Abstract
A measurement apparatus (10) for determining a shape of an optical surface. An illumination module (16) produces an illumination wave (34), an interferometer (18) splits the wave into a test wave (50), which is directed onto the optical surface, and a reference wave (52). The relative tilt between the waves produces a multi-fringe interference pattern (66) in a detection plane (62) of the interferometer when the waves are superposed. A pupil plane (28) of the illumination module is arranged in a Fourier plane of the detection plane and the illumination module is configured to produce the illumination wave so that the intensity distribution thereof in the pupil plane includes at least one spatially isolated and contiguous surface region (38) such that a rectangle (74) with the smallest possible area fitted to the surface region or the totality of surface regions has an aspect ratio of at least 1.5:1.
Claims
1. A measurement apparatus for determining a shape of an optical surface of a test object by interferometry, comprising: an illumination module configured to produce an illumination wave, an interferometer configured to split the illumination wave into a test wave, which is directed onto the optical surface, and a reference wave with a tilt relative to one another such that a multi-fringe interference pattern is produced in a detection plane of the interferometer by superposition of the test wave and the reference wave, wherein the illumination module has a pupil plane that is arranged in a Fourier plane of the detection plane and the illumination module is configured to produce the illumination wave such that an intensity distribution of the illumination wave in the pupil plane comprises at least one spatially isolated and contiguous surface region that is configured such that a rectangle with a smallest possible area that is fitted to the at least one surface region has an aspect ratio of at least 1.5:1.
2. The measurement apparatus as claimed in claim 1, wherein the intensity distribution in the pupil plane comprises the at least one contiguous surface region and wherein the contiguous surface region is embodied as a fringe.
3. The measurement apparatus as claimed in claim 1, wherein the intensity distribution in the pupil plane comprises a plurality of the contiguous surface regions and a covering surface that is adapted in form to a totality of the surface regions is embodied as a fringe.
4. The measurement apparatus as claimed in claim 2, wherein the fringe is an arcuate fringe.
5. The measurement apparatus as claimed in claim 4, wherein a pupil of the illumination module assigned to the pupil plane is delimited by a ring-shaped edge and the arcuate fringe is configured such that there is at least a tangent on the fringe which subdivides the pupil into two parts, and wherein respective areas of the parts differ from one another by no more than a factor of twenty.
6. The measurement apparatus as claimed in claim 5, wherein each tangent on at least one portion of the fringe comprising a total of at least 20% of the fringe subdivides the pupil respectively into two parts, and wherein respective areas of the parts differ from one another by no more than a factor of twenty.
7. The measurement apparatus as claimed in claim 2, wherein the fringe is a straight fringe.
8. The measurement apparatus as claimed in claim 2, wherein a pupil of the illumination module assigned to the pupil plane is delimited by a ring-shaped edge and the fringe extends transversely to the edge of the pupil.
9. The measurement apparatus as claimed in claim 2, wherein the intensity distribution in the pupil plane comprises a plurality of the fringes.
10. The measurement apparatus as claimed in claim 2, wherein a path length difference of a pupil point in the pupil plane for a field point in the detection plane is defined by a difference between a test path length and a reference path length, wherein the test path length is the path length run through by the radiation of the test wave from the pupil point to the field point in the detection plane and the reference path length is the path length run through by the radiation of the reference wave from the pupil point to the field point in the detection plane, and wherein the fringe extends along a level curve of the path length difference of the field point.
11. The measurement apparatus as claimed in claim 10, wherein a plurality of the fringes extend in the pupil plane along level curves of the path length difference of the field point, and wherein the level curves differ by integer multiples of the wavelength of the illumination wave.
12. The measurement apparatus as claimed in claim 1, wherein the interferometer is configured to merge the test wave post interaction with the optical surface and the reference wave in a superposed beam path, in which the reference wave is tilted in relation to the test wave by a tilt angle , such that:
>100.Math./D where is the wavelength of the illumination wave and D is the beam diameter of the reference wave at the location of merging into the superposed beam path with the test wave.
13. The measurement apparatus as claimed in claim 1, wherein the interferometer comprises a splitting element configured to split the illumination wave into the test wave and the reference wave and said interferometer is further configured to merge the test wave post interaction with the optical surface and the reference wave into a superposed beam path in which the reference wave is tilted in relation to the test wave by a tilt angle , and the illumination module is configured such that the intensity distribution in the pupil plane in at least one direction has an extent L.sub.Bel, such that: where is the wavelength of the illumination wave, f is a distance between the pupil plane of the illumination module and an adaptation optical unit of the interferometer configured to adapt the wavefront of the illumination wave to an intended form of the optical surface of the test object, and I is a distance between the splitting element and the optical surface of the test object.
14. The measurement apparatus as claimed in claim 1, wherein the intensity distribution in the pupil plane is configured such that the multi-fringe interference pattern has a contrast of at least 50% in the at least one region.
15. The measurement apparatus as claimed in claim 1, wherein the intensity distribution is configured such that no more than 70% of the pupil, assigned to the pupil plane, of the illumination module is illuminated.
16. The measurement apparatus as claimed in claim 1, wherein the illumination module comprises a spatial light modulator configured to produce the intensity distribution in the pupil plane.
17. The measurement apparatus as claimed in claim 1, wherein the intensity distribution of the illumination wave in the pupil plane comprises a plurality of the contiguous surface regions and the rectangle with the smallest possible area that is fitted to the totality of the surface regions has an aspect ratio of at least 1.5:1.
18. A method for determining a shape of an optical surface of a test object by interferometry, comprising: producing an illumination wave with an illumination module, and splitting the illumination wave with an interferometer into a test wave, which is directed onto the optical surface, and a reference wave, which are tilted relative to one another such that a multi-fringe interference pattern is produced in a detection plane of interferometer by superposition of the test wave and the reference wave, wherein the illumination wave is produced such that an intensity distribution of the illumination wave in a pupil plane arranged in a Fourier plane of the detection plane comprises at least one spatially isolated and contiguous surface region that is configured such that a rectangle with a smallest possible area that is fitted to the at least one surface region has an aspect ratio of at least 1.5:1.
19. The method as claimed in claim 18, wherein the intensity distribution of the illumination wave in the pupil plane arranged in the Fourier plane of the detection plane comprises plural spatially isolated and contiguous surface regions that are configured such that a rectangle with a smallest possible area that is fitted to a totality of the surface regions has an aspect ratio of at least 1.5:1.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The above and further advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments according to the invention with reference to the accompanying schematic drawings. In the figures:
(2) FIG. 1 shows an exemplary embodiment of a measurement apparatus according to the invention for determining a shape of an optical surface of a test object by interferometry,
(3) FIG. 2 shows an illustration of a distribution of a path length difference between test object path length and a reference path length in the measurement apparatus according to FIG. 1,
(4) FIG. 3 shows an exemplary embodiment of intensity distribution according to the invention in an illumination pupil plane of the measurement apparatus according to FIG. 1,
(5) FIG. 4 shows an intensity curve in a multi-fringe interferogram produced with the intensity distribution according to FIG. 3,
(6) FIG. 5 shows a further exemplary embodiment of an intensity distribution according to the invention in the illumination pupil plane of the measurement apparatus according to FIG. 1 and an intensity curve in a multi-fringe interferogram produced with this intensity distribution,
(7) FIG. 6 shows a further exemplary embodiment of an intensity distribution according to the invention in the illumination pupil plane of the measurement apparatus according to FIG. 1 and an intensity curve in a multi-fringe interferogram produced with this intensity distribution,
(8) FIG. 7 shows a further exemplary embodiment of intensity distribution according to the invention in the illumination pupil plane of the measurement apparatus according to FIG. 1,
(9) FIG. 8 shows a further exemplary embodiment of intensity distribution according to the invention in the illumination pupil plane of the measurement apparatus according to FIG. 1,
(10) FIG. 9 shows a further exemplary embodiment of intensity distribution according to the invention in the illumination pupil plane of the measurement apparatus according to FIG. 1,
(11) FIG. 10 shows a further exemplary embodiment of intensity distribution according to the invention in the illumination pupil plane of the measurement apparatus according to FIG. 1,
(12) FIG. 11 shows an exemplary embodiment of an illumination module for the measurement apparatus according to FIG. 1,
(13) FIG. 12 shows a further exemplary embodiment of an illumination module for the measurement apparatus according to FIG. 1, and
(14) FIG. 13 shows comparison examples for intensity distributions in the illumination pupil plane and intensity curves for multi-fringe interferograms produced therewith, where FIG. 13A shows a large central illumination disk and a low contrast, FIG. 13B shows a medium central illumination disk and a medium contrast, and FIG. 13C shows a small central illumination disk and a high contrast.
DETAILED DESCRIPTION
(15) In the exemplary embodiments embodiment variants described below, elements which are functionally or structurally similar to one another are provided with the same or similar reference signs as far as possible. Therefore, for understanding the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the invention.
(16) In order to facilitate the description, a Cartesian xyz-coordinate system is indicated in the drawing, from which system the respective positional relationship of the components illustrated in the figures is evident. In FIG. 1, the x-direction runs perpendicular and into the plane of the drawing, the z-direction toward the right, and the y-direction upwardly.
(17) FIG. 1 illustrates an interferometric measurement apparatus 10 in one embodiment according to the invention. The measurement apparatus 10 is suitable for determining a deviation of an actual shape from an intended shape of an optical surface 12 of a test object, from which the actual shape of the optical surface 12 arises. By way of example, the test object 14 can be present in the form of an optical lens or a mirror, in particular a projection lens of a microlithographic projection exposure apparatus. In the case of a mirror, this can relate to an optical element of an EUV projection exposure apparatus. The test object 14 is assembled using a holder that is not illustrated in the drawing.
(18) The measurement apparatus 10 comprises an illumination module 16, an interferometer 18 and an evaluation device 20. The illumination module 16 comprises a radiation source 22 for producing measurement radiation 24, for example in the form of a laser such as, for instance, a helium-neon laser for producing a laser beam. The measurement radiation 24 has sufficiently coherent light for carrying out an interferometric measurement. In the case of a helium-neon laser, the wavelength of the measurement radiation 24 is approximately 633 nm. However, the wavelength of the measurement radiation also may have different wavelengths in the visible and non-visible wavelength range of electromagnetic radiation.
(19) The measurement radiation 24 is focused onto a mechanical illumination stop 30 arranged in a pupil plane 28 of the illumination module 16 in such a way by way of a focusing optical unit 26 that a divergent, substantially spherical illumination wave 34 emanates from the illumination stop 30. In principle, the illumination stop 30 has an aperture region that is provided, in principle, for the passage of the measurement radiation 24, said aperture region defining the pupil 31 of the illumination module 16 and being circular in the shown case. Furthermore, a diffusion screen 32, which is rotated about an axis of rotation 33 during the measurement operation, is arranged in the region of the pupil plane, i.e., in the direct vicinity of the plane of the illumination stop 30. It serves to randomize the alternating phase between various points of the pupil 31.
(20) The interferometer 18 is designed as a Fizeau interferometer and comprises a beam splitter 40, an adaptation optical unit 42 in the form of a focusing optical unit or a collimator, a splitting element 46 and a detection module 54 in the form of a camera.
(21) The divergent beam of the illumination wave 34 initially passes through the beam splitter 40 and it is thereupon collimated by the adaptation optical unit 42 in such a way that the wavefront obtains a shape that is adapted to the intended shape of the optical surface 12 to be tested, i.e., substantially corresponds to or approximates the intended shape. Hence, the wavefront of the illumination wave 34 can have, for example, a plane or spherical shape after passing through the adaptation optical unit 42. The adaptation optical unit 42 can also contain diffractive optical elements in order to provide the wavefront of the illumination wave with, for example, an aspherical shape. The illumination wave 34 propagates along an optical axis 44 of the interferometer 18, said optical axis extending in the z-direction in FIG. 1.
(22) Thereupon, the illumination wave 34 is incident on the splitting element 46 in the form of a Fizeau element with a Fizeau surface 48. Some of the radiation of the illumination wave 34 is reflected at the Fizeau surface 48 as a reference wave 52. In FIG. 1, the reference wave 52 is illustrated on the basis of an exemplary beam of the reference wave. The radiation of the illumination wave 19 passing through the splitting element 46 is incident on the optical surface 12 of the test object 14 as a test wave 50. Preferably, this is implemented within the scope of autocollimation, and so the test wave 50 substantially runs back along itself post interaction with the optical surface 12. In the case illustrated in FIG. 1, in which the test object 14 is embodied as a mirror, the interaction with the optical surface 12 can be implemented by reflection on the optical surface 12. In the case of an embodiment of the test object as a lens element, the interaction can be implemented by way of a twofold passage therethrough and a back-reflection with an additional reflective element.
(23) The splitting element 46 has a tilted arrangement. The tilt is such that the Fizeau surface 48 is tilted by the tilt angle /2 with respect to the normal plane 44N in relation to the optical axis 44. In the present embodiment, the tilt denoted by the tilt angle /2 relates to a tilt about a tilt axis that is arranged at a 45 angle to both the x-axis and the y-axis; i.e., the tilt angle /2 shown in FIG. 1 also has an equally large y-component in addition to the x-component visible in the figure. Here, the x-component and y-component of an angle is understood to mean that angle component that relates to an angle rotation about the x-axis and the y-axis, respectively. The returning test wave 50 post interaction with the optical surface 12 passes through the splitting element 46 without experiencing a directional change in the process and it is consequently merged with the reference wave 52 in a superposed beam path, in which the reference wave 52 is tilted by the tilt angle in relation to the returning test wave 50 on account of the aforementioned tilt of the Fizeau surface 48. According to one embodiment, the following applies to the tilt angle :
>100.Math./D.
(24) Here, is the wavelength of the illumination wave 34 and D is the beam diameter of the reference wave 52 at the location of merging into the superposed beam path with the test wave 50, i.e., at the location of the splitting element 46. In the embodiment according to FIG. 1, D corresponds to the diameter of the adaptation optical unit 42. According to a numerical example, =633 nm, D=200 mm and hence >0.32 mrad.
(25) Together with the tilted reference wave 52, the test wave 50 returning post interaction with the optical surface 12 is steered by the beam splitter 40 into the detection module 54. The detection module 54 comprises an imaging stop 56 that is arranged in a pupil plane of the detection module 54, a camera lens 58 and a two-dimensional resolving detector 60. The returning test wave 50 interferes with the reference wave 52 on a capturing surface of the detector 60 that is arranged in a detection plane 62. On account of the tilt of the reference wave 52 in relation to the returning test wave 50 through the tilt angle , as caused by the oblique position of the splitting element 46, the intensity distribution I.sub.D (x,y) produced on the capturing surface of the detector 60 is a multi-fringe interference pattern 66.
(26) In this text, a multi-fringe interference pattern should be generally understood to mean an interference pattern that comprises at least one full period of alternating fringes of constructive and destructive interference. A full period should be understood to mean that the phase difference between the interfering waves adopts all values between 0 and 2 along the multi-fringe interference pattern. Expressed differently, a multi-fringe interference pattern should be understood to mean an interference pattern having at least two fringes, when the fringes can be bright fringes (constructive interference) or dark fringes (destructive interference). In the multi-fringe interference pattern 66 shown in FIG. 1, more than 30 bright and dark fringes are contained in each case. On the basis of the multi-fringe interference pattern 66, the evaluation device 20 establishes the deviation of the shape of the optical surface 12 of the test object from the wavefront of the test wave 34, known in advance, and hence the actual shape of the optical surface 12.
(27) The intensity distribution I.sub.P(u,v) in the pupil plane 28 of the illumination module 16 is configured by a corresponding design of the illumination stop 30. As is clear from the exemplary representation of I.sub.P(u,v) contained in FIG. 1, the latter has a spatially isolated, contiguous surface region 38 in the form of an arcuate fringe 38-1 with an intensity that exceeds a predetermined threshold. Thus, the illumination stop 30 only passes the measurement radiation 24 in the region of the arcuate fringe 38-1; by contrast, the measurement radiation is blocked by the illumination stop 30 in the remaining area of the pupil 31 that defines the maximum aperture. Like the plane of the imaging stop 56 of the detection module, too, the pupil plane 28 of the illumination module 16 is arranged in a Fourier plane of the detection plane 62. As emerges from the representation of the intensity distribution I.sub.D(u,v) within an aperture region 64 of the imaging stop 56 contained in FIG. 1, the intensity profile of the reference wave 52 is shifted obliquely downward in relation to the intensity profile of the returning test wave 50 in the pupil plane of the detection module 54. This is due to the tilt about the tilt angle , described above, of the reference wave 52 in relation to the returning test wave 50.
(28) The configuration of the intensity distribution I.sub.P(u,v) with the arcuate fringe facilitates the production of a high contrast in the multi-fringe interference pattern 66 while simultaneously suppressing disturbances traced back to defects on the optical surfaces of the interferometer 18, as described in more detail below.
(29) FIGS. 3, 5, 6, 7, 8, 9 and 10 show further specific embodiments according to the invention of the intensity distribution I.sub.P(u,v) in the pupil plane 28 of the illumination module 16, which may take the place of the intensity distribution illustrated in FIG. 1. The intensity distribution I.sub.P(u,v) according to FIG. 3 has a spatially isolated, contiguous surface region 38 in the form of an arcuate fringe 38-1, which extends along a level curve of a path length difference distribution OP(u,v) illustrated in FIG. 2. Here, the arcuate fringe 38-1 extends with both ends in each case to a corresponding portion of the ring-shaped edge 31R of the pupil 31.
(30) The path length difference distribution OP(u,v) illustrated in FIG. 2 shows the distribution of a path length difference in the pupil 31 for a predetermined field point 62p in the detection plane 62, said field point being defined by the difference between a test object path length and a reference path length. Here, the test object path length is the path length passed through by the radiation of the test wave 50 from a given point of the pupil 31 to the field point 62p in the detection plane 62 and the reference path length is the path length passed through by the radiation of the reference wave 52 from the aforementioned point of the pupil 31 to the field point 62p in the detection plane. Thus, the respective path length extends from the pupil plane 28 to the detection plane 62, wherein the portion in the region between the pupil plane 28 and the splitting element 46 is passed by the illumination wave 34, which respectively supplies radiation for the test wave 50 and the reference wave 52. In the path length difference distribution OP(u,v) illustrated in FIG. 1, level curves 67, i.e., lines with the same path length difference, are plotted for path length differences of integer values.
(31) The arcuate fringe 38-1 of the intensity distribution I.sub.P(u,v) of the illumination wave 34 in the pupil plane 28 illustrated in FIG. 3 extends along the level curve 67 with the path length difference 2 of the path length difference distribution OP(u,v) according to FIG. 2. Here, the arcuate fringe 38-1 is configured in such a way that any tangent to the fringe 38-1 subdivides the pupil 31 into two parts, the areas of which differ by no more than a factor of 3. In order to describe these circumstances, tangents t1 to t5 tangential to the outer edge of the fringe 38-1 in each case are plotted in exemplary fashion in FIG. 3. Here, the tangents t1 and t5 are arranged at the respective ends of the fringe 38-1 and the tangent t3 is arranged in the center thereof. The tangents t2 and t4 are arranged at the respective ends of a central portion 38-1m of the fringe 38 that comprises 20% of the fringe 38-1.
(32) As furthermore illustrated in FIG. 3, the tangent t1 divides the pupil 31 into an upper portion with the area A1 and a lower portion with the area A2, wherein the ratio A1/A2 is approximately 1:2.7. The same ratio emerges for t5. For the central tangent t3, the ratio is approximately 1:1.2. Hence, the ratio A1/A2 for any tangent on the fringe 38 lies in the range between approximately 1:1.2 and 1:2.7; i.e., the areas differ by a factor that lies between 1.2 and 2.7. The range is further restricted for the central portion 38-1m. According to a further embodiment, the arcuate fringe 38 is characterized in that there is at least one tangent, for which the areas A1 and A2 differ by no more than a factor of 20. According to a further embodiment, the arcuate fringe 38-1 is characterized in that the areas A1 and A2 differ by no more than a factor of 20 for each tangent on a portion comprising at least 20% of the fringe 38-1, for example the central portion 38-1m.
(33) According to one embodiment, the following applies to an extent L.sub.Bel of the arcuate fringe 38-1 in the pupil plane 28 in at least one direction:
(34)
(35) Here, is the wavelength of the illumination wave 34, f is the distance between the pupil plane 28 and the adaptation optical unit 42 and l is the distance between the splitting element 46 and the optical surface 12 of the test object. According to a numerical example, =633 nm, f=1020 mm, l=1000 mm, =22mrad and hence L.sub.Bel>0.2973 mm.
(36) Furthermore, FIG. 3 plots a rectangle 74 with the smallest possible area that is fitted to the arcuate fringe 38-1. Expressed differently, the rectangle 74 is the smallest possible rectangle in terms of area that completely comprises the arcuate fringe 38-1; i.e., it is the rectangle that is fitted to the fringe 38-1 to the best possible extent in terms of area. The rectangle 74 has a length that corresponds to the aforementioned extent L.sub.Bel of the arcuate fringe 38 and has a width d.sub.Bel. The aspect ratio of the rectangle 74 that is defined by the ratio L.sub.Bel/d.sub.Bel is approximately 3:1 in the illustrated case and hence larger than 1.5:1.
(37) FIG. 4 shows the intensity curve along the x-axis in a central region of the multi-fringe interferogram 66 for the intensity distribution I.sub.P(u,v) in the pupil plane 28 shown in FIG. 3. The contrast of this intensity curve is approximately 67%. Hence, this multi-fringe interferogram 66 has a signal-to-noise ratio which facilitates a highly accurate evaluation and hence a highly accurate determination of the form of the optical surface 12 of the test object 14. Furthermore, with approximately 12%, the illumination of the pupil 31 of the illumination module 16 is substantially higher than in the case of, for example, an intensity distribution I.sub.P(u,v) of a punctiform illumination in the pupil plane 28 as shown under (c) in FIG. 13. In this intensity distribution, which is illustrated as a comparison example, a contrast of approximately 60%, which is almost just as high, is obtained, even though the illumination is only approximately 1%.
(38) As already mentioned above, the illumination is a measure for how well disturbances that are traced back to defects on the optical surface of the interferometer 18 can be suppressed. Hence, the embodiment according to FIG. 3 facilitates a substantially improved defect suppression in relation to the comparison example shown in FIG. 13C. The defect suppression can be improved if the central illumination disk is enlarged. An illumination of approximately 8%, which is approximately as good as in the embodiment according to the invention as per FIG. 3, arises in the embodiment of the intensity distribution I.sub.P(u,v) illustrated in FIG. 13B as a further comparison example, in which the central illumination has a disk-shaped embodiment; however, the contrast in this case drops to value of approximately 11% and hence drops to far below the value that is obtainable with the embodiment according to the invention as per FIG. 3. If the central illumination disk from the intensity distribution illustrated in FIG. 13B is increased further, the contrast drops further, for example to a value of 4%, as in the further comparison example illustrated in FIG. 13A for the illumination of the entire pupil.
(39) FIG. 5 shows a further embodiment of the intensity distribution I.sub.P(u,v) according to the invention in the pupil plane 28 of the illumination module 16. This intensity distribution has a plurality of arcuate fringes 38-1, which extend along level curves of the path length difference distribution OP(u,v) illustrated in FIG. 2. Here, an arcuate fringe 38-1 is assigned to each level curve with an integer wavelength difference in this embodiment. According to further embodiment variants, the arcuate fringes 38-1 according to FIG. 5 each can have features that are listed above in respect of the arcuate fringe 38-1 described on the basis of FIG. 3. The illumination of the pupil 31 is approximately 60% in the intensity distribution according to FIG. 5, as a result of which the defect suppression is once again significantly improved in relation to the embodiment according to FIG. 3. Here, the contrast of the intensity curve in the central region of the multi-fringe interferogram 66 is only slightly reduced at approximately 62%.
(40) FIG. 6 shows a further embodiment of the intensity distribution I.sub.P(u,v) according to the invention in the pupil plane 28 of the illumination module 16. The latter differs from the intensity distribution according to FIG. 5 in that the arcuate fringes have a narrower embodiment such that an illumination of approximately 20% arises. As a result, the contrast of the intensity profile in the central region of the multi-fringe interferogram 66 can be increased significantly, to be precise to approximately 90%.
(41) FIG. 7 shows a further embodiment of the intensity distribution I.sub.P(u,v) according to the invention in the pupil plane of the illumination module 16. The intensity distribution I.sub.P(u,v) according to FIG. 7 has a spatially isolated, contiguous surface region 38 in the form of a zigzag-shaped fringe 38-2. The zigzag-shaped fringe 38-2 has two rising and two falling portions. Furthermore, FIG. 7 plots a rectangle 74 with the smallest possible area that is fitted to the zigzag-shaped fringe 38-2. Analogous to the rectangle 74 according to FIG. 3, the rectangle 74 has a length L.sub.Bel and a width d.sub.Bel. The aspect ratio of the rectangle 74 according to FIG. 7 that is defined by the ratio L.sub.Bel/d.sub.Bel is approximately 3:1 in the illustrated case and hence larger than 1.5:1. The zigzag-shaped fringe 38-2 is arranged in such a way that the longer axis of symmetry of the rectangle 74 assigned thereto extends substantially parallel to a mean direction of a level curve of a path length difference distribution OP(u,v) illustrated in FIG. 2.
(42) FIG. 8 shows a further embodiment of the intensity distribution I.sub.P(u,v) according to the invention in the pupil plane of the illumination module 16. The intensity distribution I.sub.P(u,v) according to FIG. 8 has a spatially isolated, contiguous surface region 38 in the form of a straight fringe 38-3, i.e., a fringe having the shape of a straight line. A rectangle with the smallest possible area that is fitted to the straight fringe 38-3 corresponds to the fringe 38-3 itself. The fringe 38-3 or the rectangle has a length L.sub.Bel and a width d.sub.Bel. The aspect ratio of the straight fringe 38-3 according to FIG. 8 that is defined by the ratio L.sub.Bel/d.sub.Bel is approximately 3.5:1 in the illustrated case and hence larger than 1.5:1. The straight fringe 38-3 is arranged in such a way that longitudinal extent thereof extends substantially parallel to a mean direction of a level curve of a path length difference distribution OP(u,v) illustrated in FIG. 2. In the shown embodiment, the longitudinal extent of the straight fringe 38-3 is oriented transversely to the pupil edge 31R. Here, the straight fringe 38-3 in the shown embodiment extends centrally within the pupil 31 over a length region of approximately 30-40% of the diameter of the pupil 31. In other embodiments, the straight fringe can also extend over smaller or larger regions of the pupil 31, in particular over the entire pupil 31, i.e., from a region of the pupil edge 31R to an opposite region of the pupil edge 31R.
(43) FIG. 9 shows a further embodiment of the intensity distribution I.sub.P(u,v) according to the invention in the pupil plane of the illumination module 16. The intensity distribution I.sub.P(u,v) according to FIG. 9 has a plurality of spatially isolated, contiguous surface regions 38. In the illustrated embodiment, six such surface regions are present, in each case in the form of a circular surface region 38-4.
(44) Furthermore, FIG. 9 plots a rectangle 74 with the smallest possible area that is fitted to the totality of the surface regions 38-4. Expressed differently, the rectangle 74 is the smallest rectangle in terms of area that completely comprises the totality of the surface regions 38-4. The rectangle 74 has a length that corresponds to the aforementioned extent L.sub.Bel of the arcuate fringe 38 and has a width d.sub.Bel. The aspect ratio of the rectangle 74 that is defined by the ratio L.sub.Bel/d.sub.Bel is approximately 3:1 in the illustrated case and hence larger than 1.5:1.
(45) Furthermore, FIG. 9 plots a covering surface in the form of an arcuate fringe 76 that is adapted in form to the totality of the surface regions 38-4. Expressed differently, the arcuate fringe 76 corresponds to an area which covers the surface regions and which is fitted to the shape of the arrangement of the surface regions 38-4. The shape of the arrangement of the surface region 38-4 can be an extrapolated or abstracted arrangement form, in particular. In the present case, the arcuate fringe 76 defined by the covering surface adapted to the form corresponds to the arcuate fringe 38-1 according to FIG. 3, which was explained above. The aforementioned rectangle 74 is also the rectangle with the smallest possible area that is fitted to the arcuate fringe 76, and therefore corresponds to the rectangle 74 according to FIG. 3.
(46) FIG. 10 shows a further embodiment of the intensity distribution I.sub.P(u,v) according to the invention in the pupil plane of the illumination module 16. The intensity distribution I.sub.P(u,v) according to FIG. 10 has a plurality of spatially isolated, contiguous surface regions 38. In the illustrated embodiment, two such surface regions are present, in each case in the form of a circular surface region 38-4. Consequently, the shown intensity distribution I.sub.P(u,v) is dipole-shaped.
(47) Furthermore, FIG. 10 plots a covering surface in the form of a straight fringe 78 that is adapted in form to the totality of the surface regions 38-4. Expressed differently, the straight fringe 78 corresponds to an area which covers the surface regions and is fitted to the shape of the arrangement of the surface regions 38-4. In the present case, the straight fringe 78 defined by the covering surface adapted to the form corresponds to the straight fringe 38-3 according to FIG. 8, which was explained above. Furthermore, the circumference of the fringe 78 forms a rectangle with the smallest possible area that is fitted to the totality of the surface regions 38-4. Like the straight fringe 38-3 according to FIG. 8, the latter has an aspect ratio which is defined by the ratio L.sub.Bel/d.sub.Bel and which is approximately 3.5:1 in the illustrated case.
(48) FIGS. 11 and 12 show further embodiments of an illumination module 16, which can be used instead of the illumination module 16 shown in FIG. 1. In the embodiment according to FIG. 11, the mechanical illumination stop 30 is arranged in a plane 36 of this conjugate to the pupil plane 28. The plane 36 is imaged on the rotatable diffusion screen 32, which is still arranged in the pupil plane 28, using a 4f imaging optical unit 70. In the embodiment according to FIG. 12, a spatial light modulator 68 is used instead of a mechanical stop for the purposes of producing the intensity distribution in the pupil 31. In the shown embodiment, the spatial light modulator 68 is operated in reflection and, to this end, it is irradiated at an oblique angle with the measurement radiation 24 by the radiation source 22. Thereupon, the variably reflected radiation passes through a 2f imaging optical unit, with which the surface of the light modulator 68 is imaged onto the rotatable diffusion screen 32 arranged in the pupil plane 28.
(49) The above description of exemplary embodiments is to be understood to be by way of example. The disclosure effected thereby firstly enables the person skilled in the art to understand the present invention and the advantages associated therewith, and secondly encompasses alterations and modifications of the described structures and methods that are also apparent to and understood by the person skilled in the art. Therefore, the applicant seeks to cover also all such alterations and modifications, insofar as they fall within the scope of the invention as defined by the accompanying claims and equivalents thereof.
LIST OF REFERENCE SIGNS
(50) 10 Measurement apparatus 12 Optical surface 14 Test object 16 Illumination module 18 Interferometer 20 Evaluation unit 22 Radiation source 24 Measurement radiation 26 Focusing optical unit 28 Pupil plane 30 Illumination stop 31 Pupil of the illumination module 31R Edge of the pupil 32 Diffusion screen 33 Axis of rotation 34 Illumination wave 36 Conjugate plane 38 Surface region 38-1 Arcuate fringe 38-1m Central portion 38-2 Zigzag-shaped fringe 38-3 Straight fringe 38-4 Circular surface region 40 Beam splitter 42 Adaptation optical unit 44 Optical axis 44N Normal plane to the optical axis 46 Splitting element 48 Fizeau surface 50 Test wave 52 Reference wave 54 Detection module 56 Imaging stop 58 Camera lens 60 Detector 62 Detection plane 62p Field point 64 Aperture region 66 Multi-fringe interference pattern 67 Level curve 68 Spatial light modulator 70 4f imaging optical unit 72 2f imaging optical unit 74 Rectangle with the smallest possible area 76 Arcuate fringe 78 Straight fringe
