Abstract
A mirror arrangement (30) includes: a substrate (31) with a front side (31a) having a mirror face (32a) reflecting radiation (5), and a rear side (31b) facing away from the front side and on which at least one actuator (27) generating deformations of the mirror face is arranged. A water vapor (36)-sorbing material (33, 42) is formed on the rear side (31b) and forms an adhesive layer (33) for securing the actuator. The layer extends into interspaces (35) between the actuators (27). A surface (33a, 42a) of the water vapor-sorbing material is covered at least partly by a coating (37) which forms a water vapor diffusion barrier.
Claims
1. A mirror arrangement comprising: a substrate which has a front side having a mirror face for reflecting radiation, and a rear side facing away from the front side and on which at least one actuator for generating deformations of the mirror face is disposed, wherein a water vapor sorbing material is formed on the rear side of the substrate, wherein a surface of the water vapor-sorbing material is covered at least partly by a coating which forms a water vapor diffusion barrier, and wherein the diffusion barrier comprises at least one material selected from the group consisting essentially of: Al.sub.2O.sub.3, SiO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, HfO.sub.2, ZrO.sub.2, Nb.sub.2O.sub.5, La.sub.2O.sub.3, Ta, Ti, Ru, Si.sub.xC.sub.yO.sub.zH.sub.v, parylenes, fluoropolymers, and mixtures thereof.
2. The mirror arrangement as claimed in claim 1, wherein the water vapor sorbing material is an organic material, forming an adhesive layer for securing the at least one actuator on the rear side of the substrate, the layer extending into interspaces between the actuators, wherein a surface of the adhesive layer is covered at least partly by a coating which forms a water vapor diffusion barrier.
3. The mirror arrangement as claimed in claim 1, wherein the coating comprises at least one diffusion barrier layer having thickness and having a water vapor transmission rate which is lower than the water vapor transmission rate of a layer of same thickness and which consists of the water vapor-sorbing material.
4. The mirror arrangement as claimed in claim 1, wherein the diffusion barrier is a diffusion barrier layer comprising at least one material selected from the group consisting essentially of: Al.sub.2O.sub.3, SiO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, HfO.sub.2, ZrO.sub.2, Nb.sub.2O.sub.5, La.sub.2O.sub.3, Ta, Ti, Ru, Si.sub.xC.sub.yO.sub.zH.sub.v, halogenated parylenes, polytetrafluorethylene, and also mixtures thereof.
5. The mirror arrangement as claimed in claim 1, wherein a surface of the coating and/or the material of the diffusion barrier are/is hydrophobic.
6. A mirror arrangement comprising: a substrate which has a front side having a mirror face for reflecting radiation, and a rear side facing away from the front side and on which at least one actuator for generating deformations of the mirror face is disposed, wherein a water vapor sorbing material is formed on the rear side of the substrate and forms an adhesive layer, wherein a surface of the water vapor-sorbing material is covered at least partly by a coating which forms a water vapor diffusion barrier, and wherein a surface of the adhesive layer is covered at least partly by a filling material having an elasticity modulus (E.sub.1) that is less than the elasticity modulus (E.sub.2) of the adhesive layer.
7. The mirror arrangement according to claim 6, wherein interspaces between the actuators are covered at least partly by the filling material.
8. The mirror arrangement as claimed in claim 6, wherein the elasticity modulus (E.sub.1) of the filling material is less than 1500 MPa.
9. The mirror arrangement as claimed in claim 6, wherein the filling material is hydrophobic.
10. The mirror arrangement as claimed in claim 6, wherein the filling material is selected from the group consisting essentially of: rubber, liquid rubber, wax, grease, oil, or a liquid other than oil.
11. A mirror arrangement comprising: a substrate which has a front side having a mirror face for reflecting radiation, and a rear side facing away from the front side and on which at least two actuators for generating deformations of the mirror face are disposed, wherein a water vapor sorbing material is formed on the rear side of the substrate and forms an adhesive layer, wherein a surface of the water vapor-sorbing material is covered at least partly by a coating which forms a water vapor diffusion barrier, and wherein the surface of the water vapor-sorbing material is covered at least partly by a flexible material.
12. The mirror arrangement according to claim 11, wherein interspaces between the actuators are covered at least partly by the flexible material.
13. The mirror arrangement as claimed in claim 11, wherein the flexible material is a film that protrudes into a respective interspace and covers a depression of the adhesive layer.
14. A mirror arrangement comprising: a substrate which has a front side having a mirror face for reflecting radiation, and a rear side facing away from the front side and on which at least two actuators for generating deformations of the mirror face are disposed, wherein a water vapor sorbing material is formed on the rear side of the substrate and forms an adhesive layer, wherein a surface of the water vapor-sorbing material is covered at least partly by a coating which forms a water vapor diffusion barrier, and wherein the adhesive layer in interspaces between the actuators does not completely cover the rear side of the substrate.
15. A mirror arrangement comprising: a substrate which has a front side having a mirror face for reflecting radiation, and a rear side facing away from the front side and on which at least two actuators for generating deformations of the mirror face are disposed, wherein a water vapor sorbing material is formed on the rear side of the substrate and forms an adhesive layer, wherein a surface of the water vapor-sorbing material is covered at least partly by a coating which forms a water vapor diffusion barrier, and wherein the adhesive layer in interspaces between the actuatorsprojects over the actuators.
16. The mirror arrangement as claimed in claim 15, wherein the actuators are configured as piezo actuators or as electrostrictive actuators.
17. An optical arrangement in and extreme ultraviolet (EUV) lithography apparatus or a deep ultraviolet (DUV) lithography apparatus, comprising: at least one mirror arrangement as claimed in claim 1.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:
[0038] FIG. 1 shows a schematic representation of an EUV lithography unit having a mirror which comprises a plurality of actuators for correcting aberrations,
[0039] FIG. 2 shows a schematic representation of a DUV lithography unit having a mirror which comprises a plurality of actuators for correcting aberrations,
[0040] FIG. 3 shows a schematic sectional representation of the mirror arrangement of FIG. 1 with an adhesive layer for securing the actuators on a substrate,
[0041] FIG. 4 shows a schematic representation of a detail of the mirror arrangement with a coating which is applied to a surface of the adhesive layer and which forms a water vapor diffusion barrier,
[0042] FIG. 5 shows a schematic representation analogous to FIG. 4, in which the surface of the adhesive layer is covered by an elastic filling material,
[0043] FIG. 6 shows a schematic representation analogous to FIG. 4, in which a depression covered by a film is formed in the adhesive layer,
[0044] FIG. 7 shows a schematic representation analogous to FIG. 4, in which the adhesive layer is interrupted in the interspaces between the actuators,
[0045] FIG. 8 shows a schematic representation analogous to FIG. 4, in which the adhesive layer projects over the actuators in an interspace,
[0046] FIG. 9 shows a schematic representation of the adhesive layer, in which the film represented in FIG. 6 is combined with the projecting adhesive layer of FIG. 8, and
[0047] FIG. 10 shows a schematic representation of the adhesive layer of FIG. 9, with a coating serving as a water vapor diffusion barrier being applied on the exposed surface of said layer.
DETAILED DESCRIPTION
[0048] In the following description of the drawings, identical reference signs are used for equivalent or functionally equivalent components.
[0049] FIG. 1 schematically shows the construction of an apparatus for EUV lithography in the form of an EUV lithography unit 1, specifically of a so-called wafer scanner. The EUV lithography unit 1 comprises an EUV light source 2 for generating EUV radiation, which has a high energy density in the EUV wavelength range below 50 nanometers, in particular between approximately 5 nanometers and approximately 15 nanometers. The EUV light source 2 can be configured for example in the form of a plasma light source for generating a laser-induced plasma. The EUV lithography unit 1 shown in FIG. 1 is designed for an operating wavelength of the EUV radiation of 13.5 nm. However, it is also possible for the EUV lithography unit 1 to be configured for a different operating wavelength in the EUV wavelength range, such as 6.8 nm, for example.
[0050] The EUV lithography unit 1 further comprises a collector mirror 3 in order to focus the EUV radiation of the EUV light source 2 to form a bundled illumination beam 4 and in this way to increase the energy density further. The illumination beam 4 serves for the illumination of a structured object M with an illumination system 10, which in the present example has five reflective optical elements 12 to 16 (mirrors).
[0051] The structured object M can be for example a reflective photomask, which has reflective and non-reflective, or at least less reflective, regions for producing at least one structure on the object M. Alternatively, the structured object M can be a plurality of micro-mirrors, which are arranged in a one-dimensional or multi-dimensional arrangement and which are optionally movable about at least one axis, in order to set the angle of incidence of the EUV radiation on the respective mirror.
[0052] The structured object M reflects part of the illumination beam 4 and shapes a projection beam path 5, which carries the information about the structure of the structured object M and is radiated into a projection lens 20, which generates a projected image of the structured object M or of a respective partial region thereof on a substrate W. The substrate W, for example a wafer, comprises a semiconductor material, for example silicon, and is disposed on a mounting, which is also referred to as a wafer stage WS.
[0053] In the present example, the projection lens 20 has six reflective optical elements 21 to 26 (mirrors) in order to generate an image of the structure that is present at the structured object M on the wafer W. The number of mirrors in a projection lens 20 typically lies between four and eight; however, only two mirrors can also be used, if appropriate.
[0054] In addition to the reflective optical elements 3, 12 to 16, 21 to 26, the EUV lithography apparatus 1 also comprises non-optical components, which can be for example carrying structures for the reflective optical elements 3, 12 to 16, 21 to 26, sensors, actuators, etc. Represented illustratively in FIG. 1 are a plurality of actuators 27 which are mounted on the rear side of the sixth mirror 26 of the projection lens 20 in order to carry out targeted deformation of this mirror and, in so doing, to compensate aberrations which occur when the structure is imaged onto the wafer W with the projection lens 20. For the targeted driving of the actuators 27, the EUV lithography unit 1 comprises a control device 28 in the form, for example, of a control computer, which communicates with the actuators 27 via signal lines which are not represented in the image.
[0055] FIG. 2 shows a schematic view of a of a DUV projection exposure unit 100, which comprises a beam shaping and illumination device 102 and a projection lens 104. In this case, DUV stands for “deep ultraviolet” and denotes a wavelength of the working light of between 30 nm and 370 nm. The DUV projection exposure unit 100 comprises a DUV light source 106. For example, an ArF excimer laser that emits radiation 108 in the DUV range at 193 nm, for example, may be provided as the DUV light source 106.
[0056] The beam shaping and illumination device 102 shown in FIG. 2 directs the DUV radiation 108 onto a photomask 120. The photomask 120 is formed as a transmissive optical element and may be arranged outside the beam shaping and illumination device 102 and the projection lens 104. The photomask 120 has a structure which is projected onto a wafer 124 or the like with the projection lens 104.
[0057] The projection lens 104 has a number of lens elements 128, 140 and/or mirrors 130 for projecting the photomask 120 onto the wafer 124. In this case, individual lens elements 128, 140 and/or mirrors 130 of the projection lens 104 may be arranged symmetrically in relation to the optical axis 126 of the projection lens 104. It should be noted that the number of lens elements and mirrors of the DUV projection exposure unit 100 is not restricted to the number shown. More or fewer lens elements and/or mirrors may also be provided. Furthermore, the mirrors are generally curved on their front side for beam shaping.
[0058] An air gap between the last lens element 140 and the wafer 124 may be replaced by a liquid medium 132 which has a refractive index of >1. The liquid medium 132 may be for example high-purity water. Such a set-up is also referred to as immersion lithography and has an increased photolithographic resolution.
[0059] FIG. 3 shows the mirror 26 of FIG. 1 or the mirror 130 from FIG. 2 as part of a corresponding mirror arrangement 30, in a detailed representation. The mirror arrangement 30 comprises a substrate 31 which in the example shown is formed of titanium-doped fused silica (ULE) or fused silica. Other materials which have a low coefficient of thermal expansion or a very low dependence of the coefficient of thermal expansion on the temperature may likewise be used as substrate materials. Applied to a front side 31a of the substrate 31 is a reflective coating 32 configured in such a way that incident EUV radiation 5 or DUV radiation 108 is reflected with a comparatively high reflectivity in a narrow spectral range around the operating wavelength of 13.5 nanometers or, for example, 193.4 nanometers for a specified range of angles of incidence.
[0060] In the case of the EUV mirror 26 of FIG. 1, the reflective coating 32 is configured such that it acts as an interference layer system for the EUV radiation 5 to be reflected. In this case the reflective coating 32 comprises, in alternation, first layers of a first layer material in the form of silicon and second layers of a second layer material in the form of molybdenum. Different first and second layer materials, in the form of molybdenum and beryllium, for example, are likewise possible, depending on the operating wavelength.
[0061] The mirror arrangement 30 shown in FIG. 3 may also be formed, correspondingly, on a mirror 130, illustrated in FIG. 2, of the DUV projection exposure unit 100. A mirror 130 of this kind likewise comprises a substrate 31 and a reflective coating 32. The substrate 31 in this case may consist, for example, of fused silica. The reflective coating 32 in this case is configured such that incident DUV radiation 5 is reflected with a comparatively high reflectivity in a narrow spectral range around the operating wavelength of the DUV light source 106, of 157 nm, 193 nm, 248 nm or 360 nm, for example, for a specified range of angles of incidence. In this case as well, the reflective coating 32 may be configured as a multilayer system, in order, through interference effects, to generate a maximum reflectivity of the mirror 130. The DUV mirror 130 may alternatively be a metal mirror, a protective metal mirror or a dielectric-enhanced metal mirror.
[0062] Serving for the targeted local deformation of a mirror face 32a formed on the reflective coating 32 are the actuators 27, which in the case of the example shown in FIG. 3 are configured as piezo actuators. The actuators 27 are secured with an adhesive layer 33 on the rear side 31b of the substrate 31. The adhesive layer 33 may be applied directly to the rear side 31b of the substrate 31; in the example shown, however, there is an interlayer 34 applied to the rear side 31b of the substrate 31, and the adhesive layer 33 is applied to this interlayer 34. The material of the interlayer 34 may comprise, for example, Cr, V, Si, Al, Ta, Ti, Al.sub.2O.sub.3, Ta.sub.2O.sub.5, TiO.sub.2, chromium oxide, vanadium oxide, La.sub.2O.sub.3, ZrO.sub.2.
[0063] In the example shown the adhesive layer 33 (except in the region of the actuators 27) has a constant thickness D and is applied two-dimensionally to the rear side 31b of the substrate 31. The actuators 27 are partly embedded into the adhesive layer 33 and project over the adhesive layer 33 at their top side or end-face side. The actuators 27 are adhered on the rear side 31b of the substrate 31, spaced apart from one another in a two-dimensional grid. To simplify the illustration, only two directly adjacent actuators 27 are shown in FIG. 3, with an interspace 35 formed between them. It will be appreciated that in the grid, generally, a significantly larger number of actuators 27 are present, with each pair of adjacent actuators 27 separated from the others by an interspace 25. The interspaces 25 between the actuators 27 generally have the same width, meaning that the actuators 27 are distributed uniformly over the rear side 31b of the substrate 31. The actuators 27 may also form an assembly which has gaps or interspaces 25, or a plurality of actuator assemblies may be applied on the rear side 31b of the substrate 31.
[0064] The adhesive layer 33 extends not only under the actuators 27 or between the actuators 27 and the rear side 31b of the substrate 31, but also into a respective interspace 35 between two adjacent actuators 27. In the case of the example illustrated in FIG. 3, the thickness D of the adhesive layer 33 is selected such that the interspace 35 is filled almost completely by the adhesive layer 33. The thickness D of the adhesive layer 33 may alternatively be selected such that it is not greater than the distance between the bottom side of the actuators 27 and the rear side 31b of the substrate 31, or the interlayer 34. The adhesive layer 33 may also be thinner or may have an irregular thickness D.
[0065] The reflective optical elements 3, 12 to 16 of the illumination system 10 and also the reflective optical elements 21 to 26 of the projection lens 20 of the EUV lithography unit 1 of FIG. 1 are disposed in vacuum surroundings in which there is a residual gas atmosphere. The reflective elements in the illumination system 102 and also the reflective elements 130 in the projection lens 104 of the DUV lithography unit 100 of FIG. 2 are arranged in purged surroundings having a residual partial pressure of H.sub.2O. In the surroundings of the mirror arrangement 30 and hence also in the surroundings of the adhesive layer 33 there is a small fraction of water vapor 36, the concentration of which is substantially constant in steady-state operation of the EUV lithography unit 1 or of the DUV lithography unit 100. A portion of the water vapor 36 diffuses via the surface 33a out of the adhesive layer 33, meaning that at constant ambient humidity the adhesive layer 33 or the surface 33a thereof is in dynamic equilibrium. In the dynamic equilibrium, the amount of water vapor diffusing into the adhesive layer 33 by way of the surface 33a is the same as the amount of water vapor 36 emerging from the surface 33a. The inward and outward diffusion is indicated in FIG. 3 by two double arrows running vertically.
[0066] In the event of a change in humidity in the surroundings of the mirror arrangement 30, the dynamic equilibrium is disturbed. This is the case in particular when there is an abrupt rise in the water partial pressure in the surroundings of the mirror arrangement 30, as may be the case, for example, in the course of maintenance, repair, etc. of the EUV lithography unit 1 or of the DUV lithography unit 100. In the event of an abrupt change in the ambient humidity, there is a short-term adhesive drift during which the adhesive or the adhesive layer 33 rapidly sorbs or releases water by way of the surface 33a, this leading to an expansion or a contraction of the adhesive of the adhesive layer 33, thereby generating stresses in the adhesive layer 33. In the event of an abrupt increase in the ambient humidity, the adhesive layer 33 sorbs water in its upper part, and this causes the adhesive of the adhesive layer 33 to expand in a lateral direction, i.e., substantially parallel to the surface 33a of the adhesive layer, and in this process stresses are generated in the adhesive layer 33, as indicated in FIG. 3 by double arrows running horizontally.
[0067] The stresses in the adhesive layer 33 which are indicated by the double arrows in FIG. 3 and which run parallel to the surface 33a of the adhesive layer 33 lead to a corresponding input of force onto the mutually facing side faces of the two adjacent actuators 27. This input of force generates a lever effect on the actuators 27, which results in unwanted tilting of the actuators 27 and hence unwanted deformations of the substrate 31.
[0068] In the case of the mirror arrangement 30 shown in FIG. 3, not only is the adhesive layer 33 formed of a water vapor-sorbing, generally organic material; instead, an organic material 42 is formed on the actuators 27, and is connected to the top side or to the side of a respective actuator 27. The organic material 42 may comprise, for example, protective layers for the actuators 27, insulator layers, embedding material for (electrical) lines, conductive adhesive or adhesive for supply lines. With the water vapor 36-sorbing organic material 42 as well, the problem exists that, in the event of a change in the humidity in the surroundings of the mirror arrangement 30, water vapor diffuses into/out of the organic material 42 by way of the surface 42a, and this may lead to deformations of the substrate 31.
[0069] There are various possibilities for avoiding deformations of the substrate 31 because of the material drift; a number of such possibilities are described in more detail below with reference to FIG. 4 to FIG. 10, illustratively, on the basis of the adhesive layer 33.
[0070] In the case of the example shown in FIG. 4, a coating 37 which forms a diffusion barrier is applied to the surface 33a of the adhesive layer 33 in the respective interspaces 35. In the interspaces 35, the coating 37 covers the surface 33a of the adhesive layer 33 completely. In FIG. 4 the coating 37 consists of an individual diffusion barrier layer 38, which has a water vapor transmission rate TW.sub.1 that is less than the water vapor transmission rate TW.sub.2 of a layer which consists of the material of the adhesive layer 33 and has the same thickness as the diffusion barrier layer 38 (TW.sub.1<TW.sub.2). In the example shown, the water vapor transmission rate TW.sub.1 of the diffusion barrier layer 38 is less than 1 g/(m.sup.2 d); the water vapor transmission rate TW.sub.2 of a layer which consists of the material of the adhesive layer 33 and has the same thickness as the diffusion barrier layer 38 is typically at least several orders of magnitude higher.
[0071] The diffusion barrier layer 38 in the example shown is formed of Al.sub.2O.sub.3. Alternatively or additionally, the diffusion barrier layer 38 may also comprise other materials, which may be selected, for example, from the group comprising: SiO.sub.2, Ta.sub.2O.sub.5, Al, Ta, Ti, Ru, Si.sub.xC.sub.yO.sub.zH.sub.v, TiO.sub.2, HfO.sub.2, ZrO.sub.2, La.sub.2O.sub.3, Nb.sub.2O.sub.5, parylenes, fluoropolymers, in particular polytetrafluoroethylene, and also mixtures thereof. The coating 37 or the diffusion barrier layer 38 may be applied in different ways to the surface 33a of the adhesive layer 33—for example, by deposition from the gas phase, i.e., by PVD, CVD, such as by plasma-enhanced CVD, by ALD, by plasma ALD, by sputtering, by arc coating, or else by spin coating or in a sol-gel process. The process parameters when applying the coating 37 or the diffusion barrier layer 38 are typically selected such that it can be applied with a high density and as far as possible without pinholes to the surface 33a of the adhesive layer 33.
[0072] In order to make the coating 37 more impervious to the penetration of water vapor 36, the surface 38a of the diffusion barrier layer 38, which forms the top side of the coating 37, may undergo a surface treatment. The surface treatment may be, for example, a plasma treatment or an irradiation of the surface 38a with short wave, optionally pulsed radiation, in the UV wavelength range, for example, or may be produced by a coating or a surface termination. A particular possible effect of the surface treatment is that the surface 38a of the diffusion barrier layer 38 has hydrophobic properties—that is, that it is water-repellent. Additionally or alternatively, the diffusion barrier layer 38 may itself be formed of a hydrophobic material, such as of a fluoropolymer, for example, including in particular a Teflon-containing fluoropolymer—it may be formed of Teflon AF from Dupont, for example.
[0073] In order to increase the effect as a diffusion barrier, the coating 37 may have two or more diffusion barrier layers 38 made of different materials, optionally applied with different processes. This is advantageous in particular if a coating 37 composed of a single diffusion barrier layer 38 were to have a thickness sufficient as to pose a risk of delamination of the diffusion barrier layer 38. Typical thicknesses of the coating 37 or of the diffusion barrier layer 38 are situated in the order of magnitude of around 10 nm to 100 μm. It may also be advantageous to apply a plurality of diffusion barrier layers 38 one above another, since pinholes or defects in one layer are not continued in pinholes or defects in the following layer.
[0074] FIG. 5 shows an example of a mirror arrangement 30 wherein the surface 33a of the adhesive layer 33 is covered completely, in the interspace 35 between the two adjacent actuators 27, by a filling material 39 which has an elasticity modulus E.sub.1 that is lower than the elasticity modulus E.sub.2 of the adhesive layer 33. In the example shown, the elasticity modulus E.sub.1 of the filling material 39 is less than 1500 MPa, preferably less than 1000 MPa, in particular less than around 500 MPa. Conversely, the elasticity modulus E.sub.2 of the adhesive of the adhesive layer 33, based on epoxy resin, for example, is situated typically in the order of magnitude of around 3000 MPa to around 6000 MPa. In the example shown, the filling material 39 is a solid, more specifically an elastomer (rubber). In the example shown in FIG. 5, the penetration of water into the filling material 39 is indeed not prevented, but because of its lower elasticity modulus E.sub.1, the elastic filling material 39 transmits considerably less force to the two adjacent actuators 27 than is the case for the adhesive layer 33. The filling material 33 may optionally likewise form a diffusion barrier; in other words, in addition to the comparatively low elasticity modulus E.sub.1, this material may have a water vapor transmission value TW.sub.1 which is lower than the water vapor transmission value TW.sub.2.
[0075] Instead of rubber, the filling material 39 may also comprise a different elastic and/or ductile material, such as, for example, wax or grease—vacuum grease for example. The filling material 39 may also be a liquid, in particular a nonpolar liquid—an oil, for example. It is advantageous if the filling material 39 is itself hydrophobic, as is generally the case with grease or with oil, for example. The surface 39a of the filling material 39 may optionally be rendered hydrophobic by a surface treatment, as for example by a plasma treatment or by irradiation with UV radiation.
[0076] FIG. 5 shows an example of a mirror arrangement 30 wherein a flexible material in the form of a film 40 is introduced into the interspace 35 between adjacent actuators 27. The film 40 protrudes into the interspace 35, meaning that said film extends from the top side of the two actuators 27 downward in the direction of the rear side 31a of the substrate 31. In the example shown in FIG. 5, the film 40 covers a pot-shaped depression 41 formed in the adhesive layer 33. In the example shown in FIG. 5, the depression 41 extends in the interspace 35 until approximately the level of the bottom side of the actuators 27. As a result of the depression 41, the volume of adhesive located within the interspace 35 is reduced. This is advantageous in order to reduce the so-called long-term adhesive drift, in which the entire volume of the adhesive layer 33 sorbs or releases water and leads to stresses in the substrate 31.
[0077] In the example shown in FIG. 5 the film 40 is not connected to the two adjacent actuators 27. In this way, the free ends of the film 40, which project upward slightly over the surface 33a of the adhesive layer 33, are able to move toward one another and away from one another when an adhesive drift occurs, this being indicated in FIG. 5 by two horizontal double arrows. The film 40 may likewise form a diffusion barrier, meaning that it may consist of or comprise a material which prevents or reduces the penetration of water vapor into the adhesive layer 33. The film 40 may comprise, for example, an aluminum-laminated polymer film. The film 40 may also be formed of a hydrophobic material or may have a hydrophobic surface 40a in order to reduce the sorption of water. The film 40 may have a hydrophobic coating, of Teflon AF, for example, in particular on the surface 40a, this coating acting additionally as a diffusion barrier.
[0078] The application of a film 40 to the adhesive layer 33 may also be useful in the event that the adhesive layer 33, in contrast to the illustration in FIG. 5, has a thickness D which corresponds to the distance between the bottom side of a respective actuator 27 and the rear side 31a of the substrate 31 or the top side of the interlayer 34. In this case the film 40 does typically protrude into the interspace 35, but does not form a depression in the adhesive layer 33. The film 40 in this case may completely cover, in particular, the interspace 35 or the surface 33a of the adhesive layer 33 in the interspace 35.
[0079] As well as the amount of the force exerted by the adhesive of the adhesive layer 33 on the actuators 27, another relevant factor is the force direction of the force exerted on the actuators 27; as far as the lever effect is concerned, a force direction which is oriented perpendicular to the side faces of the actuators 27 (cf. FIG. 3) is particularly deleterious. Because, in the event of short-term adhesive drift, in the context of the sorption and the release of water, the force direction runs substantially parallel to the surface 33a of the adhesive layer 33, one possibility for reducing deformations in the substrate 31 is to alter the orientation of the surface 33a of the adhesive layer 33 with respect to the actuators 27 or to the rear side 31b of the substrate 31.
[0080] In the example shown in FIG. 7, this is achieved by interruption of the adhesive layer 33 in a respective interspace 35 between the actuators 27; in other words, the layer 33 does not completely cover the interspace 35. As a result of the interruption of the adhesive layer 33, the surface 33a of the adhesive layer in the interspaces 35 is oriented diagonally to the rear side 31a of the substrate 30 and to the side faces of the actuators 27. Accordingly, the force effect of the adhesive layer 33 on sorption of water is also oriented diagonally to the side faces of the actuators 27, as is indicated in FIG. 7 by two double arrows. As a result of the interruption of the adhesive layer 33 in the interspace 35, however, the surface 33a of the adhesive layer 33 is increased in comparison to a continuous adhesive layer 33. Accordingly, in the case of the example shown in FIG. 7, it is necessary to select the geometry of the adhesive layer 33 in such a way, that in spite of the enlarged surface 33a and hence a larger absolute amount of the force, the force effect exerted by the actuators 27 on the substrate 31 is reduced by a suitably steep orientation of the surface 33a of the adhesive layer 33.
[0081] In the example described in FIG. 8 there is a decoupling between the actuators 27 and the force effect generated in the adhesive of the adhesive layer 33. This is achieved by virtue of the fact that in the interspace 35 between two adjacent actuators 27, the adhesive layer 33 projects over the actuators 27, more specifically over their top side. In contrast to the illustration in FIG. 3, therefore, the adhesive layer 33 does not have a uniform thickness D; instead, its thickness is increased in the interspaces 35 by the thickness of the projecting part of the adhesive layer 33. On sorption of water, where the adhesive layer 33 undergoes a force effect in the horizontal direction, the adhesive layer 33 may expand laterally over a respective interspace 35, as indicated in FIG. 8 by two double arrows. In this way, the transmission of force from the adhesive layer 33 to the actuators 27 and hence to the substrate 31 may be effectively reduced.
[0082] In the case of the example shown in FIG. 8, the short-term adhesive drift may be compensated, with only the upper portion of the adhesive layer 33 participating in the diffusion processes. In order to reduce the long-term adhesive drift as well, the projecting adhesive layer 33 shown in FIG. 8 may be partially covered with a film 40 which covers a depression 41 in the adhesive layer 33, as shown in FIG. 9. In the case of the mirror arrangement 30 shown in FIG. 9, therefore, the measures set out in FIG. 8 and in FIG. 6 are combined. In this case it is possible to reduce the volume of the adhesive layer 33 in the interspace 35 via the coverage with the film 40, and consequently the long-term adhesive drift of the adhesive layer 33 is reduced as well.
[0083] FIG. 10, lastly, shows a mirror arrangement 30 in which, in addition to the measures set out in FIG. 9, the projecting portion of the surface 33a of the adhesive layer 33—the portion not covered by the film 40—is covered by a coating 37, more specifically by a diffusion barrier layer 38. Where the film 40 forms a diffusion barrier as well, the surface 33a of the adhesive layer 33 is in this case protected completely against the penetration of water vapor 36.
[0084] The adhesive drift and/or the effect of the adhesive drift on the substrate can be reduced by one of the measures described earlier on above or by a combination of two or more of these measures. In this way it is possible to reduce aberrations of the EUV lithography unit 1 or of the DUV lithography unit 100 which are attributable to the adhesive drift. The start-up of the EUV lithography unit 1 or of the DUV lithography unit 100 can also be accelerated after a change in the humidity in the surroundings of the mirror arrangement 30, because the time needed to attain a steady state, in which the adhesive layer 33 is in dynamic equilibrium with the surroundings, is reduced in the manner described above.
