Abstract:
Processes for producing semiconductor components and/or other finely structured components include providing a projection objective having a mirror that is located within a predetermined proximity to a pupil surface of a projection objective. In one variant, an image of a pattern is projected onto a light-sensitive substrate in multiple exposures, in which a first pupil filter function is set on the mirror during a first exposure and, during a subsequent, second exposure, a different, second pupil filter function is set by local changes of geometric reflective properties of the mirror in a locally resolving manner.

Description:
This is a Continuation of application Ser. No. 10/975,496, filed Oct. 29, 2004, which is a Continuation Application of PCT Application No. PCT/EP03/04013, filed Apr. 17, 2003, which was published under PCT Article 21(2) in German and which claims priority from German Patent Application DE 102 20 324.5, filed on Apr. 29, 2002. The entire disclosures of application Ser. No. 10/975,496, of Application number PCT/EP03/04013 and of Application DE 102 20 324.5 are hereby incorporated into the present application by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of imaging a pattern arranged in the object plane of an optical imaging system into the image plane of the imaging system, and an imaging system for carrying out the method. The preferred area of application of the invention is projection objectives for microlithography. 
     2. Description of the Prior Art 
     Catadioptric or catoptric projection objectives are used in projection exposure installations for the production of semiconductor components and other finely structured components. They have at least one concave mirror and are used to image patterns of photomasks or graduated plates, which in the following text will generally be designated masks or reticles, onto an object arranged in the image plane of the projection objective and coated with a light-sensitive layer, at the highest resolution and on a reducing scale. 
     Since the resolving power of optical imaging systems is proportional to the wavelength λ of the light used and inversely proportional to the image-side numerical aperture (NA) of the optical imaging system, in order to produce finer and finer structures, the aim is firstly to enlarge the image-side numerical aperture of the projection objectives and secondly to use shorter and shorter wavelengths, preferably ultraviolet light with wavelengths of less than about 260 nm. 
     In addition to the resolving power, the depth of focus (DOF) that can be achieved during the imaging plays an important role for imaging that is faithful to the original. The depth of focus is likewise proportional to the wavelength used but inversely proportional to the square of the numerical aperture. Therefore, any increase in the numerical aperture without suitable measures for providing an adequate depth of focus is expedient only to a limited extent. 
     In order to improve the resolution and the depth of focus of microlithographic projection objectives, it is known to use pupil filters. The use of pupil filters is sometimes also designated optical filtering or apodisation. Here, pupil filter designates a spatial filter which is arranged in the region of a pupil surface of a projection system. This pupil surface, which is generally flat, is a Fourier-transformed surface in relation to the object plane and to the image plane. This means, for example, that a specific angle of incidence of light in the image plane of the projection objective corresponds to a specific radial coordinate in the pupil plane. With the aid of locally resolving filtering in the region of the pupil surface, it is therefore possible to exert an influence on the angular spectrum of the rays contributing to the imaging. 
     U.S. Pat. No. 5,222,112 shows a purely reflective catoptric projection objective for soft X radiation in which, in the region of a pupil, there is arranged a convex mirror whose mirror surface has a reflectance which decreases from the center toward the edge. As a result, an amplitude filter having a fixedly predefined, rotationally symmetrical filter function is created. In addition to amplitude filters which, for example, mask out specific different diffraction orders in the region of the pupil surface, phase filters are also known which, according to a predefined local filter function in the pupil surface, effect phase shifting of the light passing through in specific zones of the pupil surface. An explanation of the functioning of pupil filters and also examples of projection objectives having such filters are shown, for example, in EP 0 485 062 B1 or U.S. Pat. No. 5,144,362 and in the texts cited there. 
     In order to achieve the optimum benefit of pupil filtering, it is expedient to adapt the mode of action of a pupil filter, determined by the construction of the pupil filter, to the type of reticle structures to be imaged. Accordingly, pupil filters are optimized for specific reticle structures (for example contact holes, grid structures with one or more periodicity directions). Since reticles with an extremely wide range of structures are intended to be imaged by a projection objective, it is desirable to be able to use pupil filters with different effects as desired. For this purpose, U.S. Pat. No. 5,610,684 discloses a projection objective which has a changer for interchanging pupil filters in the pupil plane of the projection objective. The changer comprises displacement devices for displacing lenses close to the pupil, in order to provide sufficient space for the interchanging process. In EP 0 638 847 B1 (corresponding to U.S. Pat. No. 5,448,336), a projection objective having a pupil filter interchanging device is shown, the operation of which requires no movement of lenses close to the pupil. The technical implementation of interchanging devices of this type is very complicated in high-performance projection objectives, since close tolerances for material, fit and thickness of the optical components used and high requirements on the positioning accuracy and, possibly, gas tightness are imposed. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method of pupil filtering and a corresponding optical imaging system which make it possible to permit the use of pupil filters that act differently without any intervention in the optical system. It is another object to provide pupil filtering which in particular is adapted to the conditions in catadioptric or catoptric projection objectives for microlithography. 
     As a solution to these and other objects, this invention, according to one formulation, provides a method of imaging a pattern arranged in the object plane of an optical imaging system into the image plane of the imaging system with the aid of an imaging system which has a plurality of optical elements, which are arranged along an optical axis and comprise at least one mirror arranged in the region of a pupil surface of the imaging system, the method having the following steps: locally resolving filtering of the light passing through the imaging system in the region of the mirror in accordance with a variable filter function. According to another formulation there is provided an optical imaging system for imaging a pattern arranged in the object plane of the imaging system into the image plane of the imaging system, the imaging system having a plurality of optical elements, which are arranged along an optical axis and comprise at least one mirror arranged in the region of a pupil surface of the imaging system the mirror being subdivided into a plurality of mirror segments and at least some of the mirror segments being in each case assigned a drive device for moving the mirror element relative to other mirror elements. According to another formulation there is provided a mirror having a carrier structure to be incorporated in a fixed location, and a mirror layer applied to a mirror substrate, a drive device being arranged between the carrier structure and the mirror layer and being formed in such a way that, by driving the drive device, a distance between the carrier structure and the mirror layer can be changed in a controlled manner in predefined regions of the mirror. 
     Advantageous developments are specified in the dependent claims. The wording of all the claims is hereby incorporated by reference in the content of the description. 
     The method according to the invention of imaging a pattern applied in the object plane of an optical imaging system into the image plane of the imaging system makes use of an imaging system which has a plurality of optical elements, which are arranged along an optical axis and comprise at least one mirror arranged in the region of a pupil surface of the imaging system. Locally resolving filtering of the light passing through the imaging system in the region of the mirror in accordance with a variable or changeable filter function is proposed. In particular, the mirror itself is used as a variable pupil filter. The method can be applied in particular in all catoptric or catadioptric projection objectives in which at least one imaging, concavely or convexly curved mirror is provided and located in the region of a pupil surface. In this case, the mirror surface can substantially coincide with the pupil surface but it is also possible, at least locally, for there to be a distance between mirror surface and pupil surface. Said distance can be matched to the desired local resolution during the pupil filtering. By means of specifically changing reflective characteristics of the incorporated mirror, the filter function can be changed without replacement of the pupil filter being needed. 
     The mirror can be formed as an amplitude filter. This can be achieved, for example, by the reflective power of the mirror used varying over the mirror surface (locally resolving) and by it being possible to switch between different reflectance curves. 
     Embodiments in which locally resolving phase filtering is carried out in the region of the mirror are preferred. For this purpose, according to a preferred development, the mirror or the mirror surface used optically is subdivided into a number of mirror segments which can at least partly be moved relative to one another. At least some of the mirror segments are in each case assigned a drive device for moving the mirror element relative to other mirror elements, in particular with a movement component that can be oriented parallel to an optical axis. By means of controlled movement of mirror segments relative to one another with movement components parallel to the optical axis, the mirror geometry is varied locally differently or in a locally resolving manner and it is possible for optical path lengths of light rays passing through the imaging system to be varied relative to one another in a locally resolving manner in accordance with a predefined filter function by means of the movement of the mirror elements in the region of the pupil surface. This is because, by controlling the axial position of the segments, specific path differences or optical path length differences between rays reflected on different segments can be set. 
     By means of specific control of the axial position of various mirror segments that are directly adjacent or adjacent at a distance, continuous or stepwise setting and adjustment of the filter function is possible, the local components of the filter function being defined by position, shape and size of the mirror segments and it being possible for the achievable path differences to be adjusted continuously or step by step by means of the extent of the relative movement of the segments in relation to one another. 
     Tilting individual mirror segments or groups of segments can be used for the purpose of masking out light region by region or in a locally resolving manner, which means that the function of an amplitude filter can be achieved. 
     The great advantage as compared with the prior art is thus that, for the setting of different filter functions, it is not necessary to interchange with one another pupil filters each having a fixedly predefined filter function. Instead, the achievable filter function in the incorporated filter element can be set by means of specific relative movement of the mirror segments with respect to one another. A variable pupil filter is therefore provided. 
     In one development, the mirror comprises a number of annular or annular-segment-shaped mirror segments, which are arranged concentrically in relation to a center which preferably, in the incorporated mirror, coincides with the optical axis of the imaging system. This embodiment is adapted for rotationally symmetrical filter functions. The radial width of annular zones or the radius of a circular core zone can be adapted to the desired spatial resolution in the radial direction. In the most highly illuminated region of the pupil, there can be, for example, between two and ten to twenty or more annular zones. 
     In other embodiments, provision is made for the mirror to have a number of polygonal mirror segments which, for example, have a triangular, square or hexagonal segment shape. The mirror segments preferably lie so close to one another that they substantially fill the area of the illuminating region of the mirror, so that gaps which may possibly be present make up no more than about 1% of the total area. A hexagonal honeycomb shape of the mirror segments is beneficial to producing different spatial distributions of path differences. In principle, each mirror segment can be viewed as an image element or “pixel” in the Fourier domain. The number of “pixels” and the pixel size can be matched to the requirements. The number of polygonal segments can, for example, lie between about 10 and about 30 to 100 or more. 
     Dividing up the effective mirror surface into such mirror segments also permits the setting of non-rotationally symmetrical and/or eccentric filter functions in order, for example, to implement a linear course of a path difference transversely across the pupil filter or other spatial filter functions. For example, it is possible to set radially non-symmetrical pupil filter functions and/or, in the case of multiple exposure, to change quickly and simply between different pupil functions. Suitable filter functions are, in particular, on functions or function combinations which are described in the articles “Printing Isolated Features with K 1 =0.2 using Multiple-Pupil Exposure” by R. M. von Bünau and H. Fukuda, Jpn. J. Appl. Phys. Vol. 35 (1996) pages 6400-6403 and “Effects of radially non-symmetric pupil filters and multiple-pupil exposure” by R. M. von Bünau, H. Fukuda and T. Terasawa, SPIE Vol. 2726, pages 375-385. The disclosure content of these articles in this respect is incorporated by reference in the content of this description. Non-rotationally symmetrical filters can be advantageous, for example, if the mirror is operated at an angle differing from the perpendicular incidence of light or if contact holes in a dense grid with different grid spacings are arranged in mutually perpendicular spatial directions. A multiple exposure with a change between rotationally symmetrical and non-rotationally symmetrical filter function can, for example, improve resolution when imaging very thin lines. Superimposed exposure with a change between symmetrical or antisymmetrical functions with respect to two orthogonal axes can result in fourfold symmetry of the imaging properties, which can provide improvements with respect to edge sharpness or corner sharpness for specific structures. 
     In order to permit a wide range of variation when setting different filter functions, provision is made in preferred embodiments for a plurality of mirror elements that are capable of being driven separately from one another, in order to be able to carry out different movements. In particular, each of the movable mirror elements can be assigned a drive device that can be driven separately for the controlled movement of the mirror segment. Embodiments in which a drive device for a mirror segment has at least one piezoelectric crystal are particularly beneficial. For example, between a substrate to be incorporated in a fixed position or a carrier of the mirror and the mirror layer responsible for the reflection, a control layer or drive layer consisting of piezoelectric material can be arranged, being formed in such a way that, by means of driving the control layer electrically, its layer thickness and therefore the distance between the substrate or the carrier and the mirror layer can be varied in a controlled manner. In addition, other effects which, as a reaction to an electric or magnetic signal, effect a change in the dimension of a material, for example magnetostriction, can be used to construct drive devices or control layers of mirrors according to the invention. Drive devices for moving mirror segments can also operate in accordance with other principles, for example by means of an electric motor or fluid mechanics. 
     The invention can be used particularly advantageously in catadioptric or catoptric projection objectives for microlithography but also for other imaging systems, for example microscopes. 
     The above and further features emerge from the description and the drawings as well as from the claims, it being possible for the individual features to be implemented in each case on their own or in a plurality in the form of some sub-combinations in an embodiment of the invention and in other fields and to represent embodiments which are advantageous and intrinsically capable of protection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a microlithography projection exposure installation constructed as a wafer stepper and having a catadioptric projection objective according to one embodiment of the invention; 
         FIG. 2  is an axial plan view of the mirror surface of a concave mirror subdivided into concentric rings for a catadioptric projection objective; 
         FIG. 3  is a section along the line III-III in  FIG. 2 ; 
         FIG. 4  is an enlarged illustration of the region IV from  FIG. 3 ; 
         FIG. 5  is an axial section through another embodiment of a concave mirror according to the invention; 
         FIG. 6  is a schematic axial plan view of a mirror surface of a concave mirror according to another embodiment of the invention; 
         FIG. 7  is a sectional illustration of the concave mirror in  FIG. 6  along the line VII-VII. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows in schematic form a microlithography projection exposure installation in the form of a wafer stepper  1 , which is provided for the production of highly integrated semiconductor components. The projection exposure installation comprises as light source an excimer laser  2 , which emits light with an operating wavelength λ which is 248 nm in the example and, in other embodiments, can also be below this, for example 193 nm or 157 nm. An illumination system  4  arranged downstream produces a large, sharply delimited and homogeneously illuminated image field, which is matched to the telecentric requirements of the projection objective  5  connected downstream. The projection objective  5  is a preferred embodiment of an optical imaging system according to the invention. The illumination system has devices for selecting the illumination mode and, for example, can be switched over between conventional illumination with a variable degree of coherence, annular field illumination and dipole or quadrupole illumination. Arranged behind the illumination system is a device  6  for holding and manipulating a mask  7 , in such a way that the mask (reticle) lies in the object plane  8  of the projection objective and can be moved in this plane in a direction of movement  9  (y direction) with the aid of a scanner drive for the purpose of scanner operation. 
     Behind the mask plane  8  there follows the projection objective  5 , which acts as a reduction objective and projects an image of the mask on a reduced scale, for example on a scale of 1:4 or 1:5, onto a wafer  10  which is covered with a photoresist layer and which is arranged in the image plane  11  of the reduction objective  5 . Other embodiments which are designed for coarser initial structures, for example for mask-less lithography, can have greater reductions, for example between 1:20 and 1:200. The wafer  10  is held by a device  12  which comprises a scanner drive, in order to move the wafer synchronously with the reticle  7  and parallel to the latter. All the systems are controlled by a control unit  13 . 
     The projection objective  5  is a catadioptric projection objective with geometric beam splitting. Between its object plane (mask plane  8 ) and its image plane (wafer plane  11 ), it has a catadioptric first objective part  15  comprising a concave mirror  16 , a geometric beam splitter  17  and, behind the latter, a dioptric second objective part  18 . The beam splitter  17 , formed as a mirror prism, has a flat first mirror surface  19  for deflecting the radiation coming from the object plane toward the concave mirror  16  and a second mirror surface  20  for deflecting the radiation reflected from the concave mirror in the direction of the purely refractive second objective part  18 . The catadioptric objective part is designed in such a way that a freely accessible real intermediate image is located at a distance behind the second deflection mirror  20 , in the region of an intermediate image plane  21 , and is projected into the image plane  11  by the following lenses of the dioptric objective part. The optical axis  24  of the projection objective is folded at the surfaces  19 ,  16  and  20 . 
     The object plane  8 , the intermediate image plane  21  and the image plane  11  are mutually optically conjugate field planes of the imaging system  5 . Between these there are in each case located flat pupil surfaces which are Fourier-transformed in relation to the reticle plane  8  and to the image plane  11 . A first, flat pupil surface  3  lies in the region of the imaging concave mirror  16 . The pupil plane  22  following the intermediate image plane  21  and closest to the wafer is freely accessible. The adjustable system aperture stop (not shown) of the projection objective is situated in this region. 
     The exposure installation  1  is designed to achieve resolutions of 0.1 μm or better and high throughputs, and has an image-side numerical aperture (NA) between about 0.65 and about 0.85 or higher. The basic structure of the projection objective can correspond to the structure of the projection objective which is shown in EP-A-0 989 434. Other embodiments operate with physical beam splitting, it being possible for a beam splitter cube (BSC) with a polarization-selective beam splitter layer, for example, to be provided instead of the geometric beam splitter  17 . There are also intermediate-image-free embodiments of catadioptric projection objectives according to the invention in which a concave mirror is arranged in the region of a pupil surface. Possible embodiments are shown, for example, in DE 42 03 464 or U.S. Pat. No. 5,537,260. 
     One special feature of the projection objective  5  is that the concave mirror  16  is designed as a variable pupil filter with an electrically adjustable filter function. A preferred embodiment having a rotationally symmetrical filter function will be explained by using  FIGS. 2 to 4 . The axial plan view of the surface of the concave mirror  16  in  FIG. 2  shows that the mirror or the mirror surface has a number of, for example, ten annular mirror segments  30  which surround a central, circular mirror segment  31 . The central axis  32  of this rotationally symmetrical arrangement, when the mirror  16  is incorporated, coincides with the point of intersection between the optical axis  24  of the projection objective and the mirror surface and runs symmetrically between the parts of the optical axis which run toward one another at an acute angle on the mirror. The annular mirror surfaces of the individual mirror segments fill up the entire area of the mirror  16  virtually without gaps, so that only in the region of the contact lines  33  lying between the mirror segments do non-reflective regions remain, of which the total area is considerably less than about 1% of the total area of the mirror. Instead of complete rings, the mirror segments can also be shaped like annular segments. 
     As the vertical sections of  FIGS. 3 and 4  show, the mirror has a mirror substrate  34  which, for example, can consist of a glass ceramic such as Zerodur (registered trademark), a silicon carbide ceramic, a titanium silicon glass such as ULE (registered trademark) or another torsionally rigid material with a low thermal coefficient of expansion. The mirror substrate has a substrate surface  35  which is provided for the application of a reflective layer and whose concavely curved shape substantially corresponds to the desired shape of the mirror surface  36  to be applied. A layer  37  of a piezoelectrically active material is applied to the smoothly polished substrate surface  35 , being, for example, a piezoceramic based on PbZrTiO 3 , which can be applied in a sol-gel process. When electrically conductive materials are used as a substrate material, an insulating layer consisting of electrically insulating material is provided between the mirror substrate  34  and the piezoelectric layer  37  and can be dispensed with in the exemplary embodiment, since the mirror substrate  34  consists of insulating glass ceramic. A protective layer  38 , which consists of quartz in the example, is applied to the possibly polished free surface of the piezoceramic layer  37 . The protective layer  38  is used as a base for the mirror layer  39  applied to it, for which reason the surface of the protective layer that faces away from the substrate  34  must be polished to the desired shape of the mirror surface. The reflective layer  39  can be constructed as an alternating layer pack having dielectric materials of different reflective indices. A mirror layer of electrically conducting material, for example of aluminum, is likewise possible. In order to increase the reflectivity, this can further be coated with one or more dielectric layers, if appropriate. 
     The layer structure built up in this way is divided along concentric lines, which can correspond to the contact lines  33  in  FIG. 2 , by means of deep cuts  40  which lead from the mirror surface  36  into the substrate  34 . As a result, mirror segments  30  are produced which are separated electrically and physically from one another and in each case have a layer structure in which a piezoceramic layer  37  is arranged between the substrate  34  to be incorporated in a fixed position and the respective mirror surface  36 . 
     To each of the mutually electrically insulated piezoceramic layer sections there leads a hole which leads from the rear of the substrate to the substrate surface  35  and in which an electric feed line  41  that makes contact with the associated piezoelectric layer  37  is led. The piezoelectric layer  37  and the feed lines are configured in such a way that, in the event of electrical activation of the piezoelectric layer via the feed lines  41 , the piezoelectric layer changes its layer thickness on account of the inverse piezoelectric effect, as a result of which the distance between the respective reflective surface  36  of the mirror segment and the substrate  34  becomes specifically capable of variation. The arrangement is designed electrically and mechanically such that the achievable stroke in the direction parallel to the central axis  32  of the mirror arrangement is at least of the order of magnitude of half the operating wavelength, that is to say for example of the order of magnitude of about 100 nm. 
     The structure explained is preferably produced in such a way that, first of all, the mirror substrate  34  is provided with the holes for the feed lines  41  and the latter are mounted in the holes. The upper side of the substrate  35  is then polished and the piezoelectric layer  37  and the quartz layer  38  located above and also, if appropriate, further layers are applied one after another. Then, for example by a lithographic process, concentric rings or other structures are cut into the protective layer  38  and the piezoelectric layer  37 . The surfaces remaining outside the cuts  40  form the later phase-shifting areas of the mirror filter. In the last operation before the use of the mirror, the reflective layer  39  is applied. 
     The mode of operation of the mirror, which can be used as a phase-shifting pupil filter, will be explained in connection with  FIG. 4 . This shows two light rays  45  (dashed) and  46  (dotted), illustrated schematically, which, parallel to each other and substantially parallel to the optical axis  24  of the projection objective, fall on to two adjacent mirror segments  30 ,  30 ′ separated from each other by an incision  40 . In a basic configuration of the mirror, in which the adjacent mirror surfaces lie substantially at one level in relation to the substrate, the rays  45 ,  46  do not experience any difference in path during reflection at the mirror surfaces, so that after the reflection they have the same phase difference as in the light path before the reflection. If then, for example, the piezoelectric layer associated with the mirror segment  30 ′ shown on the right is energized electrically in such a way that its thickness is increased, the distance between the substrate  34  and the associated mirror surface  36 ′ is also enlarged. This situation, with the mirror surface  36 ′ raised, is shown dashed. For the two light rays  45 ,  46 , the result is now a path difference that depends on the stroke of the mirror surface  36 ′, that is to say a difference in the optical path length, since the optical path length of the ray  46  is shortened in comparison with that of the ray  45 , as a result of the lifting movement of the mirror surface  36 ′. In this way, a concentrically annular phase filter can be created which, in specific annular zones of the object pupils, effects a sudden phase shift in the light of 180° (or a sudden phase shift of another magnitude). The filter action can be set as desired by means of specific activation of the mirror segments or switched off and also, if appropriate, varied continuously or in steps. The parts of the piezoceramic layer  37  assigned to the respective mirror segments serve here as drive devices that can be electrically driven separately for the independent movement of the individual mirror segments. 
       FIG. 5  shows another embodiment of a concave mirror  50  which can be used as a phase-shifting pupil filter with a rotationally symmetrical filter function. In this embodiment, the mirror substrate  51 , consisting of a glass ceramic, is fixed to a baseplate  52  by adhesive bonding or wringing or in another way. The material of the baseplate  52  has substantially the same thermal expansion characteristics as the mirror substrate material and can be fabricated from the same material. The concavely curved and polished surface of the mirror substrate bears the reflective layer  53 . The mirror substrate has a number of concentric ring segments  55 , for example four, which surround a circular center piece  56  and are separated by a narrow gap going through to the baseplate  52 . In the example, in each second ring segment  55  a continuous annular layer  59  of piezoelectric material is applied between the baseplate  52  and the associated ring  55  of the substrate, which can be fixed to the piezoelectric layer located underneath, for example by means of wringing or of adhesive bonding. The feed lines  60  for the power supply to the piezoelectric rings  59  serving as drive devices of the mirror segments are led through the baseplate  52  here. The effect of this arrangement is that, in the event of electrical activation of the piezoceramic rings  59 , as a result of the associated thickness change of the layers  59 , the associated movable mirror segments  55  are raised or lowered relative to the radiantly adjacent, respectively immovable mirror segments. In this way, a concentrically angular phase filter can be created which, in specific angular zones of the objective pupils, can effect a sudden phase shift of the light to a predetermined extent (for example 180°). 
     Since, in embodiments of this type, the drive devices  59  for the movable mirror segments are not integrated in the sensitive layer structure close to the surface but are applied between relatively rugged parts of the subconstruction, specifically between the mirror substrate  51  and baseplate  52 , the piezoceramic drive devices  59  can be replaced particularly simply by other types of drive devices which permit a controlled lifting drive of the mirror segments  58 . Possible, for example, are micromotors which can be driven electrically or in another way or piston-cylinder drives that can be operated with a working fluid. 
     The embodiment of a phase-shifting concave mirror  70  explained by using  FIGS. 6 and 7  is an example of a design in which the size and shape of the phase-shifting regions can be set with great variability. Here, the complete mirror  70  is built up from hexagonal honeycombs  71  which form the mirror segments which fill up the entire mirror area of the mirror virtually without gaps. The structure of the subconstruction resembles the structure in  FIG. 5 . Between the mirror substrate  72 , which bears the reflective layer  73  on its polished, concave surface, and a baseplate  74 , in each of the mirror segments  71 , a six-sided piezoelectric crystal  75  corresponding to the honeycomb shape is provided as a drive device. All the honeycombs are built up on the baseplate through which the feed lines  76  for the piezoelectric crystals  75  which form the electric lifting drives of the honeycomb mirror segments lead. The parts  74 ,  75  and  72  can be permanently connected to one another, for example by means of wringing or adhesive bonding. The mirror layer is coated after the mounting of the honeycomb elements. 
     All the honeycomb mirror segments  71  can be moved separately from one another at right angles to the plane of the baseplate  74 , it being possible for a lifting movement or lowering movement to be achieved by means of electrical activation of the associated piezoelectric drives  75 . In the case of a mirror  70  incorporated close to the pupil, each honeycomb mirror surface forms a reflective image element or “pixel” in the Fourier domain. By means of synchronous activation of predefined groups of adjacent, physically contiguous, honeycomb mirror elements, it is possible to set filter functions with various spatial distributions of the desired path differences, so that even pupil filters with a non-rotationally symmetrical filter function are possible. The pixel size and therefore the spatial resolution of the pupil filter can be predefined through the size of the individual honeycombs. 
     The invention has been explained by way of example using catadioptric microlithographic projection objectives, which represent a preferred area of application of the invention. The invention can also be used in other optical imaging systems, for example in catoptric projection objectives in which all the optical elements are mirrors, or in microscopes. In order to achieve the filter action, if appropriate, an adaptive mirror having a profile of the mirror surface which can be varied continuously within certain deflection limits can also be used to an approximation. However, this has the disadvantage that sudden phase shifts, such as are present in theoretically ideal filters, cannot be implemented. Pupil filters according to the invention can alternatively or additionally also be designed as amplitude filters. For example, a mirror subdivided into mirror segments can contain mirror segments that can be tilted, so that, by tilting the segments, the proportion of light respectively incident on the latter is masked out and hence does not contribute to the production of an image. In this case, a plurality of actuators that can be driven independently can be assigned to a mirror segment, if appropriate. 
     The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.