In particular, the mirror of a microlithographic projection exposure apparatus

Abstract

The invention relates in particular to a mirror of a microlithography projection exposure apparatus, comprising an optical surface, a mirror substrate (10, 21, 31, 41, 50, 70, 80), a reflective layer stack (28, 38, 48, 75, 85) for reflecting electromagnetic radiation incident on the optically active surface, at least one first electrode arrangement (24, 34, 44), at least one second electrode arrangement (23, 33, 43) and an actuator layer system located between the first and second electrode arrangements, the mirror substrate (10, 21, 31, 41, 50, 70, 80) and an actuator layer system disposed between the first electrode configuration (70, 80) and the reflective layer stack (28, 38, 48, 75, 85), the actuator layer system including a piezoelectric layer (25, 35, 45) and responsive to a voltage applied between the first electrode configuration and the second electrode configuration, exhibiting a deformation response (11, 51) characterizing a linear deformation of the actuator layer system in a direction perpendicular to the optically active surface for a predetermined value of the voltage, the deformation response relating to the mirror having a local change in PV value of 20% or more for a predetermined voltage that is spatially constant across the piezoelectric layer.

Description

Technical Field

[0001] This application claims the priority of German Patent Application No. 10 2020 207 699.5 filed on June 22, 2020. The content of this application is incorporated herein by reference.

[0002] The present invention relates in particular to a mirror of a microlithography projection exposure apparatus.

Background Art

[0003] Microlithography is used, for example, in the manufacture of microstructured components such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus including an illumination device and a projection lens. In this case, an image of a mask (reticle) illuminated by the illumination device is projected onto a substrate (for example, a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection lens by the projection lens, so as to transfer the mask structure to the photosensitive coating of the substrate.

[0004] In the case of projection lenses designed for the EUV region, that is, for example, at wavelengths of about 13 nm or about 7 nm, since suitable light-transmitting refractive materials are not available, mirrors are used as optical components for the imaging process.

[0005] In this case, it is also known to configure one or more mirrors of an EUV system as an adaptive mirror having an actuator layer made of a piezoelectric material. In that case, by applying a voltage to electrodes arranged on both sides of this piezoelectric layer, an electric field with a locally varying intensity is generated in the piezoelectric layer. When the piezoelectric layer is locally deformed, as a result, the reflective layer stack of the adaptive mirror is also deformed. For example, imaging aberrations (and in some cases, imaging aberrations that change over time) can be at least partially compensated by appropriate driving of the electrodes.

[0006] Figure 10a schematically shows the structure of a conventionally possible adaptive mirror 110. The mirror 110 particularly includes a mirror substrate 111 and a reflective layer stack 120, and in this example has a piezoelectric layer 115 made of lead zirconate titanate (Pb(Zr,Ti)O3, PZT). Electrode structures are respectively located above and below the piezoelectric layer 115, and an electric field that generates locally variable deformation can be applied to the mirror 110 by these electrode structures. Among the above electrode structures, the second electrode structure on the substrate 111 side is configured as a continuous planar electrode 113 with a certain thickness, while the first electrode structure has a plurality of electrodes 119, and a voltage with respect to the electrode 113 can be applied to each of them by a lead wire 118. The electrodes 119 are embedded in a common smoothing layer 117, and the smoothing layer 117 is made of, for example, quartz (SiO2) and serves to level the electrode structure formed from the electrodes 119. Further, the mirror 110 has an adhesive layer 112 (for example, made of titanium Ti) between the mirror substrate 111 and the lower electrode 113 on the mirror substrate 111 side, and a buffer layer 114 (for example, made of LaNiO3) disposed between the electrode structure on the substrate 111 side and the piezoelectric layer 115. The buffer layer 114 further supports the growth of PZT with an optimal crystal structure and ensures consistent polarization characteristics of the piezoelectric layer over the service life.

[0007] During the operation of the mirror 110 or an optical system including the mirror 110, when voltages are applied to the electrodes 113 and 119, the piezoelectric layer 115 bends due to the formed electric field. In this way, for example, it is possible to achieve the driving of the mirror 110 for compensating for optical aberration due to thermal deformation when EUV radiation is incident on the optically effective surface 110a, for example.

[0008] According to FIG. 10a, the mirror 110 further has a mediator layer 116. The mediator layer 116 is in direct electrical contact with the electrodes 119 (shown in a plan view in FIG. 1 only for illustration). The mediator layer 116 acts to "mediate" between the electrodes 119 with respect to potential, and as a result has only a low conductivity (preferably less than 200 Siemens / meter), so that the potential difference between adjacent electrodes 119 substantially drops in the mediator layer 116.

[0009] When the voltage is applied to the electrode structure of the adaptive mirror, a current is generated in the mediator layer 116, and as a result, undesirable parasitic heat generation occurs due to the power generated thereby. Therefore, in principle, it is desirable to limit the power by setting a sufficiently high electrical resistance (e.g., 100 kΩ) for the mediator layer 116. Such a configuration may be suitable for specific situations of using an adaptive mirror, such as correcting the influence of deformation of an optical element such as a mirror or lens element thermally induced by radiation absorption. However, in practice, there are also situations where the desired surface shape of the adaptive mirror must be set on a significantly short time scale, e.g., within milliseconds (ms). In situations where the potential propagation becomes too slow in the mediator layer having the high resistance for power limitation, for example, consideration of thermally induced mask deformation in a lithography process is included. In that case, the mask forms irregular "mountainous" regions due to absorption exceeding about 30% of EUV light, ultimately resulting in focus fluctuations in the lithography imaging process. Consideration of the above-described variation in the surface shape of the mask by corresponding settings of the adaptive mirror 110 must be performed during the scanning operation (e.g., lasting about 100 ms) in the lithography process, and thus on a relatively short time scale in units of milliseconds (ms). Although the corresponding driving of the electrodes of the adaptive mirror can be easily realized, reducing the electrical resistance of the mediator layer causes thermal problems due to power interdependence in this regard. It can be seen that the implementation of the mentioned small time constant is difficult with respect to the mediator layer.

[0010] Regarding the prior art, refer to Patent Document 1 and Patent Document 2 as mere examples.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0012] The object of the present invention is to provide a mirror for a microlithography projection exposure apparatus that enables aberration correction while reducing the complexity of the structure and at least partially avoiding the above problems related to parasitic heat generation, in particular.

MEANS FOR SOLVING THE PROBLEM

[0013] This object is achieved by the features of the independent claims.

[0014] The mirror according to the present invention comprises an optically effective surface, a mirror substrate, a reflective layer stack that reflects electromagnetic radiation incident on the optically effective surface, at least one first electrode structure, at least one second electrode structure, an actuator layer system located between the first electrode structure and the second electrode structure, and an actuator layer system arranged between the mirror substrate and the reflective layer stack The actuator layer system includes a piezoelectric layer and exhibits a deformation response that characterizes a linear deformation of the actuator layer system in a direction perpendicular to the optically effective surface with respect to a predetermined value of the voltage in response to a voltage applied between the first electrode structure and the second electrode structure. For the above deformation response, the PV value locally changes by 20% or more for a predetermined voltage that is spatially constant throughout the piezoelectric layer.

[0015] According to one embodiment, the deformation response locally changes by 50% or more, particularly 70% or more, more specifically 90% or more of the PV value (PV = "peak to valley") with respect to a predetermined voltage that is spatially constant throughout the piezoelectric layer. In this case, the PV value refers to the difference between the maximum value and the minimum value within the local distribution of the deformation response, and the percentage criterion here relates to the maximum (i.e., the maximum value) of these values.

[0016] In particular, in the case of an adaptive mirror including a piezoelectric layer to which an electric field can be applied by an electrode structure, the present invention is based on the concept that the piezoelectric layer or the actuator layer system including the piezoelectric layer is configured such that even when a constant voltage is applied by the electrode structure, the deformation response changes in the actuator layer system or the entire adaptive mirror, thereby significantly reducing the complexity of the mirror structure.

[0017] In other words, in particular, in order to introduce local variations in the deformation profile of the adaptive mirror, the present invention does not necessarily apply a locally varying voltage by the electrode structure, but rather includes the principle of achieving the desired local variations in the deformation profile of the adaptive mirror from the beginning by the local variations in the deformation response according to the present invention.

[0018] With this configuration according to the present invention, as a result, the need for local variations in the voltage applied to the piezoelectric layer or the actuator layer system including the piezoelectric layer by the electrode structure is eliminated or only very slightly (i.e., only a slight local change in the voltage value) is required. As a result, in particular, the electrode structure can be realized in a continuous planar shape, and the aforementioned mediator layer (which is used for mediating the potential between electrodes that are otherwise different) is not required.

[0019] Therefore, it is also possible to avoid the above-described problems regarding the contradictory requirements regarding heat generation on the one hand and the speed of components on the other hand associated with the use of such a mediator layer. As a result, the present invention provides an adaptive mirror characterized by both substantially no parasitic heat generation and high speed with respect to setting the desired surface shape.

[0020] In this case, by eliminating the use of an electrode structure composed of a plurality of electrodes that can be driven independently of each other, the present invention achieves the above advantages in addition to reducing the structural complexity, in exchange for accepting a reduction in the flexibility or performance of the adaptive mirror regarding the provision of different deformation profiles.

[0021] As yet another major advantage of the configuration of the present invention, by eliminating a plurality of electrodes that can be driven independently of each other, or by replacing the electrodes with a few adjacent electrodes that cover substantially the entire surface of one continuous planar electrode or piezoelectric layer, it is possible to realize leads that supply voltage to the electrode(s) exclusively "from the side" (i.e., substantially in the plane of the corresponding electrode or perpendicular to the stacking direction of the layer structure of the adaptive mirror), that is, in other words, it is possible to avoid routing the leads in the "stacking direction" of the layer structure of the adaptive mirror, which is problematic from the perspective of production engineering. In particular, as will also be described below, even when the corresponding electrode structure is subdivided into several electrodes and those electrodes have a relatively large surface area, each of the electrodes can select an appropriate geometric shape that allows access to the leads from the side in the above sense.

[0022] The abandonment of the need for a mediator layer has already been described above and is advantageously achieved by the present invention. However, as will be described in even more detail below with regard to the configuration of the piezoelectric layer or the actuator layer system according to the present invention including the piezoelectric layer, by paying attention to ensuring that the deformation response (especially the d 33 constant) at each boundary between adjacent electrodes is small, and thus the lack of voltage driving of the piezoelectric layer in the above boundary region is not so important, this abandonment is still possible even in the case of the above configuration having a relatively small number of electrodes.

[0023] The coefficient characterizing the linear expansion of the material of the piezoelectric layer obtained depending on voltage is referred to as the "d 33 constant" and corresponds to the corresponding component of the dielectric tensor involved in the linear expansion in the direction perpendicular to the optically effective surface.

[0024] According to one embodiment, the piezoelectric layer has a d 33 constant with a PV value locally changing by 20% or more throughout the piezoelectric layer.

[0025] As far as the specific realization of the local variation of the deformation response according to the present invention of the piezoelectric layer or the actuator layer system including the piezoelectric layer is concerned, the above local variation can be achieved in various ways. In this regard, in an embodiment, the actuator layer system can have a thickness that locally varies, particularly with a local variation where the PV value is 20% or more. In particular, the above local variation can be 50% or more, more specifically 70% or more, and more specifically 90% or more.

[0026] In particular, in an embodiment, it is possible to provide a thickness variation of the piezoelectric layer suitable for imparting a desired local variation of the deformation response. Alternatively or additionally, in order to achieve the desired deformation response, the stoichiometric composition of the piezoelectric layer can also be appropriately changed.

[0027] In a further embodiment, a dielectric layer having a thickness profile that varies according to the desired local variation of the deformation profile can also be used in combination with the piezoelectric layer. In that case, the piezoelectric layer can have a constant thickness. In a corresponding embodiment, the actuator layer system according to the present invention that brings about a locally varying deformation response is formed by a combination of the piezoelectric layer and the above dielectric layer.

[0028] Therefore, according to one embodiment, the actuator layer system includes a dielectric layer in addition to the piezoelectric layer, and the dielectric layer can particularly have a locally varying thickness.

[0029] According to one embodiment, the first electrode structure and the second electrode structure each cover more than 99% of the optical use region of the piezoelectric layer.

[0030] According to one embodiment, the first electrode structure and the second electrode structure each have a voltage lead that extends perpendicular to the surface normal of the mirror.

[0031] According to one embodiment, the mirror has a stack of a plurality of such actuator layer systems respectively positioned between two electrode assemblies. In this case, the different actuator layer systems of the stack can induce different deformation modes at the wavelength of the light reflected by the mirror. The different deformation modes can particularly correspond to different Zernike deformations.

[0032] Therefore, the present invention is not limited to the use of a single piezoelectric layer or a single actuator layer system that provides a local variation in deformation response. Rather, a plurality of such piezoelectric layers (or actuator layer systems each including a piezoelectric layer) can also be provided, and each shape change of the adaptive mirror can induce a desired wavelength change (e.g., a Zernike deformation of the wavelength) by each of the piezoelectric layers or actuator layer systems. Further, the piezoelectric layer or the actuator layer system including the piezoelectric layer can be provided in a stack within the same adaptive mirror or in different mirrors.

[0033] In this case, each of the actuator layer systems of the stack can be configured to provide each desired local variation in deformation response or wavelength change (i.e., for example, with local thickness variation of the piezoelectric layer, local variation in the stoichiometric composition of the piezoelectric layer, or local thickness variation of an additional dielectric layer) in the same manner as in the above embodiment.

[0034] The present invention further relates to a mirror having an optically effective surface, particularly a mirror of a microlithographic projection exposure apparatus, a mirror substrate, a reflective layer stack that reflects electromagnetic radiation incident on the optically effective surface, a stack of actuator layer systems respectively arranged between two electrode assemblies, the stack being arranged between the mirror substrate and the reflective layer stack and each of the actuator layer systems includes a piezoelectric layer and exhibits a deformation response that characterizes a linear deformation of each actuator layer system in a direction perpendicular to the optically effective surface with respect to a predetermined value of the voltage in response to a voltage applied between the electrode assemblies. The mutually different actuator layer systems of the stack also relate to a mirror that induces different deformation modes at the wavelengths of the light reflected by the mirror.

[0035] Each actuator layer system (or the related piezoelectric layer or dielectric layer) is mutually different with respect to the deformation mode respectively given at the wavelength of the light reflected by the mirror (i.e., for example, induces mutually different Zernike deformations on the wavefront). As a result, in principle, it is possible to set any desired deformation pattern with an adaptive mirror using a number of stacked actuator layer systems accordingly. In this case, the contributions of the individual piezoelectric layers or actuator layer systems can be correspondingly cumulative, and the resulting total deformation can be described, for example, as a linear superposition of the involved Zernike deformations.

[0036] The provision of any of the above-mentioned desired deformation patterns can further be carried out without the need for a voltage profile that varies in the lateral direction of the mirror (like the conventional structure from FIG. 10a). Rather, for driving the individual piezoelectric layers or actuator layer systems, similar to the above-described embodiments according to the present invention, here too, a simple planar electrode that can be realized particularly exclusively "from the side" (i.e., substantially in the plane of the corresponding electrode or perpendicular to the stacking direction of the layer structure of the adaptive mirror) is sufficient. Also in this case, the routing of the above-mentioned lead wires in the "stacking direction" of the layer structure of the adaptive mirror, which is problematic from the perspective of production engineering, is avoided.

[0037] The mirror can in particular be a mirror of a microlithography projection exposure apparatus. However, the present invention is not limited thereto. In further applications, the mirror according to the present invention can also be used or utilized, for example, in an apparatus for mask measurement.

[0038] According to one embodiment, the mirror is designed for an operating wavelength of less than 30 nm, particularly less than 15 nm. However, since the present invention is not limited thereto, in further applications, the present invention can also be advantageously realized in an optical system having an operating wavelength in the VUV region (for example, 200 nm).

[0039] The invention further relates to an optical system of a microlithography projection exposure apparatus, in particular an illumination device or a projection lens, each comprising at least one mirror having the above characteristics and in any case comprising at least one actuator layer system located between two electrode assemblies, and also relates to a microlithography projection exposure apparatus.

[0040] The piezoelectric layer or the actuator layer system comprising the piezoelectric layer used according to the invention can also be provided on different mirrors. Thus, according to one embodiment, the optical system comprises a plurality of mirrors having the above characteristics, and the actuator layer systems are mutually different for each mirror with respect to the deformation modes induced at the wavelength of the light reflected by each mirror.

[0041] Further configurations of the invention can be obtained from the description and the dependent claims.

[0042] The invention will be explained in more detail below based on the exemplary embodiments shown in the accompanying drawings.

Brief Description of the Drawings

[0043]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

DETAILED DESCRIPTION OF THE INVENTION

[0044] A common point of the embodiments of the adaptive mirror described below is that the desired deformation profile generated by applying a voltage to the piezoelectric layer by the electrode structure is, with respect to its local variation, not the local voltage variation introduced by the electrode structure (shown in Fig. 10b for a conventional adaptive mirror), but rather is derived from the local variation in the deformation response of the piezoelectric layer or the actuator layer system including the piezoelectric layer. In Figs. 1 and 10b, the deformation responses of each adaptive mirror are schematically illustrated and indicated by "11" and "125" respectively, and the substrates of the corresponding adaptive mirrors are indicated by "10" and "111" respectively. The profile of the electric field strength generated by the electrodes (not shown in Figs. 1 and 10b) is represented by arrows.

[0045] Due to the local variation of the deformation response (in particular, the d constant of the piezoelectric layer) according to the present invention, according to Fig. 1, it is possible to achieve a desired locally varying surface profile of the adaptive mirror with a constant profile of the electric field strength (as shown in Fig. 1). Therefore, the electrode structure used to generate the corresponding electric field in the region of the piezoelectric layer can also be configured as a single continuous planar electrode on both sides of the piezoelectric layer. However, since the present invention is not limited thereto, it is intended to include configurations having a specific number of (although relatively small compared to conventional adaptive mirrors) electrodes (as will be described in more detail with reference to Fig. 6). 33

[0046] ​As will be described below with reference to FIGS. 2 to 4, the above-described locally varying deformation response can be realized in accordance with the present invention in various ways.

[0047] FIG. 2 schematically and briefly shows a possible structure of an adaptive mirror 20 according to the present invention, which can be, in particular, a projection lens or an illumination device of an optical system, especially an EUV mirror. When electromagnetic EUV radiation is incident on the optically effective surface of the mirror 20 during operation of the optical system, non-uniform volume changes of the mirror substrate 21 can occur due to the temperature distribution resulting from the absorption of the radiation incident non-uniformly on the optical surface. In order to correct such undesired volume changes or to correct other aberrations that occur during operation of the microlithographic projection exposure apparatus, the mirror 20 is of an adaptive design, as will be described in more detail below.

[0048] The mirror 20 includes a mirror substrate 21 made of any suitable mirror substrate material. One suitable mirror substrate material is, for example, titanium dioxide (TiO2)-doped quartz glass, such as the material sold under the trade name ULE (registered trademark) (from Corning Inc.). Yet another suitable mirror substrate material is, for example, lithium aluminum silicon oxide glass ceramic, such as the material sold under the trade name Zerodur (registered trademark) (from Schott AG).

[0049] On the mirror substrate 21 of the mirror 20, a functional layer stack 22 (which may include, for example, a smoothing layer, a substrate protection layer, a stress relaxation layer, and optionally further functional layers) and electrode assemblies 23, 24 for applying a voltage to the piezoelectric layer 25 located therebetween are arranged. "27" refers to yet another functional layer stack, on which a reflective layer stack 28 and an upper capping layer 29 follow in a layer structure. By way of example only, the reflective layer stack 28 may include a molybdenum-silicon (Mo-Si) layer stack. Without limiting the present invention to a specific configuration of this layer stack, a suitable configuration by way of example only may include about 50 pairs or layer packets of an actuator layer system including a molybdenum (Mo) layer with a layer thickness of 2.4 nm and a silicon (Si) layer with a layer thickness of 3.3 nm each.

[0050] In an exemplary embodiment, the piezoelectric layer 25 is made of lead zirconate titanate (Pb(Zr,Ti)O3, PZT). In yet other embodiments, the piezoelectric layer 25 can be made of some other suitable material (e.g., aluminum nitride (AlN), aluminum scandium nitride (AlScN), lead magnesium niobate (PbMgNb), or vanadium-doped zinc oxide (ZnO)). The piezoelectric layer 25 can have a thickness of less than, for example, 5 μm, and more particularly in the range of 1 μm to 4 μm.

[0051] According to FIG. 2, the mirror 20 has a locally varying thickness profile in the piezoelectric layer 25 in order to give a locally varying deformation response (to the electric field generated by the electrode assemblies 23, 24) described with reference to FIG. 1. The electrode assemblies 23, 24 are each realized as electrodes that are continuous over their entire surfaces and locally generate a substantially constant electric field in the region of the piezoelectric layer 25 when a voltage is applied. Thus, the thickness profile of the piezoelectric layer 25 is such that the d 33 constant local variations result in the desired (target) deformation profile D(x,y)=U×d 33 (x,y) is accurately selected so as to be applied to the mirror 20.

[0052] The thickness variations of the piezoelectric layer 25 can be set as desired in production engineering by corresponding adaptation of the residence time in the coating process, and the resulting surface defects can be compensated for by an additional smoothing layer or polishing layer 26, for example as shown in FIG. 2. As shown in FIG. 2, the smoothing layer or polishing layer 26 is arranged on the side opposite the substrate 21 of the electrode assembly 24 (upward in the direction of the optically effective surface), so that the electrode assembly 24 itself still follows the thickness profile of the piezoelectric layer 25. In yet another embodiment, the resulting surface defects described above can also be possible on the side of the substrate 21.

[0053] As an alternative to or in addition to the thickness variation of the piezoelectric layer, its stoichiometric composition can also be changed as desired to achieve the respective desired local profiles of the deformation response. The stoichiometric composition of the piezoelectric layer can be set as desired by the target stoichiometric composition in the coating process, the gas filling in the coating chamber, and the setting of the temperature at the growth site. Further, in order to achieve the respective desired local profiles of the deformation response, the morphological structure of the piezoelectric layer (e.g., regarding the pillar structure, pillar diameter, and pillar transition region) can also be appropriately designed to be locally variable.

[0054] FIG. 3 shows a corresponding exemplary embodiment, and components similar to or substantially functionally identical to those in FIG. 2 are denoted by reference numerals with "10" added. In this case, the piezoelectric layer 35 shown in FIG. 3 has a geometrically constant thickness, and the local variation of the deformation response is achieved here only by the variation of the stoichiometric composition of the piezoelectric layer 35 as described above.

[0055] FIG. 4 shows yet another possible configuration of the adaptive mirror 40, and components similar to or substantially functionally identical to those in FIG. 3 are further denoted by reference numerals with "10" added. According to FIG. 4, the local variation of the deformation response according to the present invention is achieved by using yet another dielectric layer 46a having an appropriate thickness variation in combination with the piezoelectric layer 45. Due to the dielectric layer 46a, the voltage drop occurring on the side of the piezoelectric layer 45 when a voltage is applied to the electrode structures 43, 44 changes according to a non-linear characteristic curve. Also here (similarly to FIG. 2 in this regard), the resulting surface defects are compensated by an additional smoothing layer or polishing layer 46b disposed on the side opposite to the substrate 41 of the electrode 44 (upward in the direction of the optically effective surface), so that the electrode structure 44 itself still follows the thickness profile of the dielectric layer 46a.

[0056] Merely by way of example, the dielectric layer 46a can be made of titanium dioxide (rutile, TiO2). Other dielectric materials can be used as well, and the relative permittivity is preferably in the range of 20 to 200, more preferably in the range of 50 to 150. The thickness variation can be carried out particularly in the range of 5 nm to 1000 nm, more specifically in the range of 10 nm to 300 nm.

[0057] The value of the voltage applied by each electrode structure of the above-described embodiments can be set, for example, in the range of 0 V to 200 V, particularly in the range of 0 V to 100 V.

[0058] In the above-described embodiments, in each case, a locally substantially constant electric field is generated in the region of each piezoelectric layer, and for this purpose, in each case, only a single continuous planar electrode is on both sides of the piezoelectric layer (or of the actuator layer system formed from the piezoelectric layer 45 and the dielectric layer 46a according to FIG. 4), but the present invention is not limited thereto. In this regard, in order to give more freedom regarding the deformable profiles that can be set for the adaptive mirror, as shown in FIG. 5, the electrode structure can be composed of several (particularly less than 20, more specifically less than 10) planar electrodes each, and adjacent electrodes can be separated from each other by electrically insulated separation regions. In order to minimize the (piezoelectrically non-activated) grooves caused by the separation regions, the separation regions preferably have a width of less than 5 mm, more specifically less than 1 mm, more specifically less than 0.5 mm.

[0059] With this configuration, the electric field generated in the region of the piezoelectric layer by the electrode structure can still vary even with a low local resolution (in accordance with the subdivision of the planar electrodes in regions 52, 53, 54) as shown in FIG. 5, so that a high flexibility is achieved regarding the deformable profiles that can be set for the corresponding adaptive mirror compared to the embodiments of FIGS. 1 to 4 (which can only set a predetermined deformable profile with a continuously adjustable amplitude).

[0060] FIG. 6 schematically shows a possible geometric arrangement of a few electrodes 61 to 66 obtained by subdividing one of two electrode structures for generating an electric field in the operation of the piezoelectric layer 60. As is apparent from FIG. 6, firstly, substantially the entire upper surface of the piezoelectric layer 60 is covered with an electrode material. Secondly, the geometric arrangement of the individual electrodes 61 to 66 is appropriately selected such that each of the electrodes 61 to 66 is accessible laterally to a lead wire necessary for applying a voltage, that is, routing of the lead wire in the stacking direction of the layer structure (that is, in the z direction with respect to the illustrated coordinate system) is avoided.

[0061] FIGS. 7 and 8 show still other embodiments of the adaptive mirror. This mirror firstly includes, in each case, a mirror substrate 70 and 80, functional layer stacks 71 and 74, and 81 and 84, reflective layer stacks 75 and 85, and capping layers 76 and 86, respectively, in the same manner as in the above-described embodiments. However, different from the above-described embodiments, according to FIGS. 7 and 8, a plurality of actuator layer systems 73a, 73b,... and 83a, 83b,... are stacked on one another, each of which is configured in the same manner as in FIG. 2, FIG. 3, or FIG. 4, that is, each includes a piezoelectric layer in combination with a dielectric layer similar to FIG. 4 in some cases. Further, electric fields can be applied independently of each other to the actuator layer systems 73a, 73b,... and 83a, 83b,... shown in FIGS. 7 and 8, respectively, by the electrode structures 72 and 82. The number of each of the stacked actuator layer systems 73a, 73b,... and 83a, 83b,... is merely an example in the figure and can be selected as desired or according to specific requirements in principle. Further, although the actuator layer systems 73a, 73b,... and 83a, 83b,... in FIGS. 7 and 8 are illustrated merely schematically and with uniform hatching for reasons of explanation, as described below, the profiles of the deformation responses are different, which can be achieved, for example, by locally varying layer thickness profiles and / or locally varying stoichiometric compositions of each piezoelectric layer.

[0062] The piezoelectric layers provided in the stacked arrangement according to the embodiments of FIGS. 7 and 8 are individually driven by the electrodes applied to the corresponding electrode structures 72 and 82, respectively, and the above voltage, and thus the electric fields generated in the regions of the individual piezoelectric layers, can be selected independently of each other.

[0063] Accordingly, the mirrors shown in FIGS. 7 and 8 each include, in a stacked manner, a plurality of actuator layer systems 73a, 73b,... and 83a, 83b,... respectively located between two electrode structures 72 and 82. As a result of the local variations in the deformation responses of the individual actuator layer systems 73a, 73b,... and 83a, 83b,... in the corresponding stack being different from each other, the different actuator layer systems of the stack induce different deformation modes at the wavelength of the light reflected by the mirror. The different deformation modes can correspond, for example, to different Zernike deformations (although the present invention is not limited thereto) such that the local variations in the deformation responses or d 33 constants in the actuator layer system all correspond to Zernike modes in any case.

[0064] In this regard, reference may be made to the publications "Zernike-based matrix model of deformable mirrors: Optimization of aperture size" by J. Alda and G. D. Boreman (Appl. Opt. 32 (1993) 2431-2438) and "Zernike polynomials: A guide" by V. Lakshminarayanan, Andre Fleck (Journal of Modern Optics 58 (2011) 545-561, DOI: 10.1080 / 09500340.2011.554896).

[0065] In this case, the embodiments of FIGS. 7 and 8 are based on the consideration that, as an effect of the stacked arrangement of each of the plurality of piezoelectric layers or actuator layer systems 73a, 73b, … and 83a, 83b, … including the piezoelectric layer, the contributions of the individual piezoelectric layers or actuator layer systems accumulate correspondingly, and the resulting total deformation can be described as a linear superposition of the deformation modes (e.g., Zernike deformation) involved.

[0066] In the example of Zernike deformation, the contribution of each of the actuator layer systems 73a, 73b, … and 83a, 83b, … to the total deformation is proportional to each Zernike polynomial, and the amplification corresponds to each Zernike coefficient. For the accurate reproduction of a freeform surface, the sum of an infinite number of Zernike polynomials is theoretically required, but in practice, in many cases, only a few modes are sufficient to achieve the desired shape correction or aberration correction.

[0067] As a result, according to the embodiments of FIGS. 7 and 8, in principle, it is possible to set an arbitrary desired deformation pattern with an adaptive mirror by using a correspondingly large number of stacked actuator layer systems.

[0068] It should be pointed out that, instead of Zernike polynomials, other suitable (orthogonal or non-orthogonal) systems can also be used as the basis for the configuration of the piezoelectric layer or its deformation response. Suitable functions can be, for example, Zernike functions scaled according to, for example, an elliptical use surface, Legendre polynomials, spline-based functions having local signatures (e.g., shapes approximately following a Gaussian bell shape with a changing center), or sine / cosine profiles.

[0069] The electrode assemblies 72 and 82 are each configured as electrodes continuous over the entire surface, similar to the embodiments described above with reference to FIGS. 2 to 4, and as described above, the lead wires for voltage supply can be realized exclusively “from the side” (i.e., substantially in the plane of the corresponding electrode or perpendicular to the stacking direction of the layer structure of the adaptive mirror).

[0070] The difference between the embodiments of FIGS. 7 and 8 is that, according to FIG. 7, in any case, one electrode structure 72 is shared by a series of actuator layer systems 73a, 73b,... in the stacking direction, while according to FIG. 8, in any case, separate electrode structures 82 are assigned to a series of actuator layer systems 83a, 83b,.... This is achieved according to FIG. 8 in that a series of electrode structures 82 in the stacking direction, which are assigned to separate the actuator layer systems 83a, 83b,..., are separated from each other by the dielectric layer 87.

[0071] The configuration shown in FIG. 7 can be selected especially when each of the individual actuator layer systems 73a, 73b,... includes a piezoelectric layer having a constant thickness and a varying stoichiometric composition. In contrast, in the configuration shown in FIG. 8, the dielectric layer 87 provided to separate the series of electrodes 82 and actuator layer systems 83a, 83b,... can also function to smooth the thickness profile that can be within the actuator layer systems 83a, 83b,... (for example, the associated piezoelectric layer) at the transition to the actuator layer systems that respectively follow in the stacking direction.

[0072] In still other embodiments, the piezoelectric layer or the actuator layer system including the piezoelectric layer can also be provided on different mirrors.

[0073] The local variation of the deformation response according to the present invention can be carried out in various ways in the individual piezoelectric layers or the associated actuator layer systems shown in FIGS. 7 and 8, similar to the above embodiments of FIGS. 2 to 4. Therefore, in particular, the individual piezoelectric layers can have a locally varying thickness profile and / or stoichiometric composition.

[0074] Coating processes suitable for realizing such local profiles can, for example, use a deposition source with a defined locally varying residence time. Suitable deposition concepts can be based, for example, on magnetron sputtering, spatial atomic layer deposition, or FEBID ("focused electron beam induced deposition"). Using a single magnetron source, for example, any desired layer thickness profile of each piezoelectric layer can be realized. However, two simultaneously operating deposition sources can be used to fabricate a piezoelectric layer having a locally varying stoichiometric composition. When using the ALD method, each precursor composition can vary over time such that the use of one deposition source is sufficient. In this regard, reference is made to U.S. Patent No. 4,533,449 and the publication "Focused electron beam induced deposition: A perspective" by M. Huth et al. (Beilstein Journal of Nanotechnology 2012, 3, 597 - 619).

[0075] Figure 9 shows a schematic view of an exemplary projection exposure apparatus designed for operation with EUV and capable of realizing the present invention.

[0076] According to Figure 9, the illumination device of the projection exposure apparatus 90 designed for EUV includes a field facet mirror 93 and a pupil facet mirror 94. Light from a light source unit including a plasma light source 91 and a collector mirror 92 is directed to the field facet mirror 93. A first telescope mirror 95 and a second telescope mirror 96 are arranged in the optical path downstream of the pupil facet mirror 94. A deflection mirror 97 is arranged downstream of the optical path, and the deflection mirror directs the incident radiation to the object field of the object plane of a projection lens including six mirrors 101 - 106. At the location of the object field, a mask 99 carrying a reflective structure is arranged on a mask stage 98, and the mask is imaged onto an image plane using the projection lens. At the image plane, a substrate 108 coated with a photosensitive layer (photoresist) is located on a wafer stage 107.

[0077] In principle, all of the mirrors 101 to 106 of the projection lens can be configured in accordance with the present invention. In particular, the adaptive mirror according to the present invention can be arranged in the vicinity of the field of view, in the vicinity of the pupil, or in an intermediate position, that is, between the field plane and the pupil plane.

[0078] Although the present invention has been described based on specific embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example, by combining and / or exchanging the features of the individual embodiments. Therefore, it goes without saying that such variations and alternative embodiments are also included in the present invention, and the scope of the present invention is limited only within the meaning of the appended claims and their equivalents.

Claims

1. A mirror having an optically effective surface, in particular a mirror of a microlithographic projection exposure apparatus, comprising: a mirror substrate (10, 21, 31, 41, 50, 70, 80); a reflector stack (28, 38, 48, 75, 85) for reflecting electromagnetic radiation incident on the optically effective surface; at least one first electrode structure (24, 34, 44); at least one second electrode structure (23, 33, 43); an actuator layer system located between the first electrode structure (24, 34, 44) and the second electrode structure (23, 33, 43), the actuator layer system being arranged between the mirror substrate (10, 21, 31, 41, 50, 70, 80) and the reflector stack (28, 38, 48, 75, 85); wherein the actuator layer system includes a piezoelectric layer (25, 35, 45) having a locally varying thickness profile, and exhibits a deformation response (11, 51) characterizing a linear deformation of the actuator layer system in a direction perpendicular to the optically effective surface in response to a voltage applied between the first electrode structure (24, 34, 45) and the second electrode structure (23, 33, 43); when the ratio indicated by the PV value is the ratio of the maximum value of the difference between the maximum value and the minimum value of the local distribution of the deformation response, the deformation response (11, 51) is a mirror that locally varies by 20% or more in PV value with respect to a predetermined voltage that is spatially constant over the entire piezoelectric layer (25, 35, 45) set as a reference.

2. A mirror having an optically effective surface, in particular a mirror of a microlithographic projection exposure apparatus, comprising: a mirror substrate (10, 21, 31, 41, 50, 70, 80); a reflector stack (28, 38, 48, 75, 85) for reflecting electromagnetic radiation incident on the optically effective surface; at least one first electrode structure (24, 34, 44); at least one second electrode structure (23, 33, 43); an actuator layer system located between the first electrode structure (24, 34, 44) and the second electrode structure (23, 33, 43), the actuator layer system being arranged between the mirror substrate (10, 21, 31, 41, 50, 70, 80) and the reflector stack (28, 38, 48, 75, 85); comprising, wherein the actuator layer system includes a piezoelectric layer (25, 35, 45), and shows a deformation response (11, 51) characterizing linear deformation of the actuator layer system in a direction perpendicular to the optically effective surface in response to a voltage applied between the first electrode structure (24, 34, 45) and the second electrode structure (23, 33, 43), When the ratio indicated by the PV value is set as the ratio to the maximum value of the difference between the maximum value and the minimum value of the local distribution of the deformation response, the deformation response (11, 51) locally changes by 20% or more in PV value with respect to a predetermined voltage that is spatially constant throughout the piezoelectric layer (25, 35, 45) set as a reference, The actuator layer system includes a dielectric layer (46a) in addition to the piezoelectric layer (45), and is a mirror characterized thereby.

3. In the mirror according to claim 1 or 2, the deformation response locally changes by 50% or more in PV value with respect to a predetermined voltage that is spatially constant throughout the piezoelectric layer (25, 35, 45), and is a mirror characterized thereby.

4. In the mirror according to any one of claims 1 to 3, the piezoelectric layer (25, 35, 45) has a d constant that locally changes by 20% or more in PV value throughout the piezoelectric layer (25, 35, 45). ３３ A mirror characterized by having such a constant.

5. In the mirror according to any one of claims 1 to 4, the actuator layer system has a thickness that locally changes with a local variation of 20% or more, particularly in PV value, and is a mirror characterized thereby.

6. In the mirror according to claim 2, the dielectric layer (46a) has a locally changing thickness, and is a mirror characterized thereby.

7. In the mirror according to any one of claims 1 to 6, the piezoelectric layer (25, 35, 45) has a locally changing stoichiometric composition, and is a mirror characterized thereby.

8. In the mirror according to any one of claims 1 to 7, the first electrode structure (24, 34, 44) and the second electrode structure (23, 33, 43) each cover 99% or more of the optically used area of the piezoelectric layer (25, 35, 45), and are mirrors characterized thereby.

9. In the mirror according to any one of claims 1 to 8, the first electrode structure (24, 34, 44) and the second electrode structure (23, 33, 43) each have a voltage lead wire that extends perpendicular to the surface normal of the mirror, and are mirrors characterized thereby.

10. In the mirror according to any one of claims 1 to 9, the mirror is characterized by having a plurality of said actuator layer systems (73a to 73f, 83a to 83f) respectively positioned between two electrode structures (72, 82) in a stacked manner.

11. In the mirror according to claim 10, the different actuator layer systems (73a to 73f, 83a to 83f) of the stack induce different deformation modes in the wavefront of the light reflected by the mirror.

12. A mirror having an optically effective surface, in particular for a microlithography projection exposure apparatus, a mirror substrate (70, 80), a reflective layer stack (75, 85) that reflects electromagnetic radiation incident on the optically effective surface, a stack of actuator layer systems (73a to 73f, 83a to 83f) respectively arranged between two electrode structures (72, 82), and a stack arranged between the mirror substrate (10, 21, 31, 41, 50, 70, 80) and the reflective layer stack (28, 38, 48, 75, 85) comprising, each of the actuator layer systems (73a to 73f, 83a to 83f) including a piezoelectric layer having a locally varying thickness profile, and showing a deformation response characterizing the linear deformation of each of the actuator layer systems (73a to 73f, 83a to 83f) in a direction perpendicular to the optically effective surface in response to a voltage applied between the electrode structures (72, 82), the different actuator layer systems (73a to 73f, 83a to 83f) of the stack induce different deformation modes in the wavefront of the light reflected by the mirror.

13. A mirror having an optically effective surface, in particular for a microlithography projection exposure apparatus, a mirror substrate (70, 80), a reflective layer stack (75, 85) that reflects electromagnetic radiation incident on the optically effective surface, a stack of actuator layer systems (73a to 73f, 83a to 83f) respectively arranged between two electrode structures (72, 82), and a stack arranged between the mirror substrate (10, 21, 31, 41, 50, 70, 80) and the reflective layer stack (28, 38, 48, 75, 85) comprising, each of the actuator layer systems (73a to 73f, 83a to 83f) including a piezoelectric layer, and showing a deformation response characterizing a linear deformation of each of the actuator layer systems (73a to 73f, 83a to 83f) in a direction perpendicular to the optically effective surface in response to a voltage applied between the electrode assemblies (72, 82), the different actuator layer systems (73a to 73f, 83a to 83f) of the stack inducing different deformation modes in the wavefront of the light reflected by the mirror, the actuator layer system including a dielectric layer (46a) in addition to the piezoelectric layer, the mirror being characterized thereby.

14. The mirror according to any one of claims 11 to 13, wherein the different deformation modes correspond to different Zernike deformations.

15. The mirror according to any one of claims 1 to 14, wherein the mirror is designed for an operating wavelength of less than 30 nm.

16. The mirror according to any one of claims 1 to 15, wherein the mirror is a mirror of a microlithography projection exposure apparatus.

17. An optical system of a microlithography projection exposure apparatus, in particular an illumination device or a projection lens, comprising at least one mirror according to any one of claims 1 to 16, in each case including at least one actuator layer system located between two electrode assemblies.

18. The optical system according to claim 17, wherein the optical system comprises a plurality of mirrors according to any one of claims 1 to 16, and the actuator layer systems are different from each other for each mirror with respect to the deformation modes induced in the wavefront of the light reflected by each mirror.

19. A microlithography projection exposure apparatus comprising an illumination device and a projection lens, comprising the optical system according to claim 17 or 18.

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