METHOD AND DEVICE FOR CHARACTERIZING THE SURFACE SHAPE OF AN OPTICAL ELEMENT

Abstract

Methods for characterizing the surface shapes of optical elements include the following steps: carrying out, in an interferometric test arrangement, at least a first interferogram measurement on the optical element by superimposing a test wave, which has been generated by diffraction of electromagnetic radiation on a diffractive element and has been reflected at the optical element, carrying out at least one additional interferogram measurement on in each case one calibrating mirror for determining calibration corrections, and determining the deviation from the target shape of the optical element based on the first interferogram measurement carried out on the optical element and the determined calibration corrections. At least two interferogram measurements are carried out for the at least one calibrating mirror, which differ from one another with regard to the polarization state of the electromagnetic radiation.

Claims

1. A method for characterizing the surface shape of an optical element, the method comprising: a) carrying out at least one first interferogram measurement on the optical element in an interferometric test arrangement by superposing a reference wave on a test wave generated by diffraction of electromagnetic radiation at a diffractive element and reflected at the optical element, wherein the reference wave has not been reflected at the optical element; b) carrying out at least one further interferogram measurement, in each case on a calibration mirror for determining calibration corrections; and c) determining a figure of the optical element based on the first interferogram measurement carried out on the optical element and based on the determined calibration corrections; wherein at least two of the interferogram measurements are carried out for the at least one calibration mirror, said interferogram measurements differing from one another in polarization state of the electromagnetic radiation.

2. The method as claimed in claim 1, wherein said determining of the calibration corrections comprises a determination of parameters characterizing a three-dimensional structure of the diffractive element.

3. The method as claimed in claim 2, wherein the parameters characterizing the three-dimensional structure of the diffractive element include at least one of: etching depth, slope angle, edge rounding, and duty cycle.

4. The method as claimed in claim 2, wherein the determination of the parameters characterizing the three-dimensional structure of the diffractive element is implemented with simulations.

5. The method as claimed in claim 1, wherein the interferogram measurements carried out in said step b) are carried out on at least two calibration mirrors.

6. The method as claimed in claim 5, wherein at least two of the interferogram measurements, which differ in respect of the polarization state of the electromagnetic radiation, are carried out for each of the calibration mirrors.

7. The method as claimed in claim 1, wherein in each case at least three interferogram measurements, which differ in respect of the polarization state of the electromagnetic radiation, are carried out for at least one of the calibration mirrors.

8. The method as claimed in claim 1, wherein the figure of the optical element is determined based on subtracting interferogram phases respectively obtained during the interferogram measurements.

9. The method as claimed in claim 1, wherein the figure of the optical element is determined based on an averaging of interferogram phases respectively obtained during the interferogram measurements.

10. The method as claimed in claim 1, wherein a plurality of the interferogram measurements, which differ in respect of the polarization state of the electromagnetic radiation, are carried out on the optical element.

11. A method for characterizing the surface shape of an optical element, the method comprising: a) carrying out a first interferogram measurement on the optical element in an interferometric test arrangement by superposing a reference wave on a test wave generated by diffraction of electromagnetic radiation at a diffractive element and reflected at the optical element, wherein the reference wave has not been reflected at the optical element; b) carrying out at least one second interferogram measurement on the optical element by superposing a reference wave on a test wave generated by diffraction of electromagnetic radiation at the diffractive element and reflected at the optical element, wherein the reference wave has not been reflected at the optical element, the first and the second interferogram measurements differing from one another in polarization state of the electromagnetic radiation; and c) determining parameters characterizing a three-dimensional structure of the diffractive element based on subtracting interferogram phases respectively obtained during the interferogram measurements, wherein the parameters comprise at least one of etching depth, slope angle, edge rounding and duty cycle.

12. The method as claimed in claim 11, wherein at least three of the interferogram measurements, which differ in the polarization state of the electromagnetic radiation, are carried out on the optical element.

13. The method as claimed in claim 11, wherein a plurality of the interferogram measurements which differ from one another in the polarization state of the electromagnetic radiation are carried out in a prior calibration on a calibration sample that is different than the optical element being characterized with regard to the surface shape.

14. The method as claimed in claim 13, characterized in that calibration corrections are further determined based on the interferogram measurements carried out on the optical element or the calibration sample.

15. The method as claimed in claim 11, wherein the interferogram measurements are carried out using electromagnetic radiation with a linear input polarization.

16. The method as claimed in claim 11, further comprising determining calibration corrections using at least one polarization correction element for reducing a component caused by polarization coupling between the diffractive element and the interferometric test arrangement in the interferogram phases respectively obtained during the interferogram measurements.

17. The method as claimed in claim 11, wherein the interferogram measurements are carried out multiple times using respectively differing diffractive elements.

18. The method as claimed in claim 1, further comprising determining the figure of the optical element based on an additional evaluation of contrast respectively obtained during the interferogram measurements.

19. The method as claimed in claim 1, wherein a plurality of the interferogram measurements, which differ in wavelength of the electromagnetic radiation, are carried out on the optical element and/or for at least one calibration mirror.

20. The method as claimed in claim 1, wherein the diffractive element is a computer-generated hologram (CGH).

21. The method as claimed in claim 1, wherein the optical element is a mirror.

22. The method as claimed in claim 1, wherein the optical element is configured for an operating wavelength of less than 30 nm.

23. The method as claimed in claim 1, wherein the optical element is an optical element of a microlithographic projection exposure apparatus.

24. A device for characterizing the surface shape of an optical element, configured to carry out a method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In the drawings:

[0056] FIG. 1 shows a schematic illustration for explaining an exemplary configuration of an interferometric test arrangement that can be used in a method according to the invention;

[0057] FIG. 2 shows a flowchart for explaining an exemplary embodiment of a method according to the invention;

[0058] FIG. 3 shows a schematic illustration for explaining a further exemplary configuration of an interferometric test arrangement that can be used in a method according to the invention; and

[0059] FIG. 4 shows a schematic illustration of a projection exposure apparatus designed for operation in EUV.

DETAILED DESCRIPTION

[0060] FIG. 4 firstly shows a schematic illustration of an exemplary projection exposure apparatus which is designed for operation in EUV and which comprises mirrors which are testable with methods according to the invention.

[0061] In accordance with FIG. 4, an illumination device in a projection exposure apparatus 410 designed for EUV comprises a field facet mirror 403 and a pupil facet mirror 404. The light from a light source unit comprising a plasma light source 401 and a collector mirror 402 is directed at the field facet mirror 403. A first telescope mirror 405 and a second telescope mirror 406 are arranged in the light path downstream of the pupil facet mirror 404. A deflection mirror 407 is arranged downstream in the light path, said deflection mirror directing the radiation that is incident thereon at an object field in the object plane of a projection lens comprising six mirrors 421-426. At the location of the object field, a reflective structure-bearing mask 431 is arranged on a mask stage 430, said mask being imaged with the aid of the projection lens into an image plane in which a substrate 441 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 440.

[0062] The optical element which is tested by a method according to the invention in an interferometric test arrangement described below can be e.g. an arbitrary mirror of the projection exposure apparatus 410.

[0063] FIG. 2 shows a flowchart illustrating multiple steps S210-S260 of an exemplary embodiment of a method according to the invention.

[0064] FIG. 1 shows a schematic illustration for describing an exemplary configuration of an interferometric test arrangement for testing a mirror using a CGH.

[0065] In accordance with FIG. 1, the illumination radiation generated by a light source (not illustrated) and emerging from the exit surface of an optical waveguide 101 emerges as an input wave 105 having a spherical wavefront, passes through a beam splitter 110 and is then incident on a complexly coded CGH 120. The CGH 120 generates a total of four output waves from the input wave 105 in transmission in the example in accordance with its complex coding, one output wave of which impinges as a test wave on the surface of the optical element to be characterized with regard to its surface shape in the form of a mirror 140 with a wavefront adapted to the target shape of the surface of said mirror 140. Furthermore, the CGH 120 generates three further output waves from the input wave 105 in transmission, each of which further output waves is incident on a respective further reflective optical element 131, 132 and 133. In this case, any combination of in each case two of the reflective optical elements 131, 132 and 133 can be chosen in order to generate a reference wave in each case and a calibration wave in each case (that is to say, in principle each of the elements 131, 132 and 133 can be used alternatively as a reference mirror for generating the reference wave or as a calibration mirror for generating a calibration wave). The reference wave reflected at the respective reference mirror is made to interfere with the test wave reflected by the mirror 140 or with a calibration wave reflected by the respective calibration mirror. In this context, individual optical elements can temporarily be coupled out of the optical beam path by way of suitably designed shutters (of which only one shutter 135 is indicated schematically). The CGH 120 also serves for superposing the waves reflected by the reflective optical elements 131-133, which are incident as convergent beams again on the beam splitter 110 and are reflected from the latter in the direction of an interferometer camera 160 designed as a CCD camera, in the course of which they pass through an eyepiece 150. The interferometer camera 160 captures an interferogram generated by the interfering waves, the actual shape of the optical surface of the mirror 140 being determined from said interferogram by way of an evaluation device (not shown).

[0066] A polarization-influencing element 170, indicated schematically in FIG. 1, serves to set a desired input polarization in targeted fashion, said element being able to be designed in any desired suitable way and being able to be arranged variably within the optical beam path. In embodiments, it is possible to use a suitable polarizer for setting linear polarization in combination with a half-wave plate for switching between the respective polarization directions. In further embodiments, it is possible to use a suitable polarizer in combination with a rotatable half-wave plate and a rotatable quarter-wave plate for setting orthogonal linear and circular input polarizations.

[0067] In FIG. 1, a further polarization-influencing element can be used instead of the shutter 135, or in addition to this shutter, in the beam path upstream of the reference mirror. In a manner similar to the element 170, the polarization-influencing element 135 also serves to manipulate the polarization with the object of in this way obtaining additional information for determining the calibration corrections to be considered when determining the figure. In further embodiments, use can be made of a suitable polarizer or a retardation element in the form of a rotatable and sufficiently thin plate that has surfaces that are as plane and as parallel as possible. If the reference mirror is embodied as a plane mirror, a plane wavefront is incident on the polarization-influencing element 135. As a result, the angular load and hence also an additional polarization error introduced by the element 135 are small. Geometric phase errors, which are consequently independent of the polarization and which are caused by surface defects or refractive index inhomogeneities of the element 135, for example, can be removed by calculation together with a figure defect of the reference mirror. The element 135 can also be used to implement a phase modulation in the interferogram measurements (instead of a phase modulation as a result of displacing the reference mirror, for example).

[0068] Now, one insight associated with the invention is the concept, known per se, of also including calibration corrections when determining the figure of the mirror 140, said calibration corrections being implemented on the basis of calibration measurements on the reflective optical elements 131-133 serving as calibration mirrors in this respect. According to the invention, interferogram measurements, especially on these calibration mirrors and possibly also on the mirror to be characterized in respect of its surface shape, are respectively carried out not only once but multiple times with different input polarizations in each case in order thus to obtain additional information for determining the calibration corrections to be included when determining the figure.

[0069] The following mathematical consideration shows that an increase in the system of equations (within the meaning of providing additional equations) is obtained in this way, said system of equations relating the respectively measured interferogram phases with known quantities and with unknown structure defects of the employed diffractive element to be determined on the basis of solving the system of equations. Accordingly, it is ultimately possible to determine a greater number of structure defects of the diffractive element. With the additional equations it is possible in general to deduce polarization-dependent defects (and not only structure defects of the CGH). According to the invention, it is possible in particular to additionally determine parameters which are characteristic for the three-dimensional structure of the diffractive element or CGH, for example etching depth, slope angle, edge rounding and duty cycle or degree of fill.

[0070] Starting point for the mathematical consideration is that on the basis of three calibration mirrors on which a respective interferogram measurement is carried out it is possible to determine three unknown quantities in accordance with the following system of equations of three equations (1)-(3):

[0071] An interpolation is carried out during the calibration according to the invention, with the wavenumber vectors (“k-vectors”) of the calibration waves spanning a tetrahedron following the diffraction of the light coming from the light source, said tetrahedron including the direction of the wavenumber vector of the test wave (such that the wavenumber vector of the test wave is located within the tetrahedron). The unknown quantities or errors in the interferogram phases of the test object and calibration mirrors, which are independent of the input polarization, which are added to the phase component actually to be determined (corresponding to the surface shape or figure of the test object) and which are determined by calibration can be decomposed into a constant component or error c.sub.0 and two (error) components c.sub.x and c.sub.y that are linear in terms of the wavenumber vectors, the following interpolation scheme arising:

[00001]$\begin{array}{cc}{\phi }_{K1}={\phi }_{K1}^P+2k_{x,K1}^{\prime }.Math.c_x+k_{y,K1}^{\prime }.Math.c_y+c_0& \left(1\right)\\ {\phi }_{K2}={\phi }_{K2}^P+2k_{x,K2}^{\prime }.Math.c_x+k_{y,K2}^{\prime }.Math.c_y+c_0& \left(2\right)\\ {\phi }_{K3}={\phi }_{K3}^P+2k_{x,K3}^{\prime }.Math.c_x+k_{y,K3}^{\prime }.Math.c_y+c_0& \left(3\right)\end{array}$

[0072] The errors c.sub.0, which yield phase components that are independent of the input polarization, constant in terms of wavenumber vectors and identical for the test surface and the calibration mirrors, for example are, inter alia, figure defects on the reference mirror, and the errors c.sub.x and c.sub.y, which yield phase components that are independent of the input polarization and linear in terms of wavenumber vectors, are in particular lateral structure offset defects in the diffractive element or CGH.

[0073] In addition to the phase component actually to be determined (corresponding to the surface shape or figure of the test object) and, in particular, in addition to figure defects on the reference mirror and lateral structure offset defects in the diffractive element or CGH, the interferogram phase contains further phase components, specifically a scalar phase component of the diffractive element or CGH, and polarization-induced phase components of both the diffractive element or CGH on its own and on account of the coupling of the diffractive element or CGH with the remaining optical system.

[0074] The present invention now targets a reduction in the phase component in the interferogram phase generated by the diffractive element or CGH. According to the invention, this is achieved by virtue of the aforementioned calibration being extended in respect of the determination of structure defects of the diffractive element or CGH by virtue of a plurality of interferogram measurements (that is to say at least two measurements) with in each case a different polarization state of the electromagnetic radiation being carried out for each calibration mirror. The more accurate knowledge of the diffractive structure of the diffractive element or CGH obtained according to the invention as a result in turn facilitates a more accurate determination of the phase components in the interferogram caused thereby and subtraction thereof from the obtained interferogram phase.

[0075] Overall, the following system of equations arises:

[00002]$\begin{array}{cc}{\phi }_{K,p}-{\phi }_K^P-{\phi }_{K,p,0}^C=2{k^{\prime }}_{x,K}.Math.c_x+2{k^{\prime }}_{y,K}.Math.c_y+c_0+\underset{m=1}{\overset{M}{.Math.}}{\phi }_{K,p,m}^C.Math.\delta c_m& \left(4\right)\end{array}$

[0076] In this case, φ.sub.K,p denotes the interferogram phases measured for the respective calibration mirrors in the case of input polarization p, φ.sub.K.sup.p denotes the figure of the respective calibration mirrors in the case of input polarization p, φ.sub.K,p,0.sup.C denotes the nominal phase or phase calculated in the optical design (with the inclusion of rigorous simulations) and φ.sub.K,p,m.sup.C denotes the sensitivities of the defects of the diffractive element or CGH (optionally likewise calculated with the inclusion of rigorous simulations). Further, quantities dependent on the polarization are denoted by an additional index p.

[0077] Then, the figure of the optical element to be characterized in terms of its surface shape can be calculated according to the following equations:

[00003]$\begin{array}{cc}{\phi }_S^P={\phi }_{S,p}-{\phi }_{S,p,0}^C-2{k^{\prime }}_{x,S}.Math.c_x-2{\left(k^{\prime }\right)}_{y,S}.Math.c_y+c_0-\underset{m=1}{\overset{M}{.Math.}}{\phi }_{S,p,m}^C.Math.\delta c_m& \left(5\right)\end{array}$

[0078] In this case, δc.sub.m denotes the unknown quantities (rigorous defects of the diffractive element or CGH such as, e.g., etching depth, slope angle, edge rounding and duty cycle and, e.g., quantities of a polarization correction element), φ.sub.S,p,0.sup.C and φ.sub.S,p,m.sup.C denote the phases for the nominal and disturbed system calculated in the optical design (with the inclusion of rigorous simulations) in the case of input polarization p, and φ.sub.S,p denotes the interferogram phases measured for the optical element or the test object to be characterized in respect of its surface shape, in the case of input polarization p.

[0079] Instead of the calculation method consisting of the system of equations (4) and equation (5), it is also possible to set up the common system of equations for the calibration mirrors and the test surface (in the case of three calibration mirrors here in exemplary fashion)

[00004]$\begin{array}{cc}{\phi }_{K1,p}-{\phi }_{K1}^p-{\phi }_{K1,p,0}^C=2{k^{\prime }}_{x,K1}.Math.c_x+2{k^{\prime }}_{y,K1}.Math.c_y+c_0+\underset{m=1}{\overset{M}{.Math.}}{\phi }_{K1,p,m}^C.Math.\delta c_m{\phi }_{K2,p}-{\phi }_{K2}^p-{\phi }_{K2,p,0}^C=2{k^{\prime }}_{x,K2}.Math.c_x+2{k^{\prime }}_{y,K2}.Math.c_y+c_0+\underset{m=1}{\overset{M}{.Math.}}{\phi }_{K2,p,m}^C.Math.\delta c_m{\phi }_{K3,p}-{\phi }_{K3}^P-{\phi }_{K3,p,0}^C=2{k^{\prime }}_{x,K3}.Math.c_x+2{k^{\prime }}_{y,K3}.Math.c_y+c_0+\underset{m=1}{\overset{M}{.Math.}}{\phi }_{K3,p,m}^C.Math.\delta c_m{\phi }_{S,p}-{\phi }_{S,p,0}^C=2{k^{\prime }}_{x,S}.Math.c_x+2{k^{\prime }}_{y,S}.Math.c_y+c_0+\underset{m=1}{\overset{M}{.Math.}}{\phi }_{S,p,m}^C.Math.\delta c_m+{\phi }_S^P& \left(6\right)\end{array}$

by virtue of the figure of the test surface φ.sub.S.sup.p being formulated to be the quantity to be determined by solving this system of equations. In the case of three calibration mirrors and N.sub.p different input polarizations in the interferogram measurements for the calibration mirrors and the test surface, a total of 4.Math.N.sub.p equations are available, as a result of which 4.Math.N.sub.p−4 further unknowns δc.sub.m,which may be for example rigorous defects of the diffractive element or CGH or other polarization errors, can be determined in addition to the unknowns φ.sub.S.sup.p, c.sub.x, c.sub.y and c.sub.o. Thus, four e.g. rigorous errors δc.sub.m can be determined in the case of two different input polarizations, this even increasing to 12 in the case of four different input polarizations. In the case of two or four different input polarizations, it is advantageous to use linear input polarizations in the horizontal and vertical directions and in the two diagonal directions in the interferogram measurements since linear input polarizations reduce the phase component due to polarization coupling between the diffractive structure of the diffractive element or CGH and the remaining system.

[0080] As a result of the introduction of a suitable virtual polarization correction element, it is possible to determine correction quantities which reduce the phase component due to polarization coupling between the diffractive structure of the diffractive element or CGH and the remaining system. In the process, polarization errors in the remaining optical system are “removed by calibration” in addition to the (CGH) structure defects, as a result of which the actual structure quantities of the diffractive element or CGH can be reconstructed more accurately by the calibration, and as a result of which the figure of the test object can be determined more accurately.

[0081] Such a virtual polarization correction element describes a polarization effect by way of a Jones matrix directly in front of the structured side of the diffractive element or CGH and may represent a combination of a pure dichroic element and a purely retarding element, this being able to be based on a linear dichroic and linear retarding effect in particular. The corresponding Jones matrices for linear dichroism and linear retardation are as follows:

[00005]$\begin{array}{cc}{J}_D={\sigma }_0+\tanh \left(\mu \right)\left[\cos \left(2{\beta }_D\right){\sigma }_1+\sin \left(2{\beta }_D\right){\sigma }_2\right]={\sigma }_0+d_1{\sigma }_1+d_2{\sigma }_2& \left(7\right)\\ J_R={\sigma }_0+i\tan \left(\delta \right)\left[\cos \left(2{\beta }_R\right){\sigma }_1+\sin \left(2{\beta }_R\right){\sigma }_2\right]={\sigma }_0+i{r}_1{\sigma }_1+i{r}_2{\sigma }_2& \left(8\right)\end{array}$

with the magnitudes and axis orientations for dichroism and retardation of

[00006]$\begin{array}{cc}\tanh \left(\mu \right)=\sqrt{d_1^2+d_2^2},\tan \left(2{\beta }_D\right)=\frac{d_2}{d_1}& \left(9\right)\\ \tanh \left(\delta \right)=\sqrt{r_1^2+r_2^2},\tan \left(2{\beta }_R\right)=\frac{r_2}{r_1}& \left(10\right)\end{array}$

[0082] The Jones matrix of the virtual polarization correction element now is the product of the two Jones matrices for linear dichroism and linear retardation, where quadratic and hence also circular components in the product may be considered negligible under the approximation that the magnitudes of dichroism and retardation are small.

[00007]$\begin{array}{cc}{J}_{\mathrm{PCE}}=J_D.Math.J_R\approx {\sigma }_0+d_1{\sigma }_1+d_2{\sigma }_2+i{r}_1{\sigma }_1+i{r}_2{\sigma }_2& \left(11\right)\end{array}$

[0083] Now, the error quantities

[00008]$\begin{array}{cc}{d}_1,d_2,r_1,r_2=>\mu ,{\beta }_D,\delta ,{\beta }_R& \left(12\right)\end{array}$

are determined in an extended calibration.

[0084] In embodiments, a plurality of polarization correction elements may be virtually inserted into the system at suitable locations. Eight parameters to be corrected are present if two polarization correction elements are used. In embodiments, in particular, a virtual polarization correction element can be used directly in front of the diffractive structure of the diffractive element or CGH, and only in the forward direction, in order to capture the polarization errors from the illumination to the diffractive structure, and a second virtual polarization correction element can be used behind an AR layer of the interferometer camera, the polarization errors in the optical system from the diffractive structure to the interferometer camera being captured by way of the latter polarization correction element.

[0085] In embodiments, it is also possible to carry out measurements on the same test structure with a plurality of different diffractive elements or CGHs. To determine the polarization-induced phase component as a result of polarization coupling of the (CGH) diffraction structure with the remaining system, it is possible in this case to exploit that when only the CGH is replaced, the structure of the interferometric test arrangement up to the diffractive structure of the CGH, and from there to the interferometer camera, remains unchanged in each case. Thus, if the measurements from one and the same test structure are evaluated for a plurality of different CGHs, it is possible as a matter of principle to determine the errors in the polarization (within the meaning of the difference to the nominal system) for the remaining system (without CGH diffraction structure).

[0086] When exchanging the CGH, it should be observed that this changes not only the polarization effect of the diffractive structure but also the polarization effect of the CGH substrate. By way of example, stress birefringence in the substrate may be problematic in this case. This stress birefringence remains in the calculation as an unknown and leads to errors as a result. By measurements with different CGHs and, as a rule, the same reference and calibration mirrors, it is possible to merge the respective systems of equations and the parameters of a virtual polarization correction element can be determined as a common unknown. Expressed differently, the equations for all CGHs can be combined into a larger system of equations, with the correction quantities for the polarization correction elements being the same for all CGHs, and so effectively there is a greater number of equations available for fewer unknowns. This not only provides the option of determining the parameters of the virtual polarization correction element (“PCE parameters”) more accurately but also of formulating further unknowns in the calibration step.

[0087] Moreover, measurements for calibration and test mirrors on the same test structure with a plurality of different diffractive elements or CGHs in the case of the same test surface are advantageous because this, in a manner analogous to the explanation above, increases the number of equations by merging the systems of equations for the respective CGHs since in addition to, e.g., the parameters of a virtual polarization correction element the figure of the test surface φ.sub.S.sup.p to be characterized is a common unknown. In this case, the use of an input polarization can be advantageous in the interferogram measurements for the calibration mirrors and the test surface itself because this, by way of solving a system of equations, allows the determination of more error quantities that are independent of the polarization.

[0088] In further embodiments, it is also possible to carry out measurements on calibration CGHs with specific known structure defects and a comparatively small polarization effect (in particular, a low stress birefringence) of the CGH substrate such that advantageously structure disturbances can also be partly detected by measurement (and not by way of the simulation).

[0089] In further embodiments, measurements can also be carried out on specific calibration polarization elements that have been introduced into the test system and that alter the polarization such that advantageously errors in the polarization (within the meaning of the difference to the nominal system) for the remaining system (without CGH diffraction structure) can also be partly detected by measurement (and not by way of the simulation).

[0090] In embodiments, the contrast and the intensity in the interferogram (for calibration mirrors and test object and for different input polarizations) can also be evaluated in addition to an evaluation of the phase in the interferogram, with the contrast and the intensity likewise having components dependent on the polarization and independent thereof. Since, to the lowest order, the test object figure does not contribute to the contrast, additional equations arise with the evaluation of the contrast for the test object and for the calibration mirrors. In the case of three calibration mirrors and interferogram measurements for the test object and the calibration mirrors for in each case N.sub.p different input polarizations, 4.Math.N.sub.p additional equations arise. In addition to the figure of the test surface, the number of quantities determinable by solving the system of equations, for example when using three calibration mirrors with two different input polarizations increases from 4.Math.2−1=7 to 7+4.Math.2=15, and increases from 4.Math.4−1=15 to 15+4.Math.4=31 equations in the case of four different input polarizations.

[0091] In embodiments of the invention (and in the case of a sufficient temporal stability of the interferometric test arrangement), the interferogram measurements on the calibration mirrors can be carried out before the interferogram measurements on the optical element to be characterized in terms of its surface shape or on the test object. Moreover, all interferogram measurements for all utilized input polarizations can be carried out for a certain test object, with only one measurement with a single input polarization then being carried out for further test objects and it being possible to convert to the interferogram phases with the remaining input polarizations on the basis of the interferogram measurement carried out in the case of the first test object.

[0092] FIG. 3 shows as an alternative to FIG. 1 a further exemplary configuration of an interferometric test arrangement.

[0093] In accordance with FIG. 3, in a Fizeau arrangement, an interferogram is generated between a reference wave reflected at a reference surface 302 (“Fizeau plate”) and a test wave reflected at a mirror 301. In this case, the measurement light is shaped by a CGH 303 to form a wavefront that corresponds mathematically exactly to the “test object shape” (i.e. the shape of the relevant mirror 301) at a target distance. The wavefronts reflected firstly from the reference surface 302 and secondly from the relevant mirror 301 or test object interfere with one another in an interferometer 304 comprising, in accordance with FIG. 3, a light source 305, a beam splitter plate 306, a collimator 307, a stop 308, an eyepiece 309 and a CCD camera 310. An interferogram of the respective mirror 301 is recorded with the CCD camera 310.

[0094] Here, too, the corresponding input polarization is set using a polarization-influencing element 350, which is indicated merely schematically in FIG. 3 and which, analogously to FIG. 1, can be configured and arranged variably in the optical beam path in any suitable way.

[0095] Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to a person skilled in the art, for example through combination and/or exchange of features of individual embodiments. Accordingly, persons skilled in the art will view such variations and alternative embodiments as being concomitantly encompassed by the present invention, and consider the scope of the invention to be restricted only within the meaning of the appended patent claims and the equivalents thereof.