NL2021359A - A lithographic apparatus and method - Google Patents

Abstract

A lithographic apparatus comprising an illumination system configured to condition a radiation beam having a wavelength λ; wherein the illumination system comprises an optical element; wherein the optical element comprises a bilayer having a first layer and a second layer; and wherein the bilayer is configured such that radiation incident on the bilayer at an angle of incidence (aoi) is reflected from the 5 bilayer such that the s polarization component of the reflected radiation is increased with respect to the s polarization component of the incident radiation.

Description

FIELD [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.

BACKGROUND [0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g,, a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0003] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0004] EUV radiation is generally not produced polarized because the EUV radiation from a plasma source is incoherent. This is in contrast with, e.g. Deep ultraviolet (DUV) radiation which can be produced from a radiation source already polarized. There is a desire to polarise EUV radiation for use in a lithographic apparatus. EUV radiation is relatively highly absorbed by matter (e.g. a vacuum is required for transmission) and so obtaining polarized EUV radiation is problematic as a lot of the EUV radiation would be lost.

SUMMARY [0005] According to a first aspect of the present invention, there is provided a lithographic apparatus comprising an illumination system configured to condition a radiation beam having a wavelength λ; wherein the illumination system comprises an optical element; wherein the optical element comprises a bilayer having a first layer and a second layer; and wherein the bilayer is configured such that radiation incident on the bilayer at an angle of incidence (aoi) is reflected from the bilayer such that the s polarization component of the reflected radiation is increased with respect to the s polarization component of the incident radiation.

[0006] The thickness of the bilayer may be substantially equal to k/(2cos(aoi)) such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized for the aoi.

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

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

[0009] The thicknesses of the first and second layers may have a ratio of at least 1:1.5 and up to a ratio of 1.5:1.

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

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

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

[00013] The bilayer may comprise MoSi or RuSi.

[00014] The illumination system may comprise a reflector located substantially in a plane perpendicular to the plane of incidence of the optical element such that p polarized radiation transmitted through the optical element is s polarized with respect to the reflector plane and the reflector may be configured to reflect the radiation.

[00015] The optical element may be positioned in a 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 comprising an optical element, the method comprising: directing radiation having a wavelength λ such that the radiation is incident at an angle of incidence (aoi) on the optical element comprising a bilayer having a first layer and a second layer; reflecting radiation from the bilayer such that the s polarization component of the reflected radiation is increased with respect to the s polarization component of the incident radiation.

[00019] The method may further comprise reflecting radiation from the bilayer having thickness being substantially equal to k/(2cos(aoi)) such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized for the aoi.

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

[00021] The method may further comprise reflecting radiation from the bilayer at an angle of incidence (aoi) corresponding to Brewster's angle, the thickness of the bilayer being configured such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized at Brewster’s angle.

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

[00023] The method may further comprise reflecting radiation transmitted through the optical element from a reflector located substantially in a plane perpendicular to the plane of incidence of the optical element such that p polarized radiation transmitted through the optical element is reflected as s polarized radiation with respect to the reflector plane.

[00024] The radiation may be EUV radiation.

BRIEF DESCRIPTION OF THE DRAWINGS [00025] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source in accordance with an embodiment of the present invention;

Figure 2 depicts a radiation beam passing through a lens onto a substrate;

Figure 3 depicts a multilayer in accordance with an embodiment of the present invention;

Figure 4 depicts graphs of reflection, transmission and loss of radiation related to the multilayer in accordance with an embodiment of the present invention;

Figure 5 depicts graphs of reflection, transmission and loss of radiation related to the multilayer in accordance with an embodiment of the present invention;

Figure 6 depicts graphs of reflection, transmission and loss of radiation related to the multilayer in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION [00026] Figure 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 the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00027] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the 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 faceted field mirror device 10 and faceted pupil mirror device 11. In this example, the faceted field mirror device includes at least one multilayer 30 (i.e. an optical element) to produce a reflected EUV radiation beam B that is at least partially polarized. This will be described in more detail later.

[00028] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[00029] Tlie substrate W may include previously formed patterns. Where 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, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00031] Tlie 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 that is capable of generating EUV radiation.

[00032] Figure 2 shows a radiation beam 16 which has been provided with a pattern 18, the radiation beam 16 being refracted by a lens 20 onto a substrate W. The radiation beam 16 has p polarized components (radiation polarized parallel to the plane of incidence of the lens 20) as shown. In particular, the radiation beam 16 is shown with a ray having a wave vector ki and an electric field vector Ei in a plane parallel to the plane of incidence and a ray having a wave vector kz and an electric field vector Ez in a plane parallel to the plane of incidence.

[00033] The lens 20 has a relatively high numerical aperture (NA) which means the rays having electric field vectors Ei and Ez ate both refracted at a relatively large angle from the lens 20. This means that when the rays reach the wafer W (see portion in partial ellipse 22), the rays are in almost opposing directions. Further, the electric field vectors Ei and Ez are in almost opposing directions (see enlarged view' of partial ellipse 22 at bottom of Figure 2). This means that the p polarized radiation (waves) will tend to cancel each other out at the substrate W. This is because the p polarized radiation w'ave interference is incomplete at the considerable propagation angle difference. Thus, high NA systems require some form of polarization control. DUV high NA systems generally use highly polarized radiation to avoid this. However, as mentioned above, it is problematic to obtain highly polarized EUV radiation (due to the EUV plasma source producing incoherent radiation and the high absorption of EUV radiation).

[00034] Figure 3 shows the multilayer 30 (i.e. optical element) with an incident EUV radiation beam B having a wavelength λ and a reflected EUV radiation beam B’ being reflected from the multilayer 30. In this respect the multilayer 30 may be considered to be a multilayer mirror. Multilayer mirrors are periodic stacks of layer materials with different refractive index in alternating arrangement. At each interface part of the incoming radiation is reflected, while the rest penetrates deeper into the stack and is partly reflected at other interfaces. All partly reflected coherent beams interfere with each other. The multilayer 30 may be located on or be supported by a membrane (i.e. a substrate) - not shown. The size and thickness of the membrane may be dependent on the particular situation (such as the number of layers in the multilayer) as described in more detail below.

[00035] The incident radiation beam B has an angle of incidence (aoi) with respect to the multilayer 30 as shown. The aoi is the angle between the surface normal of the multilayer 30 and the direction of propagation of the incident radiation beam B. The plane of incidence ol' the multilayer 30 is the plane which contains the surface normal 31 of the multilayer 30 and the wavevector (k) of the incident radiation beam B. The phase of the reflected radiation beam B’ is shown, i.e. a wave with a phase φ of 0 and a wave with a phase φ of 180. The incident radiation beam B is shown similarly.

[00036] The multilayer comprises a plurality of bilayers 32. The bilayers 32 include a first layer 34 and a second layer 36, the radiation being incident on the first layer 34 before then being incident on the second layer 36. The bilayers 32 are adjacent to each other with alternating first layers 34 and second layers 36 throughout the multilayer 30 as shown.

[00037] In this example, the first layer 34 is made from Molybdenum and the second layer is made from Silicon, i.e. the bilayer 32 is MoSi. The MoSi multilayer 30 starts with a capping layer of Ruthenium followed by Silicon. In other examples, the bilayer (the first and second layer) may be made from other materials, such as RuSi (Ruthenium Silicon). A RuSi multilayer starts with Ru. Ru is a capping layer but also functions much like Mo. Ru has a higher reflection at the SiRu interface but also a higher absorption than Mo, so it is a trade-off. The first layer 34 made from Mo has a refractive index of less than 1 for the EUV radiation. The second layer 36 made from Si has a refractive index approximately equal to 1 for the EUV radiation. The Si second layer 36 thus has a refractive index close to the refractive index of vacuum. The Si second layers 36 may be considered to be spacers used to space apart the Mo first layers 34, the Si second layer 36 providing relatively small absorption of the radiation. The difference in refractive index between the first layer 34 and the second layer 36 provides the desired reflection and transmission of the EUV radiation.

[00038] The first layer 34 has a thickness dl. The second layer 36 has a thickness d2. The total thickness of the first layer 34 and the second layer 36 is D. Thus, the total thickness of the bilayer 32 is D. That is, D~dl+d2. The thickness of the layers is measured in the z direction. The layers of the multilayer 30 also extend in the x direction and the y direction. The multilayer 30 may form a square or rectangular shape when viewed from above, i.e. in the z direction. In practice, a real mirror may also have a capping layer and intermetallic layers in place of the sharp transition between the first layer and the second layer but the principle of operation is not affected by this.

[00039] As shown in Figure 3, the incident radiation beam B is partially reflected and transmitted by the bilayers 32 of the multilayer 30. In particular, the reflections may occur at the interface between the first layer 34 and the second layer 36 of the bilayer 32. The reflections may also occur at the interface between the second layer 36 of one bilayer 32 and the first layer of the adjacent bilayer 32 and so on.

[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 radiation wave. Resolving each of the polarization direction into components along directions mutually perpendicular to each other, unpolarized radiation can be considered as two perpendicular plane polarized beams with equal magnitude. Thus, the incident radiation beam B may be considered to have a p polarization component (radiation polarized parallel to the plane of incidence of the multilayer 30) and an s polarization component (radiation polarized perpendicular to the plane of incidence of the multilayer 30).

[00041] Following the reflection from the multilayer 30, the s polarization component of the reflected radiation beam B’ is increased with respect to the s polarization component of the incident radiation beam B. In other words, the reflected radiation is more polarized than the incident radiation following the reflection from the multilayer 30. This is because comparatively more of the s polarization component is reflected from the multilayer 30 while comparatively more of the p polarization component is transmitted into the multilayer 30. In some examples, the incident radiation may also be partially polarized (e.g. partially s polarized) before being incident on the multilayer 30 but, following the reflection from the multilayer 30, the reflected radiation may be more polarized than the incident radiation.

[00042] The maximum reflection of the radiation is reached for the condition k_z(dl+d2)=% where k_z-kcos(aoi). Using the wave number equation 1<+2π/λ, then: dl+d2~Z/(2cos(aoi)). Thus, the thickness D of the bilayer 32 is substantially equal to Z/(2cos(aoi)), i.e. D~Z/(2cos(aoi)). This means that the reflectivity of the s polarized radiation reflected from the bilayer 32 is maximized for the aoi. The maximum s polarization radiation reflected from the multilayer is aoi independent as long as the bilayer is matched to the aoi using the equation D~Z/(2cos(aoi)). For decreasing thickness the reflectivity will drop to zero. For s polarized radiation, the maximum reflection can be made to occur at any angle of incidence provided that the bilayer 32 thickness is scaled as l/cos(aoi).

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

EUV radiation may be different and the thicknesses of the bilayer may be scaled accordingly.

[00044] As shown in the reflection graph of Figure 4, the radiation reflected from the multilayer 30 is substantially fully s polarized (TE - transverse electric) at an aoi of approximately 44 degrees and there is substantially no p polarized radiation (TM - transverse magnetic) reflected. Thus, the aoi of 44 degrees may be considered to be Brewster’s angle for the multilayer 30. Thus, the bilayers 32 (and thus the multilayer 30) are configured such that radiation incident at an aoi corresponding to Brewster’s angle (here ~44 degrees) is reflected from the multilayer such that the reflected radiation is substantially fully s polarized. This means that a multilayer mirror 30 has a Brewster angle that can be employed to create a polarizing mirror for EUV radiation. The membrane that the multilayer 30 is located on may be relatively thick in the case of a purely reflective device. In this case, the membrane may absorb all the 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 bilayer is 1:1.5. That is the first layer 34 is 40% of the thickness of the whole bilayer 32. The less the thickness of the Mo layer, the less the absorption of the radiation, as Si absorbs much less than Mo. However the ratio should stay close to 50% to have enough reflection at the Si/Mo interface. Less reflection means a deeper penetration into the multilayer 30 so more absorption. Thus, there are competing effects. The Mo layer being 40% of the total thickness of the bilayer may be considered to be the optimal value. In some examples, the thicknesses of the first and second layers may have a ratio of at least 1:1.5 and up to a ratio of 1.5:1. That is the thickness of the first layer may be in a range of 40% thickness of the total thickness of the bilayer to 60% thickness of the total thickness of the bilayer.

[00046] The thickness D of the bilayers 32 (i.e. dl+d2) is chosen such that the reflectivity of the s polarized radiation reflected from the bilayer 32 is maximized at the incident radiation angle of incidence being Brewster’s angle. As shown in the reflection graph of Figure 4, the reflectivity of the s polarized radiation is approximately 75% of the total amount of s polarized radiation incident on the multilayer 30. Thus, there is a loss of 25% (ideally) of s polarized radiation on reflection from the multilayer 30 at Brewster’s angle. In contrast, the reflectivity of the p polarized radiation is 0% of the total amount of p polarized radiation incident on the multilayer 30. Thus, there is a loss of 100% (ideally) of p polarized radiation on reflection from the multilayer 30 at Brewster’s angle.

[00047] The transmission graph of Figure 4 show's that there is no transmission of s polarized radiation at Brewster’s angle and there is some transmission of p polarized radiation. The loss graph of Figure 4 shows that there is relatively little absorption of the s polarized radiation, about 25%, but relatively high absorption of the p polarized radiation in the multilayer 30.

[00048] The loss of s polarized radiation being 25% and the loss and transmission of all of the p polarized radiation means that the overall efficiency with respect to a “standard” multilayer mirror operating near 6 degree aoi is about 50%. The multilayer 30 being used in place of a standard pupil mirror would provide polarized radiation at 50% of the total incident radiation which is a relatively liigh efficiency for EUV radiation. Using the multilayer mirror 30 with an angle of incidence (aoi) of the radiation being Brewster’s angle of approximately 44 instead of the aoi of 6 degrees may require modifying the optical design of the EUV lithographic apparatus LA.

[00049] 50 bilayers 32 were chosen as it protides the maximum reflection of the radiation, i.e. the full 75% of the s polarized radiation. In this example, any more bilayers than 50 would not provide any further reflection of the s polarized radiation and would just increase the loss of the radiation and increase complexity of the multilayer 30. In another example, the multilayer 30 may have a reduced number of bilayers 32, i.e. less than 50. 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, there may be a plurality of optical elements having one or more bilayers, the optical elements being positioned adjacent to each other. For example, 10 parallel mirrors, each having one bilayer, may have the same effect as a single multilayer mirror of 10 bilayers, if positioned at the correct inter-mirror distance for positive interference of the reflected light. In practice, such a device would require spacers.

[00050] Figure 5 shows reflection, transmission and loss results over a range of aoi for a multilayer 30 having 10 MoSi bilayers 32. The thickness of the MoSi bilayers 32 is the same as for Figure 4, i.e. with d 1 being 3.92nm and d2 being 5.88nm for a single plane wave. In the same way as with Figure 4, the radiation reflected from the multilayer 30 is fully s polarized radiation but the reflectivity is less,

i.e. approximately 65%, when the radiation is incident at the Brewster’s angle of ~44 degrees. There is substantially no p polarized radiation reflected from the multilayer 30 hating 10 MoSi bilayers 32 when the radiation is incident at the Brewster’s angle of -44 degrees.

[00051] In some examples, the transmission of the p polarized radiation through the multilayer 30 may be recovered to be used as s polarized radiation. This recovered radiation may then be used in the lithographic apparatus LA, e.g. may be combined with the s polarized radiation that has been reflected from the multilayer 30. The illumination system IL may include a reflector (not shown) located substantially in a plane perpendicular to the plane of incidence of the multilayer 30 such that p polarized radiation transmitted through the optical element is s polarized with respect to the reflector plane and the reflector is configured to reflect the 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 then be considered to be s polarized radiation with respect to a plane that is perpendicular to the plane of incidence of the multilayer 30. S polarized radiation can be reflected over any angle with, ideally, 75% transmission. Thus, reflecting this s polarized radiation (with respect to the plane perpendicular to the plane of incidence of the multilayer 30) using the reflector enables a higher amount of EUV radiation to be s polarized for use in the lithographic apparatus LA.

[00052] The membrane that the multilayer 30 is located on may have a thickness that allows radiation to be transmitted through it. For example, the membrane may be a relatively thin slice of silicon, similar to a pellicle used for protecting the substrate W from contamination of particles etc. For example, the membrane may have a 40nm thickness of silicon and a lOnm capping of S13N4 against corrosion. The membrane may be smaller than a pellicle and may be similar in size to a field mirror,

e.g. 20 cm². The membrane will cause additional loss in transmission.

[00053] As mentioned above, in other examples, different numbers of bilayers may be used. For example, I bilayer may also give 100% reflected s-polarization radiation, but the reflectivity may be less than 10%. 5 bilayers may give a reflectivity of s-polarization radiation of 41%. 15 bilayers may give 72% s-polarization reflectivity but only 45% p-polarized radiation transmission. Therefore, having the number of bilayers as 10 or around 10 may provide a good trade off of reflectivity of s-polarized radiation and transmission of p polarized radiation.

[00054] Figure 6 shows the reflection, transmission and loss graphed results for a range of aoi when the multilayer 30 thicknesses are chosen to optimally reflect at 34 degrees aoi. In this example, dl=5.04nm and d2=3.36nm. There are 50 MoSi bilayers 32 in this multilayer 30. In other examples, there could be more or less bilayers than 50. In this example, 34 degrees is not the Brewster’s angle and so there is some p polarized radiation reflected from the multilayer 30. In this example, l/cos(aoi)=1.2. Thus, using the equation dl+d2~V(2cos(aoi)), the aoi that provides the highest reflectivity (for both s polarized radiation and p polarized radiation) is 34 degrees.

[00055] The reflectivity of the s polarized radiation is 75% and the reflectivity of the p polarized radiation is 38% at the optimum aoi of 34 degrees. Thus, the reflected radiation is not fully polarized. However, the reflectivity of the reflected radiation is relatively high (i.e. it is maximised for that aoi) and the reflected radiation is partially polarized (i.e. the s polarization component of the reflected radiation is higher than the p polarization component of the reflected radiation). Thus, this partially polarized radiation may still be useful in the lithographic apparatus LA. At 34 degrees aoi only 2.5% of the p polarized radiation is transmitted and 0% of the s polarized radiation is transmitted.

[00056] The multilayer 30 may be part of the illumination system IL. In some examples, the multilayer 30 may replace one or more of the facetted field mirror devices 10 in the Illumination system IL. That is, the multilayer 30 may be positioned in a field plane of the illumination system IL. In other examples, the multilayer 30 may replace one or more of the facetted pupil mirror devices 11 in the illumination system. In some examples, the illumination system IL may comprise a plurality of multilayers 30.

[00057] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[00058] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus.

Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00059] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[00060] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may 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 (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[00061] While specific embodiments of the invention have been described above, it will he appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses.

1. A lithographic apparatus comprising:

an illumination system configured to condition a radiation beam having a wavelength λ; wherein the illumination system comprises an optical element;

wherein the optical element comprises a bilayer having a first layer and a second layer;

and wherein the bilayer is configured such that radiation incident on the bilayer at an angle of incidence (aoi) is reflected from the bilayer such that the s polarization component of the reflected radiation is increased with respect to the s polarization component of the incident radiation.

2. The lithographic apparatus according to clause 1, wherein the thickness of the bilayer is substantially equal to X/(2cos(aoi)) such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized for the aoi.

3. The lithographic apparatus according to any preceding clause, wherein the bilayer is configured such that radiation incident on the bilayer at an angle of incidence (aoi) corresponding to Brewster’s angle is reflected from the bilayer such that the reflected radiation is substantially fully s polarized.

4. The lithographic apparatus according to any preceding clause, wherein the thickness of the bilayer is configured such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized at Brewster’s angle.

5. The lithographic apparatus according to any preceding clause, wherein the thicknesses of the first and second layers have a ratio of at least 1:1.5 and up to a ratio of 1.5:1.

6. The lithographic apparatus according clause 5, wherein the ratio of the thickness of the first layer to the thickness of the second layer in the bilayer is substantially 1:1.5.

7. The lithographic apparatus according to any preceding claim, wherein the optical element comprises a multilayer comprising a plurality of bilayers.

8. The lithographic apparatus according to clause 7, wherein the optical element comprises at least 10 bilayers.

9. The lithographic apparatus according to any preceding clause, wherein the bilayer comprises MoSi or RuSi.

10. The lithographic apparatus according to any preceding clause, wherein the illumination system comprises a reflector located substantially in a plane perpendicular to the plane of incidence of the optical element such that p polarized radiation transmitted through the optical element is s polarized with respect to the reflector plane and the reflector is configured to reflect the radiation.

11. The lithographic apparatus according to any preceding clause, wherein the optical element is positioned in a field plane of the illumination system.

12. The lithographic apparatus according to any preceding clause, wherein the illumination system comprises a plurality of optical elements.

13. The lithographic apparatus according to any preceding clause, wherein the lithographic apparatus is an EUV lithographic apparatus.

14. A method of polarizing radiation for a lithographic apparatus having an illumination system comprising an optical element, the method comprising:

directing radiation having a wavelength λ such that the radiation is incident at an angle of incidence (aoi) on the optical element comprising a bilayer having a first layer and a second layer;

reflecting radiation from the bilayer such that the s polarization component of the reflected radiation is increased with respect to the s polarization component of the incident radiation.

15. The method according to clause 14, further comprising reflecting radiation from the bilayer having thickness being substantially equal to X/(2cos(aoi)) such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized for the aoi.

16. The method according to either of clauses 14 or 15, further comprising reflecting radiation from the bilayer at an angle of incidence (aoi) corresponding to Brewster's angle such that the reflected radiation is substantially fully s polarized.

17. The method according to any of clauses 13 to 16, further comprising reflecting radiation from the bilayer at an angle of incidence (aoi) corresponding to Brewster’s angle, the thickness of the bilayer being configured such that the reflectivity of the s polarized radiation reflected from the bilayer is maximized at Brewster’s angle.

18. The method according to any of clauses 13 to 17, further comprising reflecting radiation from a multilayer comprising a plurality of bilayers.

19. The method according to any of clauses 13 to 18, further comprising reflecting radiation transmitted through the optical element from a reflector located substantially in a plane perpendicular to the plane of incidence of the optical element such that p polarized radiation transmitted through the optical element is reflected as s polarized radiation with respect to the reflector plane.

20. The method according to any of clauses 13 to 19, wherein the radiation is EUV radiation.

Claims (6)

CONCLUSION A lithography apparatus comprising: an illumination device adapted to provide a radiation beam; 5 a carrier constructed for supporting a patterning device, which patterning device is capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a The target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device. 1/6 a, LL 2/6 3/6 4/8 (%) SSO | Θ (deg) & (deg) Θ (deg) y 5/6 (%) SSO ^ 6/6 (%) SSOj Q Θ (deg) Θ (deg) Θ (deg) y