MIRROR ASSEMBLY HAVING A HYDROGEN BARRIER AND OPTICAL ASSEMBLY

Abstract

A mirror arrangement (30) includes: a substrate (31), which has a front side (31a) having a mirror face (32a) for reflecting radiation (5), and a rear side (31b) facing away from the front side (31a), as well as at least one actuator (27) arranged to generate deformations of the mirror face (32a). The at least one actuator (27) is secured on the rear side (31b) of the substrate (31), and the mirror arrangement (30) has a hydrogen barrier (38) which is configured to protect a hydrogen-sensitive material (M) on the rear side (31b) of the substrate (31), in particular on the at least one actuator (27), from the attack by hydrogen (37) from the surroundings (36) of the mirror arrangement (30). An associated optical arrangement, in particular an EUV lithography apparatus (1), incorporating such a mirror arrangement (30) is also disclosed.

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, at least one actuator for generating deformations of the mirror face, wherein the at least one actuator is secured with an adhesive layer on the rear side of the substrate, and a hydrogen barrier which is configured to protect a hydrogen-sensitive material on the rear side of the substrate from hydrogen damage from hydrogen surrounding the mirror arrangement.

2. The mirror arrangement as claimed in claim 1, wherein exposed surface regions of the adhesive layer are protected by the hydrogen barrier from the damage from the hydrogen.

3. The mirror arrangement as claimed in claim 2, wherein the hydrogen barrier forms a water vapor diffusion barrier protecting the adhesive layer from water vapor.

4. The mirror arrangement as claimed in claim 1, wherein a surface of the hydrogen barrier that faces the surrounding hydrogen is hydrophobic and/or wherein the hydrogen barrier comprises at least one hydrophobic material.

5. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier has a hydrogen diffusion coefficient of less than 5×10.sup.−14 m.sup.2/s.

6. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier comprises at least one material and/or a material combination which has a lower solubility for hydrogen than does the hydrogen-sensitive material.

7. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier comprises at least one oxygen-containing chemical compound material having a free enthalpy of formation of less than −400 kJ/mol O.sub.2.

8. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier comprises at least one nitrogen-containing chemical compound material having a free enthalpy of formation of less than −200 kJ/mol N.sub.2.

9. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier comprises at least one metal oxide which is preferably selected from the group consisting essentially of: Al.sub.2O.sub.3, MgO, CaO, La.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Ce.sub.2O.sub.3, and compounds thereof.

10. The mirror arrangement as claimed in claim 9, wherein the at least one metal oxide is selected from the group consisting essentially of: Al.sub.2O.sub.3, MgO, CaO, La.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Ce.sub.2O.sub.3, and compounds thereof.

11. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier comprises at least one material selected from the group consisting essentially of: Al, Au, Ag, Zn, Mo, Si, W, Ti, Sn, Sb, Pt, Ni, Fe, Co, Cr, V, Cu, Mn, Pb, their oxides, borides, nitrides and carbides, and also C and B.sub.4C.

12. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier has a coating or forms a coating which covers the hydrogen-sensitive material at least partly.

13. The mirror arrangement as claimed in claim 11, wherein the coating comprises at least one hydrogen barrier layer.

14. The mirror arrangement as claimed in claim 13, wherein the at least one hydrogen barrier layer is applied on a further layer.

15. The mirror arrangement as claimed in claim 1, wherein the hydrogen barrier has a protective film or forms a protective film which covers the hydrogen-sensitive material at least partly.

16. The mirror arrangement as claimed in claim 15, wherein the protective film has a surface comprising at least one hydrogen barrier layer.

17. The mirror arrangement as claimed in claim 15, wherein the protective film projects into an interspace between adjacent actuators and covers a depression in the adhesive layer.

18. The mirror arrangement as claimed in claim 1, wherein the actuators are configured as piezo actuators or as electrostrictive actuators.

19. An optical arrangement in a 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 with a mirror which comprises a plurality of actuators for correcting aberrations;

[0039] FIG. 2 shows a schematic sectional representation of a mirror arrangement with the mirror of FIG. 1 and with an adhesive layer for securing the actuators on a substrate;

[0040] FIG. 3 shows a schematic representation of a detail of the mirror arrangement of FIG. 2, in which the adhesive layer projects over the actuators and a depression is formed between the actuators, this depression being covered by a hydrogen-protective film; and

[0041] FIGS. 4A and 4B show schematic representations analogous to FIG. 3, with a coating applied to one surface of the adhesive layer and also to the actuators, said coating formed either as a single, thick hydrogen barrier layer (FIG. 4A) or as a thin hydrogen barrier layer and a further, comparatively thick covering layer (FIG. 4B).

DETAILED DESCRIPTION

[0042] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0043] 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.

[0044] 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 bundle illumination beam 4 and to increase the energy density further in this way. The illumination beam 4 illuminates a structured object M with an illumination system 10, which in the present example has five reflective optical elements 12 to 16 (mirrors).

[0045] 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.

[0046] The structured object M reflects part of the illumination beam 4 and shapes a projection beam 5, which carries the information about the structure of the structured object M and is irradiated into a projection lens 20, which generates an image of the structured object M or of a respective subregion 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.

[0047] 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.

[0048] In addition to the reflective optical elements 3, 12 to 16, 21 to 26, the EUV lithography unit 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. FIG. 1 represents, illustratively, a plurality of actuators 27 mounted on the rear side of the sixth mirror 26 of the projection lens 20 in order to carry out targeted deformation of this lens and, in the process, 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 of a control computer, for example, which communicates via signal lines—not shown in the image—with the actuators 27.

[0049] FIG. 2 shows the mirror 26 of FIG. 1 as part of a corresponding mirror arrangement 30 in a detailed representation. The mirror arrangement 30, more specifically the mirror 26, comprises a substrate 31, which in the example shown is formed of fused silica or titanium-doped fused silica (ULE). Different materials having a low thermal expansion coefficient, or a very low dependence of the thermal expansion coefficient on the temperature, examples being certain glass-ceramics, may likewise be used as substrate materials. Applied to a front side 31a of the substrate 31 of the mirror 26 is a reflective coating 32 configured such that incident EUV radiation 5 is reflected in a narrow spectral range around the operating wavelength of 13.5 nanometers for a specified range of incident angles, with a comparatively high reflectivity. The reflective coating 32 is configured such that it acts as an interference layer system for the EUV radiation 5 to be reflected. 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.

[0050] Serving for 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. 2 are configured as piezo actuators or electrostrictive 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, but in the example shown an interlayer 34 is 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, Ru, Cu, 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, etc.

[0051] In the example shown, the adhesive layer 33 has a constant thickness D and is applied over the area of the rear side 31b of the substrate 31. The actuators 27 are mounted or embedded onto the adhesive layer 33 and project over the adhesive layer 33. The actuators 27 are glued at a distance from one another in a two-dimensional grid on the rear side 31b of the substrate 31. For simplification of the representation, FIG. 2 shows only two, directly adjacent actuators 27, with an interspace 35 formed between them. It will be appreciated that in the grid in general there is a much larger number of actuators 27, with pairs of adjacent actuators 27 separated from one another by an interspace 35. The interspaces 35 between the actuators 27 generally have the same width; in other words, the actuators 27 are distributed evenly over the rear side 31b of the substrate 31. The actuators 27, or a plurality of actuators 27, may also form an assembly which has gaps or interspaces 35, or a plurality of actuator assemblies may be applied to the rear side 31a of the substrate 31. A single actuator or actuator assembly 27 may optionally be applied to the rear side 31b of the substrate 31.

[0052] 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 represented in FIG. 2, the thickness D of the adhesive layer 33 is selected such that the adhesive layer 33 does not project into the interspace 35, but instead finishes flush with the bottom side of the actuators 27 at surface regions 33a-c which are exposed to the surroundings. The thickness D of the adhesive layer 33 may alternatively be selected such that the interspace 35 is filled wholly or partly by the adhesive layer 33. The thickness D of the adhesive layer 33 may in particular also be selected to have a size such that the adhesive layer 33 covers the top side of the actuators 27. The adhesive layer 33 may also be thinner or have an irregular thickness.

[0053] The reflective optical elements 3, 12 to 16 of the illumination system 10 and the reflective optical elements 21 to 26 of the projection lens 20 of the EUV lithography unit 1 of FIG. 1 are arranged in a vacuum environment which has a residual gas atmosphere. In the surroundings 36 of the mirror arrangement 30 and hence also in the surroundings of the adhesive layer 33, at least during the cleaning cycles, there is hydrogen 37, which may be present in different excitation states, as for example in the form of molecular hydrogen (H.sub.2), of excited molecular or atomic hydrogen, in the form of hydrogen ions, etc.

[0054] The components mounted on the rear side 31b of the substrate 31, especially the actuators 27, generally have hydrogen-sensitive material M, i.e. material which degrades on contact with the hydrogen 37, on their surface facing the surroundings 36. The hydrogen-sensitive material M may be, for example, the material of the housing of the actuators 27 and also may be conductor tracks, conductor cables, insulator layers applied to the actuators 27 and optionally to the interlayer 34, etc. Conductor tracks and insulator layers are generally produced from plastics materials which have a comparatively high solubility to hydrogen 37 and which degrade on chemical reaction with hydrogen. The material of the adhesive layer 33 and also, where appropriate, the material of the substrate 31 are also generally not chemically inert to the attack by hydrogen 37.

[0055] In order to protect the hydrogen-sensitive material M on the rear side 31a of the substrate 31 against the attack by hydrogen 37 from the surroundings 36 of the mirror arrangement 30, the mirror arrangement 30 has a hydrogen barrier 38, which may be configured in various ways, as described in more detail below with reference to FIG. 3 and to FIGS. 4A,B. FIG. 3 and FIGS. 4A,B each show only a detail of the mirror arrangement 30 or of the hydrogen barrier 38, respectively, though it generally extends over the entire rear side 31a of the substrate 31.

[0056] The hydrogen barrier 38 described in more detail below may also serve as a water vapor diffusion barrier for protecting the adhesive layer 33 against the penetration or the inward diffusion of water vapor 39 (cf. FIG. 2). The diffusion of water vapor 39 into the adhesive layer 33 or out of the adhesive layer 33 is a particular problem if the concentration of water vapor 39 in the surroundings 36 of the mirror arrangement 30 changes, since the expansion or contraction of the adhesive in the adhesive layer 33 is dependent on the amount of water vapor 39 absorbed, and leads to stresses in the adhesive layer 33 that are transferred to the substrate 31 and that may lead to unwanted deformations at the mirror face 32a. This is also the case for other organic materials connected mechanically to the mirror.

[0057] The hydrogen barrier 38 shown in FIG. 3 comprises a protective film 40 made of a flexible film material. Applied to the protective film 40, on its top side 40a facing away from the substrate 31, is a hydrogen barrier layer 41, which in the example shown consists of Al. Aluminum has a low hydrogen diffusion coefficient DW of less than around 5×10.sup.−14 m.sup.2/s. Aluminum, moreover, is a chemically inert material, which has a lower solubility for hydrogen 37 than the material of the adhesive layer 33.

[0058] Different materials as well, having on the one hand a low hydrogen diffusion coefficient DW of, for example, less than around 5×10.sup.−14 m.sup.2/s, preferably of less than 1×10.sup.−17 m.sup.2/s, in particular of less than 1×10.sup.−21 m.sup.2/s, may be applied as a hydrogen barrier layer 41 to the protective film 40, examples being Au, Ag, Zn, Mo, Si, W, Ti, Sn, Sb, Pt, Ni, Fe, Co, Cr, V, Cu, Mn, Pb, their oxides, borides, nitrides and carbides, C, B.sub.4C, and compounds thereof. The hydrogen barrier layer 41 may also comprise at least one metal oxide or consist of a metal oxide. In particular, metal oxides which have a high (negative) free enthalpy of formation of less than around −300 kJ/mol O.sub.2, preferably of less than −800 kJ/mol O.sub.2, more preferably of less than −1000 kJ/mol O.sub.2, are generally inert toward a chemical reaction with hydrogen 37. In particular Al.sub.2O.sub.3, MgO, CaO, La.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Ce.sub.2O.sub.3 and compounds thereof have proven advantageous materials for the hydrogen barrier layer 41. Nitrides as well, metal nitrides for example, which have a free enthalpy of formation of less than −200 kJ/mol N.sub.2, preferably of less than −350 kJ/mol N.sub.2, more preferably of less than −600 kJ/mol N.sub.2, are generally inert toward a chemical reaction with hydrogen 37 and may therefore be used as materials for the hydrogen barrier layer 41.

[0059] In the case of the example shown in FIG. 3, the protective film 40 is adhered over the full area of the top side 33a of the adhesive layer 33. The adhesive layer 33 in this case projects upwardly over the top side of the actuators 27. Between pairs of adjacent actuators 27, the protective film 40 projects into the interspace 35, meaning that it extends from the top side of the two actuators 27 downward in the direction of the rear side 31a of the substrate 31. The protective film 40 covers a pot-shaped depression 42 in the adhesive layer 33, in the case of the example shown in FIG. 3. In the example shown in FIG. 3, the depression 42 extends in the interspace 35 until approximately the level of the bottom side of the actuators 27. The depression 42 reduces the volume of adhesive located within the interspace 35. This is advantageous in order to reduce stresses in the adhesive layer 33 when water vapor 39 is taken up/given off, and in this way to reduce stresses in the substrate 31.

[0060] In the case of the example shown in FIG. 3, the film 40 is not connected directly to the actuators 27, but is connected only to the surface 33a of the adhesive layer 33. The mutually opposing sides of the protective film 40 can therefore move toward one another or away from one another in the event of an adhesive drift, in the region of the pot-shaped or groove-shaped depression 42, in order to counteract the incidence of stresses in the adhesive layer 33.

[0061] In the example shown, the protective film 40 forms a water vapor diffusion barrier, meaning that it consists of or comprises a material which prevents or counteracts the penetration of water vapor 39 into the adhesive layer 33. For this purpose the protective film 40 is formed of a material of low water diffusivity or comprises a material having low water diffusivity. The protective film 40 may be a two-ply film, for example, having a first ply of Al.sub.2O.sub.3 as a material with low water diffusivity, which acts as a water vapor diffusion barrier and which is applied to a second ply, e.g., a self-adhesive ply. Alternatively or additionally, the protective film 40 may have a hydrophobic surface 40a, which may be generated, for example, by a plasma treatment or termination. The surface 41a of the water vapor barrier layer 41 (e.g., with Al as layer material) may also be rendered hydrophobic with suitable surface treatment. In the event that the water vapor diffusion barrier in the form of the protective film 40 is itself insensitive to hydrogen, it may be advantageous to switch the sequence, so that the hydrogen barrier layer 41 is applied on the bottom side 40b of the protective film 40 that faces the substrate 31. Also possible is the application of a respective hydrogen barrier layer 41 to the top side 40a and to the bottom side 40b of the protective film 40.

[0062] FIGS. 4A and 4B show a hydrogen barrier in the form of a coating 38 which is applied to the rear side 31b of the substrate 31 and to the components provided there, in other words, in particular, to the actuators 27 and to the adhesive layer 33. In the case of the example shown in FIG. 4A, the coating 38 consists of a single hydrogen barrier layer 44, which is applied not only to the top side of the actuators 27 but also, in the respective interspace 35, to the side walls of the actuators 27 and to the top side of the adhesive layer 33. In the example shown, the hydrogen barrier layer 44 is formed of Al.sub.2O.sub.3, but may also comprise one or more of the materials described earlier on above, or different materials, which first have a low hydrogen diffusion coefficient DW and secondly have a low solubility for hydrogen 37.

[0063] FIG. 4B shows a hydrogen barrier 38 in the form of a coating which likewise completely covers the hydrogen-sensitive material M on the rear side 31b of the substrate 31. In contrast to the coating 38 shown in FIG. 4A, the coating 38 shown in FIG. 4B has a comparatively thin hydrogen barrier layer 44, which is applied to a further, comparatively thick, covering layer 43. The layer 43 covering the hydrogen-sensitive material M has the function of compensating irregularities and providing better growth conditions for the hydrogen barrier layer 44. In the example shown, SiO.sub.2 is present in the covering layer 43. The covering layer 43 typically has a thickness of more than 100 nm; the hydrogen barrier layer 44 typically has a thickness lower than 100 nm. In contrast to what is shown in FIG. 4B, the hydrogen barrier 38 may comprise a plurality of double layers each composed of a covering layer 43 and of a hydrogen barrier layer 44 applied thereto.

[0064] The hydrogen barrier layer 44 may be formed in particular of one or of two or more of the materials described earlier on above. The hydrogen barrier layer 44 may also comprise a hydrophobic material or its surface 44a may have hydrophobic properties so as to serve as a water vapor diffusion barrier.

[0065] In place of an individual hydrogen barrier layer 41, 44 as shown in FIG. 3 and in FIGS. 4A and 4B, the hydrogen barrier 38 may be configured as a coating having two or more hydrogen barrier layers applied one over another. In this way, for example, in the case of a double layer system, in other words of a coating 38 which comprises a pair of hydrogen barrier layers, the pinholes or defects in one individual layer are not continuous pinholes or defects in the following individual layer. In a coating 38 of this kind, therefore, different materials from among those already identified earlier on above are preferably applied in alternation, i.e. as a double layer. It is advantageous to apply two or more such double layers one over another. The materials of the hydrogen barrier layers may be, for example, alternately applied nitrides and carbides, in particular MAX phases, i.e., layered hexagonal nitrides and carbides. It will be appreciated that it is also possible for three or more of the above-stated materials to be used in a coating 38 in the form of a multilayer system.

[0066] The coating 38, in particular the hydrogen barrier layer 44, may be applied in various ways to the surface 33a of the adhesive layer 33 and to the actuators 27—for example, by deposition from the gas phase, i.e. by PVD, CVD, for example by plasma-enhanced CVD or PVD, by ALD, in particular by plasma-enhanced ALD, by sputtering, in particular by magnetron sputtering, by electron beam evaporation, etc. The operating parameters when applying the coating 38 or the hydrogen barrier layer 44 are typically selected such that it may be deposited with a high density and with as far as possible no pinholes.

[0067] The hydrogen barrier 38 shown in FIG. 3 may be combined, optionally, with the hydrogen barrier 38 shown in FIG. 4A or 4B by covering a section or a subregion on the rear side 31a of the substrate 31 with a hydrogen barrier 38 in the form of the protective film 40 (coated with the hydrogen barrier layer 41), and covering a further section or subregion on the rear side 31b of the substrate 31 with a hydrogen barrier 38 in the form of a coating in accordance with FIG. 4A or 4B. In the event that materials which are not hydrogen-sensitive are mounted on the rear side 31b of the substrate 31, it is also possible optionally to dispense with the provision of the hydrogen barrier 38 on the rear side 31b of the substrate 31 in those regions in which these materials are provided.