WO2013057046A1

WO2013057046A1 - Mirror with piezoelectric substrate, optical arrangement therewith and associated method

Info

Publication number: WO2013057046A1
Application number: PCT/EP2012/070251
Authority: WO
Inventors: Christian Thiele
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2011-10-17
Filing date: 2012-10-12
Publication date: 2013-04-25
Also published as: DE102011084649A1

Abstract

The invention relates to a mirror (13), comprising a substrate (20) and a reflective coating (21), wherein the substrate (20) comprises a piezoelectric material selected from the group comprising PMN-PT and PZN-PT. The invention also relates to an optical arrangement comprising at least one such mirror (13) and a device (25) for generating an electric field (26) in the piezoelectric material of the substrate (20) in order to influence a thickness (d) of the reflective coating (21), the device (25) being designed to set the thickness (d) of the reflective coating (21) such that the reflectivity of the reflective coating (21 ) is maximal at an operating wavelength (λ_Β) of the optical arrangement (1). The invention also relates to an associated method for setting the maximum wavelength of the mirror (13).

Description

Mirror with piezoelectric substrate, optical arrangement therewith and associated method

Cross-reference to related applications

This application claims priority under 35 U.S.C. 199(a) to German Patent Application No. 10 2011 084 649.2, filed on October 17, 2011 , the entire contents of which are hereby incorporated by reference in the disclosure of this application.

Background of the invention

The invention relates to a mirror comprising a substrate and a reflective coating, in particular for use in a microlithography projection exposure apparatus, to an optical arrangement comprising at least one such mirror, and to an associated method.

Microlithography projection exposure apparatuses serve for producing microstructured components by means of a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a

photosensitive layer with the aid of a projection optical unit. The minimum structure size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The smaller the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Imaging light having the wavelength of 193 nm or imaging light having a wavelength in the range of the extreme ultraviolet (EUV), i.e. 5 nm - 30 nm, is principally used nowadays. When using imaging light having a wavelength of 193 nm, both refractive optical elements and reflective optical elements are used within the microlithography projection exposure apparatus. When using imaging light having a wavelength in the range of 5 nm - 30 nm, by contrast, exclusively reflective optical elements are used, which are designated hereinafter as mirrors.

The reflective coating typically comprises a plurality of layer pairs formed from two layers having different refractive indices. If the radiation to be reflected at the mirror has a wavelength in the range of approximately 13.5 nm, then the layers usually consist of molybdenum and silicon. Other material combinations such as e.g. molybdenum and beryllium, ruthenium and beryllium or lanthanum and B₄C are likewise possible. The layer thickness of the layer pairs can be identical in each case and is typically approximately half of the wavelength of the radiation to be reflected, wherein the exact value is also dependent, inter alia, on the angle of incidence (Bragg condition). The layer design, i.e. in particular the thickness of the layer pairs, thus defines the wavelength at which the mirror has a reflectivity maximum. The wavelength of this maximum of the reflectivity, which hereinafter is also designated as the maximum wavelength, should in this case correspond to the operating wavelength of the optical arrangement in which the mirror is operated.

During the operation of the mirror in the optical arrangement, however, undesired shifts in the maximum wavelength of the mirror can occur, which can be caused by the heating of the mirror (i.e. thermally), for example, or which are brought about, if appropriate, by strains in the mirror material.

In order to compensate for such an undesired shift in the maximum wavelength during the operation of the mirror in an optical arrangement, it is known to use active materials, in particular piezoelectric or pyroelectric materials for the layers of the reflective coating. A dynamic correction of the wavefront is also intended to be possible with the aid of these materials, cf. "Smart Multilayer Interactive Optics for Lithography at Extreme UV Wavelengths" (SMILE), (http://www.rijnhuizen.nl/en/node/2463).

The production of lattice distortions in thin layers by piezoelectric materials is also described in the thesis "Piezoelektrisch steuerbare Gitterverzerrungen in Lanthanmanganat-Dunnschichten" ["Piezoelectrically controllable lattice distortions in thin layers of lanthanum manganate"] by Christian Thiele, TU Dresden, 2006.

DE 10 2006 040 277 A1 describes a piezoelectric component comprising a substrate and a magnetic (thin) layer, which component can be used as a resistant component, for example. The electrical and magnetic properties of the magnetic layer are intended to be able to be modified by mechanical expansion, such that, by way of example, the resistance value of the

component can be altered by applying a voltage.

Object of the invention

It is an object of the invention to provide a mirror, an optical arrangement comprising such a mirror and a method which make possible reversible adaptation of the properties of the reflective coating, in particular a shift in a maximum wavelength of the reflective coating.

Subject matter of the invention

This object is achieved by means of a mirror, comprising a substrate and a reflective (multilayer) coating, which mirror is characterized in that the substrate comprises a piezoelectric material or consists of a piezoelectric material selected from the group comprising PMN-PT and PZN-PT. As a result of an electric field being generated in the piezoelectric substrate material, the latter, on account of the inverse piezoelectric effect, is expanded or contracted parallel to the field lines of the applied field and contracted or expanded in a direction perpendicular thereto. This expansion or contraction also affects the reflective coating applied on the substrate, such that an expansion or contraction in the thickness direction of the coating occurs.

By varying the field strength of the electric field, it is possible to adapt the degree of expansion or contraction of the substrate and thus the thickness of the reflective coating, thus resulting in a shift in the wavelength at which the reflective coating has its maximum wavelength. The influence of an expansion or contraction of the reflective coating in one direction on a corresponding contraction or expansion in a direction perpendicular thereto is dependent on the Poisson ratio of the layer materials used and is generally (approximately) identical to the mechanical expansion of the substrate in the thickness direction.

The use of a piezoelectric substrate allows the thickness and thus also the maximum wavelength of the reflective coating to vary over a comparatively large range of, for example, up to approximately 1.2%. With the use of layers of the reflective coating which consist of piezoelectric material, as is the case e.g. in the SMILE project mentioned above, by contrast, only comparatively small mechanical expansions or contractions can be obtained and, consequently, only comparatively small shifts in the maximum wavelength can be achieved.

Appropriate piezoelectric materials for the substrate include piezoelectric lead oxide compounds, e.g. so-called PMN-PT having the chemical empirical formula (1-x) [Pb(Mgi/₃Nb2/3)0₃] - x[PbTi0₃] (e.g. for x = 0.28) or PZN-PT having the chemical empirical formula (1-x) Pb(Zm/3Nb2/3)03 - x[PbTi03] (0≤x<1). In the case of these materials, by generating an electric field which is typically of the order of magnitude of approximately 100 kV/cm, it is possible to achieve very high expansion values which, if appropriate, can be up to approximately 1 % - 2%. Lead zirconate titanate (PZT) having the chemical empirical formula Pb[Zr_xTi_1-x]0₃ (e.g. for x = 0.52) may possibly be used as a piezoelectric substrate material as well.

In one embodiment, at least one electrically conductive layer is arranged on a side of the substrate which faces away from the reflective coating. Said electrically conductive layer serves as a bottom (surface) electrode for applying a voltage to the substrate and can be formed from a metallic material, for example from gold (Au) or from platinum (Pt).

In a further embodiment, the reflective coating comprises at least one electrically conductive layer. With the use of electrically conductive layer materials, the reflective coating itself can serve as a top electrode for generating an electric field in the substrate, contact being made with said reflective coating laterally, for example, for this purpose.

In one embodiment, at least one electrically conductive layer is arranged between the reflective coating and the substrate and/or on a side of the reflective coating which faces away from the substrate. At least one electrically conductive layer between substrate and reflective coating, which can e.g.

likewise consist of a metallic material, can also serve as a top electrode. An electrically conductive layer can also be applied to the top side of the reflective coating. If appropriate, a metallic capping layer, e.g. composed of ruthenium, which is present anyway and serves for avoiding oxidation can serve as a top electrode. It goes without saying that one or more electrically conductive layers can also be provided in the reflective coating itself if the latter's (pairs of) layers themselves are not or not sufficiently electrically conductive.

In a further embodiment, the reflective coating comprises a plurality of pairs of layers having an identical thickness. In the case of this layer design, which is also designated as periodic, the layer thickness of the pairs of layers is typically approximately half of the wavelength of the radiation to be reflected by the mirror (wherein the exact value is dependent on the angle of incidence). By scaling the thicknesses of all the layers of the coating by a common factor brought about by an expansion or contraction of the substrate, it is therefore possible to set the maximum wavelength to a desired value.

In a further embodiment, the coating is embodied such that it is reflective to radiation in the EUV wavelength range. For this purpose, the layer thicknesses and the layer materials of the layers or pairs of layers of the reflective multilayer coating are chosen such that the reflectivity is maximal at a wavelength in the EUV range. This wavelength of maximum reflectivity predetermined by the coating design can be shifted reversibly both to higher and to lower

wavelengths by applying a voltage to the piezeoelectric substrate. It goes without saying that such a shift in the maximum wavelength is also possible, if appropriate, in the case of mirrors comprising multilayer coatings which are designed for reflecting radiation at other wavelengths, e.g. in the DUV range above 150 nm.

In one development, the piezoelectric material of the substrate is

monocrystalline or polycrystailine. The above-described substrate materials can either be present as a single crystal or have a polycrystailine microstructure. In particular for the case where the above-described materials are present as single crystals, particularly high expansion values can be achieved.

The invention also relates to an optical arrangement, for example a projection exposure apparatus for microlithography, e.g. for EUV lithography, comprising: at least one mirror embodied as described above, and a device for generating an electric field in the piezoelectric material of the substrate in order to influence or set the thickness of the reflective coating, the device being designed to set the thickness of the reflective coating such that the maximum wavelength of the reflective coating is at an operating wavelength of the optical arrangement. Depending on the polarity of the applied voltage, the substrate expands parallel to the direction of the electric field and contracts in the directions perpendicular thereto, or vice versa. This has the effect that the reflective coating is also contracted or expanded in the thickness direction, such that the thickness of the pairs of layers of the reflective multilayer coating, the periodicity of the coating and thus also the maximum wavelength of the reflectivity of the mirror vary.

By means of the device, which is typically designed for generating an electric field having a (within certain limits) continuously variable field strength, the position of the maximum wavelength of the reflectivity can thus be influenced in a targeted manner and reversibly. It goes without saying that alternatively or additionally, if appropriate, a correction of the wavefront can also be effected if a field having an inhomogeneous field strength is generated in the substrate. In general, however, the electrodes of the device for generating the electric field are of whole-area design, such that an electric field having a (virtually) constant, homogeneous field strength is generated in the substrate.

Optical arrangements such as, for example, projection exposure apparatuses for microlithography are typically operated at a so-called operating wavelength, at which the intensity distribution of the radiation supplied by a light source or a ray generating system has a sharp intensity maximum. The device can dynamically control the thickness of the reflective coating by open-loop or closed-loop control such that the maximum wavelength of the reflective coating corresponds as exactly as possible to the operating wavelength, i.e. the deviation between the operating wavelength and the maximum wavelength of the reflective coating is reduced by the generation of an electric field in the substrate with a suitably chosen field strength.

The device can, in particular, also have sensors which detect e.g. the

temperature of the mirror or of the substrate. On the basis of a known

relationship between the temperature of the mirror and the change in the layer thickness, a compensation of the thermally governed change in the maximum wavelength can be effected in this case.

It goes without saying that, if appropriate, other measures can also be implemented in order to be able to maximise the reflectivity of the mirror, e.g. by determining at least one indicator for the reflectivity of the mirror. As such an indicator it is possible, for example to measure the reflectivity of the mirror during operation. For this purpose, it is possible to use, in particular, radiation at a different wavelength from the operating wavelength, for example radiation in the UV or in the visible wavelength range in the case of an EUV mirror.

In one embodiment, the device comprises at least one electrode spaced apart from the substrate. In order to generate the electric field it is not necessary for direct electrical contact to be made with the substrate or the mirror. Rather, an electric field can also be generated contactlessly, i.e. the substrate or the entire mirror is operated in an electric field generated by two or more electrodes not conductively connected to the mirror. However, providing the electrodes on the mirror itself has been found to be particularly advantageous since the voltages for generating a desired field strength in the substrate are comparatively low in this case and no undesirable side effects such as the deflection of charged gas particles in the vicinity of the mirror can occur.

The invention also relates to a method for setting the maximum wavelength of the reflectivity of a mirror which comprises a substrate and a reflective coating, the substrate comprising a piezoelectric material, the method comprising:

varying a field strength of an electric field generated in the piezoelectric material of the substrate for setting the maximum wavelength. In this case, the setting is effected as described above in connection with the optical arrangement.

In one variant of the method, the mirror in an optical arrangement is operated with radiation at an operating wavelength and the field strength is chosen such that the maximum wavelength corresponds to the operating wavelength. As has already been explained further above in connection with the optical

arrangement, the reflectivity of the mirror can be maximized (in particular dynamically) in this way.

In one variant, the piezoelectric material of the substrate is selected from the group comprising PMN-PT and PZN-PT. As indicated above, piezoelectric lead oxide compounds may be used to achieve very high expansion values of the substrate.

In a further variant, at least one electrically conductive layer is arranged on a side of the substrate which faces away from the reflective coating. The electrically conductive layer may serve as a (bottom) electrode.

In one variant, the reflective coating is selected to comprise at least one electrically conductive layer which may be used as a top electrode for applying a voltage to the substrate.

In yet another variant, at least one electrically conductive layer is arranged between the reflective coating and the substrate and/or on a side of the reflective coating which faces away from the substrate. The electrically conductive layer may also serve as a top electrode.

In a further variant, the reflective coating is selected to comprise a plurality of pairs of layers having an identical thickness. As indicated above, the periodic design of the coating facilitates setting the maximum wavelength to a desired value.

The reflective coating may be selected such that it is reflective to radiation in the EUV wavelength range by appropriately selecting appropriate layer materials and layer thicknesses. The piezoelectric material of the substrate may be chosen to be monocrystalline or polycrystalline. Particularly high expansion values may be achieved when piezoelectric materials in the form of single crystals are used.

Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be realized in each case individually by themselves or as a plurality in any desired combination in a variant of the invention.

Drawing

Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

Figure 1 shows a schematic illustration of a projection exposure apparatus for microlithography,

Figure 2 shows a schematic illustration of a mirror comprising a

piezoelectric substrate and a reflective coating,

Figure 3 shows a schematic illustration of a further mirror comprising a piezoelectric substrate and comprising a capping layer serving as an electrode,

Figure 4 shows a schematic illustration of a further mirror comprising an electrode spaced apart from the substrate, and

Figure 5 shows a schematic illustration of the reflectivity of a mirror as a function of the wavelength. Figure 1 schematically shows a projection exposure apparatus 1 for EUV lithography. The projection exposure apparatus 1 comprises a ray generating system 2, an illumination system 3 and a projection system 4 which are accommodated in separate vacuum housings and arranged successively in a beam path 6 proceeding from an EUV light source 5 of the ray shaping system 2. By way of example, a plasma source or a synchrotron can serve as EUV light source 5. The radiation in the wavelength range of between approximately 5 nm and approximately 20 nm that emerges from the light source 5 is firstly concentrated in a collimator 7. With the aid of a downstream monochromator 8, the desired operating wavelength λ_Β, which is approximately 13.5 nm in the present example, is filtered out by variation of the angle of incidence, as indicated by a double-headed arrow. The collimator 7 and the monochromator 8 are embodied as reflective optical elements.

The radiation treated with regard to wavelength and spatial distribution in the ray generating system 2 is introduced into the illumination system 3, which has (by way of example) a first and second mirror 9, 10. The two mirrors 9, 10 direct the radiation onto a photomask 1 1 as further reflective optical element, which has a structure that is imaged onto a wafer 12 on a reduced scale by means of the projection system 4. For this purpose, a third and fourth mirror 13, 14 are provided in the projection system 4.

The mirrors 9, 10, 13, 14 each have an optical surface 9a, 10a, 13a, 14a that is subjected to the EUV radiation 6 from the light source 5. In this case, the mirrors 9, 10, 13, 14 are operated under vacuum conditions in a residual gas atmosphere 16, as is shown by way of example for the projection system 4 in Figure 1 .

The construction of a mirror 13 of the projection exposure apparatus 1 is explained by way of example below in association with Figure 2. The mirror 13 comprises a substrate 20, which consists of a piezoelectric material in the present case. A reflective multilayer coating 21 is situated on the piezoelectric substrate 20, said coating comprising a plurality of pairs of alternating individual layers 22a, 22b wherein the thickness of a respective pair of layers is

approximately half of the wavelength for which the reflective multilayer coating 21 is intended to have its maximum reflectivity.

If the radiation to be reflected at the mirror 13 has a wavelength in the range of approximately 13.5 nm, then the individual layers 22a, 22b usually consist of molybdenum and silicon. Other material combinations such as e.g.

molybdenum and beryllium, ruthenium and beryllium or lanthanum and B₄C are likewise possible. In addition to the individual layers described, the reflective coating 21 can also comprise intermediate layers for preventing diffusion or capping layers for preventing oxidation and corrosion. The illustration of such auxiliary layers has been omitted in Figure 2. If the mirror 1 is operated with imaging light at wavelengths of more than 150 nm, the reflective coating 5 generally likewise comprises a plurality of individual layers which consist alternately of materials having different refractive indices.

In the case of the example shown in Figure 2, a functional layer 23 composed of an electrically conductive material, typically a metal, e.g. gold or platinum, is mounted between the substrate 20 and the reflective coating 21. In this case, the functional layer 23 serves as a first of two electrodes 23, 24 of a device 25 for generating an electric field 26 in the substrate 20. A further metallic layer 24, which is mounted on the side facing away from the reflective coating 21 , serves as second electrode.

The device 25 for generating the electric field 26 comprises a voltage source 25a, which is designed for applying a variable voltage to the electrodes 23, 24 in order to be able to continuously vary the field strength E in the substrate 20. In the present case, on account of the inverse piezoelectric effect, the electric field 26 in the substrate 20 brings about an expansion of the substrate 20 parallel to the field lines of the electric field 26, i.e. in the thickness direction of the reflective coating 21. Although the mechanical expansion of the substrate 20 in this direction leads to an alteration of the thickness of the substrate 20, it initially has no influence on the thickness of the reflective coating 21.

The increase in the thickness of the substrate 20 leads, however, to a

contraction in a direction perpendicular thereto, i.e. parallel to the plane of the mirror surface, as is indicated by horizontal arrows in Figure 2. This leads to a corresponding contraction of the reflective coating 21 applied to the substrate 20 parallel to the mirror surface or to the plane of the mirror layers 22a, 22b. As in the case of the substrate 20, in the case of the reflective coating 21 , too, a mechanical contraction in one direction leads to a mechanical expansion in a second direction, perpendicular to the first, i.e. in a thickness direction perpendicular to the planes of the mirror layers 22a, 22b, as is indicated by a double-headed arrow in Figure 2. In this case, the magnitude of this mechanical expansion is dependent on the Poisson ratio of the layer materials 22a, 22b of the reflective coating 21 and on the Poisson ratio of the substrate 20 and can be approximately identical to the expansion of the substrate 20.

As a result of the mechanical expansion of the reflective coating 21 , the thickness d thereof and thus also the thickness of the mirror layers 22a, 22b vary. Since the thickness of a respective pair of layers is typically approximately half of the wavelength to be reflected, the maximum wavelength of the mirror 13 can be set in a continuously variable manner and in particular reversibly by the variation of the voltage applied to the piezoelectric substrate 20.

By applying a voltage having opposite polarity, it is possible, if appropriate, to reverse the relations with regard to expansion and contraction, i.e. the thickness d of the reflective coating 21 decreases, as a result of which the maximum wavelength can be shifted in the opposite direction, i.e. towards smaller wavelengths. Such a reversal is typically only possible when using fields having a comparatively low field strength (small-signal range). At higher field strengths (large-signal range), typically the entire crystal is subjected to polarity reversal, with the result that no reversal of the relations with regard to expansion and contraction occurs.

The piezoelectric substrate 20 can be either a monocrystalline or polycrystalline material, in particular PMN-PT or PZN-PT. Monocrystalline piezoelectric materials, in particular, enable large expansions in the range of up to

approximately 1 - 2% by application of a voltage.

For making electrical contact with the substrate 20, it is not necessary for an electrically conductive functional layer 23 to be introduced between the substrate 20 and the reflective coating 21 , as shown in Figure 2. Rather, as illustrated in Figure 3, an electrically conductive capping layer 27 applied to the reflective coating 21 can also be electrically contact-connected in order to serve as an electrode. Capping layers of this type are applied to the reflective coating 21 in order to prevent oxidation and/or corrosion of the layers 22a, 22b situated underneath, and often consist of electrically conductive materials, in particular of metals, for example of rhodium or ruthenium.

With the use of electrically conductive layer materials 22a, 22b it is also possible to make contact with the reflective coating 21 laterally rather than at the top side. In this case, the use of an electrically conductive capping layer or a functional intermediate layer can also be completely dispensed with. If the materials of the layers 22a, 22b of the reflective coating 21 are not or not sufficiently electrically conductive, it is also possible, if appropriate, to introduce one or more electrically conductive intermediate layers as functional layers between the layers 22a, 22b of the reflective coating 21. In the case of the mirror 13 illustrated by way of example in Figure 4, a second electrode 28 is provided, which is spaced apart from the underside of the substrate 20 and which serves for generating an electric field between said electrode 28 and a functional intermediate layer 23, which is mounted between the substrate 20 and the reflective coating 21 as in Figure 2. It goes without saying that the first electrode too, if appropriate, in contrast to the illustration in Figure 3, need not be mounted on the mirror 13 itself, but rather can be arranged in a manner spaced apart from the mirror 13. If appropriate, as an alternative, the electrodes or electrically conductive layers can also be mounted on the side surfaces of the substrate 20, such that the field lines of the electric field run parallel to the plane of the individual layers 22a, 22b; however, this variant requires a higher electrical voltage in comparison with the variant shown in Figure 2.

Typically, when applying the reflective coating 21 , the thickness of the individual layers 22a, 22b is defined such that the coating 21 is optimized for the operating wavelength λ_Β of the projection exposure apparatus 1 , i.e. the coating 21 should have a wavelength-dependent reflectivity R, the maximum

wavelength AM of which corresponds to the operating wavelength λ_Β, as is the case for the reflectivity curve 30 illustrated by a solid line in Figure 5.

However, as a result of mechanical strains or as a result of thermal influences during the operation of the mirror 13 in the projection exposure apparatus 1 , an undesired change in the thickness d of the reflective coating 21 can occur, which leads to an undesired change in the reflectivity curve 30, which converts the latter into the reflectivity curve 31 illustrated by a broken line in Figure 5, the maximum wavelength being shifted to lower wavelengths in the case of said reflectivity curve 31 .

In order to ensure that the maximum wavelength λ_Μ always corresponds to the operating wavelength λ_Β during the operation of the mirror 13, the maximum wavelength λ_Μ can be suitably (dynamically) influenced or shifted by a variation of the electric field strength of the electric field E in the substrate 20. However, such a dynamic adaptation requires knowledge of the instantaneous maximum wavelength AM or the instantaneous reflectivity of the mirror 13.

In the case of the example shown in Figure 4, a temperature sensor is provided on the substrate 20, said temperature sensor making it possible to determine the instantaneous temperature of the substrate 20 and thus indirectly the temperature of the reflective coating 21 . On the basis of a known relationship between the temperature and the thickness of the reflective coating, which relationship is e.g. determined experimentally or calculated on the basis of the coefficient of thermal expansion, it is possible to determine the instantaneous thickness d (based on thermal effects) of the reflective coating 21 .

It goes without saying that alternatively or additionally the reflectivity R or at least one indicator for the reflectivity of the mirror 13 can also be determined directly. For this purpose, by way of example, radiation at wavelengths outside the EUV wavelength range, e.g. in the visible wavelength range, can be radiated onto the mirror 13 and the radiation reflected by the mirror 13 can be detected. In this case, a control device 1 7 (cf. Figure 1 ) provided in the projection exposure apparatus can communicate the value obtained during a reflection measurement to the device 25 for generating the electric field 26, which sets the voltage applied to the substrate 20 or the electric field strength E such that the value obtained during the reflection measurement at the operating wavelength λ_Β becomes as large as possible, i.e. is maximized.

It goes without saying that configurations that deviate from the mirrors 13 shown above in association with Figure 2 to Figure 4 are also possible.

Although the mirrors 13 shown therein each have a planar surface, the latter was chosen thus merely in order to simplify the illustration, i.e. the mirror 13 can also have a (slightly) curved surface form, wherein e.g. concave surface forms or convex surface forms are possible, which can be embodied spherically and also aspherically. In this case, too, the thickness d of the reflective coating 21 can be set in the manner described above in order to suitably set the maximum wavelength k_M.

It furthermore goes without saying that the reflective coating 21 need not necessarily be embodied periodically, i.e. it is possible to depart from a periodic structure in order to optimize further mirror properties, e.g. in order to increase the broadband characteristic. In this case, too, the change in the thickness d of the reflective coating 21 can be influenced in order to compensate for mismatches of the maximum wavelength k_M.

The layer materials of the individual layers 22a, 22b generally have different Poisson ratios. An expansion/contraction of the coating 21 therefore typically leads to a different change in thickness of the individual layers 22a, 22b and thus to a change in the so-called Γ value, which is defined as the ratio of the thickness of the higher refractive index (high-Z) layer material to the sum of the thicknesses of the two individual layers 22a, 22b (in the case of silicon and molybdenum as layer materials, for example Γ = d(Mo)/d(Mo+Si) holds true). Since the Γ-value likewise influences the position of the maximum wavelength AM, depending on the layer materials used the change in thickness as a result of the voltage applied to the piezoelectric substrate 20 should turn out not to be excessively large. In this case, it has proved to be advantageous if the applied electrical voltage is chosen such that the Γ value changes by not more than approximately +/- 0%.

As explained further above, the reflective coating can also comprise further functional layers serving, for example, for avoiding diffusion. Further functional layers can also be provided between the reflective coating and the substrate, for example layers which can be polished easily, in order to be able to apply the reflective coating with a high surface quality. Since such functional layers have no or only a negligibly small influence on the reflectivity of a mirror, an expansion or contraction of these layers generally does not lead to undesirable side effects.

In general, however, it should be taken into consideration that, for the reflective coating in the present application, only materials should be used which do not degrade, or only degrade slightly, in the case of the contractions or expansions that occur here, i.e. layer materials with which cracking or delamination of individual layers does not occur in the case of mechanical contraction or expansion of approximately 1 - 2%.

Claims

Patent Claims

1. Mirror (13), comprising a substrate (20) and a reflective coating (21 ),

characterized

in that the substrate (20) comprises a piezoelectric material selected from the group comprising PMN-PT and PZN-PT.

2. Mirror according to Claim , wherein at least one electrically conductive layer (24) is arranged on a side of the substrate (20) which faces away from the reflective coating (21 ).

3. Mirror according to Claim 1 or 2, wherein the reflective coating (21 )

comprises at least one electrically conductive layer (22a, 22b).

4. Mirror according to any of the preceding claims, wherein at least one

electrically conductive layer (23, 27) is arranged between the reflective coating (21 ) and the substrate (20) and/or on a side of the reflective coating (21 ) which faces away from the substrate (20).

5. Mirror according to any of the preceding claims, wherein the reflective

coating (21 ) comprises a plurality of pairs of layers (22a, 22b) having an identical thickness.

6. Mirror according to any of the preceding claims, wherein the coating (21 ) is reflective to radiation in the EUV wavelength range.

7. Mirror according to any of the preceding claims, wherein the material of the substrate (20) is monocrystalline or polycrystalline.

8. Optical arrangement (1 ), comprising:

at least one mirror (13) according to any of the preceding claims, and a device (25) for generating an electric field (26) in the piezoelectric material of the substrate (20) for setting the thickness (d) of the reflective coating (21 ), wherein the device (25) is designed to set the thickness (d) of the reflective coating (21 ) such that the reflectivity of the reflective coating (21 ) is maximal at an operating wavelength (λ_Β) of the optical arrangement (1 ).

9. Optical arrangement according to Claim 8, wherein the device (25)

comprises at least one electrode (28) spaced apart from the substrate (20).

10. Method for setting the maximum wavelength (λ_Μ) of the reflectivity (R) of a mirror (13) which comprises a substrate (20) and a reflective coating (21 ), the substrate (20) comprising a piezoelectric material, the method comprising:

varying a field strength (E) of an electric field (26) generated in the piezoelectric material of the substrate (20) for setting the maximum wavelength (λ_Μ).

1 1 . Method according to Claim 10, wherein the mirror (13) is operated in an optical arrangement (1 ) with radiation (6) at an operating wavelength (λ_Β) and the field strength (E) is chosen such that the maximum wavelength (λ_Μ) corresponds to the operating wavelength (λ_Β).

12. Method according to Claim 10 or 1 1 , wherein the piezoelectric material of the substrate (20) is selected from the group comprising PMN-PT and PZN- PT. 3. Method according to any one of Claims 0 to 12, wherein at least one electrically conductive layer (24) is arranged on a side of the substrate (20) which faces away from the reflective coating (21 ).

14. Method according to any one of Claims 0 to 13, wherein the reflective coating (21 ) is selected to comprise at least one electrically conductive layer (22a, 22b).

15. Method according to any one of Claims 10 to 14, wherein at least one

16. Method according to any one of Claims 10 to 15, wherein the reflective

coating (21 ) is selected to comprise a plurality of pairs of layers (22a, 22b) having an identical thickness.

17. Method according to any one of Claims 10 to 16, wherein the reflective

coating (21 ) is selected such that it is reflective to radiation in the EUV wavelength range.

18. Method according to any one of Claims 10 to 17, wherein the material of the substrate (20) is chosen to be monocrystalline or polycrystalline.