Abstract:
A lithographic system includes a projection system for projecting an object field through a projection system&#39;s pupil onto an image field. The projection system includes an optical element located at the projection system&#39;s pupil. The projection system&#39;s pupil is manipulable with respect to normalized pupil heights by the optical element. Related processes are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of, and claims benefit under 35 USC 120 to, international application PCT/EP2010/055419, filed Apr. 23, 2010. International application PCT/EP2010/055419 is hereby incorporated by reference in its entirety. 
     
    
     FIELD  
       [0002]    The disclosure relates to a lithographic system, a process of operating a lithographic system and a process of manufacturing a lithographic system. 
       BACKGROUND  
       [0003]    Projection exposure systems for microlithography, for short “lithographic systems” include highly sophisticated optical projection systems, for short “projection systems”. Such projection systems include a great number of individual optical elements such as lenses or mirrors, such as disclosed in, for example, US20080174858A1. The use of a great number of optical elements is mainly driven by the desire to provide high resolution imaging over a large object field. 
       SUMMARY  
       [0004]    In some instances, it is desirable to project structures at a resolution of, for example, 45 nm structure size from a reticle onto a wafer. The resolution limit R is given by 
         [0000]    
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       k 
                       1 
                     
                      
                     
                       λ 
                       NA 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where λ is the wavelength of projection light, NA=n image  sin(i image,max ) is the numerical aperture at the image side which is the product of the index of refraction of the medium at the image side of the projection system n image  and the sine of the maximum aperture angle at the image side i image,max . The constant k 1  is a process factor which includes miscellaneous factors which influence the resolution limit R, such as, for example, properties of the resist at the wafer. 
         [0005]    It is also generally desirable to provide high throughput. This means that it is often desirable for the projection to be performed all over an object field which is desirably as large as possible. 
         [0006]    As a result, it is commonly desirable to image an object field, e.g., 57.5 mm with an image side numerical aperture, e.g., minimum of 0.9 in the case of a dry projection system and image aberrations of, e.g., less than a few percent λ wavefront rms. Here λ is the wavelength of the projection light, such as, for expel, approximately 248 nm, 193.3 nm or 13.5 nm. 
         [0007]    A “dry projection system” is understood herein to be a projection system which has a gaseous medium as the very last medium before the image plane of the projection system, where the wafer is located. An “immersion projection system” is understood to mean a projection system having a liquid as the very last medium before its image plane. An immersion projection system can have an image side numerical aperture which is approximately 1.5 times larger than the image side numerical aperture for a corresponding dry projection system because the very last medium in an immersion projection system has an index of refraction of approximately 1.5 instead of approximately 1.0. 
         [0008]    Often, to provide a projection system, there are three main issues to handle:
       1. Find an optical design for the projection system with desired large image field, desired large numerical aperture at the image side and desired low image aberrations.   2. Build up this optical design into a real world projection system within error budgets for its optical components that provide that the desired low image aberrations hold for the real world projection system, too.   3. Provide a mechanism to customize this real world projection system to the desired properties of a client and to provide this performance during operation of the projection system for its entire lifetime.       
 
         [0012]    The definitions of the following aberrations and other technical terms in optical design are taken from the Handbook of Optical Projection systems, Vol. 1, Wiley-VHC, Berlin 2005. 
         [0013]    To address item 1, the designer may start with the definition of appropriate target values for the size of the object field and the image side numerical aperture. Thereafter the designer can arrange an initial design of the projection system as a starting point for subsequent improvements to reduce the overall image aberrations of the projection system. 
         [0014]    It is commonly desirable to reduce these aberrations of the optical design in the so-called “aplanatic correction” which provides that the angular dependency of the intensity of projection light within the entrance pupil of an arbitrary point in the object field is proportional to the angular dependency in its exit pupil. In other words, the magnification of the projection system is invariant with respect to object high and pupil high, or, more geometrically speaking, the projection system has desirably fulfills the so-called “sine condition” over the entire object field as well as possible. 
         [0015]    However the sine condition is a demand that relates entrance pupils with exit pupils. There is generally no demand to fulfill the sine condition with respect to other pupils of the projection system. 
         [0016]    In what follows, any pupil P of the projection system different from the entrance pupil and the exit pupil will be called a “projection system pupil”. The fact that the sine condition may be contravened with respect to a fixed projection system&#39;s pupil P relative to the entrance pupil is the “offence against the sine condition with respect to P” in what follows. Note that in this definition the exit pupil may replace the entrance pupil since in general there is no offence against the sine condition with respect to the exit pupil in the relevant projection systems under consideration. 
         [0017]    This offence against the sine condition with respect to a projection system pupil P effects a non-constant distortion of the projection system pupil P. More mathematically speaking pupil highs within the entrance pupil and pupil highs within the exit pupil are mapped one-to-one by a linear function but in general there is only a monotonous (but in general nonlinear) functional relationship 
         [0000]        r=φ   P (ρ)   (2)
 
         [0000]    mapping pupil highs ρ of the entrance pupil onto pupil highs r of the projection system pupil P. Hence the offence against the sine condition with respect to the projection system pupil P can be quantified by the non-linear function φ P . Since for any projection system pupil P its pupil heights can be mapped one-to-one by a linear function onto the interval [0,1] in what follows φ P  can be assumed to be a function mapping [0,1] onto itself, i. e. φ P : [0,1]→[0,1] and pupil heights are always normalized in what follows, i. e. parameterized by the interval [0,1], and for short: φ P  is the to be the offence against the sine condition with respect to the projection system pupil P. 
         [0018]    To address the item 2 above, a potentially important way to build up a projection system within its error budget is the local correction of the surface-shapes of its optical elements. The so-called “ion beam figuring” is an abrasive profile shaping technique suitable to provide surface deformations to optical elements within a nanometer range, such as disclosed, for example, in US20030174297A1 and US20040042094A1. An important location within the projection system to provide such a local correction to an optical element of the projection system is at a projection system pupil since each correction mean provided at a projection system pupil results in a correction of the exit pupil which is independent of the image height. Therefore in what follows the term “exit pupil” without reference to a fixed image point can be used if a correction is made at a projection system pupil. 
         [0019]    As referred to herein, an optical element of the projection system is “located at a projection system pupil P” or, for short “at a projection system pupil P” provided there is no second optical element of the projection system different from the optical element which is closer to the projection system pupil P. Furthermore a surface of an optical element is “at a projection system pupil P” provided the optical element is at the projection system pupil P and the surface is closer to the projection system pupil P than any other surface of the optical element. This definition extends naturally to multiple optical elements: a set of optical elements of the projection system is the to be “located at a projection system pupil P” provided the elements are closer to the projection system pupil P than any other optical element of the projection system. 
         [0020]    In a nutshell, an appropriate location to relax the error budget by a correction mechanism is a projection system pupil. However in view of the above non-linear function φ P  of (2) such a correction mechanism does not correct the exit pupil in an undisturbed manner, e. g., a deformation of a surface of an optical element which is located at a projection system pupil and follows a shape of, for example, an astigmatism r 2  cos(2φ), 0≦r≦1,0≦φ&lt;2π (see, for example, Handbook of Optical Projection systems, Vol. 1, Wiley-VHC, Berlin 2005, section 11.5.4) turns out a deformation of the exit pupil which consists of the astigmatism r 2  cos(2φ) accompanied by higher astigmatic terms, e.g. (4r 4 −3r 2 )cos2φ. 
         [0021]    To address the above item 3, an important approach to customizing the projection system during its operation is to shut down its aperture stop, i. e., to decrease the diameter of the projection systems diaphragm. This affects an increasing depth of focus accompanied by reduced aberrations. The drawback to be paid is a reduced resolution (see, for example, (1)). However, in view of the above non-linear function φ P  of (2) there is a non-linear relationship between the diameter of the aperture stop and the image side aperture. For example, 10% shutting down does not result in a 10% smaller aperture in general. 
         [0022]    Moreover any correction mechanism/manipulation of the projection system which is located at a projection system pupil suffers from this non-linear function φ P . Such mechanisms are manipulators which drive an optical element, such as disclosed in, for example, US20030063268A1, U.S. Pat. No. 6,191,898B1, U.S. Pat. No. 6,104,472, or which modify the shape of the index of refraction of an optical element, such as disclosed, for example, in WO2008034636A2, U.S. Pat. No. 5,805,273, US20060244940A1. 
         [0023]    To summarize: the desire to minimize the offence against the sine condition with respect to entrance and exit pupils generally results in a serious offence against the sine condition φ P  with respect to the other projection system pupils P. Hence, to improve mechanisms of the projection system which are located at a projection system pupil P it is desirable to customize those mechanisms with respect the offence against the sine condition φ P  at the projection system pupil P. 
         [0024]    In general such a mechanisms are optical elements which manipulate the wavefront within the projection system pupil P, for short “which manipulate the system pupil P”. Since the offence against the sine condition affects only the radial component within a pupil only such optical elements are of interest whose manipulation varies with respect to a radial component, for short “the optical element manipulates the projection system pupil P with respect to its pupil heights”. For example, a stop or a deformation of a surface of the optical element located at a projection system pupil P manipulates the projection system pupil P with respect to its pupil heights and a polarizing optical element which changes the polarization state of the entire wavefront independent of the pupil height within the projection system pupil P will not take advantage from the invention. Without loss of generality the pupil heights can be normalized to be parametrized by the unit interval [0,1]. 
         [0025]    In one aspect, the disclosure provides a process of operating a lithographic system wherein the lithographic system includes a projection system for projecting an object field through a projection system pupil P onto an image field, the projection system includes an optical element located at the projection system pupil P, the projection system pupil P being manipulable by the optical element with respect to normalized pupil heights r. 
         [0026]    The process includes projecting the object field through the projection system pupil P onto the image field and manipulating the projection system pupil P by the optical element at normalized pupil heights r=ƒ P (ρ) in order to manipulate the exit pupil at normalized pupil heights ρ. At this, ƒ P : [0,1]→[0,1] is non-linear function approximating the offence against the sine condition φ P  with respect to the projection system pupil P. 
         [0027]    At this, “approximation” has to be understood to be a best approximation within some fixed set of functions, e.g., polynomials, and with respect to some fixed norm, e.g., root-mean-square rms. 
         [0028]    Such a process of operating a lithographic system does not ignore the offence against the sine condition φ P  with respect to the projection system pupil P. Since ƒ P  approximates φ P  the process allows a manipulation of the exit pupil approximately at their undisturbed pupil heights. 
         [0029]    For example, U.S. Pat. No. 6,104,472 discloses at column 5, row 9 manipulating a projection system pupil P via a pair of plane glass plates, sometimes called Alvarez Plates. The first embodiment in U.S. Pat. No. 6,104,472 tries to correct axial astigmatism at this projection system pupil P without adversely affecting other optical characteristics (see, for example, col. 5, lines 54 et seq.). However, the aspherization of these plates in the form of polynomials up to the 3 rd  order (see, for example, the first embodiment of U.S. Pat. No. 6,104,472) effects a correction of the axial astigmatism at the place of this fixed projection system pupil P. In general, the pupil distortion between this fixed projection system pupil P and the exit pupil of an image point will adversely effect astigmatic terms of higher order. 
         [0030]    In some embodiments, the non-linear function r=ƒ P (ρ) is a polynomial ƒ P (ρ)=Σ i=1   m b i ρ i , where m≦5 or preferably m=2. 
         [0031]    Such a process of operating a lithographic system can provide an easy to calculate non-linear function ƒ P  since a polynomial is an easy to calculate function. A polynomial of 2 nd  degree is the minimum requirement to follow the non-linearity of φ P . Polynomials of higher orders up to the 5 th  degree provide an even more general set of functions to approximate within and the approximation turns out to be more accurate than that by a polynomial of 2 nd  order. 
         [0032]    In certain embodiments, the manipulable optical element is a projection system aperture stop, and the projection system pupil is manipulated by stopping down the stop proportional to r=ƒ P (ρ) in order to stop down the exit pupil proportional to ρ. 
         [0033]    Such a process of operating a lithographic system can have the advantage that stopping down proportional to r=ƒ P (ρ) turns out a controlled, proportional decreasing of the aperture. 
         [0034]    In some embodiments, the projection system pupil P is manipulated by the optical element in order to manipulate the wavefront Σ i α i Z i (ρ,φ) in the exit pupil and this manipulation influences significantly a finite subset of Zernike coefficients {α i ; i=i 1 , . . . , i k }. 
         [0035]    A definition of the term “significantly” is given accompanying the description of the figures. 
         [0036]    At this the wavefront in the exit pupil is expanded into an orthogonal expansion Σ i α i Z i (ρ,φ)of Zernike polynomial Z i (ρ,φ) with coefficients α i  (see, for example, Handbook of Optical Projection systems, Vol. 1, Wiley-VHC, Berlin 2005, section 11.5.4). 
         [0037]    Such a process of operating a lithographic system like statement can have the advantage of decoupling the Zernike coefficients α i  within the exit pupil into a set of affected Zernike coefficients and a set of unaffected Zernike coefficients. This is advantageous since a crosstalk of any finite set of Zernike coefficients α i , i=i 1 , . . . , i k  to additional terms provides a correction mean for this finite set together with the side action of disturbing additional Zernike coefficients. 
         [0038]    In one aspect, the disclosure provides a process of manufacturing a lithographic system wherein the lithographic system includes a projection system for projecting an object field through a projection system pupil P onto an image field, the projection system includes an optical element having a surface located at the projection system pupil P, the process including of providing the surface with a surface deformation Σ i   n α i Z i (r, φ) in order to correct the wavefront in the exit pupil by Σ i   n α i Z i (ρ,φ), wherein r=ƒ(ρ) and ƒ P ::[0,1]→[0,1] is a non-linear function approximating the offence against the sine condition φ P  between the normalized exit pupil and the normalized projection system pupil P. 
         [0039]    A definition of a surface deformation like Σ i   n α i Z i (r,φ) is given accompanying the description of the figures. 
         [0040]    Such a process of manufacturing a lithographic system can provide that any finite set of Zernike coefficients in the wavefront of the exit pupil can be corrected without side effect to additional coefficients. This is advantageous since the error budget of the wavefront is generally mainly driven by some single lower Zernike coefficients. Other—especially higher—coefficients in general are summed up in form of an rms and such an rms is related to a single budget. Ignoring the offence against the sine condition with respect to the projection system pupil P turns out a crosstalk from each error budget of a single Zernike coefficient to the above error budget of the rms. Since this holds for any single Zernike coefficient all these crosstalks end up in a common single error budget which may run out of control. 
         [0041]    In some embodiments, the surface deformation is provided by ion beam figuring. 
         [0042]    Such a process of manufacturing a lithographic system makes use of the so-called ion beam figuring at a surface located at the projection system pupil P. This is a well-known technique which is disclosed, for example, in US2004/0169836A1. However in US2004/0169836A1 the surface deformation is calculated by measurement and ray tracing (see, for example, paragraphs [0022] and [0023] of US2004/0169836A1) and is determined for each single projection system to be manufactured. The approximation ƒ P  of the offence against the sine condition φ P  is determined only once and is understood to be an a priori assembling correction mean. However, US2004/0169836A1 may put the manufacturer in position to correct errors which are present after assembly, whereas the processes disclosed herein may put the manufacturer of the projection system in position to relax the error budget for the assembly by shifting a part of it from post-assembly to pre-assembly. 
         [0043]    In certain embodiments, the image side aperture of the projection system is not less than 0.9, preferably not less than 0.95, in case of a dry projection system, or the image side numerical aperture of the projection system is not less than 1.35, preferably not less than 1.425, in the case of an immersion projection system and/or the diameter of the image field is not less than 56 mm. 
         [0044]    In some embodiments, the projection system is a catadioptric projection system and/or the projection system is an immersion projection system. 
         [0045]    In certain embodiments, the projection system includes two biconvex lenses located at the projection system pupil P. 
         [0046]    In some embodiments, the projection system has an optical axis, and the projection is achieved via rays of projection light where the outermost rays of the projection light at the projection system pupil P are parallel to the optical axis. 
         [0047]    A process as disclosed herein can localize a projection system whose system pupils contravene the sine condition. 
         [0048]    In certain embodiments, the optical element is a lens or a mirror. 
         [0049]    The disclosure can be used to identify those projection systems where ignoring the offence against the sine condition φ P  with respect to the projection system pupil P effects a serious drawback of the effectiveness of any correction element, e. g., a manipulable optical element located at the projection system pupil P. 
         [0050]    In some embodiments, manipulation is a deformation of the optical element, or a loading with heat and/or cold, or a shift or rotation of an optical element. 
         [0051]    In certain embodiments, manipulable optical elements take the benefit of less crosstalk if they are located at the projection system pupil P and are driven with regard to the offence against the sine condition φ P  with respect to the projection system pupil P. 
         [0052]    Manipulation can be achieved in different ways. US20030063268A1 discloses driving an optical element in the direction of the optical axis of the projection system. U.S. Pat. No. 6,191,898B1 discloses driving an optical element in a direction orthogonal to the optical axis. U.S. Pat. No. 6,104,472 discloses driving Alvarez-Plates or modifying the shape or the index of refraction of an optical element. WO2008034636A2 discloses providing an optical element with a grid of electrical resisting wires which can be charged with electrical current to modify the refractive index of a lens. WO2008037496A2 discloses deforming a lens. US20060244940A1 discloses charging a lens with infrared radiation to modify the refractive index of the lens together with its shape. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The disclosure will be illustrated with respect to exemplary embodiments and accompanying figures, in which: 
           [0054]      FIG. 1  shows a lithographic system. 
           [0055]      FIG. 2  shows a projection system of a lithographic system. 
           [0056]      FIG. 3  shows two lenses of the projection system of  FIG. 2 . 
           [0057]      FIG. 4  is a spot diagram of the entrance pupil of the projection system of  FIG. 1 . 
           [0058]      FIG. 5  is a spot diagram of a pupil of the projection system of  FIG. 1 . 
           [0059]      FIG. 6  shows normalized pupil heights. 
           [0060]      FIG. 7  shows the crosstalk of a surface deformation on a surface of a lens of the projection system of  FIG. 1 . 
           [0061]      FIG. 8  shows the crosstalk of a surface deformation on a surface of a lens of the projection system of  FIG. 1 . 
           [0062]      FIG. 9  shows the wavefront in the exit pupil of an off-axis field point of the projection system of  FIG. 1 . 
           [0063]      FIG. 10  shows the wavefront in a pupil of the projection system of  FIG. 1  which is corrected with respect to the sine condition. 
       
    
    
     DETAILED DESCRIPTION 
       [0064]      FIG. 1  shows a lithographic system  100 . The lithographic system  100  includes a light source  101  which produces illumination light  102 . The illumination light  102  is prepared by an illumination system  103  to provide prepared illumination light  104 . An important difference between illumination light  102  and prepared illumination light  104  is that prepared illumination light  104  exhibits etendue. Illumination system  103  illuminates a reticle (not shown) located at an object field  105  with prepared illumination light  104 . Structures (not shown) located on the reticle diffract the prepared illumination light  104  into diffracted light  106  which enters the projection system  107 . Diffracted light  106  is projected by the projection system  107  into projected light  108  which exposes the wafer (not shown) which is located at an image field  109 . 
         [0065]      FIG. 2  shows projection system  107 . Diffracted light  106  enters projection system  107  at object field  105  and arrives at the first surface of an optical element  202  of projection system  107  and subsequently passes in succession a first pupil  203  of projection system  107 , a second pupil  204  of projection system  107 , and a third  205  pupil of projection system  107 . The maximum pupil height of the entrance pupil (not shown) corresponds to the maximum angle of a ray of diffracted light  106  which passes the entire projection system  107 . Rays having greater angles are stopped via a stop  206  which is located in between lenses  215  and  216 . As used herein, the definition of the pupils is understood to be different to the Handbook of Optical Projection systems, Vol. 1, Wiley-VHC, Berlin 2005. The entrance pupil is the image of stop  206  into the object space, i.e. the image of the projection systems stop generated by all optical elements of the projection system located before the projection systems stop. Note that in  FIG. 2  the rays are running from the left hand side to the right hand side. The exit pupil is the image of stop  206  into the image space, i.e. the image of the projection systems stop generated by all optical elements of the projection system located after stop  206 . More generally, a pupil of projection system  107  is defined to be the image of stop  206  generated by a consecutive sequence of optical elements of projection system  107 . Of potentially most interest are those pupils which are real images of stop  206 . Those are the entrance pupil, the exit pupil and the projection system pupils. The projection system pupils are located at positions  203 ,  204  and  205 . Projection system pupil  205  is stop  206  itself and can be understood to be the image of stop  206  by an empty set of optical elements. Projection system pupil  204  is located between mirrors  207  and  208  and is the image of  206  generated by the optical elements of the group  213  together with mirror  208 . Projection system pupil  205  is located in between lenses  210  and  211  and is the image of  206  generated by the optical elements of groups  213  and  212  together with mirrors  207  and  208 . The entrance pupil is located at infinity at the left hand side of the object plane  105  and is the image of stop  206  generated by the optical elements of groups  213 ,  212  and  211 , together with mirrors  207  and  208 . Finally, the exit pupil is located at infinity at the right hand side of the image plane and is the image of  206  generated by the optical elements of group  214 . 
         [0066]      FIG. 3  shows lenses  215  and  216  located before and after stop  206 . The numerical aperture of the projection system at its image is 1.35, and its maximum object height is 3.5 mm Projection system  107  is an immersion projection system having an index of refraction of 1.436 of the optical medium just in front of the image field. Furthermore the magnification of projection system  107  is equal to −0.25 which turns out that the largest angle of a ray  230  of diffracted light  106  which passes the projection system at the largest pupil height of the entrance pupil of the projection system  107  is approximately 10.5°. That ray is the outermost ray  230  at the projection systems stop  205 . There are other rays  301  shown passing the lenses  215  and  216 . These rays do not exhibit equidistant heights despite the fact that they have equidistant pupil heights within the entrance pupil of the projection system  107 . This illustrates the offence against the sine condition with respect to pupil  205 . 
         [0067]      FIGS. 4 and 5  show penetration points of rays within pupils.  FIG. 4  shows the entrance pupil. The points are equidistant with respect to the radial coordinate.  FIG. 5  shows pupil  205 . The points are not equidistant with respect to the radial coordinate. This illustrates the offence against the sine condition with respect to pupil  205 . 
         [0068]    The graph of  FIG. 6  quantifies the offence of the sine condition with respect to pupil  205 . The abscissa parameterizes equidistant pupil heights within the entrance pupil. The ordinate parameterizes equidistant pupil heights within pupil  205 . Both pupils are normalized, i. e. their maximum pupil heights are equal to one. The dotted graph is the offence against the sine condition φ P : [0,1]→[0,1] mapping pupil heights of equidistant rays within the entrance pupil to pupil heights of the same rays in pupil  205 . The dotted graph can be interpolated by a continuous smooth function, e.g. a spline of grade  2 , c.f. the continuous graph in  FIG. 6 . It can also be approximated by a polynomial of e. g. 5 th  order, i.e. π P (ρ)=0.3252 ρ 5 −0.0135 ρ 3 +0.6851ρ, ρ∈[0,1] (not shown). Apparently interpolation, approximation as well as the offence against the sine condition φ P :[0,1]→[0,1] itself are nonlinear functions. 
         [0069]      FIG. 7  illustrates the crosstalk to additional orders of Zernike coefficients in case of the invention is not used. A surface  209  of optical element  219  which is located at pupil  205  is provided with a surface deformation of  10  nanometers of the Zernike polynomial Z 49 (π,φ). More precisely on the unit circle, Z 49 (ρ,φ) is given by 
         [0000]        Z   49 (ρ,φ)=924ρ 12 −2772ρ 10 +3150ρ 8 −1680ρ 6 +420ρ 4 −42ρ 2 +1.
 
         [0070]    On the unit circle, Z 49 (ρ,φ) has a maximum value of 1. Its radial variable ρ has to be put in proportionality to the real diameter of a surface  209  of lens  219  and the maximum value of 1 has to be put in proportionality to the real maximum surface deformation of 10 nanometers. If such a surface deformation is provided by a local abrasive technique like ion beam figuring, the real maximum surface deformation is doubled because only glass material is removed. 
         [0071]    In  FIG. 7  the coefficient spectrum of the wavefront expansion within the exit pupil of the projection system is shown up to an order of 100 coefficients. This spectrum is understood to be the optical effect of the pure deformation of the lens surface  209  as a manipulation mean, i.e. the difference of the spectrum of the wavefront within the exit pupil of the projection system with deformed lens surface  209  and the spectrum of the wavefront within the exit pupil of the projection system without deformation. One can identify a strong peak of about 2.3 nanometer at the 49 th  coefficient which shows that the surface deformation of the lens surface  209  at pupil  205  effects a 49 th  Zernike polynomial in the exit pupil, too. However on the one hand the absolute value of the 49 th  coefficient is not as high as expected in case of a pure Z 49 -deformation of the wavefront, i.e. one would expect approximately 5.0 Nanometer instead of approximately 2.3 Nanometer. On the other hand there is a crosstalk to higher orders Z 64 , Z 81 , Z 100  as well as to lower orders Z 36 , Z 25 , Z 16 , Z 9  and Z 4 . E.g. the coefficient of Z 36  is even higher than that of Z 49  itself. Such strong crosstalk is caused by the offence against the sine condition φ P  at pupil  205  which is equal to φ P  of  FIG. 6 . Therefore one observes the coefficient spectrum of Z 49 ∘φ P  within the exit pupil. At this ∘ is to be understood to be the composition of functions. 
         [0072]    In  FIG. 8  the same coefficient spectrum is shown for the surface deformation of Z 49 ∘π P   −1  instead of Z 49  where π P   −1  is understood to be the inverse of π P  where π P  is the polynomial approximation up to the 5 th  order of the offence against the sine condition φ P  at pupil  205 . One can observe a lower crosstalk to additional Zernike coefficients since in this case Z 49 ≈Z 49 ∘π P   −1 ∘φ P  instead of Z 49 ∘φ P  can be observed within the exit pupil. 
         [0073]    In general the same holds true for arbitrary Zernike polynomials other than Z 49 . Since π P  effects only the radial variable within the unit circle one can observe that the crosstalk generally effects only coefficients having the same azimuthal order. I.e. a deformation with the shape of a Zernike polynomial having an azimuthal order m effects only coefficients having the azimuthal order m, too. More illustrative the crosstalk remains within a single line of the table of  FIG. 11-19  of Handbook of Optical Projection systems, Vol. 1, Wiley-VHC, Berlin 2005. 
         [0074]    To be more general a manipulation of an optical element is the to be “influencing” a set of Zernike coefficients {α i ; i∈I} if and only if 
         [0000]      0≠α i =α i1 −α i2   , i∈I    (3)
 
         [0000]    where Σ i α i1 Z i (ρ,φ) is the wavefront expansion in the exit pupil where the optical element is manipulated and Σ i α i2 Z i (ρ,φ) is the wavefront expansion in the exit pupil where the optical element is not manipulated. E.g.  FIG. 7  shows that the Z 49 -deformation of the surface  209  of optical element  219  influences the set of Zernike coefficients where I consists of all square integers which means that precisely all rotational symmetric Zernike polynomials are affected by the manipulation (see, for example, Handbook of Optical Projection systems, Vol. 1, Wiley-VHC, Berlin 2005). 
         [0075]    The 0 in formula (3) is more or less theoretically. To quantify it a set of Zernike coefficients {α i ; i∈I} is the to be “significantly influenced” by a manipulation if and only if 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                   &lt; 
                   0.1 
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where the α i  are given by (1) and there is no subset of I which holds (2), too. In words the set of all Zernike coefficients divides into a smallest subset I and its complement such that any coefficient of the complement is less than 1% in absolute value of any element of I. E.g. if the coefficients are given by 
         [0000]    
       
         
           
             
               a 
               i 
             
             = 
             
               1 
               i 
             
           
         
       
     
         [0000]    there is no significantly influencing set of coefficients. In case of the  FIG. 7  and the Z 49 -deformation of surface  209  of the lens  219  there is no significantly influencing set of coefficients, too. More precisely there is no finite significantly influencing set. If the offence against the sine condition with respect to pupil  205  is taken into account, i. e. surface  209  of the lens  219  is deformed according to the shape Z 49 ∘π P   −1  one observes the spectrum of  FIG. 8  and I={49} is the significantly influenced set. 
         [0076]    Since deformations and more general manipulation mechanisms can be superimposed the invention provides a mean to influence significantly finite sets of Zernike coefficients of the wavefront within the exit pupil by taken into account the offence against the sine condition with respect to pupil  205 . 
         [0077]    Neither the system of Zernike polynomials does limit the invention nor does so the value 0.1 on the right hand side of formula 4. Any system of functions a wavefront can be expanded within will serve well as analogue to describe the benefit of the invention and values other than 0.1 to quantify the crosstalk of the coefficients may be used. To take into account the offence against the sine condition φ P  with respect to pupil  205  and its countermean, the transformation of the radial variable ρ by π P   −1  is the root of the matter. 
         [0078]    Moreover the deformation of a surface of a lens is just an example to illustrate the invention. The invention provides a countermean against the crosstalk to additional Zernike coefficients as intended to influence for any type of manipulator located at an arbitrary projection system pupil within the projection system. 
         [0079]      FIG. 9  shows a Z 28 -deformed wavefront  901  which has been generated by a manipulation mean  222  like one of those described in U.S. Pat. No. 6,104,472, or WO2008034636A2, or WO2008037496A2, or US20060244940A1 (see, for example, above for a list of such mechanisms). Such mechanisms can be located at pupil  203 . To generate this Z 28 -deformation at the exit pupil a wavefront  1001  as in  FIG. 10  has to be provided by the manipulation mean  222  at pupil  203 . 
         [0080]    In case of the manipulation mean  222  deforms a lens  210  as in WO2008037496A2 there are torques provided at the periphery of the lens  210 . To provide a deformation of the wavefront  1001  the torques and forces at the periphery of the lens have to be modified with respect to the torques and forces to be provided to generate a deformation of the wavefront like  901 . 
         [0081]    In case of the manipulation mean  222  consists of a pair of Alvarez plates  223  as in U.S. Pat. No. 6,104,472 one has to calculate a primitive with respect to a coordinate of movement, say x. The primitive to be calculated is that of the wavefront deformation to be generated. Following the invention one has to calculate the primitive of the function given by  FIG. 10  rather than by  FIG. 9 . More mathematically speaking one has to provide the Alvarez plates with the surface shape of 
         [0000]    
       
         
           
             
               ∫ 
               0 
               x 
             
              
             
               
                 
                   Z 
                   28 
                 
                  
                 
                   ( 
                   
                     
                       
                         Π 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         ρ 
                         ) 
                       
                     
                     , 
                     ϕ 
                   
                   ) 
                 
               
                
               
                  
                 x 
               
             
           
         
       
     
         [0000]    where ρ=√{square root over (x 2 +y 2 )} and φ=arcsin x. The integral can be calculated numerically. 
         [0082]    The information disclosed herein can be used in dry projection systems as well as immersion projection systems.