, and & - selecting (010) a refractive index of the first material layer or a = refractive index range and/or profile of the second material layer, and = selecting the first or second material, and/or a thickness of the first or = second material layer based on the above selections, optionally by 1 30 utilizing the transfer matrix method. 00 3

14. The method of any of claims 11-13, wherein the antireflection coating is N formed as a patterned antireflection coating by depositing (112) the first material layer, removing (002) at least a portion of the first material layer, depositing (114) the second material layer, and removing (004) at least a portion of the second material layer or by depositing the first material layer, depositing the second material layer, and removing at least a portion of the second material layer and optionally removing at least a portion of the first material layer.

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