EUV MULTILAYER MIRROR, OPTICAL SYSTEM INCLUDING A MULTILAYER MIRROR AND METHOD OF MANUFACTURING A MULTILAYER MIRROR

Abstract

A multilayer mirror (M) reflecting extreme ultraviolet (EUV) radiation from a first wavelength range in an EUV spectral region includes a substrate (SUB) and a stack of layers (SL). The stack of layers has layers having a low index material and layers having a high index material. The low index material has a lower real part of the refractive index than does the high index material at a given operating wavelength in the first wavelength range. The stack of layers also includes a spectral purity filter on the stack of layers. The spectral purity filter is effective as an anti-reflection layer for ultraviolet (UV) radiation from a second wavelength range in a UV spectral region to increase an EUV-UV-reflectivity ratio of the multilayer mirror. The spectral purity filter (SPF) includes a non-diffractive graded-index anti-reflection layer (GI-AR) effective to reduce reflectivity in the second wavelength range.

Claims

1. A multilayer mirror configured to reflect extreme ultraviolet (EUV) radiation from a first wavelength range in an EUV spectral region, the mirror comprising: a substrate; a stack of layers on the substrate, wherein the stack of layers comprises layers comprising a low index material and layers comprising a high index material, the low index material having a lower real part of the refractive index than does the high index material at a given operating wavelength λ in the first wavelength range, a spectral purity filter on top of the stack of layers, wherein: the spectral purity filter is effective as an anti-reflection layer for ultraviolet (UV) radiation from a second wavelength range in a UV spectral region, to increase an EUV-UV-reflectivity ratio of the multilayer mirror, and the spectral purity filter comprises a non-diffractive graded-index anti-reflection layer configured to reduce reflectivity in the second wavelength range, wherein the non-diffractive graded-index anti-reflection layer comprises a sub-wavelength structure, wherein the sub-wavelength structure is a periodic structure comprising an array of tapered structural elements forming a periodic surface relief structure, and wherein a period length of the periodic sub-wavelength structure is in a range from 25 nm to 100 nm.

2. The multilayer mirror according to claim 1, wherein the wherein the structural elements have lateral dimensions smaller than the UV wavelength in the second wavelength range, wherein the sub-wavelength structure is configured with a graded refractive index that reduces an optical contrast at a radiation entry surface of the stack of layers and suppresses at least partly reflections of the UV radiation.

3. The multilayer mirror according to claim 2, wherein the sub-wavelength structure comprises an array of the structural elements with monotonically changing effective refractive index in the second wavelength range from a substrate side to a radiation incidence side.

4. The multilayer mirror according to claim 1, wherein the periodic surface relief structure has a single periodicity across an entire reflective surface of the mirror, or wherein the sub-wavelength structure is a combined periodic structure in which at least two different profile tapered structures are combined across the entire reflective surface.

5. The multilayer mirror according to claim 1, wherein the sub-wavelength structure comprises a pyramid-array surface relief structure.

6. The multilayer mirror according to claim 1, wherein the period length of the periodic sub-wavelength structure is larger than a wavelength in the first wavelength range and smaller than a wavelength in the second wavelength range.

7. The multilayer mirror according to claim 1, wherein a structural depth of the sub-wavelength structure is smaller than a wavelength in the second wavelength range.

8. The multilayer mirror according to claim 1, wherein the non-diffractive graded-index anti-reflection layer is formed of a single material having a low absorption with an extinction coefficient of less than 0.007 for EUV radiation in the first wavelength range.

9. The multilayer mirror according to claim 8, wherein the single material is selected from the group consisting of amorphous silicon (Si), hydrogenated silicon α-Si:H, and carbon (C).

10. The multilayer mirror according to claim 1, wherein the non-diffractive graded-index anti-reflection layer comprises a multilayer structure corresponding to a multilayer structure of the stack of layers.

11. The multilayer mirror according to claim 1, further comprising a protective capping layer on the non-diffractive graded-index anti-reflection layer.

12. The multilayer mirror according to claim 1, wherein the non-diffractive graded-index anti-reflection layer is arranged on a diffractive grating structure dimensioned to diffract radiation from a third wavelength range with wavelengths larger than the second wavelength.

13. The multilayer mirror according to claim 1, configured as a collector mirror for collecting EUV radiation emitted from an EUV radiation source.

14. A method of manufacturing a multilayer mirror reflecting extreme ultraviolet (EUV) radiation from a first wavelength range in an EUV spectral region, comprising: providing a substrate; forming a stack of layers on the substrate, wherein the stack of layers comprises layers comprising a low index material and layers comprising a high index material, the low index material having a lower real part of the refractive index than does the high index material at a given operating wavelength λ in the first wavelength range, and forming a spectral purity filter on top of the stack of layers, wherein: the spectral purity filter is effective as an anti-reflection layer for ultraviolet (UV) radiation from a second wavelength range in a UV spectral region, to increase an EUV-UV-reflectivity ratio of the multilayer mirror in the first wavelength range, and forming the spectral purity filter includes forming a non-diffractive graded-index anti-reflection layer configured to reduce reflectivity in the second wavelength range, wherein the non-diffractive graded-index anti-reflection layer comprises a sub-wavelength structure, wherein the sub-wavelength structure is a periodic structure comprising an array of tapered structural elements forming a periodic surface relief structure, and wherein a period length of the periodic sub-wavelength structure is in a range from 25 nm to 100 nm.

15. The method according to claim 14, wherein forming the spectral purity filter comprises: removing material from a free surface of the multilayer mirror by guiding an ion beam consisting essentially of noble gas ions having energies in a prescribed energy range onto target portions on the free surface of the multilayer mirror.

16. The method according to claim 15, wherein the noble gas ions are selected from a group consisting of argon (Ar) ions, neon (Ne) ions, krypton (Kr) ions and xenon (Xe) ions.

17. The method according to claim 15, wherein the material is removed from the free surface at ion energies in a range from 100 eV to 500 eV.

18. The method according to claim 15, wherein the removal is controlled such that after the material is removed, a surface roughness of the free surface is less than 0.5 nm rms.

19. The method according to claim 15, wherein the ion beam is generated with an independent control of ion flux and control of ion energy.

20. The method according to claim 15, wherein the noble gas ions are generated by an inductive coupled plasma source, and a capacitive coupled plasma is used to direct the ions as the ion beam towards the target portion.

21. The method according to claim 15, wherein the mirror is supported on a substrate holder and the substrate holder is cooled to a temperature less than 0° C.

22. The method according to claim 15, wherein the ions are directed onto the target portions substantially at normal incidence.

23. An EUV optical system comprising at least one multilayer mirror as claimed in claim 1.

24. The EUV optical system according to claim 23, wherein the optical system is an illumination system of a microlithograpy projection exposure apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1A shows a schematic vertical section through the layer structure of a multilayer arrangement in accordance with a first exemplary embodiment;

[0064] FIG. 1B shows a depths profile of refractive index for UV radiation in a transition region between the environment and the multilayer mirror;

[0065] FIG. 2 shows an embodiment comprising a sub-wavelength structure with pyramidal structural elements made of a single material with low absorption for EUV radiation;

[0066] FIG. 3 shows an embodiment comprising a sub-wavelength structure with pyramidal structural elements made of a sequence of high and low index refractive layers similar to the layers of a stack of layers on the substrate side thereof;

[0067] FIG. 4 shows a diagram illustrating the reflectance for ultraviolet radiation in the second wavelength range for various embodiments together with the same reflectance of a reference multilayer mirror without graded-index anti-reflection layer;

[0068] FIG. 5 shows a diagram illustrating the reflectance of multilayer mirrors in the EUV wavelength range in a comparative view for two embodiments and a reference multilayer mirror without graded-index anti-reflection layer;

[0069] FIG. 6 and FIG. 7 show schematically embodiments of multilayer mirrors having both structures suppressing reflection of UV radiation and structures redirecting IR radiation; and

[0070] FIG. 8 shows optical components of an EUV microlithography projection exposure apparatus in accordance with an embodiment.

DETAILED DESCRIPTION

[0071] Aspects of embodiments of the invention are explained below by way of example using a plurality of embodiments of EUV mirrors designed for an EUV operating wavelength of λ=13.5 nm and for angles of incidence AOI in the range 0° to 40°, i.e. normal incidence or near normal incidence. The angle of incidence (AOI) denotes the angle formed by a ray impinging on the mirror surface relative to the normal N to the surface of the mirror at point of incidence. Angle-of-incidence intervals within this range can occur, for example, in optical systems for EUV microlithography which operate with a high numerical aperture.

[0072] FIG. 1 shows a schematic vertical section through the multilayer structure of an EUV mirror M according to a first embodiment. Mirror M comprises a substrate SUB and a stack of layers, SL, formed on the substrate. The stack of layers may also be denoted as “layer stack” SL. An outermost layer of the stack of layers, SL, terminates as the free surface FS of the multilayer mirror on the radiation incidence side. In operation, electromagnetic radiation including a predominant portion of EUV radiation is incident from vacuum on the mirror. In an optical system operating at a finite numerical aperture, rays of radiation enter the mirror from various angles of incidence.

[0073] The stack of layers is formed on the surface of the substrate SUB, which is machined to optical quality to exhibit the desired surface shape, which may be plane or curved. The single layers of the layer stack SL are then deposited on the substrate using a suitable deposition technique, such as Physical or Chemical Vapor Deposition (PVD or CVD). One or more additional layers may be formed on the surface of the substrate prior to depositing the layers of the layer stack, for example to improve mechanical stress properties on the substrate side of the stack of layers.

[0074] The stack of layers comprises layers made of a “low index material” (letter “L”) and adjacent layers made of a “high index material” (letter “H”) deposited on each other in alternate fashion. In a pair of adjacent layers, the low index material is the material having the lower real part of the refractive index at a given operating wavelength than the adjacent high index material. The layer material having a higher real part of the refractive index is also called “spacer” and the layer material having relative thereto a lower real part of the refractive index is also called “absorber”. Layer pairs can be constructed e.g. with the layer material combinations molybdenum/silicon (Mo/Si) and/or ruthenium/silicon (Ru/Si). In this case, silicon respectively forms the spacer material, while Mo and/or Ru serve as absorber material.

[0075] The layer stack SL includes a large number of two-layer repetitive units each including a low index material layer and an adjacent high index material layer. Such repetitive units may also be referred to as “bilayer”. A bilayer can contain at least one further layer, in particular an interposed barrier layer, which can consist e.g. of C, B.sub.4C, Si.sub.xN.sub.y, SiC or a composition comprising one of said materials and is intended to prevent interdiffusion at the interface.

[0076] The layer structure of multilayer mirror M includes a spectral purity filter SPF on a radiation entry side of the stack of layers opposite to the substrate SUB. The free surface FS of the spectral purity filter forms the free surface of the multilayer mirror bounding to the environment, which is typically a vacuum in operation. The spectral purity filter is configured so that it is effective as an anti-reflection layer (AR layer) for ultraviolet radiation UV contained in the radiation incident on the multilayer mirror. While it is desirable that the multilayer mirror has high reflectivity for EUV radiation, the reflectivity of the portions of incident radiation in the UV range (typically from 100 nm to 400 nm) should be as small as possible to avoid significant amounts of UV radiation in the reflected radiation optically downstream of the mirror, i.e. after reflection on the mirror. The spectral purity filter SPF is effective to increase the EUV-UV-reflectivity ratio of the multilayer mirror when compared to a multilayer mirror having the same layer structure of the stack of layers, but without the spectral purity filter layer.

[0077] The spectral purity filter SPF of the embodiment comprises a non-diffractive graded-index anti-reflection layer GI-AR. The spectral purity filter may be formed entirely by the graded-index anti-reflection layer, but may also contain one or more additional layers. The graded-index anti-reflection layer GI-AR terminates as the free surface FS of the multilayer mirror.

[0078] FIG. 1B shows a schematic diagram of a refractive index depth profile in the transition region between the environment (typically vacuum) and the layer structure of the multilayer mirror in a depth direction (z-direction, parallel to the surface normal N). The refractive index axis of the diagram refers to the effective refractive index, n.sub.EFF, for radiation in the second wavelength range (UV radiation), particularly for DUV radiation. The effective refractive index on the radiation incidence side is typically at or very close to 1 (refractive index of vacuum). There may be a minor step towards relatively higher refractive index values (greater than 1) at the free surface FS, i.e. at the transition from a material free environment to a layer including solid layer material. In the example, a minor refractive index step is present between the graded-index antireflection layer GI-AR and the first continuous layer L1 of the stack of layers. The graded-index anti-reflection layer GI-AR has a distribution of material such that a gradient effective index for DUV radiation (in the second wavelength range) results, which forms a smooth, continuous transition from the refractive index n.sub.i at the incidence side (at the free surface FS) to the refractive index of the outermost layer L1 of a repetitive unit on the substrate side of the graded-index anti-reflection layer (for example, layer L1 may be made of Si with n.sub.Si=2.56 at 300 nm). In the example there is a linear increase in refractive index from about 1 to values greater than 2. Within the bilayer structure of the stack of layers the refractive index alternates in a stepwise fashion between the value n.sub.H of the high reflective index material H (corresponding to Si in the example) and the value n.sub.L of the low reflective index material L (corresponding to Mo in this example, with n.sub.Mo=2.77 at 300 nm). (Note that the relative magnitudes of refractive index may have opposite relation in the DUV range. For example, in a Mo/Si combination, Si is the material with relatively lower refractive index in the DUV range)

[0079] A gradient-effective-index layer having these general characteristics can provide a very broad anti-reflection band (reflection-reducing characteristics) to suppress undesirable out-of-band (OoB) radiation especially in the UV range, typically at least in the range from 100 nm to 400 nm, for example.

[0080] The structure of the graded-index anti-reflection layer GI-AR is further configured such that virtually no diffraction of undesirable UV radiation is generated. Quite to the contrary, the graded-index anti-reflection layer GIL effectively enhances the transmission for UV radiation in the transition region so that large portions of the undesirable UV radiation can be absorbed within the depth of the layer structure of the multilayer mirror. A portion may be absorbed in the sub-wavelength structure.

[0081] Inventors have found that very efficient graded-index anti-reflection layers can be manufactured if the graded-index anti-reflection layer comprises or is formed by a sub-wavelength structure, which is further abbreviated by “SWS” in this application. The term “sub-wavelength structure” (SWS) as used herein denotes a structure having structural elements on a spatial (lateral) scale which is substantially smaller than the (undesirable) target wavelength, i.e. the wavelengths from the second wavelength range. If the typical dimensions are small enough in the lateral direction, then the UV radiation cannot resolve the fine structure, but “sees” the SWS as a composite material having a gradient of effective index between the incidence side and the substrate side. The optical properties of a sub-wavelength structure, e.g. the optical constants thereof, are typically between that of gas or vacuum on the one side and that of the material or materials used to generate the sub-wavelength structure. The optical properties can be controlled by changing the filling ratio of the structure, which will be explained in more detail below.

[0082] Periodic sub-wavelength structures may be preferable due to their potential for very efficient suppression of undesired radiation. Preferably, three conditions should be fulfilled so that a periodic structure may effectively work as a sub-wavelength structure approximating a continuous medium with an effective index. According to the first condition, a period-to-wavelength ratio should satisfy the condition P.sub.sws/λ<1/(max (n.sub.i, n.sub.s)+n.sub.i sin θ), where P.sub.sws is the period length in the lateral direction, n.sub.i is the refractive index on the radiation incidence side, n.sub.s is the refractive index on the substrate side of the sub-wavelength structure and λ is the target wavelength (second wavelength) and θ is the angle with respect to the normal of the surface of the multilayer structure. Secondly, it is preferable that the structure is designed such that only one non-evanescent mode is able to propagate in the structure. Thirdly, a structural depth of the sub-wavelength structure should be larger than a significant fraction of the wavelength of the undesirable UV radiation in the second wavelength range.

[0083] Some embodiments of the invention are characterized by the fact that a sub-wavelength structure effective as a graded-index anti-reflection layer is integrated on the light incidence surface side of an EUV-multilayer mirror. It is thereby possible to suppress large portions or the full band of undesirable UV reflection under the premise of maintaining a high level of EUV reflectivity.

[0084] In some embodiments, the sub-wavelength structure SWS is made of a single material with low absorption for EUV radiation to reduce EUV loss. An embodiment is explained in detail in connection with FIG. 2. Other embodiments are characterized by the fact that the structural elements of the sub-wavelength structure are basically made from the same sequence of high and low index refractive layers as the stack of layers on the substrate side thereof. In that case the sub-wavelength structure may have a double effect because (i) EUV radiation may be reflected by the structural elements of the sub-wavelength structure and (ii) the structural elements will be effective to reduce reflectivity of the undesirable UV radiation. An embodiment is described in connection with FIG. 3.

[0085] FIG. 4 shows a diagram illustrating the reflectance R (UV) for ultraviolet radiation in the wavelength range from 100 nm to 400 nm for various embodiments of the present invention together with the same reflectance of a reference multilayer mirror REF having a layer stack with Mo/Si multilayers without graded-index anti-reflection layer. FIG. 5 shows a diagram illustrating the reflectance R (EUV) of multilayer mirrors in the EUV wavelength range from 12.5 nm to 15.5 nm in a comparative view for two embodiments of the present invention and a reference Mo/Si multilayer mirror REF without graded-index anti-reflection layer.

[0086] In the embodiment of FIG. 2 the sub-wavelength structure SWS formed on the radiation entry side of the stack of layers SL is made of a single material with low absorption for EUV radiation. The EUV low-absorption material can be amorphous silicon, for example, due to the absorption edge of Si at λ=12.4 nm. A hydrogenated Si, α-Si:H, can be used as an alternative to obtain even less absorption.

[0087] Alternatively the single material can be amorphous carbon, which corresponds to a material that is often present or can build up during operation as a contamination layer on the free surfaces of EUV reflecting multilayers. In order to provide sufficient material for a subsequently formed sub-wavelength structure SWS, the carbon contamination layer can be made thicker by deliberately forcing the contamination in between operation periods, e.g. by providing a carbon containing contamination source and simultaneous irradiation with EUV light in the vicinity of the surface that shall be covered with a sub-wavelength structure SWS made of carbon. After this, a focused ion beam, e.g. with hydrogen ions may be directed towards the carbon layer in a patterning fashion, such that the desired sub-wavelength structure is forming in the carbon layer. According to this method, the geometry of the sub-wavelength structure SWS can be modified in situ, i.e. without removing the optical element carrying the sub-wavelength structure SWS from the optical system.

[0088] The sub-wavelength structure SWS comprises a regular 2D array of structural elements SE having a generally tapered form with a broad base on the substrate side and a tip on the radiation incidence side. The structural elements SE in the embodiment take the form of pyramids with quadratic base. The tapered shape causes a continuous increase in filling ratio from the radiation incidence side to the substrate side. As used here, the term “filling ratio” refers to a ratio between portions of an area filled with (solid) material and the entire area under consideration in a given sectional plane extending in lateral directions at a certain level (depth) through the sub-wavelength structure SWS. Evidently, the filling ratio is small at the tip level and larger on the base level of the pyramidal sub-wavelength structure. Filling ratio equals 1 in a solid layer of material.

[0089] The sub-wavelength structure SWS may be characterized by a period length P.sub.SWS of (lateral distance between adjacent corresponding portions, for example lateral distance between directly adjacent grooves between pyramids) in the range of 100 nm to 50 nm, i.e. smaller than the (undesirable) second wavelength, but significantly larger than the (desirable) EUV radiation. A structural depth SD.sub.SWS is measured in the normal direction of the multilayer mirror between the substrate side of the sub-wavelength structure (at the base portions of the pyramids) and the light incidence side (at the level of the pyramid tips). In the embodiment, the structural height is about 80 nm, which can already achieve high suppression in the full band of the EUV range. However, other values, such as between 50 nm and 100 nm for example, would also be possible.

[0090] For period lengths P.sub.SWS=50 nm and structural depths SD.sub.SWS=80 nm (curve I in FIG. 4 and FIG. 5) the pyramid-on-multilayer system shows an average UV suppression by about factor 19 compared to the Mo/Si reference system without sub-wavelength pyramid structure. The EUV reflectance is about 68.5% (compared to about 75% for the “perfect” Mo/Si-reference system). Further, the structure shows a relative reflectance up to 91.3%, which is much higher than the relative reflectance of a free-standing membrane.

[0091] Curve II shows the reflectance of a Si pyramid SWS with P.sub.SWS=80 nm and SD.sub.SWS=80 nm.

[0092] It is worth to note that a protective capping layer can be added on top of the sub-wavelength structure. This may require further optimization regarding capping layer material and structural shape. As an example, curve III in FIG. 4 shows the UV reflectance of such sub-wavelength structure with P.sub.sws=80 nm, SD.sub.SWS=80 nm and a 2 nm thick layer of SiO.sub.2 capping layer on the surface thereof.

[0093] A sub-wavelength multilayer pyramid structure basically in accordance with FIG. 3 may provide an even higher EUV efficiency due to the fact that the structural elements SE of the sub-wavelength structure (here shaped as quadratic pyramids) are also reflective for EUV radiation due to their multilayer structure. Nevertheless, the lateral period length P.sub.sws of the sub-wavelength structure required for UV suppression should be smaller than the UV wavelength to be suppressed but still larger than the EUV wavelength.

[0094] EUV diffraction is evident for multilayer pyramids with period from about 100 nm towards smaller periods. But the diffraction angles of the higher orders are too large for sub-100 nm period so the higher orders cannot be collected. Simulations show that when period is reduced from 80 nm to 30 nm, the 0th order efficiency/reflectance is increasing, so diffraction efficiency is decreasing. Further reducing the period will help to reduce the reflectivity loss due to high orders diffraction.

[0095] For instance, a multilayer sub-wavelength structure with a period length of 30 nm and a structural depth of 84 nm (corresponding to 12 bilayers, see curve IV in FIG. 4 and FIG. 5) can still have a high EUV reflectance (0.sup.th order) of 71.1%, which means a relative efficiency of 94.8%. This value is even higher than the value discussed above for structural elements made of Si. The efficiency of the multilayer pyramid structure with regard to UV suppression is comparable to that of the single material Si pyramid sub-wavelength structure as shown in FIG. 4.

[0096] The reflectance values discussed here are based on calculations rather than measurements. All calculations were performed based on rigorous coupled wave theory (RCWT). Alternative shapes of structural elements, like tapered structures which can produce linear, cubic, quintic index profiles, or Klopfenstein tapered structures, can be used. With further optimization, they may provide better suppression effect which may then be sufficiently efficient with even less structural depth of the SWS to achieve the same suppression results as pyramid structures. Pyramid structure here is a geometric profile, not an index profile. Different groove shapes may also be optimized for the multilayer sub-wavelength structure to further reduce undesirable diffraction effects for EUV radiation.

[0097] A sub-wavelength structure effective as a graded-index anti-reflection layer for UV radiation may be integrated with structures capable of suppressing infrared (IR) radiation from being reflected in 0.sup.th order from a surface of a multilayer mirror. Exemplary embodiments of multilayer mirrors having both structures suppressing reflection of UV radiation and structures redirecting IR radiation are schematically shown in FIG. 6 and FIG. 7. Both a single material sub-wavelength structure basically in accordance with the embodiment of FIG. 2 (see FIG. 6) and a multi-layered sub-wavelength structure basically in accordance with the embodiment of FIG. 3 (see FIG. 7) may be applied. Characterizing features are now explained in connection with the embodiment of FIG. 6.

[0098] The multilayer mirror M in FIG. 6 comprises a substrate SUB and a stack of layers SL comprising alternative low index material and high index material layers are formed on the substrate. A spectral purity filter SPF comprising a 2D periodic pyramid structure made of silicon (Si) is formed on each portion of the stack of layers. Typical dimensions of the sub-wavelength structure may be the same as in the previous embodiments, for example with period lengths below 100 nm and typical structural depths in the order of 80 nm, for example.

[0099] This anti-reflection structure effective to suppress reflection of UV radiation is applied on a diffractive structure dimensioned to effectively suppress direct reflection of infrared radiation. Specifically, a 2D phase shift grating structure PSG is formed on the incidence side of the multilayer structure. In order to efficiently diffract incident IR radiation out of the optical path used for EUV radiation, a periodic grating structure is formed with lateral period lengths Paz in the order of about 1 mm, for example (typically in a range from 100 μm to 1000 μm) and a structural depth SD.sub.IR in the order of one fourth of the wavelength of infrared radiation.

[0100] The effect of the integrated structure on incident radiation including portions of extreme ultraviolet radiation (EUV), ultraviolet radiation (UV) and infrared radiation (IR) is schematically depicted by the arrows in FIG. 6 or FIG. 7. Incident EUV radiation is reflected with high reflectance by the multilayer structure of the stack of layers without being strongly influenced by either the coarse grating structure suppressing infrared radiation or the fine sub-wavelength structure having anti-reflective effect for UV radiation. Therefore, reflected intensity is a substantial portion of the incident intensity of EUV radiation. Reflection of UV radiation is efficiently suppressed by the pyramid-like sub-wavelength structure on the incidence side of the structured multilayer mirror substantially without diffracting any UV radiation in directions other than the 0.sup.th order reflection direction. Further, portions of incident radiation from the IR range are effectively diffracted by the phase shift grating structure PSG with typical period lengths in the order of the wavelengths of the infrared radiation. Mainly due to diffraction, intensity of infrared radiation in the direction corresponding to the 0.sup.th order (OR) of diffraction is substantially reduced, while a relatively larger portion of incident IR radiation is diffracted into 1.sup.st, 2.sup.nd and 3.sup.rd etc. orders of diffraction. The phase shift grating lateral structure is dimensioned such that diffracted IR radiation is directed out of the optical path intended to be used by EUV radiation. IR intensity that is left in the 0.sup.th order direction is further suppressed via destructive interference of the IR light, due to the phase shift grating vertical structure depth which corresponds to an optical path difference in the order of one fourth of the IR wavelength.

[0101] It is worthy to mention that structural elements of a sub-wavelength structure may have different shape like pyramid with four facets, or pyramid with a cone shape. Sometimes, the cone shape can give even better UV suppression.

[0102] Based on theoretical and experimental results, integrated structures of this type may provide a total EUV relative reflectance in the order of 81% or more when compared to a corresponding multilayer mirror structure without such spectral purity filter layer.

[0103] Embodiments of the invention including sub-wavelength graded-index anti-reflection layers effective to reduce reflectivity in the UV spectral range include the following advantages when compared to current spectral purity methods addressed in the introductory portion.

[0104] Firstly, a full band UV suppression can be obtained including the wavelength range from about 100 nm to 400 nm, which is considered an improvement when compared to the antireflection layer method based on destructive interference or phase shift grating

[0105] Further, there is only minor loss of EUV power. It has been found that an EUV relative reflectance after applying such surface structures is expected to be in the order of 91.3% to 94.8% normalized to a standard Mo/Si multilayer mirror without such structure. Thus, if an ideal multilayer mirror including such sub-wavelength structure is used to replace a conventional multilayer mirror not having such graded-index anti-reflection layer, it is expected that less than 9% to 5% EUV power will be lost, while at the same time the level of UV radiation intensity is significantly reduced.

[0106] Further, multilayer mirrors according to embodiments of the invention provide higher mechanical stability as compared to free-standing membranes.

[0107] Further, a better thermal stability can be expected when compared to thin films or membranes especially under high heat load.

[0108] A capping layer can be optimized and coated on the Si sub-wavelength structure in order to protect the multilayer structure against EUV plasma etching or cleaning methods.

[0109] The multilayer mirror may be a collector mirror collecting EUV radiation from a primary radiation source. The spectral purity filter comprising a non-diffractive graded-index anti-reflection layer to efficiently reduce UV reflected intensity can also be applied to any multilayer mirror utilized in an EUV exposure apparatus in order to suppress undesired wavelengths optically downstream of the collector mirror.

[0110] An embodiment of a method of manufacturing multilayer mirrors according to embodiments of the present invention includes a new physical patterning process to remove material from a free surface of the multilayer mirror by guiding an ion beam essentially consisting of noble gas ions having energies in a prescribed energy range onto target portions on the free surface of the multilayer mirror. An embodiment using argon (Ar) ions is described in detail.

[0111] In a first step, a reflective multilayer structure (multilayer composition) including a stack of layers comprising alternating high and low refractive index layers is fabricated in a conventional way. If a single material sub-wavelength structure is desired (see e.g. FIG. 2 and corresponding description) a thick layer of the single material (e.g. a 80 nm layer of Silicon) is formed in a final depositing step prior to structuring. A structuring process including targeted material removal is applied after the deposition is finished.

[0112] An inductively coupled plasma source (ICP) of argon is used to produce high ion density. A further source, namely a capacitive coupled plasma (CCP), is used in a pulsed mode to direct the ions from the coupled plasma source region towards the sample to be structured with low energy, suitable for targeted removal of layer material and therewith creating a structure according to an embodiment or the invention. The sample is placed in a fixed position on a sample holder. A cooling device is provided to cool the sample holder down to temperatures in the order of −20° C. during the process. A low working pressure (<10 mTorr) is applied to avoid collisions and to keep the directionality of ion flux. Preferably, the ions are impinged at normal incidence for this process.

[0113] Dual source systems basically suitable for performing the process are known per se so that no detailed description is given here. For example, dual source systems are known in the deep reactive ion etching field, which typically contains a ICP source and an CCP source. Such system can also work at non-reactive mode—using only ions to sputter material. Such mode can be utilized here. Details on dual source systems are described, for example, in: H. V. Jansen, M. J. de Boer et al, “Black silicon method X: a review on high speed and selective plasma etching of silicon with profile control: an in-depth comparison between Bosch and cryostat DRIE processes as a roadmap to next generation equipment”, J. Micromech. Microeng. 19 (2009) 033001.

[0114] The dual source system provides for a largely separate control of ion density/flux (controlled by setting the ICP power) and ion energy (controlled by setting CCP power). Independent control of ion density/flux and ion energy allows for generating large ion flux with low ion energy to obtain a desired etch rate and/or mask selectivity and, at the same time, low level of sub-surface damage.

[0115] This process is pure physical sputtering, which essentially has a much smaller difference of etch rate between different materials than conventional methods, such as reactive ion etching (RIE), which utilize reactive chemically reactive gas species, such as fluorine or chlorine, to support the etching process. The process using noble gas ions is capable of producing relatively uniform etching with low roughness. The process can be applied to different multi-layered structures and/or material combinations.

[0116] The process may be particularly useful as a process for patterning multilayer structures and/or for refurbishing multilayer structures and/or removal of contamination from a mirror surface. In experiments, over 94% reflectance has been maintained relatively after patterning through 15 bilayers, and the surface roughness was still as low as about 0.2 nm rms.

[0117] The reflectance of the refurbished multilayer mirror whose surface layers are etched away can be further improved by matching the end etching position in the top period with a standing wave field. As the process is pure ion sputtering, the flux and energy uniformity is mainly determined by the bias field (capacitive coupled plasma) on the sample. Therefore, the process may achieve better etching uniformity over a large area compared to processes including chemical plasma with radicals. The etch rate can also be increased by increasing the ion flux.

[0118] The process is not limited to using Ar ions, but other noble gas ions, such as neon (Ne), krypton (Kr) or xenon (Xe) may be used. The process can also be applied to different multilayers or multi-component systems with more than one material composition.

[0119] Multilayer refurbishment is usually done after surface contamination by e.g. tin, Gd, Tb, carbon, oxides or combinations thereof. A non-flat topography may cause a roughness replication problem for physical sputter. Other chemical cleaning processes can be combined or applied before this physical etching process to improve the performance of the refurbishment.

[0120] EUV mirrors of the type described in this application can be used in various optical systems, e.g. in the field of EUV microlithography.

[0121] FIG. 8 shows by way of example optical components of an EUV microlithography projection exposure apparatus WSC in accordance with one embodiment of the invention. The EUV microlithography projection exposure apparatus serves for the exposure of an radiation-sensitive substrate W arranged in the region of an image plane IS of a projection lens PO with at least one image of a pattern of a reflective patterning device or mask M, said pattern being arranged in the region of an object plane OS of the projection lens.

[0122] In order to facilitate the description, a Cartesian xyz coordinate system is indicated, which reveals the respective positional relationship of the components illustrated in the figures. The projection exposure apparatus WSC is of the scanner type. During the operation of the projection exposure apparatus, the mask M and the substrate are moved synchronously in the y-direction and thereby scanned.

[0123] The apparatus is operated with the radiation from a primary radiation source RS. An illumination system ILL serves for receiving the radiation from the primary radiation source and for shaping illumination radiation directed onto the pattern. The projection lens PO serves for imaging the structure of the pattern onto a light-sensitive substrate.

[0124] The primary radiation source RS can be, inter alia, a laser plasma source or a gas discharge source or a synchrotron-based radiation source. Such radiation sources generate a radiation RAD in the EUV range, in particular having wavelengths of between 5 nm and 15 nm. In order that the illumination system and the projection lens can operate in this wavelength range, they are constructed with components which are reflective to EUV radiation.

[0125] Radiation emerging from the radiation source typically contains portions of radiation having wavelength other than the desired EUV radiation, particularly UV radiation and infrared (IR) radiation.

[0126] The radiation RAD emerging from the radiation source RS is collected by a collector COL and guided into the illumination system ILL. The illumination system comprises a mixing unit MIX, a telescope optical unit TEL and a field forming mirror FFM. The illumination system shapes the radiation and thereby illuminates an illumination field situated in the object plane OS of the projection lens PO or in the vicinity thereof. In this case, the shape and size of the illumination field determine the shape and size of the effectively used object field OF in the object plane OS.

[0127] A reflective reticle or some other reflective patterning device is arranged in the object plane OS during the operation of the apparatus.

[0128] The mixing unit MIX substantially consists of two facet mirrors FAC1, FAC2. The first facet mirror FAC1 is arranged in a plane of the illumination system which is optically conjugate with respect to the object plane OS. Therefore, it is also designated as a field facet mirror. The second facet mirror FAC2 is arranged in a pupil plane of the illumination system that is optically conjugate with respect to a pupil plane of the projection lens. Therefore, it is also designated as a pupil facet mirror.

[0129] With the aid of the pupil facet mirror FAC2 and the imaging optical assembly which is disposed downstream in the beam path and which comprises the telescope optical unit TEL and the field forming mirror FFM operated with grazing incidence, the individual mirroring facets (individual mirrors) of the first facet mirror FAC1 are imaged into the object field.

[0130] The spatial (local) illumination intensity distribution at the field facet mirror FAC1 determines the local illumination intensity distribution in the object field. The spatial (local) illumination intensity distribution at the pupil facet mirror FAC2 determines the illumination angle intensity distribution in the object field.

[0131] The projection lens PO serves for the reducing imaging of the pattern arranged in the object plane OS of the projection lens into the image plane IS that is optically conjugate with respect to the object plane and lies parallel thereto. This imaging is effected with electromagnetic radiation from the extreme ultraviolet range (EUV) around an operating wavelength λ, which in the case of the example is 13.5 nm.

[0132] The projection lens has six mirrors M1 to M6 having mirror surfaces which are arranged in a projection beam path PR between the object plane OS and the image plane IS in such a way that a pattern arranged in the object plane or in the object field OF can be imaged into the image plane or the image field IF using the mirrors M1 to M6.

[0133] The mirrors (EUV mirrors) M1 to M6 having a reflective effect for radiation from the EUV range each have a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range and comprising a large number of layer pairs comprising alternately relatively low refractive index and relatively high refractive index layer material.

[0134] Projection exposure apparatuses and projection lenses having this or a similar construction are disclosed for example in the U.S. Pat. No. 7,977,651 B2. The disclosure of said patent is incorporated by reference in the content of this description.

[0135] In the illumination system ILL, with the exception of the field forming mirror FFM operated with grazing incidence, all mirrors can benefit from multilayer mirror structure of the type proposed here. Alternatively, or in addition, the reflective surface of the collector mirror COL may have structures reducing reflected intensity of UV radiation, in some cases in addition to structures diffracting IR radiation out of the path of EUV radiation.