Mirror arrangement for an EUV projection exposure apparatus, method for operating the same, and EUV projection exposure apparatus

Abstract

A mirror arrangement for an EUV projection exposure apparatus for microlithography comprises a plurality of mirrors each having a layer which is reflective in the EUV spectral range and to which EUV radiation can be applied, and having a main body. In this case, at least one mirror of the plurality of mirrors has at least one layer comprising a material having a negative coefficient of thermal expansion. Moreover, a method for operating the mirror arrangement and a projection exposure apparatus are described. At least one heat source is arranged, in order to locally apply heat in a targeted manner to the at least one layer having a negative coefficient of thermal expansion of the at least one mirror.

Claims

1. An arrangement, comprising: a plurality of mirrors comprising a first mirror, each of the plurality of mirrors comprising: a surface that is reflective in the EUV spectral range; and a main body; and a first heat source, wherein: the first mirror further comprises a layer comprising a material having a negative coefficient of thermal expansion; the first heat source is configured to apply heat to the layer comprising the material having the negative coefficient of thermal expansion; and the first heat source is configured to apply a positionally variable heat distribution to the layer comprising the material having the negative coefficient of thermal expansion.

2. The arrangement of claim 1, wherein: the first mirror further comprises: a thermally insulating layer; and a layer comprising a material having a positive coefficient of thermal expansion; and the thermally insulating layer separates the layer comprising the material having the positive coefficient of thermal expansion from the layer comprising the material having the negative coefficient of thermal expansion.

3. The arrangement of claim 2, wherein the material having the positive coefficient of thermal expansion comprises at least one material selected from the group consisting of Zr, Y, Nb, Mo, Si, Ge, Ru, RuO2, RuSi, Ni.

4. The arrangement of claim 2, further comprising a second heat source, wherein the second heat source is configured to apply heat to the layer comprising the material having the positive coefficient of thermal expansion.

5. The arrangement of claim 1, wherein the plurality of mirrors comprises a second mirror which is different from the first mirror, and the second mirror comprises a layer comprising a material having a negative coefficient of thermal expansion.

6. The arrangement of claim 5, wherein: during use of the arrangement, EUV radiation passes through the arrangement along a path; and the first and second mirrors are arranged at positions in the path that are conjugate or approximately conjugate with regard to their optical effect.

7. The arrangement of claim 5, further comprising a second heat source, wherein the second heat source is configured to apply heat to a layer of the second mirror.

8. The arrangement of claim 1, wherein the plurality of mirrors comprises a second mirror which is different from the first mirror, and the second mirror comprises a layer comprising a material having a positive coefficient of thermal expansion.

9. The arrangement of claim 1, wherein the first heat source is configured to apply heat to the layer comprising the material having the negative coefficient of thermal expansion from a side of the main body of the first mirror.

10. An arrangement, comprising: a plurality of mirrors comprising a first mirror, each of the plurality of mirrors comprising: a surface that is reflective in the EUV spectral range; and a main body; and a first heat source, wherein: the first mirror further comprises a layer comprising a material having a negative coefficient of thermal expansion; the first heat source is configured to apply heat to the layer comprising the material having the negative coefficient of thermal expansion; and the first heat source comprises an IR radiation source.

11. An arrangement, comprising: a plurality of mirrors comprising a first mirror, each of the plurality of mirrors comprising: a surface that is reflective in the EUV spectral range; and a main body; and a first heat source, wherein: the first mirror further comprises a layer comprising a material having a negative coefficient of thermal expansion; the first heat source is configured to apply heat to the layer comprising the material having the negative coefficient of thermal expansion; and the first heat source is configured to apply heat to the layer comprising the material having the negative coefficient of thermal expansion through the reflective layer of the first mirror.

12. The arrangement of claim 11, wherein the first mirror further comprises a thermally insulating layer which is between the main body of the first mirror and the layer comprising the material having the negative coefficient of thermal expansion.

13. The arrangement of claim 12, wherein the first mirror further comprises a layer having high thermal conductivity which is between the thermally insulating layer and the main body of the first mirror.

14. An arrangement, comprising: a plurality of mirrors comprising a first mirror, each of the plurality of mirrors comprising: a surface that is reflective in the EUV spectral range; and a main body; and a first heat source, wherein: the first mirror further comprises a layer comprising a material having a negative coefficient of thermal expansion; the first heat source is configured to apply heat to the layer comprising the material having the negative coefficient of thermal expansion; and the material having the negative coefficient of expansion comprises at least one material selected from the group consisting of ZrMo.sub.2O.sub.8, ZrW.sub.2O.sub.8, HfMo.sub.2O.sub.8, Zr.sub.2 (MoO.sub.4).sub.3, Zr.sub.2(WO.sub.4).sub.3, Hf.sub.2 (MoO.sub.4).sub.3, ScF.sub.3, ZnC.sub.2N.sub.2, ZnF.sub.2, Y.sub.2W.sub.2O.sub.12, BiNiO.sub.3.

Description

(1) Exemplary embodiments of the invention are illustrated in the drawing and are described in greater detail hereinafter with reference thereto. In the figures:

(2) FIG. 1 schematically shows an excerpt from a mirror in accordance with the prior art in section, EUV radiation being applied thereto;

(3) FIG. 2 schematically shows an excerpt from a mirror in section, to which EUV radiation is applied, wherein the mirror has a layer having a negative coefficient of thermal expansion;

(4) FIG. 3 schematically shows an excerpt from a further mirror in section;

(5) FIGS. 4a) to 4c) show a surface of the mirror in FIG. 3, wherein the surface in FIG. 4a) has a surface deformation which is intended to be changed into a surface deformation in accordance with FIG. 4b) and FIG. 4c) shows a conventional method as to how the surface deformation in accordance with FIG. 4b) is produced from the surface deformation in accordance with FIG. 4a);

(6) FIG. 5 schematically shows an excerpt from a mirror in section which has a layer having a negative coefficient of thermal expansion;

(7) FIGS. 6a) to 6c) show an alternative method with respect to FIGS. 4a) to c) in order to obtain the same optical effect via a thermal manipulator having at least one mirror in accordance with FIG. 5, in order to produce the optical effect in accordance with the surface deformation in accordance with FIG. 4b) proceeding from the optical effect of the surface deformation in FIG. 4a), wherein FIGS. 6a) and 6b) shows two surface profiles having surface deformations whose resulting optical effect of the surface deformation is shown in FIG. 6c);

(8) FIG. 7 schematically shows a resulting surface profile of a mirror arrangement for obtaining a desired optical effect on the wavefront of EUV radiation;

(9) FIGS. 8a) to 8d) show a basic schematic diagram of how the optical effect on the wavefront profile in accordance with FIG. 7 can be achieved by a mirror arrangement comprising two mirrors, wherein FIG. 8a) schematically shows an excerpt from a first mirror in section having a layer having a positive coefficient of thermal expansion, FIG. 8b) shows a surface of the mirror in FIG. 8a) after targeted local application to the layer having a positive coefficient of thermal expansion, FIG. 8c) schematically shows an excerpt from a further mirror in section having a layer having a negative coefficient of thermal expansion, FIG. 8d) shows a surface of the mirror in FIG. 8c) after targeted local application to the layer having a negative coefficient of thermal expansion;

(10) FIG. 9 schematically shows a surface profile of a mirror for obtaining a desired optical effect on the wavefront of EUV radiation;

(11) FIG. 10 schematically shows an excerpt from a mirror in section which has both a layer having a positive coefficient of thermal expansion and a layer having a negative coefficient of thermal expansion, wherein the surface profile in accordance with FIG. 9 is set by targeted local application to the layer having a positive coefficient of thermal expansion and the layer having a negative coefficient of thermal expansion;

(12) FIG. 11 schematically shows an excerpt from a mirror in section having a layer construction in accordance with a first exemplary embodiment;

(13) FIG. 12 schematically shows an excerpt from a mirror in section having a layer construction in accordance with a further exemplary embodiment;

(14) FIG. 13 shows an EUV projection exposure apparatus containing a mirror arrangement comprising a mirror pair having a mirror having a layer having a positive coefficient of thermal expansion and a mirror having a layer having a negative coefficient of thermal expansion; and

(15) FIG. 14 shows a basic illustration of a control system used for driving heat sources in a targeted manner in order to locally apply heat to mirrors of a projection lens which have at least one NTE or PTE layer, in order to set a wavefront profile of the projection lens.

(16) Referring to FIGS. 1 to 13, the principles of the present invention are described in greater detail below. Insofar as the mirrors are shown as plane mirrors in the figures, it is understood that the drawing is merely schematic and the invention can, of course, also be applied to mirrors having concave curvature and having convex curvature. Furthermore, it should be noted that layer thicknesses shown in the figures are not true to scale either on their own or in relation to one another.

(17) FIG. 1 illustrates a mirror 10 for a projection exposure apparatus for microlithography according to the prior art. The mirror 10 has a reflective layer (HR) 12 and a main body (GK) 14. Further layers are usually present in the main body 14 and the reflective layer 12, the further layers being symbolized here by an interrupted line 16 for reasons of simplification.

(18) The reflective (HR) layer 12 is, for example, a molybdenum-silicon layer system having a topmost protection layer (not illustrated) composed of ruthenium (Ru) or rhodium (Rh), for example. The main body 14 is produced from a material having no or extremely low thermal expansion, for example a ULE (ultra low expansion), glass ceramic such as is offered by the manufacturer Corning.

(19) The reflective layer 12 has a surface 18 to which EUV radiation 20 is applied during the operation of the mirror 10. The main body is thus situated on that side of the reflective layer 12 which faces away from the EUV radiation. The EUV radiation 20 can be applied to the entire surface 18 or, as shown in FIG. 1, only a section 22 of the surface 18.

(20) The EUV radiation 20 is reflected for the most part at the surface 18 of the reflective layer 12, but part of the EUV radiation is absorbed by the reflective layer 12, as a result of which the reflective layer 12 heats up. The heating of the reflective layer 12 has the effect that the reflective layer 12 expands in the region where EUV radiation is applied to it, as a result of which the surface 18 of the reflective layer 12 deforms, as is illustrated by a surface peak 24 in FIG. 1. Such an undesirable surface deformation of the surface 18 of the reflective layer 12 is a consequence of the fact that the layer construction of the mirror 10 only contains materials having a positive coefficient of thermal expansion, which also include the material of the reflective layer 12.

(21) By contrast, FIG. 2 shows a mirror 30 having a reflective (HR) layer 32, a main body (GK) 34 and, between the reflective layer 32 and the main body 34, a layer 36 comprising a material having a negative coefficient of thermal expansion (NTE, negative thermal expansion). One such material is ZrW.sub.2O.sub.8, for example. Further materials having a negative coefficient of thermal expansion will be mentioned later. A negative coefficient of thermal expansion means that the material does not expand in the event of a temperature increase, but rather, on the contrary, contracts. Conversely, a material having a negative coefficient of thermal expansion expands if heat is withdrawn from the material or if the temperature is decreased.

(22) The layer 36 could also form the main body 34 itself or a part thereof, whereas it is embodied as a separate layer in the exemplary embodiment shown.

(23) If EUV radiation 38 is applied to the mirror 30 during operation, the heat absorption of the reflective layer 32 has the effect that the heat, as is indicated by arrows 40, propagates to the layer 36 having a negative coefficient of thermal expansion, the layer 36 accordingly absorbs this heat and contracts. The contraction of the layer 36 compensates for the expansion of the reflective layer 32 precisely such that a surface 42 of the reflective layer 32 is not deformed. In order to achieve this effect, the thickness of the layer 36 and the material of the layer 36 should be adapted to the thickness and the material of the layer 32 and to the thermal expansion of the main body 34.

(24) Layers of a mirror which comprise a material having a negative coefficient of thermal expansion are designated hereinafter as NTE layers for short. An NTE layer can be used in a mirror not only for avoiding a deformation of the surface of the reflective layer, rather an NTE layer can in particular also be used for bringing about a surface deformation in the surface of the reflective layer in a targeted manner, as is described below, with the result that a thermal manipulator is provided.

(25) FIG. 3 firstly shows a mirror 50 having a reflective (HR) layer 52, a main body 54 and a layer 56 comprising a material having a positive coefficient of thermal expansion (PTE, positive temperature expansion, positive thermal expansion).

(26) Layers having a positive coefficient of thermal expansion are designated hereinafter as PTE layers.

(27) In the exemplary embodiment shown, the PTE layer 56 is a so-called surface protection layer (SPL), such as is usually used in EUV mirrors in order, for example, to protect the main body 54 against aging as a result of EUV radiation. The HR layer too, is generally a PTE layer.

(28) FIG. 4a) depicts a surface 58 of the reflective layer 52 of the mirror 50 by itself in section. As a result of heat being locally applied to the mirror 50 in a targeted manner, the surface 58 has a surface or figure deformation 60 which is spatially localized. In the case where the surface deformation 60 was set in a targeted manner by application of heat to the mirror 50, this may be caused, for example, by the fact that the surface deformation 60 is intended to result in a specific optical effect on the wavefront of the EUV radiation, in order, for example, to compensate for an imaging aberration appearing in the EUV projection exposure apparatus. The surface deformation 60 can therefore be brought about by heat being locally applied to the mirror 50 in a targeted manner. Since the mirror 50 only comprises materials having a positive coefficient of thermal expansion, the surface deformation 60 is accordingly a surface peak.

(29) It may then be desirable, for example in the event of a change in the operating mode of the EUV projection exposure apparatus, to eliminate the surface deformation 60 and in return to produce a different surface deformation 62 elsewhere on the surface 58. The surface deformation 62 is in turn a surface peak.

(30) In the case of a mirror such as the mirror 50 which only comprises PTE layers, it is necessary in conventional methods, as is shown in FIG. 4c), firstly to apply heat to the entire mirror 50 until the surface 58 is brought by thermal expansion to a surface level 58 at which the surface deformation 60 is overwritten. The raising of the level of the surface 58 to the surface level 58 is indicated by arrows 64.

(31) In order to obtain the surface deformation 62 in accordance with FIG. 4b), heat is locally applied in a targeted manner to the mirror 50 proceeding from the surface level 58, in order to form the surface deformation 62 at the surface level 58 now set, as is indicated by an arrow 66. However, this procedure has the considerable disadvantage that the mirror 50 is loaded overall with a very high heat input that can lead to layer stresses through to the destruction of the reflective layer 52. Moreover, the additional heat input disturbs the thermal budget of the projection exposure apparatus, which results in a change or impairment of the wavefronts and can thus lead to a reduction of the yield during production. Furthermore, if the mirror 50 is arranged in a position of an EUV projection exposure apparatus in which it is already subjected to a high irradiance anyway as a result of the EUV radiation, this problem becomes even more serious.

(32) On the other hand, consideration could be given to firstly eliminating the surface deformation 60 before the surface deformation 62 is introduced. Owing to the very slow thermal relaxation, however, the mirror 50 would have to be locally cooled in a targeted manner at the position of the surface deformation 60, but this cannot be realized without additional disadvantages in practice, as was discussed in the introductory part of the description.

(33) A description will now be given below of how it is possible to set the surface profile of the surface 58 with the surface deformation 62 proceeding from the surface profile of the surface 58 with the surface deformation 60 on a short time scale, without excessive heat input and without active cooling.

(34) For this purpose, according to the invention at least one mirror having at least one NTE layer is provided in the mirror arrangement of an EUV projection exposure apparatus. Such a mirror is shown schematically in the form of a mirror 70 in FIG. 5.

(35) The mirror 70 has a reflective (HR) layer 72, a main body 74 and an NTE layer 76. The layer 70 can have further layers between the reflective layer 72 and the NTE layer 76 and between the NTE layer 76 and the main body 74. As will be described later, such a mirror, in addition to the NTE layer 76, can also be provided with an additional PTE layer as well. Moreover, the NTE layer 76 can also form the main body 74 itself or a part thereof.

(36) If the mirror 70 is available in a mirror arrangement for an EUV projection exposure apparatus, then it is possible to set the surface profile of the surface 58 in accordance with FIG. 4b) proceeding from the surface profile of the surface 58 in FIG. 4a) without excessive heat input and without targeted local cooling as is shown in FIGS. 6a) and b).

(37) FIG. 6a) shows that, as a result of heat being locally applied in a targeted manner to the mirror 50 in FIG. 3, the surface deformation 62 in accordance with FIG. 4b) is set in addition to the surface deformation 60 in accordance with FIG. 4a). Therefore, in contrast to the illustration shown in FIG. 4c), the surface deformation 60 is not overwritten by global heating of the mirror 50.

(38) FIG. 6b) illustrates that a wave deformation 80 is introduced into a surface 78 of the reflective layer 72 of the mirror 70 by targeted local application to the mirror 70, more precisely the NTE layer 76, which wave deformation is positioned with regard to its optical effect at the level of the surface deformation 60 and is the inverse thereof, i.e. is a surface valley. If the two mirrors 70 and 50 are arranged in the beam path of the EUV radiation at positions at which they have similar or even identical optical effects on the wavefront of the EUV radiation, that is to say if the two mirrors 50 and 70 are arranged at positions which are optically conjugate with respect to one another as well as possible, this gives rise to a resulting optical effect on the wavefront of the EUV radiation which corresponds to that of the surface profile of the surface 58 in FIG. 4b). This surface profile, in which only the surface deformation 62 has a resulting optical effect on the wavefront of the EUV radiation, is shown in FIG. 6c).

(39) In the case of the method described above, therefore, it is not necessary for the mirror 50 to be greatly heated overall in order to eliminate the surface deformation 60 in terms of its optical effect, rather it is possible to employ purely targeted local and thus lower heat inputs because the surface deformation 80 itself can be set by targeted local application of heat and cancels the surface deformation 60 in terms of its optical effect.

(40) While the exemplary embodiment just described provides a mirror arrangement of which one mirror is provided with at least one NTE layer and at least one other mirror is provided with at least one PTE layer, the combination of NTE layer and PTE layer can also be realized in a single mirror, as will be described later.

(41) FIG. 7 shows by way of example the desired profile of the resulting optical effect of a mirror arrangement on the wavefront of the EUV radiation in a projection exposure apparatus. The profile of the optical effect has three surface peaks 82, 84, 86 and two surface valleys 88 and 90, as seen in cross section. With a conventional mirror having only PTE layers, such an optical effect can be realized at most if the mirror could be locally cooled in a targeted manner, which, however, is associated with the technical problems mentioned in the introduction.

(42) By contrast, the desired surface profile of the optical effect can easily be realized by a mirror arrangement comprising the mirrors 50 in accordance with FIGS. 3 and 70 in accordance with FIG. 5, as described below with reference to FIGS. 8a) to d).

(43) FIG. 8a) shows the mirror 50 additionally with a heat source 92, which is embodied as an infrared (IR) radiation source. Such an IR radiation source can be formed by an IR pixel diode arrangement. Via the heat source 92, heat is locally applied to the PTE layer 56 in a targeted manner as is indicated by arrows 93a, 93b and 93c. In this case, the IR rays impinge on the reflective layer 52 and through the latter heat the actual PTE layer 56 at positions 95a, 95b and 95c at which an expansion of the PTE layer is desired for producing the surface peaks 82, 84 and 86. In order to produce the surface peaks 82, 84 and 86, as shown in FIG. 7, with different amplitudes, the absolute values of heat input at the positions 95, 95b and 95c correspondingly differ from one another. This is indicated by heat arrows 97a, 97b and 97c of different lengths in FIG. 8a). For this purpose, the heat source 92 can be designed in particular such that it can apply positionally variable heat distribution to the PTE layer.

(44) As a result of the application of heat to the PTE layer 56 in accordance with FIG. 8a), a surface profile of the surface 58 as shown in FIG. 8b) is set. The surface profile accordingly has the surface peaks 82, 84 and 86.

(45) It is also possible, instead of the separate layer 56 or additionally, to use the reflective layer 52 as a PTE layer.

(46) FIG. 8c) shows the mirror 70, to which a further heat source 96 in the form of an IR radiation source 98 is assigned, in order to locally apply heat to the NTE layer 76 in a targeted manner. In this case, heat is applied to the NTE layer 76 from the side of the main body 74 or the rear side thereof, in particular through the main body 74, which for this purpose is produced from a material that is transmissive to IR radiation. A ULE glass ceramic body, in particular, can be used for this purpose as material for the main body 74. Moreover, the main body can be polished in a suitably transparent fashion.

(47) The application of heat to the NTE layer 76 is illustrated here by arrows 99a and 99b, wherein the heat is radiated locally in a targeted manner at positions 101a and 101b corresponding to the desired positions of the surface valleys 88, 90 to be produced in FIG. 7. In order to produce the surface valleys 88, 90 with different amplitudes here as well, the heat from the heat source 96 is radiated at the positions 101a and 101b with different absolute values of heat, as is illustrated by heat arrows 103a and 103b of different lengths. Here, too, the heat source 96 is designed to apply positionally variable heat distribution to the NTE layer.

(48) As a result of the application of heat to the NTE layer 76 at the positions 101a and 101b, the NTE layer accordingly contracts, as a result of which the surface valleys 88, 90 are produced, as is illustrated in FIG. 8d).

(49) With a mirror arrangement comprising the mirrors 50 and 70 in accordance with FIGS. 8a) to d) a thermal manipulator is thus provided which makes it possible to set in the combination of the surface profiles of the surfaces 58 and 78 a resulting surface profile which produces a specific desired optical effect on the wavefront of the EUV radiation, as is shown in FIG. 7.

(50) It should be pointed out at this juncture that instead of only one mirror 50 and only one mirror 70, two or three mirrors of the type of the mirror 50 and/or two mirrors of the type of the mirror 70 can be arranged in the beam path of the EUV radiation, the production of the surface peaks 82, 84, 86 then being distributed between the two or three mirrors of the type of the mirror 50, and/or the production of the surface valleys 88 and 90 then being distributed between the two mirrors of the type of the mirror 70. As a result, the heat inputs can be distributed along a plurality of mirrors, which further reduces the thermal loading of the individual mirrors and can increase the number of degrees of freedom for the manipulator.

(51) In particular it is advantageous if the mirror arrangement of the EUV projection exposure apparatus comprises at least two mirrors each having at least one NTE layer.

(52) Referring to FIGS. 9 and 10, a further exemplary embodiment of a mirror arrangement will be described, which mirror arrangement uses a mirror having at least one PTE layer and at least one NTE layer.

(53) FIG. 9 firstly shows a desired profile 102 of the optical effect on the wavefront of EUV radiation with a surface peak 104 and a surface valley 106.

(54) FIG. 10 shows a mirror 110 for an EUV projection exposure apparatus, which mirror can be used to set the profile 102 of the optical effect on the wavefront of the EUV radiation by itself. The mirror 110 has a reflective (HR) layer 112, a main body 114, a PTE layer 116, an NTE layer 118 and a thermally insulating (ISO) layer 120 between the PTE layer 116 and the NTE layer 118. In this case, the PTE layer 116 can also be part of the HR layer 112.

(55) A heat source 122, embodied as an IR radiation source 124, and also a further heat source 126, likewise embodied as an IR radiation source 128, are assigned to the mirror 110.

(56) Via the heat source 124, heat is locally applied to the PTE layer 116 in a targeted manner, to be precise at a position 130 situated at the level of a position 132 of a surface 134 of the reflective layer 112. As a result of application of heat to the PTE layer at the position 130 with heat transfer through the reflective layer 112, as is indicated by a heat arrow 136, the PTE layer expands in the region of the position 130, and a surface deformation in the form of the surface peak 104 correspondingly forms on the surface 134 of the reflective layer 112. The heat input at the position 130 should be dimensioned taking account of any possible additional thermally governed expansion of the reflective layer 112, since, specifically, heat is applied to the PTE layer 116 here through the reflective layer 112.

(57) Heat is likewise applied to the NTE layer 118, to be precise via the heat source 126 through the main body 114 at a position 138 situated at the level of a position 140 of the surface 134 of the reflective layer 112 at which the wave valley 106 is intended to be produced. The heat input into the NTE layer 118 is indicated by a heat arrow 142. As a result of the application of heat to the NTE layer at the position 138, the NTE layer contracts in the region of the position 138, wherein the deformation of the NTE layer that is caused as a result propagates through the thermally insulating layer 120, the PTE layer 116 and the reflective layer 112, such that the surface valley 106 arises at the surface 134 of the reflective layer 112.

(58) The thermally insulating layer 120 prevents or reduces a heat transfer between the PTE layer 116 and the NTE layer 118, such that the application of heat to the PTE layer 116 at the position 130 does not also lead to an application of heat to the NTE layer 118, and vice versa.

(59) By virtue of the construction of the mirror 110 both with a PTE layer and with an NTE layer and as a result of targeted local application of heat to the NTE layer independently of the PTE layer, a surface profile of the surface 134 with surface peaks and surface valleys can thus be set with only one mirror.

(60) FIG. 11 shows an example of a layer construction of a mirror 150 in accordance with the principles of the present invention.

(61) The mirror 150 has a reflective (HR) layer 152, a main body 154 and an NTE layer 156 between the reflective layer 152 and the main body 154. A thermally insulating (ISO) layer 158 is optionally situated between the reflective layer 152 and the NTE layer 156, the thermally insulating layer preferably having no or only a very low coefficient of thermal expansion. Furthermore, the mirror 150 optionally has a thermally insulating (ISO) layer 160 on that side of the NTE layer 156 which faces away from the reflective layer 152, the thermally insulating layer preferably comprising a material having no or only a very low coefficient of thermal expansion. The material for the insulating layer 160 can be produced from quartz, for example. A layer 162 having high thermal conductivity (HC) is optionally situated between the NTE layer 156 and the main body 154 and serves for introducing the residual heat that remained in the mirror 150 into the main body 154 as uniformly as possible. In this case, the thickness and the material of the layer 162 should be adapted to the thickness and material of the NTE layer 156. The HC layer 162 ideally has a low coefficient of thermal expansion. The insulating (ISO) layers, in particular the ISO layer 160 is preferably embodied in a transparent fashion in order to enable the IR rays to penetrate in particular into the NTE layer 156. The HC layer having high thermal conductivity 162 is likewise preferably embodied in a transparent fashion in order to enable the IR rays to penetrate into the ISO layer 160.

(62) The main body 154 preferably consists of ULE ceramic. The reflective layer 152 can be a molybdenum-silicon layer stack. The layer 162 having high thermal conductivity can be produced for example from copper or nickel or, in the case of a preferred transparency mentioned above, from CaF.sub.2.

(63) Table 1 below lists various materials having a negative coefficient of thermal expansion which can be used as material for the NTE layer 156. In Table 1, CTE denotes the coefficient of thermal expansion, K denotes the unit Kelvin, d denotes an exemplary thickness of the NTE layer, delta t denotes an assumed temperature difference, and delta d denotes the contraction of the NTE layer given the assumed temperature difference delta t. and in the formula n=(1)+i denote the real deviating from 1 and the imaginary part of the complex refractive index n of the respective material at a wavelength of 13.5 nm. These variables indicate the extent to which the respective material can be used within or even above the HR layer. The lower the value , the less transmission is lost through the NTE layer within or above the HR layer. The respective suitability of the material is determined by the efficiency ratio CTE/. The more negative the ratio, the more efficient the use of the respective material.

(64) TABLE-US-00001 TABLE 1 CTE delta t delta d [10e6/K] CTE/ d [nm] [K] [nm] PbTiO.sub.3 3.300 0.097 0.054 60.584 1000.000 10.000 0.033 Sc.sub.2W.sub.3O.sub.12 2.200 0.022 0.010 216.599 1000.000 10.000 0.022 Y.sub.2W.sub.3O.sub.12 4.200 0.031 0.013 325.677 1000.000 10.000 0.042 Lu.sub.2W.sub.3O.sub.12 6.800 0.059 0.032 213.688 1000.000 10.000 0.068 NbOPO.sub.4 3.700 0.099 0.032 114.294 1000.000 10.000 0.037 ZrV.sub.2O.sub.7 7.100 0.076 0.030 233.806 1000.000 10.000 0.071 ZrW.sub.2O.sub.8 8.700 0.047 0.020 443.595 1000.000 10.000 0.087 ZrMo.sub.2O.sub.8 5.000 0.069 0.020 250.279 1000.000 10.000 0.050 K.sub.5Zr(PO.sub.4).sub.3 0.500 0.010 0.003 144.829 1000.000 10.000 0.005 KZr.sub.2(PO.sub.4).sub.3 1.700 0.009 0.003 515.072 1000.000 10.000 0.017 Zn(CN).sub.2 18.1000 0.072 0.045 402.059 1000.000 10.000 0.181

(65) In selecting the materials for the at least one NTE layer, the following criteria should be taken into consideration: a precise knowledge of the coefficient of expansion in the temperature range used the isotropy of the material (otherwise stresses and the risk of microcracks arise) production as simply as possible including application during the production of the mirror negative coefficient of thermal expansion that is as high as possible no phase transitions at relatively low pressures isotropic or anisotropic contraction of the material used.

Example of a Compensation of the HR Layer by an NTE Layer

(66) The material ZrW.sub.2O.sub.8 is used for the NTE layer. The reflective layer shall be a molybdenum-silicon layer stack comprising 50 layers of a layer system composed of 2.4 nm molybdenum and 3.3 nm Si, this layer stack often being mentioned in the literature. A total thickness of 120 nm molybdenum and 165 nm silicon thus arises for the reflective layer. The average coefficient of thermal expansion of the reflective layer is 3.6110.sup.6/K, determined from the coefficient of thermal expansion of molybdenum of approximately 510.sup.6/K and of silicon of approximately 2.610.sup.6/K. A thermal expansion of the reflective layer can be compensated for with an approximately 118 nm thick NTE layer composed of ZrW.sub.2O.sub.8. Given a layer thickness of the NTE layer composed of ZrW.sub.2O.sub.8 of more than 118 nm thickness, therefore, even with the same heat input into both layers, the contraction of the NTE layer can be greater than the thermal expansion of the reflective layer, in order thus, for example, to produce a surface valley in the surface of the reflective layer, or in order to compensate for further expansion effects of the material ULE.

(67) FIG. 12 shows a further exemplary embodiment of a mirror 170 in accordance with the principles of the present invention.

(68) The mirror 170 has a reflective (HR) layer 172, a main body 174, a PTE layer 176, and an NTE layer 178 between the reflective layer 172 and the main body 174. The PTE layer 176 and the NTE layer 178 can also be interchanged. The construction of the reflective layer 172, the NTE layer 178 and the main body 174 can be as in the case of the mirror 150 in FIG. 11.

(69) The PTE layer 176 is, for example, a so-called surface protection layer, such as is usually used in the case of EUV mirrors.

(70) A thermally insulating layer 180 is situated between the PTE layer 176 and the NTE layer 178, the task of which thermally insulating layer is to minimize a heat transfer between the PTE layer 176 and the NTE layer 178, particularly if, as has already been described above, heat is applied to the PTE layer 176 and the NTE layer 178 from opposite sides of the mirror 170. By way of example, quartz can be used as material for the layer 180.

(71) A thermally insulating layer 182 and a layer 184 having a high thermal conductivity are situated between the NTE layer 178 and the main body 174, as has already been described with reference to the mirror 150 in FIG. 11.

(72) Finally, a capping layer (CAP) 186 is also applied on the reflective layer 172, and protects the reflective layer 172 toward the environment.

(73) FIG. 13 shows an EUV projection exposure apparatus for microlithography in which the principles of the present invention can be applied, the apparatus being provided with the general reference sign 200. The EUV projection exposure apparatus comprises an illumination system 202 and a projection lens 204. The illumination system 202 has an EUV radiation source 206, a mirror arrangement comprising a collector mirror 208, and further mirrors 210, 212, 214, 216 and 218. The EUV radiation generated by the EUV radiation source 206 is directed via the collector mirror 208, the mirrors 210, 212, 214, 216 and 218 into an object plane 220 in which a reticle 222 is arranged.

(74) The object plane 220 forms the object plane of the projection lens 204, which has in the direction of propagation of the EUV radiation a mirror arrangement comprising a mirror 224, a mirror 226, a mirror 228, a mirror 230, a mirror 232 and a mirror 234. The mirrors 224 and 234 image a pattern (not illustrated here) of the reticle 222 into an image plane 236, in which a wafer 238 is arranged.

(75) The mirror arrangement of the projection lens 204 formed from the mirrors 224 to 234 can comprise, then, according to the principles of the present invention, at least one mirror having at least one NTE layer and/or at least one mirror having at least one NTE layer and at least one PTE layer. In particular the two mirrors 224 and 226 are suitable here for such a configuration of the mirrors 224 to 234 of the mirror arrangement of the projection lens 204. Thus, the mirror 224 can have at least one PTE layer, as in the case for example of the mirror 50 in FIG. 3 or of the mirror 110 in FIG. 10 or else of the mirror 170 in FIG. 12. The mirror 226 can have at least one NTE layer, such as, for example, the mirror 30 in FIG. 2, the mirror 70 in FIG. 5, the mirror 110 in FIG. 10, and the mirror 150 in FIG. 11 or the mirror 170 in FIG. 12.

(76) The two mirrors 224 and 226 are suitable here as a PTE and NTE mirror pair in particular because the two mirrors 224 and 226 are arranged in the projection lens 204 at positions at which they have a similar optical effect on the wavefront of the EUV radiation. Specifically, both mirrors 224 and 226 are arranged near a pupil plane situated between the two mirrors 224 and 226.

(77) The mirrors 224 and 226 are assigned corresponding heat sources for locally applying heat in a targeted manner to the at least one PTE layer of the mirror 224 and the at least one NTE layer of the mirror 226, the heat sources not being illustrated in FIG. 13, but in this regard reference is made to FIGS. 8a) and c) and FIG. 10.

(78) Consequently, with the mirror pair comprising the mirrors 224 and 226, a thermal manipulator is provided with which the surface profile of the respective mirror is set by targeted local application to the at least one PTE layer of the mirror 224 and/or the at least one NTE layer of the mirror 226 in such a way that a resulting surface profile of the mirror arrangement comprising both mirrors 224 and 226 arises in a combination of both surface profiles, in order to set the wavefront profile of the EUV radiation reflected at the mirrors 224 to 234 in a desired manner. This setting can be performed in order to compensate for an imaging aberration, in order to counteract a deformation of the surface that is caused by the EUV radiation and/or in order to vary the wavefront in a manner dependent on an operating mode of the projection exposure apparatus 200.

(79) A further mirror pair suitable for a thermal manipulator in this sense constitutes the mirrors 232 and 234.

(80) It was described with reference to FIG. 13 that the projection lens 204 according to the principles of the present invention can comprise at least one mirror having at least one NTE layer, and/or at least one mirror having at least one NTE layer and at least one PTE layer.

(81) It was furthermore described above that it is possible to influence the wavefront of the EUV radiation via targeted local application of heat to such mirrors, e.g. the mirrors 224 and 226 of the projection lens 204 in FIG. 13.

(82) FIG. 14 then shows a basic illustration of a control system 300 which is able to drive the thermal manipulators in a suitable manner in order to obtain a desired wavefront profile of the EUV radiation.

(83) FIG. 14 shows a projection lens 302, which can be e.g. the projection lens 204 from FIG. 13. The projection lens 302 images, via EUV radiation 304, a reticle (not illustrated), arranged in a reticle plane 306 onto a wafer (not illustrated), arranged in a wafer plane 308.

(84) The projection lens 302 here comprises by way of example two heat sources 310 and 312, which, as described above, can be embodied in particular as infrared radiation sources, in particular as IR pixel diode arrangements. The heat sources 310 and 312 are able to locally apply heat, in particular a heat intensity profile, in a targeted manner to individual mirrors, not illustrated in FIG. 14, of the projection lens 302, as has already been described above. The mirrors can be e.g. the mirrors 224 and 226 of the projection lens 204 in FIG. 13.

(85) A wavefront measuring device 314 serves for measuring the wavefronts of the projection lens 302 in the wafer plane 308. The wavefronts can be measured before and/or during the operation of the projection lens 302.

(86) The wavefront measuring device 314 is connected to a calculating and wavefront measuring control unit 316 with pulse transmitter, which control unit initiates the measurements of the wavefronts by the wavefront measuring device 314 and then performs the evaluation and conditioning of the wavefronts. Instead of initiating a measurement of wavefronts, wavefronts can also be generated by the calculating and wavefront measuring control unit 316 on the basis of extrapolation or prediction models.

(87) The control system 300 furthermore comprises a computing unit 318 and a memory 320.

(88) The memory 320 stores the optical sensitivities of the mirrors that are variable by targeted local application of heat (e.g. the mirrors 224 and 226 of the projection lens 204 in FIG. 13). It is assumed here that the heat sources 310 and 312 respectively provide different heat application configurations, e.g. in the case where the heat sources 310, 312 are embodied as IR pixel diode arrays. Different heat intensity profiles are provided in this case. Furthermore, the optical effect is determined for so-called basic configurations representing specific basic patterns of driving for the IR pixel arrays.

(89) The computing unit 318 serves for computing a wavefront correction that determines, for a wavefront to be corrected that is provided via the calculating and wavefront measuring control unit 316, with the aid of the optical sensitivities stored in the memory 320, a correction in the form of actuation paths for the basic configurations mentioned above. The computing unit 318 determines from the basic configurations, by superposition, intensity profiles for the application of heat to the mirrors (e.g. the mirrors 224 and 226 of the projection lens 204 in FIG. 13), such that the mirrors to which heat is applied in a positionally dependent manner in this way alter the wavefronts of the projection lens 302 in a computed manner.

(90) The actuation paths determined by the computing unit 318 for the basic configurations are communicated by the computing unit 318 to corresponding control units 322, 324 assigned to the heat sources 310 and 312, respectively, wherein the control units 322 and 324 translate the abovementioned actuation paths of the basic configurations into actuating commands for the heat sources 310 and 312, respectively, which then locally apply heat, preferably with a corresponding heat intensity profile, in a targeted manner to the mirrors (not illustrated) to which heat is to be applied.