Abstract:
A projection optics for microlithography, which images an object field in an object plane into an image field in an image plane, where the projection optics include at least one curved mirror and including at least one refractive subunit, as well as related systems, components, methods and products prepared by such methods, are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/045,723, filed Mar. 11, 2011, now U.S. Pat. No. 8,643,824, which is a continuation of U.S. application Ser. No. 11/969,476, filed Jan. 4, 2008, now U.S. Pat. No. 7,929,114, which claims priority under 35 U.S.C. §119(e)(1) to U.S. provisional patent application Ser. No. 60/885,235, filed Jan. 17, 2007. U.S. application Ser. No. 11/969,476 also claims priority under 35 U.S.C. §119 to German patent application serial number 10 2007 003 307.0, filed Jan. 17, 2007. The contents of these applications are hereby incorporated by reference. 
    
    
     FIELD 
     The disclosure relates to a projection optics for microlithography, which images an object field in an object plane into an image field in an image plane, where the projection optics include at least one curved mirror and including at least one refractive subunit, as well as related systems, components, methods and products prepared by such methods. 
     BACKGROUND 
     Projection optics for a projection exposure system of are known. In some instances, the components of the refractive subunits contained therein are utilised asymmetrically and only incompletely. Also known are projection optics for projection exposure systems, in which the imaging beam path is guided via a beam splitter cube. 
     SUMMARY 
     In one aspect, the disclosure provides a projection optics for microlithography which images an object field in an object plane into an image field in an image plane. The projection optics includes a curved mirror and a refractive subunit. A reflection surface of the mirror is configured as a static free-form surface which cannot be described by a rotationally symmetrical function. 
     In another aspect, the disclosure provides a microlithography projection exposure system. The system includes an illumination optics configured to guide illumination light toward an object field in an object plane, a projection optics as described in the preceding paragraph. 
     In a further aspect, the disclosure provides a method that includes using a microlithography projection exposure system as described in the preceding paragraph to produce a microstructured element. 
     In some embodiments, the disclosure provides a projection optics for a projection exposure system with a relatively simple design and/or that provides effective utilisation of the components of the at least one refractive subunit of the projection optics over the cross section of these components. 
     In certain embodiments, a projection optics, having a reflection surface of at least one mirror, is configured as a static free-form surface which cannot be described by a rotationally symmetrical function. 
     It has been recognised that the use of at least one static free-form surface in the projection optics markedly can increase the degrees of freedom during guidance of the imaging light through the projection optics. The use of at least one free-form surface can allow an imaging beam path in the at least one refractive subunit of the projection optics that utilises the components contained therein more effectively than in the prior art. The use of a smaller number of refractive components accordingly can allow a predetermined image-side numerical aperture in the projection optics according to the disclosure. Conversely, a predetermined number of refractive components can allow a larger image-side numerical aperture to be produced than in the prior art. This can lead to the possibility of a projection optics having higher structural resolution. The free-form surface cannot be described by a function which is rotationally symmetrical about a marked axis normal to a surface portion of the optical surface. The free-form surface therefore cannot, in particular, be described by a conic section-aspherical equation. Although aspheres of this conic type differ from a spherical symmetry, they can be described by a rotationally symmetrical function, i.e. by a function which is dependent merely on one parameter, i.e. on the distance from an optical axis, whereas the free-form surfaces have at least two mutually independent parameters to describe the surface. Conic section-aspherical surfaces are therefore not free-form surfaces according to the disclosure. The shape of the border of the optically active surface is generally not significant in this regard. Optically active surfaces which are non-rotationally symmetrically bordered are known. Nevertheless, optically active surfaces of this type can be described by a rotationally symmetrical function, a non-rotationally symmetrically bordered detail of this optical surface being used. The term “static free-form surface” refers to a free-form surface, the shape of which is not actively changed during use of the projection optics for projection. A static free-form surface can also be displaced as a whole for the purposes of adjustment. The free-form surface can, in particular, be designed starting from a planar reference surface or basic shape, from a concave reference surface or from a convex reference. In particular, use may be made of at least one free-form surface designed starting from a curved reference surface. In this case, use is desirably made of a reference surface having a vertex curvature which is constant over the entire reference surface. A conic section-asphere can also be used as the reference surface. The at least one free-form surface differs from a rotationally symmetrical surface, which is optimally adapted to the free-form surface and does not have to correspond to a design reference surface, at most by an amount corresponding, in particular, at least to the amount of a wavelength of the imaging light. This difference from, in particular, at least the amount of a wavelength of the imaging light is in practice always much greater than production tolerances in the manufacture of optical components for microlithography, which are in absolute terms typically 0.1 nm and in relative terms typically 1/50 or 1/100 of the wavelength of the illumination light used. If DUV (deep ultraviolet) imaging light is used, this difference is therefore typically more than 100 nm, in particular even more than 500 nm or even more than 1 μm. Even much greater differences between the free-form surface according to the disclosure and an optimally adapted rotationally symmetrical surface are possible. A free-form surface according to the disclosure may be provided by a biconical surface, i.e. an optical surface having two different basic curvatures and two different conical constants in two directions perpendicular to each other, by a toric surface or by an anamorphous and at the same time, in particular, aspherical surface. A cylindrical surface is therefore also a free-form surface of this type. The free-form surfaces according to the disclosure can be mirror-symmetrical to one or more planes of symmetry. The free-form surface according to the disclosure may be a surface having an n-fold symmetry, wherein n is an integer and greater than or equal to 1. The free-form surface may have no axis of symmetry whatsoever and no plane of symmetry whatsoever. The projection optics can have at least two curved mirrors mapping the object field into the image field. 
     Various possibilities for describing optical surfaces, in particular anamorphous surfaces, are described for example in U.S. Pat. No. 6,000,798. Analytical formulae for describing non-rotationally symmetrical surfaces, in particular anamorphous aspherical surfaces, toric surfaces or biconical aspherical surfaces, are also described in WO 01/88597 A. Insofar as the disclosure of these documents relates to the mathematical description of optical surfaces, these documents are hereby incorporated by reference. Certain optical design programs such as Oslo® and Code V® also allow the description and design of optical systems via mathematical functions via which it is also possible to define non-rotationally symmetrical optical surfaces. The above-mentioned mathematical descriptions relate to mathematical surfaces. An optical surface which is actually optically utilised, i.e. a physical surface of an optical element that is acted on by an illumination beam and that can be described using a mathematical description of this type, generally contains merely a detail of the actual mathematical surface which is also referred to as the parent surface. The mathematical surface therefore also extends beyond the physical, optically active surface. Insofar as an optical system can be described with the aid of a reference axis, some or all of the optically used surface portions can be arranged outside this reference axis in such a way that the reference axis intersects the mathematical surface but not the detail of this mathematical surface that is actually optically utilised. 
     Field planes arranged parallel to one another facilitate integration of the projection optics into the design environment of the projection exposure system. This can be particularly advantageous if the projection optics is used in a scanning projection exposure system, as the scanning directions can then be guided parallel to one another. 
     Arrangements having a small object/image offset can lead to a compact projection optics and also facilitate optic tests in which the projection optics is pivoted about an axis extending centrally through the object or image field and located perpendicularly to the corresponding field plane, as in this case the object and image fields do not migrate far during swivelling of the projection optics. 
     Arrangements of the optical axis relative to the object or image field wherein the intersection of an optical axis of an refractive subunit with the object plane is located in the object field and in particular is centred in the object field, and wherein the intersection of the optical axis of the refractive subunit with the image plane is located in the image field and in particular is centred in the image field allow optimally symmetrical beam guidance of the illumination or imaging light along the optical axis of the at least one refractive subunit of the projection optics. This can be utilised for substantially rotationally symmetrical or even completely rotationally symmetrical illumination of the optical components of the refractive subunit. In the event of residual absorption of illumination light in these optical components, possible repercussions which corresponding heating of the absorbing optical components has on the imaging of the projection optics are also either substantially or completely rotationally symmetrical or have an n-fold symmetry and can therefore be compensated for or corrected relatively easily. The term “n-fold symmetry” means in this connection that rotation through an angle of 360°/n about the optical axis causes any imaging effects present, in particular imaging errors, to merge. 
     A projection optics having six mirrors can allow for effective wave front and distortion correction via the image field. Alternatively, the projection optics can also be equipped with a different number of mirrors, for example with two or four mirrors. These mirrors may include a free-form surface, although there may also be two or more free-form surfaces, of which at least one free-form surface may be configured as a curved free-form surface. The same applies to a refractive subunit having at most eight lenses. The fewer lenses the refractive subunit has, the lower the transmission losses of the refractive subunit. The more lenses the refractive subunit has, in general, the better the possibilities for wave front and distortion correction are in practice. The refractive subunit can have at least six lenses. A refractive subunit having eight lenses is in this regard a good compromise in terms of the number of lenses. Depending on the imaging demands placed on the projection optics, six or fewer lenses may also be an advantageous compromise between imaging quality and reduction of transmission losses, material and manufacturing costs. 
     A projection optics having an image-side and an object-side refractive unit of the proj ection optics, between which at least two mirrors are arranged, of which at least one has a free-form surface, may be configured symmetrically, and this provides advantages for production in the case of the projection optics. Complete symmetry can be achieved only in specific cases in which the imaging scale of the projection optics is 1:1. A reflective subunit having two or else having four mirrors may, for example, be arranged between the refractive subunits. 
     A beam guidance in a projection optics, wherein the intersection of the optical axis of the object-side refractive unit of the projection optics with the object plane is located in the object field and wherein the intersection of the optical axis of the image-side refractive subunit of the projection optics with the image plane is located in the image field, and a beam guidance in a projection optics, wherein the intersection of the optical axis of the object-side refractive subunit of the projection optics with the object plane is centred in the object field and wherein the intersection of the optical axis of the image-side refractive subunit of the projection optics with the image plane is centred in the image field, have, both for the image-side and for the object-side refractive subunit, the advantages discussed hereinbefore. 
     A beam guidance in a projection optics, wherein a principal ray of the central field point between two mirrors arranged between the two refractive subunits extends parallel to the optical axis of the two refractive subunits and set apart from the optical axis, allows a symmetrical configuration of a reflective subunit, including the at least two mirrors, of the projection optics between the two refractive subunits, and this also has advantages for production in the manufacture of the optical components of the projection optics. A beam guidance in an projection optics, wherein the optical axes of the two refractive subunits extend parallel and set apart from each other, allows, for example, a point-symmetrical configuration of the optical components of the projection optics, and this also has advantages for production. 
     A deformable mirror in the region of a pupil plane of the projection optics can compensate for drift effects occurring during operation of the projection optics, for example as a result of thermal influences. The reflection surface of the deformable mirror can be regarded as a non-static free-form surface. 
     An image field which is larger than 1 mm 2  can result in a high throughput of the projection optics. 
     An image-side numerical aperture of at least 0.5 can allow for high resolution of the projection optics. If an immersion system is used, the image-side numerical aperture can be even larger, for example larger than 1.0. 
     A telecentric projection optics on one of the group of object and image side can increase the flexibility of use thereof. Projection optics having image-side telecentry have an imaging scale which is constant over the entire focus range. 
     A light source for generating illumination light having a wavelength in the range of from 126 to 248 mm can allow for good structural resolution of the projection exposure system. Typical UV light sources which can be used have wavelengths of 126, 157, 193 and 248 nm. 
     A projection optics having in the imaging beam path between the object plane and the image plane at least one intermediate image plane can allow, due to the intermediate image plane, the imaging effects of the refractive subunit to be separated from the imaging effect of the at least one curved mirror of the projection optics. This can simplify the design of the projection optics. 
     The advantages of a projection exposure system for microlithography including a light source for illumination light, an illumination optics for guiding the illumination light toward an object field in an object plane and a projection optics according to the disclosure, can correspond to those previously listed hereinbefore with regard to the projection exposure system according to the disclosure. 
     The same applies to a manufacturing method including the steps of providing a reticle and a wafer, projecting a structure on the reticle onto a light-sensitive layer of the wafer with the aid of the projection exposure system described above, and producing a microstructure on the wafer, and to the microstructured structural part manufactured thereby. 
     Exemplary embodiments of the disclosure will be described hereinafter in greater detail with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic illustration of a projection exposure system for microlithography; 
         FIG. 2  shows a section, including imaging beam paths of field points set apart from one another, through an embodiment of a projection optics of the projection exposure system according to  FIG. 1 ; 
         FIG. 3  shows a diagram showing the field profile of the wave front error (rms value) of the projection optics according to  FIG. 2 ; 
         FIG. 4  shows a diagram showing the field profile of the distortion of the projection optics according to  FIG. 2 ; 
         FIG. 5  shows a section through a non-rotationally symmetrical free-form surface and through a rotationally symmetrical reference surface; 
         FIG. 6  shows a view similar to  FIG. 1  of a projection exposure system for microlithography; 
         FIG. 7  shows a schematic illustration of a projection optics for the projection exposure system according to  FIG. 1 ; and 
         FIG. 8  shows a schematic illustration of a projection optics for the projection exposure system according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A projection exposure system  1  for microlithography has a light source  2  for illumination light. The light source  2  is a UV light source generating light having a wavelength of approx. 193 nm. Light sources  2 , i.e. lasers, generating wavelengths of this type are known. Other wavelengths, for example 157 nm or 248 nm, which are suitable for refractive media, are also possible.  FIG. 1  is a highly schematic illustration of a beam path of the illumination light  3 . 
     An illumination optics  5  is used to guide the illumination light  3  toward an object field in an object plane  4 . The object field is mapped into an image field in an image plane  8  having a predetermined scale of demagnification using a projection optics  6 . The projection optics  6 , which is shown in detail in  FIG. 2 , demagnifies by a factor of 4. The image plane  8  is arranged in the projection optics  6  parallel to the object plane  4 . A detail, coinciding with the object field, of a mask  9  which is penetrated by radiation and is also referred to as a reticle, is imaged. The imaging is carried out onto the surface of a substrate  10  in the form of a wafer carried by a substrate holder  11 .  FIG. 1  shows schematically between the reticle  9  and the projection optics  6  a beam cluster  12 , entering the projection optics, of the illumination light  3  and between the projection optics  6  and the substrate  10  a beam cluster  13 , leaving the projection optics  6 , of the illumination light  3 . The image fieldside numerical aperture of the projection optics  6  according to  FIG. 2  is 0.80. The projection optics  6  according to  FIG. 2  is telecentric both on the object and on the image side. 
     To facilitate description of the projection exposure system  1 , the drawings show a Cartesian xyz-coordinate system producing the respective positional relationship of the components shown in the figure. In  FIG. 1 , the x-direction extends into the drawing plane perpendicularly thereto, the y-direction toward the right and the z-direction downward. 
     The projection exposure system  1  is of the scanner type. Both the reticle  9  and the substrate  10  are scanned in the y-direction during operation of the projection exposure system  1 . 
       FIG. 2  shows the optical design of the projection optics  6 . Illustrated is the beam path of the three respective individual beams  14  emanating from five object field points which in  FIG. 2  are located one above another and set apart from one another in the y-direction, the three individual beams  14 , which pertain to one of these five object field points, each being associated with three different directions of illumination. 
     Starting from the object plane  4 , the individual beams  14  are first reflected by a reflective subunit  15  having a total of six mirrors  16  to  21  which are numbered in the order of the beam path and will subsequently also be referred to, likewise in the order of the beam path, as mirrors M 1 , M 2 , M 3 , M 4 , M 5  and M 6 . The mirrors  16 ,  18 ,  19  and  21  have a concave basic shape, i.e. can be described by a concave optimally adapted surface. The mirrors  17  and  20  have a convex basic shape, i.e. can each be described by a convex optimally adapted surface. In the remainder of the description, mirrors of this type will be described simply as concave or convex. 
     All six mirrors  16  to  21  of the projection optics  6  are configured as free-form surfaces which cannot be described by a rotationally symmetrical function. Also possible are other embodiments of the projection optics  6 , in which at least one of the mirrors  16  to  21  has a free-form reflection surface of this type. At least one reflection surface is in this case configured as a static free-form surface, i.e. a surface, the shape of which cannot be purposefully altered during operation or during downtime of the projection exposure system  1 , which surface cannot be described by a rotationally symmetrical function. 
     In the exemplary embodiment shown in  FIG. 2 , planar pupil or intermediate image planes may generally not be allocated to the projection optics  6 . Both pupils and intermediate images are generated in the reflective subunit  15  on pupil surfaces or intermediate image surfaces extending transversely to the beam path of the individual beams  14  having complex topography. Surfaces of this type, which are indicated in  FIG. 2  by straight or curved lines, will be referred to hereinafter simply as pupil or intermediate image planes. 
     A first pupil plane  22  is located between the first mirror  16  and the second mirror  17  of the projection optics  6 . A first intermediate image plane  23  of the projection optics  6  is located between the fourth mirror  19  and the fifth mirror  20 . The numerical aperture in the intermediate image plane  23  is approximately 0.17. A second pupil plane  24  of the projection optics  6  is located between the fifth mirror  20  and the sixth mirror  21 . 
     A marked individual beam  14 , which connects a central object field point to a point in the pupils of the projection optics  6  in the pupil planes  22 ,  24 , will be referred to hereinafter also as the principal ray  25  of the central field point. From reflection on the sixth mirror  21 , the principal ray  25  of the central field point encloses with the image plane  8  roughly a right angle, i.e. extends also substantially parallel to the z-axis of the projection exposure system  1 . This angle is in any case greater than 85°. From reflection on the sixth mirror  20 , the principal ray  25  extends along an optical axis  26  of a refractive subunit  27 , connected to the reflective subunit  15 , of the projection optics  6 . The intersection of the optical axis  26  of the refractive subunit  27  with the image plane  8  is located centrally in the image field. 
     The refractive subunit  27  has a total of six lenses  28  to  33  which are numbered in the order of the beam path between the object plane  4  and the image plane  8 . 
     A second intermediate image plane  34  of the projection optics  6  is located between the sixth mirror  21  of the reflective subunit  15  and the first lens  28  of the refractive subunit  27 . The numerical aperture of the projection optics  6  in the intermediate image plane  34  is 0.37. 
     Located between the first lens  28  and the second lens  29  is a third pupil plane  35  of the projection optics  6 , in which for example an aperture stop can be arranged. 
     The image field of the projection optics  6  in the image plane  8  is rectangular. Parallel to the x-direction, the image field has an extension of 26 mm. Parallel to the y-direction, the image field has an extension of 6 mm. The optical axis  26  passes through the image field centrically, i.e. at the intersection of the diagonals thereof. 
     The projection optics  6  dispenses with a beam splitter cube and a planar folding mirror and thus has a particularly small number of optical components. 
       FIG. 3  shows the field profile of the wave front of the projection optics  6  according to  FIG. 2  in the image field. The scale of the y-axis is in this case shown stretched compared to that of the x-axis.  FIG. 3  shows a wave front correction to a value of at most 80 mλ. The smallest wave front error occurs at relatively high positive y-values, based on the x-axis, centrically and at relatively high negative y-values eccentrically. 
       FIG. 4  shows the profile of the distortion via the image field of the projection optics  6  according to  FIG. 2 . The scale of the x-axis and the y-axis corresponds to the scale of  FIG. 3 . The distortion is corrected to a maximum value of approximately 25 nm. This maximum value occurs at high y-values located, with respect to x, at the edge of the image field. As may be seen, the image error profile no longer extends rotationally symmetrically around the centre of the field, as is the case in conventional rotationally symmetrical systems having a centred object field and image field. 
     The generation of a free-form surface  36  from a rotationally symmetrical reference surface  37  will be described hereinafter with reference to  FIG. 5 . 
     Firstly, information for characterising the viewed free-form surface is obtained with the aid of an optical design program. The reference surface  37  may, for example, be a rotationally symmetrical asphere. The design information may include the radius of curvature of the reference surface  28 , which is also denoted as 1/c, wherein c denotes the peak curvature of the reference surface  37 . The information additionally includes a conical constant k of the reference surface  37  and polynomial coefficients describing the reference surface  37 . 
     Alternatively or additionally, information characterising the reference surface  37  can be obtained from a surface measurement of a reference mirror surface, for example using an interferometer. A surface measurement of this type produces a function z′(x′, y′) describing the reference surface  37 , wherein z′ denotes the pitch of the reference surface  37  along the z′-axis for various (x′, y′) coordinates, as shown in  FIG. 5 . 
     This first step in the design of the free-form surface  36  additionally includes determining that portion of the initially unlimited mirror surface, defined only by the surface description, that is actually utilised for the reflection of illumination or imaging light  3  during the mapping of the object field into the image field. This region is also referred to as a footprint. The footprint of the mirror can be determined, at least approximately, by ray tracing of the projection optics  6 . Examples of a possible footprint in the x-dimension are indicated in  FIG. 5 . xmin denotes the lower limit and xmax the upper limit for the exemplary footprint. The data above xmax and below xmin is also calculated within certain limits so as to avoid undesirable edge effects when determining the free-form surface  36 . 
     Once the information characterising the reference surface  37  has been determined, a local coordinate system for the reference surface  37  is introduced, in which both the decentring and the tilting of the reference surface  37  are each zero. The z′-axis is therefore the rotational axis of symmetry of the aspherical reference surface  37  or else, if the reference surface was obtained by surface measurement, the optical axis of the measuring device, for example the interferometer. The z′-axis is generally displaced and tilted parallel relative to the z-axis of the xyz-coordinate system of the projection exposure system  1 . This applies accordingly to the other coordinate axes x′, y′. This parallel displacement or tilting is defined in the initial step of the optical design of the free-form surface. 
     As an alternative to an asphere, the reference surface  37  may also be a spherical surface. The coordinate origin x c , y c , z c  for describing the spherical reference surface  37  generally differs from the origin of the xyz-coordinate system of the projection exposure system  1 . 
     Once the reference surface  37  has been determined, a local distance d i  (i=1 . . . N) between a number of points on the reference surface  37  and points on the free-form surface  36  is determined parallel to the z′-axis. The various local distances d i  are then varied until a group of secondary conditions is fulfilled. These secondary conditions are predetermined limit values for specific imaging errors and/or illumination properties of the projection optics  6 . 
     The free-form surface can be described mathematically by the following equation: 
     
       
         
           
             Z 
             = 
             
               
                 
                   cr 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             1 
                             + 
                             k 
                           
                           ) 
                         
                         ⁢ 
                         
                           c 
                           2 
                         
                         ⁢ 
                         
                           r 
                           2 
                         
                       
                     
                   
                 
               
               + 
               
                 
                   ∑ 
                   
                     j 
                     = 
                     2 
                   
                   66 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   
                     C 
                     j 
                   
                   ⁢ 
                   
                     X 
                     m 
                   
                   ⁢ 
                   
                     Y 
                     n 
                   
                 
               
             
           
         
       
       
         
           
             
               wherein 
               ⁢ 
               
                 : 
               
               ⁢ 
               
                   
               
               ⁢ 
               j 
             
             = 
             
               
                 
                   
                     
                       ( 
                       
                         m 
                         + 
                         n 
                       
                       ) 
                     
                     2 
                   
                   + 
                   m 
                   + 
                   
                     3 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     n 
                   
                 
                 2 
               
               + 
               1 
             
           
         
       
     
     Z is the pitch of the free-form surface parallel to a Z-axis which may, for example, be parallel to the z′-axis according to  FIG. 5 . 
     c is a constant corresponding to the peak curvature of a corresponding asphere. k or K corresponds to a conical constant of a corresponding asphere. C j  are the coefficients of the monomials X m Y n . Typically, the values of c, k and C j  are determined on the basis of the desired optical properties of the mirror within the projection optics  6 . The order of the monomial, m+n, can be varied as desired. A relatively high-order monomial can lead to a design of the projection optics  6  having better image error correction but is more complex to calculate. m+n may assume values between 3 and greater than 20. 
     Free-form surfaces can also be described mathematically by Zernike polynomials, an account of which is provided, for example, in the manual of the optical design program CODE V®. Alternatively, free-form surfaces can be described with the aid of two-dimensional spline surfaces. Examples of these include Bezier curves or non-uniform rational basis splines (NURBS). Two-dimensional spline surfaces can, for example, be described by a network of points in an xy-plane and associated z-values or by these points and gradients associated therewith. Depending on the respective type of spline surface, the whole surface is obtained by interpolation between the network points using, for example, polynomials or functions having specific properties in terms of their continuity and differentiability. Examples of these include analytic functions. 
       FIG. 6  is, again, a slightly modified view of the projection exposure system  1  to illustrate a further value characteristic of the projection optics  6 , i.e. the object/image offset d ois . The object/image offset is defined as the distance between a perpendicular projection of the central object point onto the image plane  8  and the central image point. Equivalent to this definition is a definition of the object/image offset as the distance between the central object point and the intersection of the optical axis  26  of the refractive subunit with the object plane. This equivalence is due to the fact that the central image point is located on the optical axis of the refractive subunit. In the projection optics  6  according to  FIG. 2 , the object/image offset d ois  is 0. In particular, the object field is in the projection optics  6  at a distance of less than 50 mm from the intersection of the optical axis  26  of the refractive subunit  27  with the object plane  4 . This intersection is located centrically in the object field. 
     Characteristic parameters of the projection optics will be recapitulated hereinafter. The wavelength of the illumination light  3  is 193.3 nm at a bandwidth of 0.3 pm. The image-side numerical aperture of the projection optics is 0.8. The image field size is 6×26 mm 2 . The projection optics is telecentric on the input and output sides. 
     The following first table sets out the radii R, i.e. the reciprocal values of the peak curvatures c, and the distances between the optical components of the projection optics  6  in the z-direction (thickness). Mirror  1  to mirror  6  denote in this case the mirrors M 1  to M 6  of the projection optics  6 . Lens  1   a  and lens  1   b  denote the object-side and the image-side surface of the lens  28 . Accordingly, the lines from lens  2   a  denote the subsequent lens surfaces up to the image-side surface of the lens  33 . 
     The subsequent second, two-line table sets out the index of refraction of the material SILUV, selected for the lenses  28  to  33 , within the bandwidth of the illumination light. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Surface 
                 Radius 
                 Thickness 
                 Mode/Material 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Object 
                 INFINITY 
                 270.081 
                   
               
               
                   
                 Mirror 1 
                 −252.965 
                 −238.081 
                 REFL 
               
               
                   
                 Mirror 2 
                 −406.220 
                 891.920 
                 REFL 
               
               
                   
                 Mirror 3 
                 −979.012 
                 −891.920 
                 REFL 
               
               
                   
                 Mirror 4 
                 1445.893 
                 891.920 
                 REFL 
               
               
                   
                 Mirror 5 
                 223.730 
                 −194.805 
                 REFL 
               
               
                   
                 Mirror 6 
                 214.750 
                 447.659 
                 REFL 
               
               
                   
                 Lens 1a 
                 917.582 
                 80.000 
                 SILUV 
               
               
                   
                 Lens 1b 
                 −395.946 
                 113.054 
               
               
                   
                 STOP 
                 INFINITY 
                 61.038 
               
               
                   
                 Lens 2a 
                 728.174 
                 80.000 
                 SILUV 
               
               
                   
                 Lens 2b 
                 −447.709 
                 72.468 
               
               
                   
                 Lens 3a 
                 1274.086 
                 80.000 
                 SILUV 
               
               
                   
                 Lens 3b 
                 −344.145 
                 16.037 
               
               
                   
                 Lens 4a 
                 287.238 
                 83.816 
                 SILUV 
               
               
                   
                 Lens 4b 
                 −560.707 
                 2.069 
               
               
                   
                 Lens 5a 
                 291.341 
                 59.824 
                 SILUV 
               
               
                   
                 Lens 5b 
                 584.886 
                 15.839 
               
               
                   
                 Lens 6a 
                 471.437 
                 55.444 
                 SILUV 
               
               
                   
                 Lens 6b 
                 INFINITY 
                 10.001 
               
               
                   
                 Image 
                 INFINITY 
                 0.000 
               
               
                   
                 Wavelength 
                 193.400 
                 193.300 
                 193200 
               
               
                   
                 SILUV 
                 1.560332 
                 1.560491 
                 1.560650 
               
               
                   
                   
               
             
          
         
       
     
     The following table sets out the coefficients C j  of the monomials X m Y n  in the above-indicated free-form surface equation for the mirrors M 1  to M 6 . 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Coefficient 
                 M1 
                 M2 
                 M3 
                 M4 
                 M5 
                 M6 
               
               
                   
               
             
             
               
                 K 
                 −7.021612E−01 
                 −2.157890E+00  
                 −7.667374E−01  
                 −1.806988E+00  
                  1.063503E+00 
                 −1.746422E−01 
               
               
                 Y 
                  0.000000E+00 
                 0.000000E+00 
                 0.000000E+00 
                 0.000000E+00 
                  0.000000E+00 
                  0.000000E+00 
               
               
                 X2 
                  9.095147E−04 
                 3.668196E−04 
                 2.973173E−04 
                 −4.649230E−05  
                 −1.183376E−03 
                 −3.419562E−04 
               
               
                 Y2 
                  6.389416E−04 
                 2.560392E−04 
                 2.978561E−04 
                 −7.671144E−05  
                 −9.182562E−04 
                 −3.007775E−04 
               
               
                 X2 
                 −8.076564E−07 
                 −6.252867E−07  
                 −3.355736E−08  
                 −6.400021E−08  
                  9.454888E−06 
                  2.940059E−07 
               
               
                 Y3 
                 −2.117970E−06 
                 −5.447745E−07  
                 −2.754244E−08  
                 −1.460071E−08  
                  6.613284E−07 
                  1.847585E−07 
               
               
                 X4 
                 −2.138970E−10 
                 −8.640102E−09  
                 3.920489E−11 
                 8.751552E−11 
                 −4.472091E−08 
                 −1.426177E−09 
               
               
                 X2Y2 
                 −6.007701E−09 
                 −2.165820E−08  
                 6.203578E−11 
                 3.851618E−11 
                  1.500646E−07 
                 −1.613361E−09 
               
               
                 Y4 
                 −9.250730E−09 
                 −6.779145E−09  
                 6.568625E−11 
                 1.229035E−10 
                 −1.127945E−07 
                 −1.408353E−09 
               
               
                 X4Y 
                 −2.056552E−11 
                 −8.895852E−12  
                 −1.220037E−14  
                 2.660699E−13 
                 −2.255937E−09 
                  4.795621E−12 
               
               
                 X2Y3 
                 −4.739414E−11 
                 5.099086E−12 
                 −6.214499E−14  
                 −8.442145E−15  
                  3.342074E−09 
                  7.024093E−12 
               
               
                 Y5 
                 −3.327059E−11 
                 2.955670E−11 
                 7.082062E−14 
                 8.855762E−14 
                 −1.023558E−09 
                 −2.144019E−13 
               
               
                 X6 
                 −1.832168E−14 
                 −1.470440E−13  
                 −5.507976E−18  
                 −3.219684E−17  
                 −1.402903E−11 
                 −2.945400E−15 
               
               
                 X4Y2 
                 −1.922635E−13 
                 −1.214485E−12  
                 9.317572E−17 
                 5.849016E−16 
                 −7.112824E−11 
                 −4.171618E−15 
               
               
                 X2Y4 
                 −1.575366E−13 
                 −3.577373E−14  
                 2.207096E−16 
                 −7.153565E−17  
                  3.488042E−11 
                 −2.806538E−14 
               
               
                 Y6 
                 −8.968203E−14 
                 7.845522E−13 
                 7.803209E−17 
                 2.198865E−17 
                  3.795039E−12 
                 −3.038365E−14 
               
               
                 X6Y 
                 −1.281116E−16 
                 −2.627145E−15  
                 −4.849826E−20  
                 −9.242214E−20  
                 −4.829253E−13 
                  1.984066E−16 
               
               
                 X4Y3 
                 −9.140726E−16 
                 −3.557075E−15  
                 3.159899E−19 
                 6.234055E−19 
                 −1.008249E−12 
                  4.813599E−16 
               
               
                 X2Y5 
                 −2.413169E−16 
                 1.757810E−15 
                 9.795658E−19 
                 −2.688674E−19  
                  1.917495E−13 
                  3.980208E−16 
               
               
                 Y7 
                 −3.754943E−16 
                 5.872039E−15 
                 −9.221194E−20  
                 −7.819713E−21  
                  1.901940E−13 
                  3.896110E−17 
               
               
                 X8 
                  5.695077E−20 
                 7.943905E−18 
                 −5.452381E−23  
                 −1.225852E−22  
                  3.587655E−15 
                 −4.740965E−19 
               
               
                 X6Y2 
                 −1.254319E−18 
                 2.689048E−16 
                 −1.335406E−22  
                 −3.678195E−23  
                 −7.156201E−15 
                 −7.811848E−19 
               
               
                 X4Y4 
                 −1.658577E−18 
                 3.240175E−16 
                 4.953597E−22 
                 4.109732E−22 
                 −6.930341E−15 
                  6.890495E−19 
               
               
                 X2Y6 
                  4.989886E−20 
                 1.087723E−17 
                 1.704149E−21 
                 −2.812559E−22  
                 −3.664911E−16 
                  2.629890E−19 
               
               
                 Y8 
                 −1.551184E−18 
                 2.695661E−17 
                 −3.595780E−22  
                 2.753738E−23 
                  1.738454E−15 
                  5.135593E−20 
               
               
                 X8Y 
                 −9.973638E−21 
                 1.930861E−18 
                 −1.667244E−25  
                 −2.536190E−25  
                  9.857871E−17 
                 −1.714248E−21 
               
               
                 X6Y3 
                 −7.606565E−21 
                 6.217397E−18 
                 −2.273189E−25  
                 1.022753E−25 
                 −6.200635E−17 
                 −6.960381E−21 
               
               
                 X4Y5 
                  4.843747E−22 
                 4.566777E−18 
                 2.515461E−25 
                 2.188836E−25 
                 −1.792880E−17 
                 −9.623084E−21 
               
               
                 X2Y7 
                  1.828007E−23 
                 −1.014580E−19  
                 1.345882E−24 
                 −1.041776E−25  
                 −9.929708E−18 
                 −5.083648E−21 
               
               
                 Y9 
                 −3.672869E−21 
                 3.548978E−20 
                 −3.911346E−25  
                 3.732796E−26 
                  6.950217E−18 
                  1.389683E−21 
               
               
                 X10 
                 −2.139041E−23 
                 3.593455E−21 
                 −1.343515E−29  
                 −1.288642E−29  
                  3.181161E−19 
                  1.291645E−23 
               
               
                 X8Y2 
                 −4.164394E−23 
                 1.984011E−20 
                 −1.655578E−28  
                 −1.291875E−28  
                  4.620193E−19 
                  8.021568E−23 
               
               
                 X6Y4 
                 −1.391966E−23 
                 3.356495E−20 
                 −1.667437E−28  
                 9.320319E−29 
                 −2.306996E−19 
                  1.145367E−22 
               
               
                 X4Y6 
                  2.959150E−24 
                 1.871129E−20 
                 4.168020E−30 
                 8.701089E−29 
                 −9.628058E−21 
                 −2.791674E−23 
               
               
                 X2Y8 
                 −1.066355E−24 
                 2.488743E−22 
                 4.086345E−28 
                 3.310765E−31 
                 −4.149211E−20 
                 −9.425279E−23 
               
               
                 Y10 
                 −3.494516E−24 
                 1.039002E−22 
                 −1.471588E−28  
                 1.304435E−29 
                  9.112470E−21 
                 −4.264041E−23 
               
               
                 Nradius 
                  1.000000E+00 
                 1.000000E+00 
                 1.000000E+00 
                 1.000000E+00 
                  1.000000E+00 
                  1.000000E+00 
               
               
                   
               
             
          
         
       
     
     The following two-line table sets out for the mirrors M 1  to M 6  the amounts in mm by which each mirror, starting from an initial design, is decentred (Y-decentre) and rotated (X-rotation). This corresponds to the parallel displacement and the tilting during the above-described free-form surface design process. Displacement is in this case carried out in the y-direction and tilting about the x-axis. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Coefficient 
                 M1 
                 M2 
                 M3 
                 M4 
                 M5 
                 M6 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Y-decentre 
                 163.446 
                 −22.380 
                 −119.285 
                 124.459 
                 36.347 
                 −37.190 
               
               
                 X-rotation 
                 29.034 
                 −0.370 
                 3.988 
                 1.234 
                 7.717 
                 3.171 
               
               
                   
               
             
          
         
       
     
     The following table sets out the aspherical constants for the curved surfaces of the lenses  28  to  33 . 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Lens 1b 
                 Lens 2a 
                 Lens 3° 
                 Lens 4a 
                 Lens 5b 
                 Lens 6a 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 K 
                 −5.320892E+00  
                  0.000000E+00 
                  0.000000E+00 
                 −4.766092E+00 
                  0.000000E+00 
                  0.000000E+00 
               
               
                 A 
                 0.000000E+00 
                 −1.988938E−08 
                 −1.463509E−09 
                  0.000000E+00 
                 −9.475075E−08 
                 −3.645569E−08 
               
               
                 B 
                 1.887087E−13 
                 −1.531838E−13 
                 −6.363694E−14 
                 −6.652848E−13 
                  7.801991E−12 
                  9.565989E−12 
               
               
                 C 
                 −2.425975E−18  
                 −1.077221E−18 
                  4.281797E−19 
                  2.251669E−17 
                 −3.331273E−16 
                 −9.233817E−16 
               
               
                 D 
                 1.942174E−24 
                 −2.420592E−23 
                 −1.251778E−23 
                 −6.275180E−23 
                  1.114370E−20 
                  7.314484E−20 
               
               
                 E 
                 1.934240E−27 
                 −8.586290E−28 
                  3.666866E−29 
                 −3.691459E−27 
                 −5.980525E−25 
                 −9.261069E−24 
               
               
                   
               
             
          
         
       
     
     K and A to E are in this case coefficients in the following aspherical equation: 
     
       
         
           
             Z 
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   
                     1 
                     - 
                     
                       
                         ( 
                         
                           1 
                           + 
                           K 
                         
                         ) 
                       
                       ⁢ 
                       
                         c 
                         2 
                       
                       ⁢ 
                       
                         h 
                         2 
                       
                     
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 Eh 
                 12 
               
             
           
         
       
     
     In this case, Z is the pitch of the aspherical surface, c is the peak curvature, K is the conicity, h denotes the respective location on the lense surface at which the pitch is calculated (h 2 =x 2 +y 2 ). The coefficients A to E are assigned to the respective even-numbered orders of h. 
       FIG. 7  shows a projection optics  38  which can be used instead of the projection optics  6  in the projection exposure system  1 . Components or reference variables corresponding to those previously described hereinbefore with reference to  FIGS. 1 to 6  have the same reference numerals and will not be discussed again in detail. The imaging beam path through the projection optics  38  is shown in  FIG. 7  merely based on the principal ray  25  of the central field point. 
     Starting from the projection plane  4 , the projection optics  38  has firstly a refractive subunit  39 . The refractive subunit is arranged in such a way that the principal ray  25  extends through the refractive subunit  39  along an optical axis  40  of the refractive subunit  39 . The refractive subunit  39  can include one or more lenses. 
     After the refractive subunit  39 , the illumination light  3  is reflected by a reflective subunit  41  of the projection optics  38 . The reflective subunit  41  has two mirrors  42 ,  43 , both of which have static reflection free-form surfaces which cannot be described by a rotationally symmetrical function. The mirrors  42 ,  43  are numbered in accordance with their order in the imaging beam path in the projection optics  38 . The mirrors  42  and  43  are concave. 
     After the mirror  43 , the illumination light  3  extends through a second refractive subunit  44  of the projection optics  38 . The principal ray  25  extends, in turn, along an optical axis  45  of the second refractive subunit  44 . The second refractive subunit  44  can have one lens or a plurality of lenses. 
     The projection optics  38  has between the object plane  4  and the image plane  8  one or more intermediate image planes. The intermediate image planes can, for example, be arranged between the first refractive subunit  39  and the reflective subunit  41  or else between the reflective subunit  41  and the second refractive subunit  44 . 
       FIG. 8  is an illustration similar to  FIG. 7  of a projection optics  46  which can be used instead of the projection optics  6  in the projection exposure system  1 . Components or reference variables corresponding to those previously described hereinbefore with reference to  FIGS. 1 to 7  have the same reference numerals and will not be discussed again in detail. 
     The illumination light  3  in the projection optics  46  is illustrated in  FIG. 8  based on three individual beams  14  emanating from an object point. The central individual beam  14  is in this case the principal ray  25 . 
     Starting from the object field  4 , the illumination light  3  first passes through a refractive subunit  47  which can be configured so as to correspond to the refractive subunit  39  according to  FIG. 7 . The principal ray  25  passes through the refractive subunit  47 , on the optical axis  48  thereof. 
     After the refractive subunit  47 , the illumination light  3  passes through a reflective subunit  49  of the projection optics  46 . The reflective subunit  49  has a total of four reflecting mirrors  50 ,  51 ,  52 ,  53  which are numbered in the order in which they are acted on in the imaging beam path. The mirrors  50  to  53  are concave. However, convex mirrors can also be used in this subunit. A pupil plane of the projection optics  46  is arranged in the region of the second mirror  51 . An intermediate image plane  54  of the projection optics  46  is arranged between the mirrors  51  and  52 . The optical axis  48  is, for example, perpendicular to the intermediate image plane  54 . Between the mirrors  51  and  52 , the principal ray  25  extends substantially parallel to the optical axis  48 . The principal ray  25  can also extend between the mirrors  51  and  52  at an angle to the optical axis  48 . 
     After the last mirror  53  of the reflective subunit  49 , the illumination light  3  passes through a second refractive subunit  55  of the projection optics  46 . 
     In relation to the intermediate image plane  54 , the optical components of the three subunits  47 ,  49  and  55  of the projection optics  46  are arranged substantially symmetrically to one another. The optical axis of the second refractive subunit  55  coincides with the optical axis  48  of the first refractive subunit  47 . A further pupil plane of the projection optics  46  is arranged in the region of the third mirror  52 . 
     The four mirrors  50  to  53  of the reflective subunit  49  all have a reflection free-form surface which cannot be described by a rotationally symmetrical function. The free-form surfaces of the mirrors  50 ,  52  and  53  are in this case static. The reflection surface of the second mirror  51  is deformable, i.e. non-static, in its configuration. Alternatively or additionally, the reflection surface of the third mirror  52  can also be deformable in its configuration. This allows, for example, drift effects, which can occur during projection exposure with the projection exposure system  1 , to be corrected. A correction mechanism including corrections sensors can be used for this purpose, as described in DE 101 20 446 C1. The deformable free-form mirror  51  can be formed from a large number of micromirror segments which can be tilted individually via actuators associated therewith. Micromirror arrays of this type are known to a person skilled in the art. The tilting is activated by the correction mechanism in accordance with defined values calculated from the values determined by the correction sensors. There is sufficient space for the actuators of the micromirror array after the mirror  51 , as no beam path extends at this location. Alternatively, a deformable free-form mirror can be configured as a monolithic mirror, there being attached to the back of the deformable free-form mirror actuators which are able to deform the mirror in the manner of a membrane. 
     Instead of the deformable free-form mirror  51 , a static free-form mirror can also be used. 
     The projection optics  46  has an object/image offset of 0. 
     To manufacture a microstructured structural part with the aid of the projection exposure system  1 , the reticle  9  and the wafer  10  are first provided. Subsequently, a structure on the reticle  9  is projected onto a light-sensitive layer of the wafer  10 . As a result of this and by subsequent machining, a microstructure is produced on the wafer  10 . 
     Other embodiments are in the claims.