Optical arrangement for EUV radiation with a shield for protection against the etching effect of a plasma

Abstract

An optical arrangement (1) for EUV radiation includes: at least one reflective optical element (16) having a main body (30) with a coating (31) that reflects EUV radiation (33). At least one shield (36) is fitted to at least one surface region (35) of the main body (30) and protects the at least one surface region (35) against an etching effect of a plasma (H+, H*) that surrounds the reflective optical element (16) during operation of the optical arrangement (1). A distance (A) between the shield (36) and the surface region (35) of the main body (30) is less than double the Debye length (λ.sub.D), preferably less than the Debye length (λ.sub.D), of the surrounding plasma (H+, H*).

Claims

1. An optical arrangement for extreme ultraviolet (EUV) radiation, comprising: at least one reflective optical element having a main body with a reflective coating that is disposed on a front side of the main body and is configured to reflect the EUV radiation and with at least one surface region on a lateral side of the main body, and at least one shield comprising a screen separated from the at least one surface region by an empty gap, and is configured to protect the at least one surface region against an etching effect of a plasma that surrounds the reflective optical element during operation of the optical arrangement, wherein the at least one shield at least partially covers the at least one surface region at a distance (A), wherein the distance (A) between the shield and the surface region of the main body is less than double the Debye length (λ.sub.D) of the surrounding plasma, and wherein the Debye length (λ.sub.D) is less than 0.5 mm.

2. The optical arrangement as claimed in claim 1, wherein the distance (A) between the shield and the surface region of the main body is less than the Debye length (λ.sub.D) of the surrounding plasma.

3. The optical arrangement as claimed in claim 1, wherein the screen, at least at a side facing the gap, has a coating composed of a hydrogen recombination material, or wherein the screen consists of a hydrogen recombination material.

4. The optical arrangement as claimed in claim 3, wherein the hydrogen recombination material forms a contamination getter material selected from the group consisting of: Ir, Ru, Pt, Pd.

5. The optical arrangement as claimed in claim 1, wherein the screen has a screen section projecting at least partly over the reflective coating, applied to the main body, outside an optically used region of the reflective optical element.

6. The optical arrangement as claimed in claim 5, wherein the screen section projects over the reflective coating over a length (L) that is greater than a width (A) of the gap, which equals the distance (A) between the shield and the surface region of the main body.

7. The optical arrangement as claimed in claim 1, wherein the Debye length (λ.sub.D) is less than 0.1 mm.

8. The optical arrangement as claimed in claim 1, wherein the material of the shield is a metallic material or a ceramic material.

9. The optical arrangement as claimed in claim 8, wherein the material of the shield is selected from the group consisting of: Cu, Co, Pt, Ir, Pd, Ru, Al, high-grade steel, AlO.sub.x, and Al.sub.2O.sub.3.

10. The optical arrangement as claimed in claim 1, wherein the main body comprises at least one material selected from the group consisting of: quartz glass, titanium-doped quartz glass, glass ceramic, silicon, aluminum, copper, silicon-containing alloys, aluminum-containing alloys, copper-containing alloys, and compounds or composites thereof.

11. The optical arrangement as claimed in claim 1, comprising: an EUV light source for generating the EUV radiation, an illumination system for illuminating a structured object with the EUV radiation, and a projection lens for imaging the structured object onto a substrate.

12. The optical arrangement as claimed in claim 11, wherein the reflective optical element is a collector mirror for focusing the EUV radiation of the EUV light source, or a reflective optical element, disposed adjacent to the structured object, of the illumination system.

13. An optical arrangement for extreme ultraviolet (EUV) radiation, comprising: at least one reflective optical element having a main body with a reflective coating that reflects the EUV radiation, and at least one shield fitted to the at least one surface region of the main body, at least one shield forming a protective film as at least one protective foil, wherein the protective film is indirectly connected to the at least one surface region of the main body and protects the at least one surface region against an etching effect of a plasma that surrounds the reflective optical element during operation of the optical arrangement, wherein the at least one surface region is arranged outside of the reflective coating and forms a lateral surface region of the main body, wherein a distance (A) between the shield and the surface region of the main body is less than double the Debye length (λ.sub.D) of the surrounding plasma, wherein a connecting material for cohesively connecting the protective film to the main body is disposed between the protective film and the protected surface region, and wherein the connecting material is an adhesive.

14. The optical arrangement as claimed in claim 13, wherein the distance (A) between the shield and the surface region of the main body is less than the Debye length (λ.sub.D) of the surrounding plasma.

15. The optical arrangement as claimed in claim 13, wherein the protective film has a maximum thickness (D) of less than 50 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic illustration of an optical arrangement in the form of an EUV lithography apparatus,

(3) FIGS. 2A-2C show, respectively, a first, a second and a third schematic sectional illustration of a reflective optical element with a shield in the form of a screen for protecting a lateral surface region of the main body against the etching effect of a surrounding plasma,

(4) FIGS. 3A and 3B show schematic illustrations of an optical element having surface regions to be protected, respectively, without and with a screen for shielding the surface regions,

(5) FIGS. 4A and 4B show, respectively, a sectional illustration and a plan view of an optical element, to the main body of which a layer serving as a shield is applied directly, and

(6) FIGS. 5A and 5B show, respectively, a sectional illustration and a plan view of an optical element with a shield in the form of a protective film connected to the main body by way of an adhesive layer.

DETAILED DESCRIPTION

(7) In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

(8) FIG. 1 schematically shows the construction of an optical arrangement for EUV lithography in the form of an EUV lithography apparatus 1, specifically of a so-called wafer scanner. The EUV lithography apparatus 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 apparatus 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 apparatus 1 to be configured for a different operating wavelength in the EUV wavelength range, such as 6.8 nm, for example.

(9) The EUV lithography apparatus 1 furthermore comprises a collector mirror 3 in order to focus the EUV radiation of the EUV light source 2 to form an illumination beam 4 and to increase the energy density further in this way. The illumination beam 4 serves for the illumination of a structured object M by an illumination system 10, which in the present example has five reflective optical elements 12 to 16 (mirrors).

(10) The structured object M can be for example a reflective photomask, which has reflective and non-reflective, or at least much 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 possibly movable about at least one axis, in order to set the angle of incidence of the EUV radiation on the respective mirror.

(11) 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 arranged on a mounting, which is also referred to as a wafer stage WS.

(12) 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 on 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.

(13) In order to achieve a high imaging quality in the imaging of a respective object point OP of the structured object M onto a respective image point IP on the wafer W, extremely stringent requirements are to be made in respect of the surface shape of the mirrors 21 to 26; and the position or the alignment of the mirrors 21 to 26 in relation to one another and in relation to the object M and the substrate W also requires precision in the nanometer range.

(14) FIGS. 2A-2C show respective sectional illustrations of the fifth optical element 16 of the illumination system 10, which comprises a main body 30 composed of aluminum, said main body being of integral design in the example shown. The main body 30 can also be formed from silicon, from copper or from a silicon-containing, aluminum-containing or copper-containing material. The main body 30 serves as a substrate for a reflective coating 31 applied to a planar front side 32a of the main body 30. At its rear side 32b, which is likewise planar, the main body 30 is areally applied to a mounting, not illustrated pictorially in FIGS. 2A-2C. The reflective optical element 16 is illustrated in a greatly simplified manner in FIGS. 2A-2C, and that said reflective optical element has a more complex geometry in practice.

(15) In the respective examples shown in FIGS. 2A-2C, the reflective coating 31 is configured for reflecting EUV radiation 33 incident with grazing incidence on the front side 32a of the main body 30, i.e. for EUV radiation 33 incident on the front side 32a of the main body 30 at an angle of incidence of more than approximately 60°. In the example shown, the reflective coating 31 is formed by a single layer, but can also be formed by a multilayer system.

(16) As is likewise evident in FIGS. 2A-2C, the reflective optical element 16 is exposed to a hydrogen plasma in the form of hydrogen ions H.sup.+ and hydrogen radicals H* that is produced in a vacuum environment 34, in which 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 are arranged.

(17) While the front side 32a and the rear side 32b of the main body 30 are protected against the hydrogen plasma H.sup.+, H* by the reflective coating 31 and respectively by the mounting, not illustrated pictorially, this is not the case for the circumferential side surface of the main body 30, with the result that said side surface forms a lateral surface region 35 that is exposed to the hydrogen plasma H.sup.+, H* in the environment of the reflective optical element 16.

(18) In order to protect the lateral surface region 35 against an etching effect of the hydrogen plasma H.sup.+, H*, in the respective examples shown in FIGS. 2A-2C, a shield in the form of a screen 36 is provided at the reflective optical element 16. For this purpose, the screen 36 covers the lateral surface region 35 of the reflective optical element 16 at a defined distance A with formation of a gap 37, whose gap width (which is constant in the examples shown) corresponds to the distance A between the screen 36 and the lateral surface region 35.

(19) In order to achieve the best possible protective effect for the lateral surface region 35 of the reflective optical element 16, the distance A between the protected surface region 35 of the reflective optical element 16 and the shield 36 should be as small as possible. In particular, the distance A should be less than double the Debye length λ.sub.D, in particular less than the Debye length λ.sub.D, of the surrounding plasma H.sup.+, H*, which is less than 5 mm, less than 0.5 mm or less than 0.1 mm in the examples shown.

(20) The shield in the form of the screen 36 is formed from a metallic material in the form of high-grade steel in the example shown in FIG. 2A. Other materials that are comparatively insensitive to an etching attack, in particular metallic or ceramic materials, can also be used as materials for the screen 36. By way of example, the screen 36 can be formed from copper (Cu), cobalt (Co), platinum (Pt), iridium (Ir), palladium (Pd), ruthenium (Ru), aluminum (Al), tungsten (W), tantalum (Ta) or a ceramic material, in particular AlOx or Al.sub.2O.sub.3. The material of the screen 36 typically has a greater resistance to the etching effect of a (hydrogen) plasma than the material of the main body 30.

(21) In the example shown in FIG. 2B, the screen 36, at a side facing the gap 37, has a coating 38 formed from a hydrogen recombination material. A hydrogen recombination material is understood to mean a material having a hydrogen recombination coefficient of 0.08 or higher. In the example shown, the hydrogen recombination material from which the coating 38 is formed is ruthenium. Ruthenium has firstly a high resistance to hydrogen etching and secondly a high recombination capability for hydrogen radicals H* and/or for hydrogen ions H.sup.+. In addition, an Ru surface of the screen acts as getter surface or as getter material, i.e. as a sacrificial layer for etching products that possibly still do arise, and in this way additionally reduces the contamination of the lateral surface region. The use of Ru or of other getter materials, e.g. of Ir, Pt, Pd, for the coating 38 thus provides protection in multiple respects for the lateral surface region 35 and for the reflective coating 31.

(22) In the example shown in FIG. 2B, the screen 36 not only covers the lateral surface region 35 of the main body 30, but additionally has an over-projecting screen section 36a, which screen section extends along the front side 32a of the main body 30, specifically likewise at a distance A which, in the example shown, is less than the Debye length λ.sub.D of the surrounding plasma H.sup.+, H*. The over-projecting screen section 36a thus also partly projects over the reflective coating 31, applied to the whole area of the front side 32a of the main body 30, and likewise at least partly covers a circumferential edge formed at the transition between the front side 32a of the main body 30 and the lateral surface region 35.

(23) The over-projecting screen section 36a can effectively prevent the hydrogen plasma, to put it more precisely hydrogen ions H.sup.+ and hydrogen radicals H*, from penetrating into the gap 37. For this purpose, it is advantageous if the over-projecting screen section 36a has a length L that is greater, in particular significantly greater, than the distance A between the lateral surface region 35 and a laterally extending section 36b of the screen 36. For the length L of the over-projecting screen section 36a, the following can hold true, in particular: L>10 A, L>20 A or L>50 A. In this case, it is advantageous, in particular, if the distance A between the over-projecting screen section 36a and the front side 32a of the main body 30 is additionally also less than the Debye length or double the Debye length λ.sub.D. As is likewise evident in FIG. 2B, the over-projecting screen section 36a is applied outside an optically used region 39 of the reflective coating 31. The beam path of the EUV radiation 33 lies within the optically used region 39, which beam path is not intended to be shielded by the screen 36.

(24) In the example shown in FIG. 2C, as in the example shown in FIG. 2A, the screen 36 does not have an over-projecting screen section 36a. In contrast to the example shown in FIG. 2A, however, a filling material 40 is introduced in the gap 37, said filling material completely filling the gap 37. The filling material 40 serves as a spacer to keep the screen 36 at the distance A from the lateral surface region 35. The filling material 40 can be embodied for example as a plate-like component part extending circumferentially in a ring-shaped fashion around the lateral surface region 35. The filling material 40 can optionally be permanently connected to the lateral surface region 35 and/or to the screen 36. By way of example, the filling material 40 can be deposited on the lateral surface region 35. However, such a permanent connection of the filling material 40 to the lateral surface region 35 and/or to the screen 36 is not absolutely necessary.

(25) As an alternative to the example shown in FIG. 2C, the gap 37 can be only partly filled with the filling material 40. By way of example, in this case the filling material 40 can be introduced into the gap 37 in a structured manner and can be deposited e.g. only locally, i.e. only at specific locations, on the lateral surface region 35. In this way it is possible to form web-shaped structures, for example, which bridge the gap 37. For the case where the screen 36 is embodied as illustrated in FIG. 2B, i.e. has an over-projecting screen section 36a, the filling material 40 can be introduced for example only between the lateral surface region 35 and the laterally extending section 36b of the screen 36, but not between the front side 32a and the over-projecting section 36a of the screen 36.

(26) A material suitable for protecting the gap 37 between the lateral surface region 35 and the screen 36 against contamination is chosen as filling material 40. The filling material can be selected for example from the group comprising: Aluminum oxide, zirconium nitride, yttrium oxide, cerium oxide, zirconium oxide, niobium oxide, titanium oxide, tantalum oxide, tungsten oxide, metals, preferably noble metals, in particular Ru, Rh, Pd, Ir, Pt, Au, and compositions thereof.

(27) FIGS. 3A and 3B show the fifth reflective optical element 16 of the illumination system 10 in an illustration without a shield (cf. FIG. 3A) and with a shield in the form of a screen 36 (cf. FIG. 3B). As is evident in FIG. 3A, the main body 30 of the reflective optical element 16 has a lateral surface region 35 having a section 35a tapering in a tongue-shaped manner, a lower section in FIG. 3B. The tongue-shaped section 35a of the lateral surface region 35 is covered laterally by a lateral screen section 36b—shown in FIG. 3B—of a bipartite screen 36 in the manner illustrated in association with FIG. 2A. A section 35b extending substantially rectangularly, an upper section in FIG. 3A, of the surface region 35 extending laterally circumferentially is covered by a part of the screen 36 that is embodied in the manner as illustrated in FIG. 2B. Regarding the part of the screen 36 covering the upper section 35b of the main body 30, only the over-projecting screen section 36a of said part is discernible in FIG. 3B.

(28) The screen 36, to put it more precisely the over-projecting screen section 36a, covers the reflective coating 31 partly outside the optically used region 39, illustrated in a dashed manner in FIG. 3A. As is evident in FIG. 3B, the over-projecting screen section 36a ends at the upper end of the reflective optical element 16 in FIG. 3B, that is to say that there the screen 36 protects only the laterally circumferentially extending surface region 35 of the main body 30, as is illustrated in FIG. 2A.

(29) In contrast to the illustration shown in FIG. 3B, it is possible for the over-projectingregion 36a of the screen 36 not only to be formed in the upper section 35b of the lateral surface region 35, but to extend along the lateral circumference of the entire reflective surface 31. In particular, the over-projecting screen section 36a can optionally serve to protect the reflective coating 31 itself at the (sharp) edge formed by the latter and/or the front side 32a of the main body 30 with the lateral surface region 35 against an etching attack.

(30) FIGS. 4A and 4B show by way of example the collector mirror 3 of the illumination system 10 in a sectional illustration and in a plan view. The collector mirror 3 has a reflective coating 31 at its front side 32a, which reflective coating, in the example shown, is a multilayer coating for reflecting EUV radiation 33 incident with normal incidence on the collector mirror 3. For this purpose, the reflective multilayer coating 31 has a plurality of layers having alternately a high and a low real part of the refractive index. The collector mirror 3 is also exposed to a hydrogen plasma H.sup.+, H* during operation of the EUV lithography apparatus 1. In order to prevent the material of the main body 30, which can be Si, SiC or SiO.sub.2 in the example shown, from being exposed to the surrounding hydrogen plasma H.sup.+, H*, a shield in the form of a coating 41 is applied to a circumferentially extending lateral surface region 35 of the main body 30. The coating 41 serving as a shield is applied to the lateral surface region 35 directly, i.e. without the formation of a gap, with the result that the distance A between the lateral surface region 35 and the shield in the form of the coating 41 is equal to zero.

(31) As materials for the coating 41 serving as a shield, the metallic or ceramic materials described further above in association with FIGS. 2A and 2B can be used, for example, which are insensitive to an etching attack of the hydrogen plasma H.sup.+, H* to the greatest possible extent since these materials do not enter into reactions with the hydrogen plasma H.sup.+, H* in which readily volatile etching products arise. In the example shown, the shield in the form of the coating 41 is formed from ruthenium, which additionally has a high recombination capability for hydrogen radicals H.sup.+, H*, with the result that this material provides protection of the main body 30 against the surrounding hydrogen plasma H.sup.+, H* in two respects (see above).

(32) The material of the coating 41 serving as a shield should ensure a maximum coverage of the lateral surface region 35 in order to ensure that the fewest possible hydrogen ions H.sup.+ or hydrogen radicals H* can penetrate as far as the lateral surface region 35. Within the meaning of this application, coverage is understood to mean that proportion of the surface which is covered with a protective coating material and thus no longer contributes to contamination on account of the etching process. The coverage by the coating 41 serving as a shield should be as great as possible and ideally more than 97%. Besides being dependent on the surface constitution of the main body 30 and the material of the coating 41 serving as a shield, the coverage is also dependent on the thickness D of the coating 41. Said thickness D should typically be not less than approximately 50 nm and not greater than hundreds of nanometers, for example less than approximately 500 nm or less than approximately 200 nm.

(33) The thickness D of the coating 41 serving as a shield, which is formed from a single layer of ruthenium in the example shown in FIGS. 4A and 4B, can optionally vary in a location-dependent manner, but is generally constant. The maximum thickness D of the coating 41 serving as a shield is limited by the layer stress, inter alia. The layer stresses in the coating 41 should not prove to be so great that they result in a significant alteration of the surface geometry of the front side 32a of the main body 30 (figure).

(34) In order to prevent this, the coating 41 serving as a shield can comprise two or more layers, the layer stresses of which compensate for one another at least partly, in particular completely. In the case of such a coating 41 comprising a plurality of layers, the outermost layer, which is exposed to the surrounding hydrogen plasma H.sup.+, H*, is formed from a material that is as insensitive as possible to an etching attack, while this property is possibly not absolutely necessary for the materials of the underlying layers. In particular, the coating 41 can have a periodic sequence of layers or plies composed of a first material and a second material in order to simplify the compensation of the layer stresses. The first material, having an intrinsic tensile stress, can be for example copper, silver, gold, chromium or a nickel-chromium alloy in which the ratio of chromium:nickel is between 30:70 (% by weight) and 70:30 (% by weight). The second material, having an intrinsic compressive stress, can be ruthenium or silicon, for example. Materials other than those mentioned can also be used for the compensation of layer stresses.

(35) The materials of the layer(s) of the coating 41 serving as a shield which are applied below the outermost layer, in order to bring about stress compensation, can be NiSi or WB4C, for example. The layer stress of the coating 41 serving as a shield can be adjusted in a targeted manner by the use of at least one stress-compensating layer. In this way, the thickness D of the outermost layer providing the protection against the surrounding hydrogen plasma H.sup.+, H*, or of the entire coating 41 serving as a shield, can prove to be greater than was indicated further above. The coating 41 serving as a shield can be applied to the main body 30 with a conventional directional or nondirectional coating method. During the application of the coating 41 serving as a shield, the front side 32a of the main body 30 and/or the reflective coating 31 should be protected suitably, for example by the use of a screen.

(36) FIGS. 5A and 5B show by way of example the second reflective optical element 22 of the projection lens 20 in a sectional illustration and in a plan view. In this case, too, the reflective coating 31 is configured for the reflection of EUV radiation 33 with normal incidence. In the example shown, the material of the main body 30 is ULE®, i.e. titanium-doped quartz glass.

(37) In the case of the reflective optical element 22 illustrated in FIGS. 5A and 5B, said reflective optical element likewise being exposed to a hydrogen plasma H.sup.+, H* present in the vacuum environment 34, a shield embodied as a protective film 42 or as a protective foil is provided at the lateral surface region 35. The protective film 42 is to the greatest possible extent impermeable to hydrogen ions H.sup.+ and insensitive to an etching attack. The protective film 42 therefore protects the lateral surface region 35 against ion etching by hydrogen ions H.sup.+. The protective film 42 is connected to the main body 30, to put it more precisely to the lateral surface region 35, by an indirect, typically cohesive connection. In order to produce the cohesive connection, a connecting material 43 in the form of an adhesive layer, for example, can be introduced between the main body 30 and the protective film 42. The use of a connecting material 43 in the form of a double-sided adhesive tape or of other suitable materials, e.g. of a solder, is also possible.

(38) The connection or the connecting material 43 should be chosen such that the transfer of mechanical stresses to the main body 30 is avoided as much as possible. This can typically be ensured by the use of an adhesive or an adhesive tape having a low (volume) shrinkage of e.g. less than approximately 0.5% during and after the production of the adhesive connection. Optionally other joining methods, for example soldering, . . . can be used to apply the protective film 42 to the main body 30. The connection or the connecting material 43 should also be as insensitive as possible to the ambient conditions prevailing in the EUV lithography apparatus 1, particularly if the connecting material 43 is not completely covered by the protective film 42.

(39) The protective film 42 itself is formed from a typically metallic or ceramic material that is resistant to an etching effect of the hydrogen plasma H.sup.+, H* or that at least has a greater resistance to the etching effect of the plasma H.sup.+, H* than the underlying material of the main body 30. In particular, the protective film 42 can comprise one or more of the materials described further above in association with the shield embodied as a screen 36. In the case of metallic materials, the protective film 42 should have a maximum thickness D that is less than approximately 50 μm, preferably less than approximately 20 μm. The (maximum) distance A between the protective film 43 and the lateral surface region 35 is less than double the Debye length λ.sub.D, preferably less than the Debye length λ.sub.D. The Debye length λ.sub.D is typically less than 5 mm, less than 0.5 mm or less than 0.1 mm.

(40) In order to increase the protective effect against the etching effect of the surrounding hydrogen plasma H.sup.+, H*, the measures illustrated in FIGS. 2A and 2B and/or in FIGS. 3A and 3B can also be combined with the measures illustrated in FIGS. 4A and 4B or in FIGS. 5A and 5B: Therefore, a shield in the form of a screen 36 and also the coating 41 serving as a shield as shown in FIGS. 4A and 4B or the protective film 42 shown in FIGS. 5A and 5B can be used at one and the same reflective optical element 16. The coating 41 serving as a shield and the protective film 42 can also be used at different surface regions of one and the same reflective optical element. Moreover, it is not absolutely necessary for the shield 36, 41, 42 to be of integral design, rather said shield can be composed of a plurality of component parts.

(41) Optionally, in addition to the resistance to the etching effect of the plasma, the materials used for the shield 36, 41, 42 can also fulfil other functions; by way of example, they can have an absorbing effect for the EUV radiation 33, as is the case e.g. for NiSi or WB4C.