JP2019197127A - Lithographic apparatus and method - Google Patents

Abstract

Polarizing radiation in a lithographic apparatus. A lithographic apparatus comprising an illumination system configured to condition a radiation beam B having a wavelength λ, the illumination system comprising an optical element 30, the optical element 30 comprising a first layer 34 and a second layer 34. Of the radiation beam B incident on the bilayer 32 at an angle of incidence (aoi) such that the s-polarized component of the reflected radiation beam B ′ is reflected by the bilayer 32. It is arranged to increase with respect to the s-polarized component of the incident radiation beam B. [Selection diagram] Fig. 3

Description

[0001] The present invention relates to a lithographic apparatus and a method for polarizing radiation in a lithographic apparatus. More particularly, polarizing EUV radiation in a lithographic apparatus.

A lithographic apparatus is a machine that is configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern on a patterning device (eg, a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

[0003] The lithographic apparatus can use electromagnetic radiation to project a pattern onto a substrate. The wavelength of this radiation determines the minimum feature size that can be formed on the substrate. With a lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4-20 nm, for example 6.7 nm or 13.5 nm, for example smaller than a lithographic apparatus using a wavelength of 193 nm Features can be formed on the substrate.

[0004] Since EUV radiation from a plasma source is incoherent, EUV radiation is generally not generated in a polarized state. This is in contrast to, for example, deep ultraviolet (DUV) radiation that can already be generated in a polarized state from a radiation source. It is desired to polarize EUV radiation for use in a lithographic apparatus. Since EUV radiation is absorbed relatively greatly by the material (eg, a vacuum is required for transmission), it is problematic to obtain polarized EUV radiation when most of the EUV radiation is lost.

[0005] According to a first aspect of the invention, there is provided a lithographic apparatus comprising an illumination system configured to condition a radiation beam having a wavelength λ. The illumination system comprises an optical element, the optical element comprising two layers having a first layer and a second layer, wherein the two layers reflect radiation incident on the two layers at an angle of incidence (aoi). The s-polarized component of the reflected radiation is configured to increase relative to the s-polarized component of the incident radiation.

[0006] The thickness of the two layers can be substantially equal to λ / (2cos (aoi)) so that the reflectivity of the s-polarized radiation reflected from the two layers is maximized for aoi.

[0007] The bilayer may be configured such that radiation incident on the bilayer at an incident angle (aoi) corresponding to the Brewster angle is reflected from the bilayer and the reflected radiation is substantially fully s-polarized.

[0008] The thickness of the two layers may be configured such that the reflectivity of the s-polarized radiation reflected from the two layers is maximized at the Brewster angle.

[0009] The ratio of the thickness of the first and second layers is at least 1: 1.5 and may be up to 1.5: 1.

[00010] The ratio of the thickness of the first layer to the thickness of the second layer in the two layers may be substantially 1: 1.5.

[00011] The optical element may comprise a multilayer comprising a plurality of two layers.

[00012] The optical element may comprise at least 10 bilayers.

[00013] The bilayer may include MoSi or RuSi.

[00014] The illumination system comprises a reflector substantially disposed in a plane perpendicular to the plane of incidence of the optical element, and p-polarized radiation transmitted through the optical element is s-polarized with respect to the plane of the reflector. And the reflector can be configured to reflect radiation.

[00015] The optical element may be positioned in the field plane of the illumination system.

[00016] The illumination system may comprise a plurality of optical elements.

[00017] The lithographic apparatus may be an EUV lithographic apparatus.

[00018] According to a second aspect of the present invention, there is provided a method of polarizing radiation for a lithographic apparatus having an illumination system with an optical element. The method directs radiation having a wavelength λ to be incident at an angle of incidence (aoi) on an optical element comprising a bilayer having a first layer and a second layer, and radiating the radiation from the bilayer. Reflecting so that the s-polarized component of the reflected radiation increases relative to the s-polarized component of the incident radiation.

[00019] The method reflects radiation from a bilayer having a thickness substantially equal to λ / (2cos (aoi)) such that the reflectivity of the s-polarized radiation reflected from the bilayer is maximized for aoi. Can further include.

[00020] The method may further include reflecting the radiation from the bilayer at an angle of incidence (aoi) corresponding to the Brewster angle so that the reflected radiation is substantially fully s-polarized.

[00021] The method can further include reflecting radiation from the two layers at an angle of incidence (aoi) corresponding to a Brewster angle, wherein the thickness of the two layers is the s-polarized radiation reflected from the two layers. The reflectivity can be configured to be maximum at the Brewster angle.

[00022] The method may further include reflecting radiation from a multilayer including a plurality of bilayers.

[00023] The method reflects radiation transmitted through an optical element from a reflector substantially disposed in a plane perpendicular to the plane of incidence of the optical element to transmit p transmitted through the optical element. It may further comprise causing the polarized radiation to be reflected as s-polarized radiation with respect to the reflector surface.

[00024] The radiation may be EUV radiation.

[00025] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings.

1 depicts a lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment of the invention. A radiation beam passing through a lens and onto a substrate is shown. Figure 3 shows a multilayer according to an embodiment of the present invention. FIG. 4 shows a graph of reflection, transmission, and radiation loss associated with multiple layers, in accordance with one embodiment of the present invention. FIG. 4 shows a graph of reflection, transmission, and radiation loss associated with multiple layers, in accordance with one embodiment of the present invention. FIG. 4 shows a graph of reflection, transmission, and radiation loss associated with multiple layers, in accordance with one embodiment of the present invention.

FIG. 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply this EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA includes an illumination system IL, a support structure MT configured to support the patterning device MA (eg, mask), a projection system PS, and a substrate table WT configured to support the substrate W; It has.

[00027] The illumination system IL is configured to condition the EUV radiation beam B before it enters the patterning device MA. Thus, the illumination system IL can include a facet field mirror device 10 and a facet pupil mirror device 11. Both facet field mirror device 10 and facet pupil mirror device 11 provide EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the facet field mirror device 10 and the facet pupil mirror device 11. In this example, the facet field mirror device includes at least one multilayer 30 (ie, an optical element) to generate a reflected EUV radiation beam B that is at least partially polarized. This will be described in more detail later.

[00028] After being adjusted in this way, the EUV radiation beam B interacts with the patterning device MA. This interaction results in a patterned EUV radiation beam B '. The projection system PS is configured to project a patterned EUV radiation beam B ′ onto the substrate W. For this purpose, the projection system PS can comprise a plurality of mirrors 13, 14 configured to project a patterned EUV radiation beam B 'onto a substrate W held by a substrate table WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B ', thereby forming an image of features that are smaller than the corresponding features in the patterning device MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS is illustrated in FIG. 1 as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (eg, 6 or 8 mirrors).

[00029] The substrate W may include a pre-formed pattern. If this is the case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B ′ with a pattern previously formed on the substrate W.

[00030] A relative vacuum, ie, a small amount of gas (eg, hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, illumination system IL, and / or projection system PS.

[00031] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL), or any other radiation source capable of producing EUV radiation.

FIG. 2 shows a radiation beam 16 provided with a pattern 18. The radiation beam 16 is refracted onto the substrate W by the lens 20. The radiation beam 16 has a p-polarized component (radiation polarized parallel to the entrance surface of the lens 20) as shown. Specifically, the radiation beam 16 includes a light beam having a wave vector k ₁ and an electric field vector E ₁ in a plane parallel to the incident plane, and a wave vector k ₂ and an electric field in a plane parallel to the incident plane. It indicated by a light ray having a vector E _2.

[00033] The lens 20 has a relatively large numerical aperture (NA). This means that rays having the electric field vectors E ₁ and E ₂ are both refracted from the lens 20 at a relatively large angle. This means that when the light beam reaches the wafer W (see the portion of the partial ellipse 22), the light beam is in approximately the opposite direction. Furthermore, the electric field vectors E ₁ and E ₂ are in approximately opposite directions (see enlarged view of the partial ellipse 22 at the bottom of FIG. 2). This means that p-polarized radiation (waves) tends to cancel each other out on the substrate W. This is because the interference of the p-polarized radiation wave is incomplete at a large propagation angle difference. Therefore, some form of polarization control is required for high NA systems. DUV high NA systems generally use highly polarized radiation to avoid this. However, as mentioned above, obtaining highly polarized EUV radiation is problematic (because the EUV plasma source produces incoherent radiation and the absorption of EUV radiation is large).

FIG. 3 shows a multilayer 30 (ie, an optical element), where the incident EUV radiation beam B has a wavelength λ and the reflected EUV radiation beam B ′ is reflected from the multilayer 30. In this respect, the multilayer 30 is considered to be a multilayer mirror. A multilayer mirror is a periodic stack in which layer materials of different refractive indices are alternately arranged. At each interface, some of the incident radiation is reflected and the rest penetrates deeply into the stack and is partially reflected at other interfaces. All partially reflected coherent beams interfere with each other. The multilayer 30 can be disposed on or supported by a membrane (ie, substrate) not shown. The size and thickness of the membrane can vary depending on the particular situation (such as the number of layers in the multilayer), as described in more detail below.

The incident radiation beam B has an incident angle (aoi) with respect to the multilayer 30 as shown. aoi is an angle between the surface normal of the multilayer 30 and the propagation direction of the incident radiation beam B. The incident surface of the multilayer 30 is a surface including the surface normal 31 of the multilayer 30 and the wave vector (k) of the incident radiation beam B. The phase of the reflected radiation beam B ', that is, a wave having a phase Φ of 0 and a wave having a phase Φ of 180 is shown. An incident radiation beam B is shown as well.

The multilayer includes a plurality of two layers 32. The bilayer 32 includes a first layer 34 and a second layer 36, and the radiation is incident on the first layer 34 and then on the second layer 36. As shown, the two layers 32 are adjacent to each other, with the first layers 34 and the second layers 36 alternating throughout the multilayer 30.

[00037] In this example, the first layer 34 is made of molybdenum, and the second layer is made of silicon. That is, the two layers 32 are MoSi. The MoSi multilayer 30 starts with a ruthenium capping layer followed by silicon. In another example, the bilayer (first and second layers) can be made from other materials such as RuSi (ruthenium silicon). RuSi multilayers start with Ru. Ru is a capping layer, but functions almost the same as Mo. Ru has a larger reflection than Mo at the SiRu interface, but has a larger absorption than Mo, and is a trade-off. The first layer 34 made of Mo has a refractive index less than 1 for EUV radiation. The second layer 36 made of Si has a refractive index approximately equal to 1 for EUV radiation. Accordingly, the second layer 36 of Si has a refractive index close to that of vacuum. The Si second layer 36 can be thought of as a spacer used to space the Mo first layer 34, and the Si second layer 36 provides relatively little radiation absorption. The difference in refractive index between the first layer 34 and the second layer 36 provides the desired reflection and transmission of EUV radiation.

[00038] The first layer 34 has a thickness d1. The second layer 36 has a thickness d2. The total thickness of the first layer 34 and the second layer 36 is D. Therefore, the total thickness of the two layers 32 is D. That is, D = d1 + d2. The thickness of these layers is measured in the z direction. The layers of the multilayer 30 extend in the x direction and the y direction. The multilayer 30 may form a square or rectangular shape when viewed from above, ie in the z direction. In reality, an actual mirror may have a capping layer and an intermediate layer instead of a sudden transition between the first and second layers, but the principle of operation is not affected thereby.

As shown in FIG. 3, the incident radiation beam B is partially reflected and transmitted by the two layers 32 of the multilayer 30. Specifically, the reflection can occur at the interface between the first layer 34 and the second layer 36 of the bilayer 32. Reflection can also occur at the interface between the second layer 36 of a certain bilayer 32 and the first layer of the adjacent bilayer 32.

[00040] The incident radiation beam B may be unpolarized radiation. Unpolarized radiation consists of polarized components in all directions perpendicular to the direction of propagation of the radiated wave. When each of the polarization directions is decomposed into components along directions perpendicular to each other, the unpolarized radiation can be regarded as two perpendicular planar polarized beams of equal magnitude. Thus, the incident radiation beam B has a p-polarized component (radiation polarized parallel to the incident surface of the multilayer 30) and an s-polarized component (radiation polarized perpendicular to the incident surface of the multilayer 30). It can be considered to have.

[00041] After reflection from the multilayer 30, the s-polarized component of the reflected radiation beam B 'increases relative to the s-polarized component of the incident radiation beam B. In other words, after reflection from the multilayer 30, the reflected radiation is more polarized than the incident radiation. This is because a relatively large amount of the s-polarized component is reflected from the multilayer 30 and a relatively large amount of the p-polarized component is transmitted into the multilayer 30. In some examples, incident radiation may be partially polarized before entering the multilayer 30 (eg, partially s-polarized), but after reflection from the multilayer 30, the reflected radiation is more polarized than the incident radiation. Can be done.

[00042] Maximum reflection of radiation is reached under the condition k _z (d1 + d2) = π. Here, k _z = k cos (aoi). Using the wave equation k = 2π / λ, d1 + d2≈λ / (2cos (aoi)). Accordingly, the thickness D of the bilayer 32 is substantially equal to λ / (2cos (aoi)). That is, D≈λ / (2cos (aoi)). This means that the reflectance of the s-polarized radiation reflected from the bilayer 32 is maximized for aoi. As long as the two layers are adapted to aoi using the formula D≈λ / (2cos (aoi)), the maximum s-polarized radiation reflected from the multilayer depends on aoi. In order to reduce the thickness, the reflectivity drops to zero. For s-polarized radiation, maximum reflection can occur at any angle of incidence if the thickness of bilayer 32 is scaled as 1 / cos (aoi).

[00043] FIG. 4 shows a multilayer having 50 MoSi bilayers 32 with d1 (Mo first layer 34) of 3.92 nm and d2 (Si second layer 36) of 5.88 nm. Figure 2 shows a related reflection, transmission and radiation loss graph for a single plane wave. The incident EUV radiation beam B can have a wavelength substantially in the range of 13.5 nm to 14 nm. In other examples, the wavelength of EUV radiation may be different and the bilayer thickness may be scaled accordingly.

[00044] As shown in the reflection graph of FIG. 4, the radiation reflected from the multilayer 30 is substantially fully s-polarized (TE) and reflected at about 44 degrees aoi. There is virtually no p-polarized radiation (TM). Therefore, aoi of 44 degrees can be regarded as the Brewster angle of the multilayer 30. For this reason, the bilayer 32 (and thus the multilayer 30) is such that the incident radiation at aoi corresponding to the Brewster angle (here ≈44 degrees) is reflected from the multilayer, and this reflected radiation is substantially completely s. It is configured to be polarized. This means that the multilayer mirror 30 has a Brewster angle that can be used to produce a polarizing mirror for EUV radiation. The film on which the multilayer 30 is placed can be relatively thick in the case of a purely reflective device. In this case, the membrane can absorb all transmitted radiation.

[00045] In this example, the ratio of the thickness of the first layer 34 to the thickness of the second layer 36 in the two layers is 1: 1.5. That is, the first layer 34 is 40% of the total thickness of the two layers 32. Since Si absorbs less than Mo, the smaller the Mo layer thickness, the less radiation is absorbed. However, this ratio should be kept close to 50% in order to have sufficient reflection at the Si / Mo interface. Smaller reflection means deeper penetration into the multilayer 30 and thus greater absorption. For this reason, there are conflicting effects. It can be regarded as an optimum value that the Mo layer is 40% of the total thickness of the two layers. In some examples, the ratio of the thicknesses of the first and second layers can be at least 1: 1.5 and up to 1.5: 1. That is, the thickness of the first layer can range from 40% of the total thickness of the two layers to 60% of the total thickness of the two layers.

[00046] The thickness D of the two layers 32 (ie, d1 + d2) is selected so that the reflectance of the s-polarized radiation reflected from the two layers 32 is maximized when the incident angle of the incident radiation is a Brewster angle. Is done. As shown in the reflection graph of FIG. 4, the reflectance of s-polarized radiation is about 75% of the total amount of s-polarized radiation incident on the multilayer 30. Thus, for reflection from the multilayer 30 at the Brewster angle, the loss of s-polarized radiation is (ideally) 25%. In contrast, the reflectance of p-polarized radiation is 0% of the total amount of p-polarized radiation incident on the multilayer 30. Thus, for reflection from the multilayer 30 at Brewster's angle, the loss of p-polarized radiation is (ideally) 100%.

[00047] The transmission graph of FIG. 4 shows that there is no transmission of s-polarized radiation at Brewster's angle and that there is some transmission of p-polarized radiation. The loss graph of FIG. 4 shows that in multilayer 30, there is a relatively small, approximately 25% absorption of s-polarized radiation, but there is a relatively large absorption of p-polarized radiation.

[00048] The loss and transmission of s-polarized radiation is 25%, and the overall loss and transmission of p-polarized radiation is about 50% overall efficiency for a “standard” multilayer mirror operating at approximately 6 degrees aoi. It means that. Using multilayer 30 instead of a standard pupil mirror provides polarized radiation at 50% of the total incident radiation, which is relatively high efficiency with EUV radiation. To use the multilayer mirror 30 with a Brewster angle of about 44 degrees instead of 6 degrees of radiation incident angle (aoi), it may be necessary to change the optical design of the EUV lithographic apparatus LA.

[00049] The 50 bilayer 32 was selected because it gives the maximum reflection of the radiation, ie, gives a maximum of 75% of the s-polarized radiation. In this example, more than 50 bilayers does not provide further reflection of s-polarized radiation, only increasing radiation loss and increasing the complexity of the multilayer 30. In another example, the multilayer 30 may have a smaller number, i.e., less than 50 bilayers 32. In some examples, the multilayer may have 10 bilayers. In other examples, the multilayer may have less than 10 MoSi bilayers. In other examples, the optical element may have a single bilayer. In other examples, a plurality of optical elements having one or more bilayers can be positioned adjacent to each other. For example, 10 parallel mirrors, each with one bilayer, have the same effect as a single multilayer mirror with 10 bilayers when positioned at the correct inter-mirror distance due to positive interference of reflected light Can have. In fact, such devices require spacers.

[00050] FIG. 5 shows a range of aoi reflection, transmission, and loss results for a multilayer 30 having ten MoSi bilayers 32. For a single plane wave, the thickness of the MoSi bilayer 32 is the same as that of FIG. 4, ie d1 is 3.92 nm and d2 is 5.88 nm. Similar to FIG. 4, if the radiation is incident at a Brewster angle of ≈44 degrees, the radiation reflected from the multilayer 30 is completely s-polarized radiation, but the reflectivity is lower, ie about 65%. If the radiation is incident at a Brewster angle of ≈44 degrees, there is substantially no p-polarized radiation reflected from the multilayer 30 having ten MoSi bilayers 32.

[00051] In some examples, transmission of p-polarized radiation through the multilayer 30 may be recovered for use as s-polarized radiation. This recovered radiation can then be used in the lithographic apparatus LA. For example, this can be combined with s-polarized radiation reflected from the multilayer 30. The illumination system IL may include a reflector (not shown) disposed substantially in a plane perpendicular to the plane of incidence of the multilayer 30 so that p-polarized radiation transmitted through the optical element is reflected by the reflector surface. S-polarized light. The reflector is configured to reflect radiation. In other words, p-polarized radiation transmitted through the multilayer is p-polarized with respect to the plane of incidence of the multilayer 30. This p-polarized radiation can be regarded as s-polarized radiation with respect to a plane perpendicular to the plane of incidence of the multilayer 30. The s-polarized radiation is ideally 75% transparent and can be reflected at any angle. Thus, a larger amount of EUV radiation is s-polarized for use in the lithographic apparatus LA by reflecting this s-polarized radiation (with respect to a plane perpendicular to the plane of incidence of the multilayer 30) using a reflector. It becomes possible.

[00052] The membrane in which the multilayer 30 is disposed may have a thickness that can transmit radiation. For example, the film can be a relatively thin slice of silicon, similar to a pellicle used to protect the substrate W from contamination such as particles. For example, the film may have 40 nm thick silicon and 10 nm capping of Si ₃ N ₄ against corrosion. The membrane can be smaller than the pellicle and can be as large as a field mirror, for example 20 cm ² . The membrane causes an additional loss in permeation.

[00053] As mentioned above, different numbers of bilayers may be used in other examples. For example, one bilayer may give 100% reflected s-polarized radiation, but the reflectivity may be lower than 10%. Five bilayers can give 41% s-polarized radiation reflectance. Fifteen bilayers can give 72% s-polarized reflectivity, but p-polarized radiation transmission can be only 45%. Thus, having a bilayer number of 10 or about 10 can give a good tradeoff between the reflectance of s-polarized radiation and the transmission of p-polarized radiation.

[00054] FIG. 6 graphically illustrates the reflection, transmission, and loss results for a range of aois when the thickness of the multilayer 30 is selected to optimally reflect at 34 degrees aoi. Yes. In this example, d1 = 5.04 nm and d2 = 3.36 nm. In this multilayer 30 there are 50 MoSi bilayers 32. In other examples, there may be more or less than 50 bilayers. In this example, 34 degrees is not the Brewster angle, so some p-polarized radiation is reflected from the multilayer 30. In this example, 1 / cos (aoi) = 1.2. Thus, using the formula d1 + d2≈λ / (2cos (aoi)), the aoi that gives the maximum reflectivity (for both s-polarized and p-polarized radiation) is 34 degrees.

[00055] At an optimal aoi of 34 degrees, the reflectance of s-polarized radiation is 75% and the reflectance of p-polarized radiation is 38%. Therefore, the reflected radiation is not fully polarized. However, the reflectance of the reflected radiation is relatively high (ie, optimized for this aoi) and the reflected radiation is partially polarized (ie, the s-polarized component of the reflected radiation is the p-polarized component of the reflected radiation). Bigger than). For this reason, this partially polarized radiation may still be useful in the lithographic apparatus LA. At 34 degrees aoi, only 2.5% of p-polarized radiation is transmitted and 0% of s-polarized radiation is transmitted.

[00056] The multilayer 30 may be part of the illumination system IL. In some examples, the multilayer 30 can replace one or more of the facet field mirror devices 10 in the illumination system IL. That is, the multilayer 30 can be positioned on the field plane of the illumination system IL. In other examples, the multilayer 30 can replace one or more of the faceted pupil mirror devices 11 in the illumination system IL. In some examples, the illumination system IL may include multiple multilayers 30.

[00057] Although this text specifically refers to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein has other uses. Other possible applications include the manufacture of integrated optical systems, magnetic domain memory guidance and detection patterns, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[00058] Although this document specifically refers to embodiments of the invention in the context of a lithographic apparatus, the embodiments herein may be used with other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). . These devices can generally be referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[00059] While the above specifically refers to the use of embodiments of the present invention in the context of optical lithography, the present invention is not limited to optical lithography where the context allows, eg, other such as imprint lithography. It will be appreciated that it can also be used for other purposes.

[00060] Embodiments of the invention can be implemented in hardware, firmware, software, or any combination thereof where the context allows. Embodiments of the invention can also be implemented as instructions stored on a machine-readable medium that can be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (eg, a computing device). For example, a machine readable medium may be a read only memory (ROM), a random access memory (RAM), a magnetic storage medium, an optical storage medium, a flash memory device, an electrical, optical, acoustic, or other form of propagated signal (eg, a carrier wave, Infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, such descriptions are merely for convenience and such actions may actually occur and occur from a computing device, processor, controller, or other device that executes firmware, software, routines, instructions, etc. In doing so, it will be appreciated that actuators or other devices may interact with the material world.

[00061] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims (20)

Comprising an illumination system configured to condition a radiation beam having a wavelength λ,
The illumination system comprises an optical element;
The optical element comprises two layers having a first layer and a second layer;
The two layers are configured such that radiation incident on the two layers at an angle of incidence (aoi) is reflected from the two layers and the s-polarized component of the reflected radiation is increased relative to the s-polarized component of the incident radiation. A lithographic apparatus.   The thickness of the two layers is substantially equal to λ / (2cos (aoi)) so that the reflectance of the s-polarized radiation reflected from the two layers is maximized for the aoi. Lithographic apparatus.   The two layers are configured such that radiation incident on the two layers at an incident angle (aoi) corresponding to a Brewster angle is reflected from the two layers and the reflected radiation is substantially completely s-polarized. A lithographic apparatus according to claim 1, wherein:   4. The thickness according to claim 1, wherein the thickness of the two layers is configured such that the reflectance of the s-polarized radiation reflected from the two layers is maximized at a Brewster angle. Lithographic apparatus.   A lithographic apparatus according to any one of the preceding claims, wherein the thickness ratio of the first and second layers is at least 1: 1.5 and at most 1.5: 1.   6. A lithographic apparatus according to claim 5, wherein the ratio of the thickness of the first layer to the thickness of the second layer in the two layers is substantially 1: 1.5.   A lithographic apparatus according to any one of the preceding claims, wherein the optical element comprises a multilayer comprising a plurality of two layers.   A lithographic apparatus according to claim 7, wherein the optical element comprises at least 10 bilayers.   A lithographic apparatus according to any one of the preceding claims, wherein the two layers comprise MoSi or RuSi.   The illumination system comprises a reflector substantially disposed in a plane perpendicular to the entrance plane of the optical element, and p-polarized radiation transmitted through the optical element is s with respect to the plane of the reflector. A lithographic apparatus according to any one of the preceding claims, wherein the lithographic apparatus is adapted to be polarized and the reflector is configured to reflect the radiation.   A lithographic apparatus according to any one of the preceding claims, wherein the optical element is positioned in a field plane of the illumination system.   A lithographic apparatus according to any one of the preceding claims, wherein the illumination system comprises a plurality of optical elements.   A lithographic apparatus according to any one of the preceding claims, wherein the lithographic apparatus is an EUV lithographic apparatus. A method of polarizing radiation for a lithographic apparatus having an illumination system with an optical element comprising:
Directing radiation having a wavelength λ to be incident at an angle of incidence (aoi) on the optical element comprising two layers having a first layer and a second layer;
Reflecting radiation from the two layers such that the s-polarized component of the reflected radiation is increased relative to the s-polarized component of the incident radiation;
Including methods.   Reflecting radiation from the two layers having a thickness substantially equal to λ / (2cos (aoi)) such that the reflectance of the s-polarized radiation reflected from the two layers is maximized for the aoi. 15. The method of claim 14, further comprising:   16. The method of any of claims 14 or 15, further comprising reflecting radiation from the bilayer at an incident angle (aoi) corresponding to a Brewster angle so that the reflected radiation is substantially fully s-polarized. The method described in 1.   Further comprising reflecting radiation from the two layers at an angle of incidence (aoi) corresponding to a Brewster angle, wherein the thickness of the two layers is such that the reflectivity of the s-polarized radiation reflected from the two layers is The method according to claim 13, wherein the method is configured to maximize the Brewster angle.   18. A method according to any of claims 13 to 17, further comprising reflecting radiation from a multilayer comprising a plurality of bilayers.   P-polarized radiation transmitted through the optical element is reflected by reflecting radiation transmitted through the optical element from a reflector substantially disposed in a plane perpendicular to the plane of incidence of the optical element. 19. A method according to any one of claims 13 to 18 further comprising allowing it to be reflected as s-polarized radiation against the surface of the reflector.   20. A method according to any one of claims 13 to 19, wherein the radiation is EUV radiation.