Patent ID: 12235097

DETAILED DESCRIPTION

In the exemplary embodiments or embodiments or 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.

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. InFIG.1, the z-direction extends perpendicularly to the plane of the drawing out of said plane, the x-direction extends toward the right, and the y-direction extends upward.

FIG.1shows an embodiment of a diffractive optical element10in the form of a computer-generated hologram (CGH). The diffractive optical element10serves for measuring a shape of an optical surface102of a test object104with a test interferometer100, as explained in greater detail below with reference toFIG.6. For this purpose, the diffractive optical element10comprises diffractive shape measuring structures16in the form of CGH structures arranged on a used surface14of the diffractive optical element10. The used surface14extends over a large portion of a surface12of the diffractive optical element10. In the present embodiment, the diffractive optical element10is configured as a circular disk and the surface12corresponds to the top side of the circular disk. In the present embodiment, the used surface14is embodied in elliptical fashion, the semiminor axis being oriented in the x-direction, such that the left and right edge sections of the surface12do not belong to the used surface14.

In the present embodiment, the used surface14is completely covered by the diffractive shape measuring structures16, with the exception of areas provided for the test fields18. The test fields18, also referred to as markers, are arranged at a multiplicity of locations of the used surface14, wherein some of the test fields18have a regular arrangement. However, so-called forbidden regions22and preferred regions20are also defined on the used surface14. While no test fields14are arranged in the forbidden regions22, test fields14are arranged particularly preferably or in particularly high density in the preferred regions20. The exact structure of the test fields14is explained in greater detail below with reference toFIGS.4and5A-5F.

FIG.6illustrates an embodiment according to the invention of the abovementioned test interferometer100for measuring a shape of an optical surface102of a test object104. In the case illustrated, the optical surface102is the surface of a mirror. Alternatively, for instance, the surface of a lens element can also be examined. The test interferometer100is embodied as a high coherence interferometer in the form of a Fizeau interferometer. The test interferometer100comprises a test radiation source106for generating a test radiation108, e.g. in the visible wavelength range. The test radiation source106can comprise a laser, for example, such as a helium-neon laser, for instance. The test radiation108propagates along an optical axis110of the test interferometer100and firstly passes through a beam splitter112.

The test radiation108thereupon impinges on a focusing lens element in order to convert the test radiation108into a plane wave, which thereupon impinges on a reference element116in the form of a Fizeau element having a Fizeau surface118. Part of the test radiation108is reflected as a reference wave120at the Fizeau surface118. In the present example, that portion of the test radiation108which passes through the Fizeau surface118has a plane wavefront and is referred to hereinafter as incoming test radiation108i. The incoming test radiation108ithereupon passes the diffractive optical element10. In this case, the wavefront of the test radiation108iis adapted to a target shape of the surface102of the test object104by the diffractive shape measuring structures16arranged on said diffractive optical element. The wave that arises in this case is referred to here as a test wave122.

The test wave122having the adapted wavefront is thereupon reflected at the surface102to be measured. The reflected test wave122rreturns in the beam path of the incoming test radiation108iin the opposite direction, in the process passes through the diffractive optical element10and the reference element116and is thereupon directed by the beam splitter112together with the reference wave120via a stop124and an eyepiece126onto the surface of a detector camera128. In this text, the arrangement comprising the reference element116, the focusing lens element114, the beam splitter112, the stop124and the eyepiece126is also referred to as an interferometry module127.

On the detector camera128, an intensity distribution in the form of an interferogram arises as a result of these two radiation portions being superimposed. In the embodiment according to the invention, the reference element116is connected to a displacement unit, e.g. in the form of a piezeoelement. The displacement unit allows the reference element116to be displaced in the direction of the optical axis110by fractions of the wavelength of the test radiation108. The phase of the reference wave120can be varied by such displacement. This has the consequence that the intensity distributions generated on the detector camera128are varied. The intensity distributions that arise for different positions of the reference element116are recorded by the detector camera128and evaluated in an evaluation unit130.

In this case, predetermined calibration values86of the diffractive optical element10are taken into account by the evaluation unit130. The calibration values86concern profile properties36of the diffractive shape measuring structures16, which characterize a profile line26of the shape measuring structures16extending transversely with respect to the used surface14. The procedure for determining the calibration values130is explained in detail below. The result of the evaluation of the recorded intensity distributions is the deviation of the actual shape of the optical surface102from the target shape thereof, from which the actual shape of the optical surface102is then derived.

FIG.2shows a detail of an embodiment of a structure pattern24of the diffractive shape measuring structures16that extends along the used surface14. In other words, the structure pattern24is understood to mean the pattern discernible in a plan view of the diffractive shape measuring structures16. In the embodiment illustrated inFIG.2, the structure pattern24of the diffractive shape measuring structures16is substantially formed by a pattern of horizontal lines, the respective shape of which is distorted in an irregular manner.

In accordance with one embodiment, the structure pattern24of the diffractive shape measuring structures16can be configured as a singly encoded CGH pattern for generating the test wave122from the incoming test radiation108iin the test interferometer100configured as a Fizeau interferometer in accordance withFIG.6. In accordance with further embodiments, the structure pattern24can also be configured as a multiply encoded CGH pattern. In the case of such a multiply encoded CGH pattern, the structure pattern24contains a superimposition of a plurality of CGH patterns, such that the incoming test radiation108iis converted into a plurality of outgoing waves simultaneously in the first order of diffraction in each case. The outgoing waves here can comprise, in addition to the test wave122, calibration waves and optionally, given the use of a type of interferometer with a reference mirror disposed downstream of the diffractive optical element10, a reference wave as well. In accordance with one embodiment variant, the structure pattern24is configured as a quintuply encoded CHG structure pattern for generating the test wave122, a reference wave and three calibration waves.

FIG.3shows a cross section through the diffractive shape measuring structures16along the line III-III′ inFIG.2and thus a profile line26of the relevant portion of the shape measuring structures16, said profile line extending transversely with respect to the used surface14. The profile line26substantially shows the profile of a trench28extending along the x-direction. The trench28has sidewalls, also called flank regions30, and a bottom region32. The width d of the trench28at the level of the surface12is designated by the reference sign37. The level difference between the surface12of the diffractive optical element10and the bottom region32is referred to as profile depth dp, (reference sign36-1) or etching depth for the illustrated case where the trench28was produced by etching technology in the material19of the diffractive optical element10. The material19can be a quartz glass, for example.

On account of the etching process, the flank regions30of the trench28do not extend completely perpendicularly, but rather are inclined in each case by a flank angle relative to the perpendicular to the surface12; in this case, the flank angle of the left flank region30is designated by α1(reference sign36-2) and the flank angle of the right flank region30is designated by α2(reference sign36-3). Furthermore, the etching process used has the undesired side effect that microtrenches34form in each case at the transition between the flank regions30and the bottom region32.

The depth dMof said microtrenches is designated by the reference signs36-4inFIG.3. The profile depth36-1of the trench28, the flank angles36-2and36-3, the depth36-4of the microtrenches34and optionally further parameters are referred to as profile properties36of the profile line26of the shape measuring structures16. If a target profile having perpendicular flank regions30, a flat bottom region without microtrenches34and a predefined profile depth is assumed for the trench28, then the profile properties36are parameters that can be used to characterize manufacturing deviations of the real profile line26of the shape measuring structures16from the target profile thereof. However, said manufacturing deviations result in unwanted aberrations in the wavefront of the test wave122in the test interferometer100in accordance withFIG.6.

A lack of correction of these aberrations in the evaluation of the intensity distributions recorded by the detector camera128in turn results in defects in the shape of the optical surface102that is determined with the test interferometer100. In order to enable a correction of the manufacturing deviations in the shape measuring structures16, however, the test fields18mentioned above with reference toFIG.1are provided on the used surface14of the diffractive optical element10. In this case, the test fields18, on the basis of test structures38contained therein, make it possible to estimate the profile properties36of the shape measuring structures16as precisely as possible and to provide them to the evaluation unit130of the test interferometer100for defect correction in the determination of the shape of the optical surface102.

FIG.4illustrates an embodiment of one of the test fields18in plan view. This test field is configured as a matrix composed of test field sections40arranged in five rows and five columns. Further embodiments of the test fields18can also comprise matrices composed of more or fewer rows and columns. In the embodiment shown, the test field sections40have a rectangular, in particular square, shape and are designated according to the scheme40-CR, where “C” stands for the relevant column and “R” stands for the relevant row in the matrix. In this regard, for instance, the test field section designated by “H1” inFIG.4is designated by the reference sign40-21. In accordance with one embodiment, the side lengths of the test fields18lie in the range of 0.1 mm to 3 mm, in particular in the range of 0.5 mm to 1.5 mm.

FIG.5Ashows the test field section40-21identified by “H1” inFIG.4in plan view. Said test field section, like the test field sections identified by “H3”, “H4”, “H5”, “H6”, “H7” and “H8”, too, is configured as a so-called horizontal line test field section and to that end comprises test structures38, the structure pattern39of which comprises parallel straight lines42arranged periodically horizontally, i.e. in the x-direction, in plan view. The lines42are separated by respective interspaces44. The lines42each have an upper edge46-1and a lower edge46-2. The structure pattern39of the horizontal line test field section40-21with the upper edges46-1and the lower edges46-2comprises in each case periodically repeating and identically oriented edges.

The periodicity of the edges46-1or respectively46-2is identified by the period p inFIG.5Aand, in accordance with one embodiment, lies below the resolution of a diffraction measuring station60operated with visible light, said diffraction measuring station being explained in greater detail below with reference toFIG.7. In accordance with one embodiment, the resolution of such a diffraction measuring station lies below 300 μm, in particular below 100 μm, below 50 μm or below 10 μm.

In accordance with various embodiment variants, the periodicity of the edges46-1or respectively46-2can lie for instance between 100 nm and 1 μm, in particular between 300 nm and 800 nm, e.g. can be approximately 500 nm. In this case, the ratio of the respective width of the lines42and the respective width of the interspaces44, the so-called width/gap ratio, can vary between 1:1, as illustrated inFIG.5A, and 1:10. The test field sections identified by “H2”, “H3”, “H4”, “H5”, “H6”, “H7” and “H8” inFIG.4comprise periodically arranged parallel straight lines42of the type illustrated inFIG.5Awith different width/gap ratios and/or different periodicities p; in particular, they can comprise lines42having different widths with the same periodicity.

If the test structures38in accordance withFIG.5Aare viewed along the line in cross section, then a profile line58of the relevant portion of the test structures38arises which corresponds structurally to the profile line58of the relevant portion of the shape measuring structures16as illustrated inFIG.3. That is to say that the profile of the sequence of interspace44, line42and further interspace44along the line in accordance withFIG.5Ais likewise the profile of a trench28extending along the x-direction. Said trench28also has flank regions30, a bottom region32and microtrenches34. The corresponding profile properties36, in particular the profile depth36-1of the trench28, the flank angles36-2and36-3and the depth36-4of the microtrenches34, can thus also be determined for the test structures38and be used as an approximate estimation of the corresponding profile properties36of the shape measuring structures16arranged in the environment of the relevant test field18.

FIG.5Bshows the test field section40-11identified by “V1” inFIG.4in plan view. Said test field section, like the test field sections identified by “V2”, “V3”, “V4”, “V5”, “V6” and “V7”, too, is configured as a so-called vertical line test field section. The vertical line test field sections comprise test structures38, the respective structure pattern39of which comprises parallel straight lines42arranged periodically vertically, i.e. in the y-direction, in plan view. The structure pattern39in accordance withFIG.5Barises here as a result of rotation of the structure pattern39in accordance withFIG.5Aby 90°. For the right edges46-3and left edges46-4of the lines42, the explanations given above with regard to the edges46-1and46-2of the horizontal line test field section40-21in accordance withFIG.5Ahold true analogously in regard to their periodicity. If the test structures38in accordance withFIG.5Bare viewed along the line III-III′ extending in the x-direction in cross section, then the profile line58illustrated in the y-z cross-sectional plane inFIG.3arises analogously for the x-z cross-sectional plane. The test field sections identified by “V2”, “V3”, “V4”, “V5”, “V6” and “V7” inFIG.4comprise periodically arranged parallel straight lines42of the type illustrated inFIG.5Bwith different width/gap ratios and different periodicities p.

FIG.5Cshows the test field section40-22identified by “B1” inFIG.4in plan view. Said test field section, like the test field sections identified by “B2”, “B3” and “B4”, too, is configured as a so-called brick pattern test field section. To that end, the test field section40-22comprises rows—arranged in the horizontal direction—of periodically arranged two-dimensional structures in the form of rectangular structures50or brick-shaped structures. Said rows are repeated in the y-direction, interrupted by linear interspaces44, with successive rows being offset in each case in the x-direction, thus resulting in a brick pattern. In an alternative characterization, the structure pattern in accordance withFIG.5Calso corresponds to the line pattern in accordance withFIG.5Awith the difference that regular interruptions48are provided in the lines42. The test field sections identified by “B2”, “B3” and “B4” inFIG.4each comprise brick patterns of the type shown inFIG.5A, but differ therefrom in particular in the orientation of the lines42, the periodicity of the lines42, the line/gap ratio, the periodicity of the interruptions48and/or the offset pattern of the rows of bricks.

FIGS.5D and5Eshow the test field sections40-33and40-43identified by “F1” and “F2”, respectively, inFIG.4in plan view. These test field sections and the test field section identified by “F3” are configured as so-called F-pattern test field sections and to that end comprise periodically arranged 2-dimensional structures in the form of the letter “F”. The structure patterns39in the various F-pattern test field sections can differ in terms of periodicity, spacing and size of the letter “F”, as is the case between the structure patterns39inFIGS.5D and5E, and/or the orientation of the letter “F”.

The structure patterns39of the test structures38in the individual test field sections40of the test fields18described above are chosen in a targeted manner such that some or all of the abovementioned profile properties36can be measured with a particularly high measurement accuracy with a measuring device provided therefor. In any case the structure patterns39of the test structures38are configured such that some or all of the profile properties36are measurable, with the measuring device provided therefor, with a measurement accuracy which is increased by comparison with a measurement accuracy achievable during a measurement of the corresponding profile properties36of the shape measuring structures16. In particular, the abovementioned diffraction measuring station60and a scanning probe microscope84explained in greater detail below with reference toFIG.8are conceivable as measuring device for measuring the profile properties36.

The brick pattern test field sections (cf.FIG.5C) identified by “B1” to “B4” inFIG.4and the F-pattern test field sections (cf.FIGS.5D and5E) identified by “F1” to “F3”, in addition to the above-described suitability for measuring the profile properties characterizing the profile line58extending transversely with respect to the used surface14, are also suitable for measuring one or more contour properties of the test structures38. The measured contour properties of the test structures38can be applied to the relevant shape measuring structures16. This is done analogously to how the profile properties36are applied as described below.

A contour property should be understood to mean a shape property of a structure pattern of the test structures38extending along the used surface14, as explained by way of example below with reference toFIGS.14A to14C. These figures each show different actual shapes50aof a rectangular structure50in accordance withFIG.5C, which deviate from a target shape50sof the rectangular structure50in different ways. These deviations are classified as contour properties. InFIG.14A, the actual shape50adiffers from the target shape50sby virtue of an isotropic displacement of the rectangle edges and a rounding of the corners. InFIG.14B, the edge offset is dependent on its immediate surroundings. Furthermore, the corner rounding is more greatly pronounced.FIG.14Cshows a particularly pronounced example of an anisotropic edge offset. The upper and lower edges are further out of position than the left and right edges. The corner roundings and edge displacements that can be seen inFIGS.14A to14Care typically attributable to diffusion and so-called proximity effects of the lithographic portion during the production of the diffractive optical element10.

An embodiment of the structure pattern39in accordance withFIG.5Aor respectivelyFIG.5Bwith lines and interspaces having a periodicity of approximately 500 nm and a width/gap ratio of 1:1 is particularly suitable for examining the flank shape of the test structures38with the diffraction measuring station60. Intensity values ascertained with the diffraction measuring station60for this embodiment of the structure pattern39show a high correlation with the flank shape of the test structures38. Thus, particularly the flank angles36-2and36-3of the profile line58can be derived from the relevant intensity values with high accuracy.

The use of 2-dimensional structures of higher complexity, such as, for instance, in the brick pattern test field sections in accordance withFIG.5Cor the F-pattern test field sections in accordance withFIG.5DorFIG.5E, serves in particular for providing geometric shapes in the structure patterns39of the test structures38, which, in addition to the straight line pattern, is approximated to other geometric shapes contained in the structure pattern24of the shape measuring structures16. In this regard, for instance, the rectangular structures50from the brick pattern test field section in accordance withFIG.5Cand the F-structures52in accordance withFIGS.5D and5Fare suitable for simulating island-shaped structures or transitions between structure elements oriented perpendicularly to one another in the structure pattern24of the shape measuring structures.

FIG.5Fshows the diffractive optical element10in the region of the test field section40-34in accordance withFIG.4, which is configured as an unstructured test field section. The diffractive shape measuring structures16are arranged on a so-called used side55of the disk-shaped diffractive optical element10. In the present embodiment, an antireflection coating56is applied on the rear side57of the diffractive optical element10, said rear side being opposite to the used side55. Said coating is adapted to the wavelength of the test radiation108iincident on the diffractive optical element10in the test interferometer100, i.e. the antireflection coating56is configured such that when the test radiation108enters the diffractive optical element10at the rear side thereof virtually no intensity is lost.

However, if test radiation64having a different wavelength is incident on the diffractive optical element10during the measurement of the test fields18with the diffraction measuring station60explained in greater detail below with reference toFIG.7, then the effect of the antireflection coating56changes. Part of the test radiation64is reflected at the rear side57of the diffractive optical element10(reflected test radiation64r). The intensity of the transmitted test radiation64tis accordingly reduced.

In this case, the antireflection coating56, depending on the wavelength of the test radiation64, can furthermore have a reflection-reducing effect (destructive interference) or even a reflection-intensifying effect (constructive interference) by comparison with the reflection at the rear side57without an antireflection coating56. The unstructured test field section40-34then serves for determining the influence of the antireflection coating56on the intensity of the transmitted test radiation64tin the diffraction measuring station60.

To that end, in the diffraction measuring station60, a measurement of the unstructured test field section40-34and a corresponding measurement without arrangement of the diffractive optical element10are carried out for the different wavelengths of the test radiation64. From these, with Fresnel's formulae being applied, the corresponding effect of the antireflection coating56on the measurements of other test field sections40of the test fields18is ascertained and correspondingly taken into account in the evaluation of these measurements.

FIG.13Ashows an example of the transmission behavior of the test radiation64having a specific wavelength used by the diffraction measuring station60at test fields18of the diffractive optical element10(cf.FIG.1) which are arranged along a line extending in the x-direction. In this case, the respective transmission value T was determined on the basis of the respective test field section40-34of the relevant test fields18. As evident fromFIG.13A, the transmission at the given wavelength decreases toward the edges of the diffractive optical element10. In order to illustrate the correction principle,FIG.13Bshows a distribution of flank angles α (36-2or36-3in accordance withFIG.3) before and after correction on the basis of the transmission behavior fromFIG.13A, said distribution being ascertained by evaluation of the measurements carried out with the diffraction measuring station60with the stated wavelength.

Furthermore, the test field18illustrated inFIG.4comprises a monitoring test field section40-32identified by the letter “K”. Said monitoring test field section contains so-called monitoring structures corresponding to the shape measuring structures16arranged in the used surface14. In other words, a segment of the pattern of the used surface14with the shape measuring structures16is arranged in the monitoring test field section40-32. The monitoring test field section40-32, which may also be referred to as a background window, is used in the diffraction measuring station60or the scanning probe microscope84for a monitoring measurement to establish whether the region in which the relevant test field18is arranged on the diffractive optical element10is representative of the regions of the used surface14that adjoin the relevant test field18, such that the profile properties36ascertained by measurement of the test field18at the test structures38can be applied to the relevant shape measuring structures16.

Furthermore, the test field18illustrated inFIG.4comprises a so-called reference test field section40-14. This test field section comprises reference structures54resolvable with an optical microscope. These reference structures54can comprise markings that can be used to check a correct alignment of the test fields both with regard to rotation and with regard to translation. Furthermore, the reference structures54can each comprise an identification number for the unambiguous assignment of the measurements established with regard to a specific test field18to design properties of the test structures38on which the measurements are based. The information from the reference test field section40-14is used in particular during the measurement of the diffractive optical element10with the scanning probe microscope84described in greater detail below.

FIG.7shows an embodiment of the abovementioned diffraction measuring station60for measuring the profile properties36of the test structures38in the test fields18of the diffractive optical element10. The diffraction measuring station60comprises a tunable test radiation source62for generating the test radiation64already mentioned above, said test radiation being monochromatic with a wavelength that is adjustable in the wavelength range between approximately 300 nm and 800 nm. Furthermore, the diffraction measuring station60comprises a first focusing lens element66for focusing the test radiation64provided by the test radiation source62, a polarizer68arranged near the focal point of the first focusing lens element66and serving for polarizing the test radiation64, and a second focusing lens element70for radiating the test radiation64in the form of a plane test wave72onto the whole area of the diffractive optical element10to be measured with regard to the profile properties36.

After passing through the diffractive optical element10, the test wave72is directed via two Fourier optical units74and80(the second Fourier optical unit80being symbolized by two lens elements inFIG.7) onto an areally measuring detector82, which can be embodied e.g. as a CCD sensor. An aperture stop76arranged between the two Fourier optical units74and80serves for eliminating radiation of a higher order of diffraction than the zero order of diffraction. In the present embodiment, the detector82serves to yield spatially resolved information about the brightness or intensity distribution provided by the diffractive optical element10in the zero order of diffraction.

The resolution of the diffraction measuring station60in the embodiment in accordance withFIG.7, in which only an intensity distribution generated by the zero order of diffraction is evaluated, lies between 10 μm and 300 μm and thus far above the periodicities contained in the structure patterns39in accordance withFIGS.5A to5E. Therefore, these structure patterns are not resolved during the measurement with the diffraction measuring station60. Rather, grayscale or intensity values arise for the individual test field sections40.

During the measurement of the profile properties36of the test structures38with the diffraction measuring station60in accordance withFIG.7, the following procedure is adopted: Through corresponding manipulation of the tunable test radiation source62and arrangement of different variants of the polarizer68, test waves72with various combinations of different wavelengths and different polarization settings are successively radiated onto the diffractive optical element10. A suitable analyzer68is arranged depending on the polarization property of the polarizer68.

In accordance with one embodiment, 7 to 12 different wavelengths are combined with 2 to 4 different polarization settings. The wavelengths used preferably lie between 300 nm and 800 nm; in this regard, for example, the wavelengths 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm and 800 nm can be used. For example, the polarization directions 0°, 45°, 90° and 135° can be used as polarization settings. In the example mentioned, test waves72with 11×4, i.e. 44, different combinations of wavelength and polarization settings are radiated onto the diffractive optical element.

For each of the different combinations of wavelengths and polarization settings, the detector82of the diffraction measuring station60records an associated intensity distribution containing the intensity values assigned to the individual test field sections40on the surface12of the diffractive optical element10.

In an evaluation unit83, all the intensity values of all recorded intensity distributions that are assigned to the individual test field sections40are processed taking into account the design information of the various structure patterns24in accordance withFIGS.5A to5Ein the individual test field sections40using an evaluation algorithm. This processing results in profile properties36of the profile lines58of the test structures38, in particular the respective profile depth36-1, the respective flank angles36-2and36-3and the respective depth36-4of microtrenches34in the profile lines58of the test structures38contained in the various test field sections40. Further profile properties36of the profile lines58which can be ascertained as the result of the evaluation of the intensity distributions comprise so-called “notching” or so-called “trenching”, for example, in which lower corners in the profile line58are indented or rounded out. Furthermore, material properties of the diffractive optical element10, such as, for instance, variations in the refractive index, surface roughness or the above-explained contour properties of the test structures38, can also be ascertained using the evaluation. In particular, structure widths of the test structures can also be determined.

In accordance with one embodiment, all vertical line test field sections40-11,40-12,40-13,40-15,40-25,40-35,40-45, all horizontal line test field sections40-21,40-31,40-41,40-51,40-52,40-53,40-54,40-55, all brick pattern test field sections40-22,40-42,40-23,40-24, all F-pattern test field sections40-33,40-43,40-44, and the monitoring test field section40-32and the unstructured test field section40-34are processed in this case.

From the profile properties36ascertained, using an evaluation algorithm based on rigorous calculations, the evaluation unit83ascertains the calibration values86already mentioned above with regard to the shape measuring structures16contained on the used surface14of the diffractive optical element10. In this case, in accordance with one embodiment variant, in the calibration values86the profile properties36which were ascertained with regard to the individual test fields18distributed over the used surface14are respectively assigned to shape measuring structures16arranged in regions of the used surface14that adjoin the respective test field18. Alternatively, the local assignment of the profile properties assigned to the shape measuring structures16can also be ascertained by interpolation of the profile properties ascertained at the individual test fields18.

In accordance with one embodiment, when ascertaining the calibration values86of the shape measuring structures16, the evaluation unit83uses a relationship—ascertained on the basis of the test fields18—between the depth dpand the width b of a trench28(cf.FIG.3). While this relationship can be based on an arbitrary function, in principle, the trench depth dpis often all the greater, the greater the width b of the trench28, as illustrated inFIG.11. This effect occurs, inter alia, during etching processes on the length scale of a few 100 nm.

Since, as mentioned above, the resolution of the diffraction measuring station60is above 10 μm, a direct detection of the effect is not possible. As mentioned above, the horizontal line test field sections40-21,40-31,40-41,40-51,40-52,40-53,40-54,40-55and the vertical line test field sections40-11,40-12,40-13,40-15,40-25,40-35,40-45comprise regularly arranged lines42having different widths. The evaluation of the intensity values recorded by the diffraction measuring station60for the individual test field sections makes it possible, then, with assignment of the profile widths b known from the design, to ascertain the relationship between the depth dpand the width b of the trenches28.

A similar procedure can be adopted in the case of the two-dimensional structures in the brick pattern test field sections40-22,40-42,40-23,40-24and the F-pattern test field sections40-33,40-43,40-44, the “trench widths” then changing there in both dimensions. This relationship is assumed to be representative of generic structures and is taken into account when ascertaining the calibration values86of the shape measuring structures16in the evaluation unit83. Analogously to the variation of the profile depth dPas a function of the lateral structure definition, variations of the structure widths such as, for instance, the trench widths b and also other profile parameters such as, for instance, depth dPof microtrenches34(cf.FIG.3) can also occur. These variations are detected and dealt with analogously in accordance with one embodiment. For two-dimensional structures it is possible, in particular, to detect relationships of the contour variation as a function of the surrounding trench widths—proximity effects of the lithography process of CGH production. Typical examples are the rounding of inner and outer corners (F-structures) and non-isotropic shape defects of the rectangles.

FIG.8describes the scanning probe microscope84(also designated as SPM), already mentioned above as an alternative measuring device for measuring the profile properties36of the test structures38. In the example illustrated, the scanning probe microscope84is a scanning force microscope or an atomic force microscope (also designated as AFM). A measuring head89is incorporated in the scanning probe microscope84.

The measuring head89is secured to a frame (not illustrated inFIG.8) of the scanning probe microscope84with a holding unit87. The frame allows the positioning of the diffractive optical element10at any arbitrary location, in any orientation. The holding unit87can be rotated about its longitudinal axis extending in the horizontal direction. A piezo-actuator88is attached to the holding unit87of the measuring head89and enables a movement of the free end of the piezo-actuator88in three spatial directions (not illustrated inFIG.8). A bending beam, which hereinafter is referred to as cantilever90, as is customary in the technical field, is secured to the free end of the piezo-actuator88.

The cantilever90has a holding plate (not illustrated inFIG.8) for securing to the piezo-actuator88. The opposite end of the cantilever90with respect to the holding plate carries a measuring probe92. In the embodiment illustrated, the measuring probe92is embodied such that it is pyramidal or conical at its free end. Furthermore, the measuring probe92can for example also be embodied such that it is cylindrical or in the shape of an inverted cone or hammer-like (also referred to as “re-entrant”).

The cantilever90and the measuring probe92can be embodied in one piece. By way of example, the cantilever90and the measuring probe92can be manufactured from a metal, such as, for instance, tungsten, cobalt, iridium, a metal alloy or from a semiconductor, such as, for instance, silicon or silicon nitride. It is also possible to manufacture the cantilever90and the measuring probe92as two separate components and to subsequently connect these to one another. This can be effected by adhesive bonding, for example. In particular, the measuring probe92can also be produced in two separate steps.

The diffractive optical element10to be measured is fixed on a sample stage94. This can be effected for example by the diffractive optical element10being placed on bearing points of the sample stage94in a vacuum or high vacuum environment.

As symbolized by arrows inFIG.8, the sample stage94can be moved by a positioning system96in three spatial directions relative to the measuring head96of the scanning probe microscope84. Furthermore, the sample stage94can be rotated about the normal to the diffractive optical element10(not shown inFIG.8). In the example inFIG.8, the positioning system96is embodied in the form of a plurality of micromanipulators. Furthermore or alternatively, the positioning system96can be equipped with stepper motors and/or linear drives for moving the diffractive optical element10. An alternative embodiment of the positioning system96might be piezo-actuators. The positioning system96is controlled by signals of a control unit. In an alternative embodiment, the control unit does not move the sample stage94, but rather the holding unit of the measuring head89of the scanning probe microscope84. It is furthermore possible for the control unit to carry out a coarse positioning of the diffractive optical element10serving as sample in terms of height (z-direction) and for the piezo-actuator88of the measuring head89to perform a precise height setting of the scanning probe microscope84.

Alternatively or additionally, in a further embodiment, the relative movement between the sample and the measuring probe92can be divided between the positioning system96and the piezo-actuator88. By way of example, the positioning system96carries out the movement of the sample in the sample plane (xy-plane) and the piezo-actuator88enables the movement of the measuring probe92in the direction of the normal to the sample.

The scanning probe microscope92can be operated in a one-dimensional or a two-dimensional measuring mode. In the one-dimensional measuring mode, the measuring probe92scans the sample in a line-like manner in a predefined measuring direction, a high spatial resolution being achieved in the scanning direction and a comparatively low spatial resolution (typically a spatial resolution lower by a factor of 100) being achieved transversely with respect to the scanning direction on account of the line spacing chosen. In the two-dimensional measuring mode, the line spacing is reduced such that a high spatial resolution is likewise achieved transversely with respect to the scanning direction, for instance a spatial resolution lower than that in the scanning direction only by a factor of 10. However, the two-dimensional measuring mode is significantly more time-consuming that the one-dimensional measuring mode and is therefore avoided if possible.

In the one-dimensional measuring mode, scanning is preferably effected perpendicularly to the plane defined from the axes of symmetry of the cantilever90and of the measuring probe92, measurement artefacts being reduced as a result. In the two-dimensional measuring mode, the measuring probe is preferably moved fast perpendicularly to that axis of symmetry, while the slower movement is effected perpendicularly thereto.

FIG.9Ashows a further detail—differing from the detail illustrated inFIG.2—of an embodiment of a structure pattern24of the diffractive shape measuring structures16that extends along the used surface14. In this case, the trench-shaped structures in cross section have very irregular shapes with flank regions having different orientations. If the structure pattern shown is then measured with the above-described scanning probe microscope84in the one-dimensional measuring mode with the horizontal scanning direction (in the x-direction) along the scanning line B-B′, the profile line26illustrated inFIG.9Bresults.

However, the exact structure of the profile line26is extremely dependent on the exact position of the scanning line in the y-direction. Furthermore, the flank angles of the measured profile line26are corrupted by the pyramid-like or cone-like shape of the measuring probe92. However, the extent of corruption is dependent on the orientation of the flank regions30in the xy-plane, that is to say that very accurate knowledge of the flank orientation along the scanning line is necessary in order to precisely work out the influence of the shape of the measuring probe92. On account of these measurement uncertainties, the measurement results ascertained during measurement of the structure patterns24of real shape measuring structures16in the one-dimensional measuring mode of the scanning probe microscope84are usually too inaccurate for the purpose of correcting the surface measurement in the test interferometer100. By contrast, the measurement in the two-dimensional measuring mode is often too complex.

In accordance with one embodiment according to the invention, then, structure patterns39of the test structures38arranged in the test fields18are measured instead of the shape measuring structures16. Referring toFIG.4, all vertical line test field sections40-11,40-12,40-13,40-15,40-25,40-35,40-45, all horizontal line test field sections40-21,40-31,40-41,40-51,40-52,40-53,40-54,40-55and all brick pattern test field sections40-22,40-42,40-23,40-24are appropriate here.FIG.10Bshows along the scanning line B-B′ the profile line58of the structure pattern39in the form of vertical straight lines illustrated inFIG.10A. Since, in this case, the orientation of the flank regions30varies independently of the position of the scanning line in the y-direction and furthermore also does not vary along the scanning line, the influence of the shape of the measuring probe92can be worked out very accurately from the measured profile line58.

The relationship between the depth dpand the width b of a trench28as described above with reference toFIG.11can be ascertained with particularly high accuracy by corresponding measurement of the relevant profiles in the horizontal line test field sections40-21,40-31,40-41,40-51,40-52,40-53,40-54,40-55and the vertical line test field sections40-11,40-12,40-13,40-15,40-25,40-35,40-45with the scanning probe microscope84. The same applies to the two-dimensional structures in the brick pattern test field sections40-22,40-42,40-23,40-24and the F-pattern test field sections40-33,40-43,40-44. During the measurement, the reference test field section40-14is preferably used for the correct alignment of the corresponding test fields18.

FIG.12Ashows a detail from a further embodiment of a test field section40having substantially the inverse structure of the brick pattern test field section40-22illustrated inFIG.5Cand rotated by 90°. In contrast to the structure pattern in accordance withFIG.5C, in which the rectangular structures50are formed by trenches, the rectangular structures50in the structure pattern in accordance withFIG.12Aare surrounded by trenches28.

FIG.12Bshows the profile line58measured along the line B-B′ inFIG.12Awith the scanning probe microscope84. Said profile line clearly reveals that the maximum trench depth is in the crossing regions between vertical and horizontal trench sections, while the trench depth has a minimum98at half the distance between the crossing regions on account of the small trench width prevailing there. The trench depth signature shown can be measured very precisely using the scanning probe microscope84and it is possible to use the corresponding relationship with the design of the structure pattern when ascertaining the calibration values86of the shape measuring structures16.

From the vertical line test field sections40-11,40-12,40-13,40-15,40-25,40-35,40-45, horizontal line test field sections40-21,40-31,40-41,40-51,40-52,40-53,40-54,40-55, brick pattern test field sections40-22,40-42,40-23,40-24and F-pattern test field sections40-33,40-43,40-44measured with the scanning probe microscope84, taking account of the monitoring test field section40-32and optionally taking account of the above-described relationships between design dimensions of the corresponding structure pattern and the trench depth, in an evaluation unit97of the scanning probe microscope84, the profile properties36of the profile lines58of the test structures38are ascertained. As in the case of the measurement with the diffraction measuring station60, the profile properties36can contain the profile depth36-1, the respective flank angles36-2and36-3and the respective depth36-4of microtrenches in the profile lines58of the test structures38contained in the various test field sections40.

Further profile properties36of the profile lines58which can be ascertained with the scanning probe microscope84comprise for example “notching” or “trenching”, already explained with regard to the diffraction measuring station60. Furthermore, using the scanning probe microscope84, it is also possible to ascertain material properties of the diffractive optical element10in the surface roughness or else with regard to contour properties of the test structures38. In particular, structure widths of the test structures can also be determined.

Analogously to the manner of operation of the evaluation unit83of the diffraction measuring station60, the evaluation unit83depicted inFIG.8ascertains, from the profile properties36ascertained, the calibration values86with regard to the shape measuring structures16contained on the used surface14of the diffractive optical element10.

In accordance with one embodiment variant according to the invention, in each case at least some of the vertical line test field sections40-11,40-12,40-13,40-15,40-25,40-35,40-45, horizontal line test field sections40-21,40-31,40-41,40-51,40-52,40-53,40-54,40-55, brick pattern test field sections40-22,40-42,40-23,40-24and F-pattern test field sections40-33,40-43,40-44are measured both with the diffraction measuring station60and with the scanning probe microscope84as described above. From the measurement results, optionally taking suitably into account the monitoring test field section40-32, the unstructured test field section40-34and the above-described relationships between design dimensions of the corresponding structure pattern and the trench depth, the profile properties36of the profile lines58of the test structures38are ascertained. In other words, the test fields are measured using a plurality of different measurement methods, in the present case using the measurement methods based on the diffraction measuring station60and the scanning probe microscope84, and the profile properties36are determined by computation of the measurement results ascertained using the different measurement methods.

Further measurement methods that can be used here comprise transmission electron measurements (TEM), measurements using a near field scanning microscope, such as, for instance, so-called TSOM (Through Focus Scanning Optical Microscope), x-ray measurements (XRT), and scatterometry methods carried out independently of the diffraction measuring station, such as goniometry, ellipsometry, reflectometry, etc.

The computation of the measurement results ascertained using the different measurement methods can be effected for example using the Bayesian approach described in the publication “Improving optical measurement uncertainty with combined multitool metrology using a Bayesian approach”, Applied Optics, Vol. 51, No. 25, September 2012, pages 6196-6206, using iteration back and forth and/or using a parameter separation. Furthermore, it is possible to use a common comprehensive model for the different measurement methods.

One exemplary embodiment of the computation of the measurement results ascertained with the diffraction measuring station60with measurement results which were ascertained by an alternative measurement method is illustrated in the flow diagram in accordance withFIG.15. In accordance with this exemplary embodiment, three different data sets are computed by nonlinear fitting using the method of least squares.

The first data set is determined by a theoretical prediction or a calculation with regard to diffraction efficiencies for various parameters {Pi}i, {Pi}iis the set of parameters describing the geometric surface of the diffractive optical element10. These parameters concern in particular geometric properties, such as profile properties and/or contour properties, of test structures38contained in the test fields18. In this regard, for example, P1=etching depth, P2=flank angle and P3=web width/contour variation.

The second data set comprises weights w(x,y) that are ascertained by measurement of the diffraction efficiencies (referred to above as intensity values) with the diffraction measuring station60, data conditioning and corresponding estimation of the weights w(x,y). The third data set comprises weights w(x,y) that are ascertained by ascertaining measurement values using the alternative measurement method, such as, for instance, the measurement method carried out with the scanning probe microscope84, data conditioning and corresponding estimation of the weights w(x,y).

The data conditioning is generally necessary since the measuring unit typically does not measure the relevant parameters directly, but rather only data related thereto. By way of example, the scanning probe microscope84measures relative height changes and absolute etching depths can then be derived by way of an external calibration sample. In a similar manner, the diffraction measuring station60carries out measurement twice, with and without a diffractive optical element10in the beam path; the resulting diffraction efficiency is the quotient of these two measurements.

The estimation of the weights of the measurement information is performed in order, in the case of the hybrid use of a plurality of measuring unit that is present here, to correctly take account of their different measurement accuracies. By way of example, the scanning probe microscope84can be implemented in particular because it can predict the etching depth particularly precisely, whereas microtrenches are measurable only very coarsely with the scanning probe microscope. In accordance with one embodiment variant, the weight is chosen reciprocally with respect to the measurement error, i.e.

wi⁡(x,y)∼1Δ⁢Pi,
with x, y indicating the location on the diffractive optical element10and ΔPibeing the measurement error of the i-th parameter.

The result of the computation of the three data sets with nonlinear fitting using the method of least squares is values for the parameters {Pi}ias a function of the location on the diffractive optical element10. Fitting using the method of least squares (also referred to as “non-linear least square fit”) is a form of nonlinear regression in which the weighted squares of the differences in the individual measurement channels are minimized:
D=({w(x,y;λ,p)|Im(x,y;λ,p)−IR(x,y;λ,p;{Pi(x,y)}i)|2},{wi(x0,y0;x,y)|Pim−Pi(x,y)|2}i)

In this case, w(x,y; p) stands for the weight at the location (x,y) of the diffractive optical element10for the measurement with the wavelength λ and the polarization p. Im(x,y; λ, p), analogously with regard to x,y, λ, p stands for the measured intensity in the zero order of diffraction. IR(x,y; λ, p; {Pi(x,y)}i) stands for the calculated intensity with variation of the manufacturing parameters Piof the diffractive optical element10at this location. The second part represents the use of a second measuring unit, which, for example, like the scanning probe microscope84, has direct access to the geometric parameters Pi. w (x0, y0; x, y) describes the possibility that such a measurement of the parameters Pidid not take place at the location (x,y) at which reconstruction is effected, but rather occurred at a somewhat more distant test field18at the position (x0, y0). Pimstands for the measured value of the parameter, while Pi, analogously to the occurrence in IR, is a variation parameter of the fit.

Since the number of measurement channels is restricted in diffraction measuring stations and a certain residual error can also occur in the measurement data, in practice with detailed CGH manufacturing defect models the situation arises that a plurality of combinations of CGH manufacturing defects can plausibly explain the measured diffraction efficiencies. This corresponds to a plurality of local minima in the merit function during profile parameter reconstruction. It is not possible to state which local minimum is the physical minimum (the profile parameters that most likely describe the CGH actually manufactured), not even by comparison of their “depth”, i.e. of the merit function values. By defining a profile parameter, a second measuring unit (e.g. scanning probe microscope, in particular AFM) can then decide which of the local minima is the physical minimum.

Furthermore, it is possible to identify specific profile parameters in the zero order of diffraction of the diffraction measurement only with difficulty or not at all. One example of this is the profile flank angle. Particularly the case of asymmetric flank angles, e.g. left flank 85° and right flank 95° (overhang), is not detectable with regard to the sign (i.e. whether left or right flank overhanging) in the zero order of diffraction for reasons of symmetry. In the phase, i.e. for the order of diffraction, used by the shape measuring structures, the sign is crucial, however. Here the measurement in the marker with a scanning probe microscope, in particular with an AFM, can correctly define the sign. Horizontal and vertical line structures considered jointly also help to detect a pronounced tilt along the CGH radius.

The above description of exemplary embodiments, embodiments or embodiment variants should 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 in the understanding of the person skilled in the art. Therefore, all such alterations and modifications, insofar as they fall within the scope of the invention in accordance with the definition in the accompanying claims, and equivalents are intended to be covered by the protection of the claims.

LIST OF REFERENCE SIGNS

10Diffractive optical element12Surface14Used surface16Diffractive shape measuring structures18Test field20Preferred region22Forbidden region24Structure pattern of the shape measuring structures26Profile line of the shape measuring structures28Trench29Material30Flank region32Bottom region34Microtrench36Profile property36-1Profile depth36-2Flank angle left36-3Flank angle right36-4Depth of the microtrench37Width of the trench38Test structures39Structure pattern of the test structures40Test field section40-11Vertical line test field section40-14Reference test field section40-21Horizontal line test field section40-22Brick pattern test field section40-32Monitoring test field section40-33F-pattern test field section40-34Unstructured test field section40-43F-pattern test field section42Straight line44Interspace46-1Upper edge46-2Lower edge46-3Right edge46-4Left edge48Interruption50Rectangular structure50aActual shape of the rectangular structure50sTarget shape of the rectangular structure52F-structure54Reference structures55Used side56Antireflection coating57Rear side58Profile line of the test structures60Diffraction measuring station62Tunable test radiation source64Test radiation64rReflected test radiation64tTransmitted test radiation66First focusing lens element68Polarizer70Second focusing lens element72Test wave74First focusing lens element76Aperture stop78Analyzer80Second Fourier optical unit82Detector83Evaluation unit84Scanning probe microscope86Calibration values87Holding unit88Piezo-actuator89Measuring head90Cantilever92Measuring probe94Sample stage96Positioning system97Evaluation unit98Minimum of the trench depth99uUncorrected flank angle distribution99kCorrected flank angle distribution100Test interferometer102Optical surface104Test object106Test radiation source108Test radiation108iIncoming test radiation110Optical axis112Beam splitter114Focusing lens element116Reference element118Fizeau surface120Reference wave122Test wave122rReflected test wave124Stop126Eyepiece127Interferometry module128Detector camera130Evaluation unit