DE102022201462A1 - Measuring device for determining the shape of an optical surface of a test piece - Google Patents

Abstract

A measuring device (10) for determining a shape of an optical surface (12) of a specimen (14) comprises: an illumination device (16) for generating an input wave (18), an interferometer (20) for splitting the input wave into one on the optical surface directed test wave (46i) and a reference wave (44), the lighting device having a diffusing disk (28) rotating during measurement operation, a modulation module (30-1, 30-2, 30-3) and a control module (34) and the control module is configured to control the modulation module in such a way that it causes a temporal modulation of a radiation distribution (22) directed onto the lens.

Description

Background of the Invention

The invention relates to a measuring device for determining the shape of an optical surface of a test piece.

Such a measuring device is, for example, in WO 2006/077145 A2 described. This measuring device comprises an illumination device for generating an input wave and a Fizeau interferometer, with which a measuring wave with a wave front adapted to the desired shape of the optical surface is generated from the input wave. After reflection on the optical surface, the wave front of the adapted measuring wave is evaluated interferometrically to determine the deviation of the actual shape of the optical surface from its target shape.

The optical element with the optical surface is, for example, an optical component such as a lens or a mirror. Such optical components are used in optical systems such as a telescope used in astronomy or in an imaging system as used in lithographic processes. The success of such an optical system is essentially determined by the accuracy with which its optical components can be manufactured and processed in such a way that their surfaces each correspond to a target shape that was specified by a designer of the optical system when it was designed. Within the scope of such a production, it is necessary to compare the shape of the processed optical surfaces with their target shape and to determine differences or deviations between the manufactured surface and the target surface. The optical surface can then be machined in those areas in which differences between the machined area and the target area exceed predetermined threshold values, for example.

In the high-precision measurement of optical surfaces by means of interferometry, measurement inaccuracies can occur that are due to poor quality of the input wave provided by the illumination device.

Underlying Task

It is an object of the invention to provide a measuring device with which the aforementioned problems can be solved and, in particular, measurement inaccuracies caused by poor quality of a measurement radiation provided by the illumination device can be reduced.

Solution according to the invention

According to a first aspect of the invention, the above-mentioned object can be achieved, for example, with a measuring device for determining the shape of an optical surface of a test specimen with an illumination device for generating an input wave, an interferometer for splitting the input wave into a test wave directed at the optical surface, and a reference wave. The illumination device has a diffusing disk rotating during measurement operation, a modulation module and a control module, the control module being configured to control the modulation module in such a way that it causes a temporal modulation of a radiation distribution directed onto the diffusing disk.

The temporal modulation of the radiation distribution directed onto the diffusing screen means that the local radiation distribution on the diffusing screen is changed over time, ie not only an intensity amplification parameter of the profile of the radiation distribution is changed over time, but the profile as such is changed over time. Furthermore, the temporal modulation of the radiation distribution can also include a phase modulation. The temporal modulation of the radiation distribution directed onto the diffusing screen results in the speckle pattern generated in the input shaft being varied by means of the diffusing screen, ie the modulation module can also be referred to as a "speckle pattern modulation module". Speckles, also known as speckle patterns or light granulation, are granular interference phenomena that can be observed with sufficiently coherent illumination of optically rough object surfaces with unevenness in the wavelength range. The temporal modulation takes place in particular during the measurement operation, and in particular during a measurement section in which measurements take place in a temporally contiguous manner, ie the measurements are not interrupted in a relevant way. The temporal modulation can take place in particular during the recording of an interferogram with the recording device and/or between the recordings if a measurement includes a number of interferograms. In what is known as phase shift interferometry, in which a series of associated individual interferograms with different global phase positions are recorded to determine the phase difference between the test wave and the reference wave, the recording of an interferogram is to be replaced by the recording of a series of individual interferograms with different phase positions. The of The input wave and speckle pattern modulation generated by the illumination device should be identical for all individual interferograms belonging to a row in order to avoid measurement errors.

The rotating diffusing disk serves to reduce coherent measurement artefacts, so-called "shooting disks", that occur in an interferogram generated by the interferometer due to a high spatial coherence of the input wave. Different coherent states can be conveyed by rotating the diffusing disk, which leads to a reduction in the number of shooting disks. The solution according to the invention is based on the finding that speckle patterns generated on the lens due to the splitting of the input wave into the test wave and the reference wave in the interferometer can lead to measurement inaccuracies, which can be attributed to an overlay of the speckle pattern of the test wave with the speckle pattern of the reference wave. This effect is particularly pronounced when the path length of the test wave differs greatly from the path length of the reference wave in the interferometer.

The influence of this speckle pattern superposition effect on the surface measurement accuracy can be reduced by mediating many speckle pattern configurations generated by the illumination module during a measurement. The mediation can take place in particular through the exposure time of the detection unit and/or through arithmetical averaging of several interferograms.

The modulation module arranged according to the invention makes it possible to significantly increase the number of speckle pattern configurations available for calculation, in particular for averaging, and thus to further increase the surface measurement accuracy.

According to one embodiment, the modulation module is configured for temporal modulation of a phase distribution of the radiation directed onto the lens. According to a further embodiment, the modulation module is configured for the temporal modulation of an intensity distribution of the radiation directed onto the lens.

According to a further embodiment, the modulation module comprises at least one further diffuser. According to one embodiment variant, the control module is configured to cause the spreading discs to rotate at different speeds. In particular, the speeds differ by a factor of at least 2, in particular by a factor of at least 10, at least a factor of 50 or at least a factor of 100.

According to a further embodiment, the two scattering disks have different scattering angle distributions. In particular, the scattering angle distributions are so different that a convolution of the scattering angle distributions results in a distribution that is as homogeneous as possible, in particular a so-called tophat distribution, in the angle space. The test object is thus illuminated as homogeneously as possible.

According to a further embodiment, the modulation module comprises a spatial light modulator. Such a light modulator is also called SLM (Spatial Light Modulator). In this case, an SLM can basically be configured for spatial intensity and/or phase modulation. The SLM is configured to switch between at least two intensity and/or phase patterns. According to one embodiment variant, the SLM is configured at least for spatial phase modulation of the radiation directed onto the lens. This means that different local phase distributions of the radiation radiated onto the lens can be set. For example, the spatial light modulator may be a reflective pure phase SLM based on Liquid Crystal on Silicon (LCOS) technology, in which a liquid crystal is used by a direct and precise voltage to modulate a light beam is controlled.

According to one embodiment, the modulation module including the spatial light modulator has a transmission of more than twice the pupil filling of the input wave generated by the illumination device for pupil fillings of less than 30%. The pupil filling of less than 30% of the maximum pupil filling can be produced, for example, by configuring the diffuser disk as ring-shaped illumination. In the present text, the geometric imaging aperture of the camera optics of the detection device is referred to as the pupil, and its image in the plane of the interferometer on the illumination side conjugated thereto, ie in the entrance of the interferometer, is referred to as the illumination pupil. According to a further embodiment, the entire section of the illumination device downstream of a measurement radiation source has a transmission of more than twice the pupil filling of the input wave generated by the illumination device for pupil fillings of less than 30%. In other words, the entire illumination device, including the spatial light modulator, works more or less "without radiation losses", ie the entire radiation provided by the measurement radiation source is redistributed to the desired pupil filling and no radiation needs to be “cut away” to reduce pupil filling.

According to a further embodiment, the measuring device is configured in such a way that the far field of the spatial light modulator is projected onto the diffuser. In particular, an optical module in a 2f configuration or an optical module that deviates only slightly from a 2f configuration can be used for this purpose.

According to a further embodiment, the measuring device is configured in such a way that the spatial light modulator is imaged onto the diffuser. In particular, an optical module in a 4f configuration or an optical module that deviates only slightly from a 4f configuration can be used for this purpose.

According to a further embodiment, the modulation module comprises at least two electro-optical phase modulators. Each of the phase modulators is configured to effect fast, spatially homogeneous phase modulation of incident light. Fast phase modulation is understood to mean, in particular, modulation with a modulation frequency in the range from 100 kHz to 100 MHz. According to one embodiment, the respective modulation frequency of the electro-optical phase modulators exceeds a rotational frequency of the diffuser by at least one order of magnitude.

According to a further embodiment, the electro-optical phase modulators are arranged in such a way that the diffusing screen is irradiated by the phase modulators with different angles of incidence in each case. This means that the radiation radiated onto the diffuser from a first of the phase modulators has a different angle of incidence on the diffuser than the radiation radiated from the second of the phase modulators. In particular, the modulation frequency of the further electro-optical phase modulator also exceeds the rotational frequency of the diffuser by at least one order of magnitude. For example, the modulation frequency is at least 100 kHz or at least 200 kHz. According to a further embodiment, the modulation module comprises at least one acousto-optical modulator.

wobei f die Brennweite eines Kollimators des Interferometers sowie L der Abstand zwischen dem Fizeau-Element und dem Prüfling ist, und die Formmessgenauigkeit der Messvorrichtung ist im quadratischen Mittel besser als 1 nm, insbesondere besser als 0,1 nm. Unter einer Formmessgenauigkeit im quadratischen Mittel von besser als 1 nm ist zu verstehen, dass das Messergebnis der Formmessung auf dem zu vermessenden Bereich im quadratischen Mittel (auch RMS - englisch für „root mean square“ - bezeichnet) weniger als 1 nm von der tatsächlichen Form des Prüflings abweicht. Insbesondere ist die Streuscheibe am Fokuspunkt des Kollimators angeordnet. Wird beispielsweise von einer Streuscheibe ausgegangen, die ungefähr n=1000 verschiedene Specklemusterkonfigurationen erzeugen kann, so zeigt sich, dass eine Formmessgenauigkeit im quadratischen Mittel von ungefähr 1 nm für eine beispielhafte kreisförmige Strahlungsverteilung am Ort der Streuscheibe ohne deren erfindungsgemäße zeitliche Variation nur für $R<\sqrt{\mathrm{0,3}\mu m\cdot \frac{f^2}{L}}$

$$

erreicht werden kann. Durch die erfindungsgemäße Variation der in der Eingangswelle erzeugten Specklemuster kann dies auch für $R>\sqrt{\mathrm{0,3}\mu m\cdot \frac{f^2}{L}}$

$$

erzielt werden.According to a further embodiment, the interferometer has a Fizeau element for splitting the input wave into the test wave and the reference wave, and the input wave has an illumination at the location of the rotating diffuser with a maximum extension 2R with: $R>\sqrt{0.3\mu m\cdot \frac{f^2}{L}},$

where f is the focal length of a collimator of the interferometer and L is the distance between the Fizeau element and the specimen, and the shape measurement accuracy of the measuring device is better than 1 nm in root mean square, in particular better than 0.1 nm. Under a mean square shape measurement accuracy better than 1 nm means that the result of the shape measurement in the area to be measured deviates by less than 1 nm from the actual shape of the test object in the root mean square (RMS). In particular, the diffuser is arranged at the focus point of the collimator. If, for example, a diffuser is assumed that can generate approximately n=1000 different speckle pattern configurations, it is found that a shape measurement accuracy in the root mean square of approximately 1 nm for an exemplary circular radiation distribution at the location of the diffuser without its temporal variation according to the invention only for $R<\sqrt{0.3\mu m\cdot \frac{f^2}{L}}$

$$

can be reached. The inventive variation of the speckle pattern generated in the input shaft, this can also be used for $R>\sqrt{0.3\mu m\cdot \frac{f^2}{L}}$

$$

be achieved.

wobei f_oku die Brennweite einer Kameraoptik der Erfassungseinrichtung und I_CCD der Abstand zwischen den Bildlagen des Prüflings und einer Referenzfläche ist, und die Formmessgenauigkeit der Messvorrichtung ist im quadratischen Mittel besser als 1 nm, insbesondere besser als 0,1 nm. Das strahlaufspaltende Element kann beispielsweise ein Fizeauelement oder ein diffraktives optisches Element sein. Die Referenzfläche ist im Fall eines Fizeauelements die Fizeaufläche, im Fall eines diffraktiven optischen Elements kann die Referenzwelle von einem Referenzspiegel reflektiert werden, sodass die Referenzfläche von dem Referenzspiegel gebildet wird.According to a further embodiment, the interferometer comprises a beam-splitting element for splitting the input wave into the test wave and the reference wave, and a detection device for detecting an interferogram formed by superimposition of the test wave after interaction with the test object with the reference wave after reflection on a reference surface. The input shaft has an illumination with a maximum expansion of 2R at the location of the rotating lens $R>\sqrt{0.3\mu m\cdot \frac{f_{Ok\mathrm{and}}{}^2}{l_{CCD}}},$

where f _oku is the focal length of a camera optics of the detection device and I _CCD is the distance between the image positions of the test object and a reference surface, and the form measurement accuracy of the measuring device is better than 1 nm, in particular better than 0.1 nm, in the root mean square. The beam-splitting element can for example, be a Fizeau element or a diffractive optical element. In the case of a Fizeau element, the reference surface is the Fizeau surface; in the case of a diffractive optical element, the reference wave can be reflected by a reference mirror, so that the reference surface is formed by the reference mirror.

Furthermore, according to a second aspect of the invention, a measuring device for determining the shape of an optical surface of a test piece is provided. This measuring device comprises: an illumination device with a diffuser rotating during measuring operation to generate an input wave, a diffractive optical element which is configured to, by diffraction from the input wave, on the one hand direct a test wave onto the optical surface with an at least partially adapted to a target shape of the optical surface adapted wavefront and on the other hand to generate a reference wave directed towards a reference mirror, as well as a detection device for detecting an interferogram which is generated by superimposing the test wave after interaction with the test object and the reference wave reflected back from the reference mirror. The path length of the test wave between the diffractive optical element and the test object differs from the path length of the reference wave between the diffractive optical element and the reference mirror by less than 10 cm, in particular by less than 1 cm.

Due to the limitation of the path length difference between the test wave and the reference wave as defined above, the measurement error caused by the speckle pattern generated on the diffusing screen turns out to be small. The speckle patterns of the test wave and the reference wave, which are superimposed on a detector of the detection device, differ little due to the small path length difference, which is why the measurement error caused by the superimposition is small.

Furthermore, according to a third aspect of the invention, a measuring device for determining the shape of an optical surface of a test piece is provided. This measuring device comprises: an illumination device with a diffuser rotating during measuring operation to generate an input wave, a diffractive optical element which is configured to, by diffraction from the input wave, on the one hand direct a test wave onto the optical surface with an at least partially adapted to a target shape of the optical surface adapted wavefront and, on the other hand, a reference wave directed at a reference mirror, as well as a detection device with camera optics characterized by a depth of focus for detecting an interferogram, which is generated in a detection plane by superimposing the test wave after interaction with the test object and the reference wave reflected back from the reference mirror becomes. An image position of the test specimen is spaced from an image position of the reference mirror within the detection device by less than the depth of field of the camera optics, in particular by less than a tenth of the depth of field of the camera optics.

Due to the limitation of the path length difference between the test wave and the reference wave as defined above, the measurement error caused by the speckle pattern generated on the diffusing screen turns out to be small. The speckle patterns of the test wave and the reference wave, which are superimposed on a detector of the detection device, differ little due to the small path length difference, which is why the measurement error caused by the superimposition is small.

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Das Quadrat des Pupillenausleuchtungsparameters β sollte im Allgemeinen nicht mit der vorstehend erwähnten Pupillenfüllung verwechselt werden. Der Pupillenausleuchtungsparameter β ist bei einer kreisförmigen (homogenen) Pupillenausleuchtung das sogenannte Setting σ, welches die kreisförmige Ausleuchtung in radialer Richtung kennzeichnet.The depth of focus (DOF) relevant for speckle propagation is defined by the quotient of the wavelength λ of the input wave and the product of the square of a pupil illumination parameter β of the input wave and the square of the numerical aperture (NA) of the camera optics: $DOf=\frac{\mathrm{\lambda }}{{\mathrm{\beta }}^2\cdot \mathrm{<figure-callout\; id="2"\; label="N\; A"\; filenames="DE102022201462A1\_0015.png"\; state="\{\{state\}\}">N</figure-callout>}{A}^{}{}^2}.$

$$

In general, the square of the pupil illumination parameter β should not be confused with the pupil filling mentioned above. In the case of a circular (homogeneous) pupil illumination, the pupil illumination parameter β is the so-called setting σ, which characterizes the circular illumination in the radial direction.

In the case of an annular (homogeneous) pupil illumination, the pupil illumination parameter is defined as follows: β ² =σ _max ² −σ _min ² , where σ _max denotes the outer boundary and σ _min denotes the inner boundary in the radial direction of the annular illumination. The depth of focus with a ring-shaped pupil illumination is thus defined as follows: $DOf=\frac{\mathrm{\lambda }}{\left({\sigma }_{max}{}^2\cdot {\sigma }_{min}{}^2\right)\cdot \mathrm{<figure-callout\; id="2"\; label="N\; A"\; filenames="DE102022201462A1\_0015.png"\; state="\{\{state\}\}">N</figure-callout>}{A}^{}{}^2}.$

According to one embodiment of the measuring device, it is configured to specifically modify speckle patterns generated in the input shaft by the temporal modulation of the radiation distribution directed onto the diffusing disk by means of the diffusing disk in such a way that the transmission of the speckle pattern during the rotation of the diffusing disk eliminates a form measurement error caused by the speckle pattern Measuring device is reduced in the root mean square to less than 1 nm, in particular to less than 0.1 nm. The root mean square shape measurement error of less than 1 nm means that the error in the measurement result of the shape measurement in relation to the actual shape of the test piece in the root mean square (also referred to as RMS) is less than 1 is nm.

According to one embodiment, the measuring device is configured to record at least two interferograms for determining the shape of the optical surface and to switch between the interferograms by means of the modulation module between a corresponding number of different radiation distributions.

According to a further embodiment, the measuring device is configured to generate a speckle contrast in the intensity distribution on the optical surface of the test object of less than 1%, in particular less than 0.1%. The speckle contrast is the quotient of the standard deviation (= root of the variance) of the intensity distribution of the input wave on the optical surface and the mean value of the intensity distribution on the optical surface.

Furthermore, according to a fourth aspect of the invention, a measuring device for determining a shape of an optical surface of a test piece is provided, which comprises an illumination device that illuminates an illumination pupil with an extension of at least 10 μm, in particular of at least 100 μm, and is configured to generate an input shaft to create. In particular, the lighting device can have at least one diffuser. Furthermore, the measuring device includes an interferometer for splitting the input wave into a test wave directed onto the optical surface and a reference wave. The measuring device is also configured to generate a speckle contrast of less than 1%, in particular less than 0.1%, in an intensity distribution generated by the test wave on the optical surface of the test object.

The solution according to the invention according to this aspect is also based on the finding that speckle patterns generated on the diffusing screen can lead to measurement inaccuracies due to the splitting of the input wave into the test wave and the reference wave in the interferometer, which is due to an overlay of the speckle pattern of the test wave with the speckle pattern of the reference wave are due. According to the fourth aspect of the invention, the influence of this speckle pattern superposition effect on the surface measurement accuracy is significantly reduced by reducing the speckle contrast in the intensity distribution on the specimen to less than 1%.

The features specified with regard to the above-mentioned embodiments, exemplary embodiments or embodiment variants, etc. of the measuring device according to the first aspect of the invention can be transferred accordingly to the measuring device according to the second, third and/or fourth aspect of the invention, and vice versa. These and other features of the embodiments according to the invention are explained in the description of the figures and the claims. The individual features can be realized either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments which are independently protectable and whose protection may only be claimed during or after the application is pending.

character list

• 1 eine erfindungsgemäße Ausführungsform einer Messvorrichtung zur Vermessung einer Form einer optischen Oberfläche eines Prüflings mit einer Beleuchtungseinrichtung in einer ersten Ausführungsform, welche mit zwei rotierenden Streuscheiben versehen ist,
• 2 eine Veranschaulichung der Streuwinkelverteilungen der beiden rotierenden Streuscheiben,
• 3 eine weitere Ausführungsform einer Beleuchtungseinrichtung zur Verwendung in der Messvorrichtung gemäß 1,
• 4 eine weitere Ausführungsform einer Beleuchtungseinrichtung zur Verwendung in der Messvorrichtung gemäß 1, sowie
• 5 eine weitere erfindungsgemäße Ausführungsform einer Messvorrichtung zur Vermessung einer Form einer optischen Oberfläche eines Prüflings mit einem diffraktiven optischen Element zur Aufspaltung einer Eingangswelle in eine Prüfwelle sowie eine Referenzwelle.

The above and other advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments according to the invention with reference to the attached schematic drawings. It shows:

• 1 an embodiment according to the invention of a measuring device for measuring a shape of an optical surface of a test object with an illumination device in a first embodiment, which is provided with two rotating diffusers,
• 2 an illustration of the scattering angle distributions of the two rotating scattering discs,
• 3 a further embodiment of an illumination device for use in the measuring device according to FIG 1 ,
• 4 a further embodiment of an illumination device for use in the measuring device according to FIG 1 , such as
• 5 a further embodiment according to the invention of a measuring device for measuring a shape of an optical surface of a test specimen with a diffractive optical element for splitting an input wave into a test wave and a reference wave.

Detailed description of exemplary embodiments according to the invention

In the exemplary embodiments or embodiments or design variants described below, elements that are functionally or structurally similar to one another are provided with the same or similar reference symbols as far as possible. Therefore, for an understanding of the features of each element of a particular embodiment, reference should be made to the description of other embodiments or the general description of the invention.

To facilitate the description, a Cartesian xyz coordinate system is given in the drawing, from which the respective positional relationship tion of the components shown in the figures. In 1 the x-direction runs perpendicular to the plane of the drawing into it, the z-direction to the right and the y-direction upwards.

In 1 an embodiment according to the invention of a measuring device 10 for measuring a shape of an optical surface 12 of a test specimen 14 is illustrated. In the case shown, the optical surface 12 is the surface of a mirror. Alternatively, the surface of a lens can also be examined. The measuring device 10 includes an illumination device 16 for generating an input wave 18 and a highly coherent interferometer 20 in the form of a Fizeau interferometer. The illumination device 16 comprises a diffuser 28 which rotates during measurement operation and from which the input shaft 18 extends during measurement operation. The rotating diffusing disk 28 serves to reduce coherent measurement artifacts, so-called “shooting disks”, that occur due to the high spatial coherence of the input shaft 18 in the interferogram generated by the interferometer 20 . Different coherent states can be conveyed by rotating the diffusing disk 28, which leads to a reduction in the number of shooting disks.

The illumination device 16 also includes a measurement radiation source, not shown in the drawing, for generating a measurement radiation 22, e.g. in the visible wavelength range, which is provided by means of a waveguide 24. The measuring radiation source can, for example, comprise a laser, such as a helium-neon laser for generating radiation with a wavelength of approximately 633 nm. The measurement radiation 22 provided by the waveguide 24 is first expanded to a beam with a circular cross section with the radius R by means of a first lens 26 . This beam impinges on a modulation module 30-1 in the form of a further diffusing disk rotating during measurement operation. The measuring radiation 22 penetrating this diffusing disk is then radiated onto the rotating diffusing disk 28 by means of optics 32 .

In the exemplary embodiment shown, the optics 32 comprise two lenses in a 4f configuration, the scattering surface of the modulation module 30-1 and the scattering surface of the diffusing disk 28 lying in planes conjugate to one another. The optics 32 can also be configured differently, for example as a 2f configuration for projecting the far field of the scattering surface of the modulation module 30-1 onto the scattering surface of the diffusing screen 28. The modulation module 30-1 enables a temporal modulation or change in the light directed onto the diffusing screen 28 radiation distribution. This can involve a temporal modulation of the intensity and/or phase distribution of this radiation.

The measuring radiation 22 hits the diffusing disk 28 analogously to the diffusing disk of the modulation module 30-1 as a beam with a circular cross-section with the radius R and thus represents an extended radiation source at the location of the diffusing disk 28. The effect of a reduction of the Interference contrast can be reduced by changing the configuration of the illumination pattern at the location of the diffuser 28. For this purpose, the use of a ring-shaped illumination pattern with an outer radius R can be considered instead of a disc-shaped illumination pattern. The respective rotational movement of the diffusing disk of the modulation module 30-1 and the diffusing disk 28 is controlled by means of a control module 34, as explained in more detail below.

The measurement radiation 22 propagates along an optical axis of the interferometer 20 and first passes through a beam splitter 36. The measurement radiation 22 then impinges on a collimator 38 in order to convert the measurement radiation 22 into a plane wave, which then hits a reference element 40 in the form of a Fizeau element with a Fizeau surface 42 impinges. A part of the measurement radiation 22 is reflected at the Fizeau surface 42 as a reference wave 44 . The Fizeau surface 42 thus serves as a reference surface. In the present example, the portion of the measurement radiation 22 passing through the Fizeau surface 42 has a plane wavefront and is referred to below as the incoming test wave 46i.

The incoming test wave 46i is then reflected on the surface 12 to be measured. The reflected test wave 46r travels back in the opposite direction in the beam path of the incoming test wave 46i, passes through the reference element 40 and is then directed by the beam splitter 36 together with the reference wave 44 to a detection device 48 . The detection device 48 includes an aperture 50, camera optics 52 in the form of an eyepiece and a two-dimensional detector 54 for detecting the above-mentioned interferogram, which is produced by the superimposition of the reflected test wave 46r with the reference wave 44.

The camera optics 52 have a focal length f _oku which corresponds to the distance between the diaphragm 50 and the camera optics 52 . in 1 In the exemplary embodiment illustrated, an image position 70 of the specimen 14 is located on the detection surface of the detector 70, ie the optical surface 12 is imaged exactly on the detection surface of the detector 70. An image layer 72 of the reference surface, ie the Fizeau surface 42, however, is located between the camera optics 52 and the detector 70.

The diffuser 28 generates a so-called speckle pattern in the input shaft 18. As a speckle pattern, also simply referred to as speckles or light granulation, granular interference phenomena are referred to which, with sufficiently coherent illumination of optically rough object surfaces, such as the surface of the diffuser 28, with bumps in the magnitude of wavelength can be observed. Since the input wave 18 is split into the test wave 46i and the reference wave 44 in the interferometer 20, two speckle patterns are superimposed in the interferogram, namely the speckle pattern of the test wave 46r with the speckle pattern of the reference wave 44. Because of the path length L, the superimposed speckle patterns are between test specimen 14 and reference element 40 are propagated to different extents, which is why the speckle pattern superimposition leads to a noise-like, high-frequency measurement error.

According to one exemplary embodiment, the illumination device 16 is configured to generate the input wave 18 in such a way that a speckle contrast of less than 1% is generated in the intensity distribution on the optical surface 12 of the specimen 14 . As already mentioned above, the speckle contrast is the quotient of the standard deviation (=root of the variance) of the intensity distribution of the input wave 18 on the optical surface 12 and the mean value of the intensity distribution on the optical surface 12 .

This measurement error (passeRMS) generated by the speckle propagation can be reduced by detecting a large number n of different speckle pattern configurations in the input shaft 18 . The multiplicity n of speckle patterns can be conveyed, for example, by setting the exposure time of the detector 54 to the time span of one revolution of the diffusing disk 28 . The resulting measurement error passeRMS is proportional to |ΔOPD| · n ^-1/2 is (passeRMS ~ |ΔOPD| · n ^-1/2 ). In a configuration of the lighting device 16 without an upstream modulation module 30 - 1 , ie only one rotating diffusing disk 28 , the number n is limited by the number of different speckle pattern configurations which occur at different rotational positions within one revolution of the diffusing disk 28 . The effective path length difference ΔOPD depends on the path length L between the test object 14 and the reference element 40 and the angular distribution of the measurement radiation 22 with which this path length L is traversed. The angular distribution in turn results from the focal length f of the collimator 38 and the spatial intensity distribution on the diffuser 28. In a similar way, the effective path length difference can also depend on the focal length f _oku of the camera optics 52, the image layer distance I _CCD between the test object 14 and the reference element 40 , and the spatial intensity distribution in the pupil 51 can be formulated.

The diaphragm 50 of the detection device 48, more precisely the passage opening of the diaphragm 50 for the measuring radiation 22, is referred to as the pupil 51. The image of the pupil 51 in the illumination-side pupil plane 33, i.e. the conjugate plane of the pupil 51 in the entrance of the interferometer 20, is referred to as the illumination pupil 51b. According to one embodiment, the illumination pupil 51b is configured in a circular shape with an extension, i.e. a diameter, of at least 10 µm. In the case of a non-circular shape of the illumination pupil 51b, the extent denotes the maximum extent of the illumination pupil 51b. In other words, the illumination device 16 illuminates the illumination pupil 51b with an extent of at least 10 μm. According to a further embodiment, the expansion of the illumination pupil 51b is at least 100 μm.

By connecting the modulation module 30-1 in advance according to the present inventive embodiment, the number n can be significantly increased. For this purpose, the rotational speed of the diffusing disk of the modulation module 30-1 is preferably set to a value that deviates from the rotational speed of the diffusing disk 28, in particular by a factor of at least 2 or more. In particular, the rotational speed of the diffusing disk of the modulation module 30-1 exceeds the rotational speed of the diffusing disk 28. According to an alternative embodiment, however, the rotational speed of the diffusing disk 28 can also exceed the rotational speed of the diffusing disk of the modulation module 30-1.

According to an exemplary embodiment with an exemplary exposure time of the detector of 200 ms, the diffusing disk of the modulation module 30-1 is operated at a rotational speed of 200 ms/revolution by appropriate control using the control module 34 (therefore f1=300 rpm), also applies, for example n1= 100. Furthermore, the diffuser 28 is operated at a rotational speed of 2 ms/revolution (thus f2 = 30,000 rpm), with n2 = 1000. This increases the number of speckle pattern configurations conveyed during the exposure time to n _tot = n1 · n2 = 1000 · 100 = 100000. The measurement error passeRMS is thus reduced by a factor of 10.

wobei f die Brennweite des Kollimators 38 des Interferometers 20 ist. Die Streuscheibe 28 ist am Fokuspunkt des Kollimators 38 angeordnet und damit entspricht f dem Abstand zwischen der Streuscheibe 28 und dem Kollimator 38 des Interferometers 20. L ist der Abstand zwischen dem Fizeau-Element 40 und dem Prüfling 10. In Kombination mit einem entsprechend ausgestalteten erfindungsgemäßen Modulationsmodul kann diese Konfiguration eine Formmessgenauigkeit der Messvorrichtung 10 im quadratischen Mittel von besser als 1 nm, insbesondere von besser als 0,1 nm, ermöglichen.According to one embodiment, the illumination at the location of the diffuser 28 has a maximum extension of 2R $$R>\sqrt{0.3\mu m\cdot \frac{f^2}{L}},$$

where f is the focal length of the collimator 38 of the interferometer 20. The diffuser 28 is arranged at the focal point of the collimator 38 and f corresponds to the distance between the diffuser 28 and the collimator 38 of the interferometer 20. L is the distance between the Fizeau element 40 and the test piece 10. In combination with a correspondingly designed inventive Modulation module, this configuration can enable a form measurement accuracy of the measuring device 10 in the root mean square of better than 1 nm, in particular better than 0.1 nm.

In 2 the scattering angle distributions of the two diffusers are illustrated in an embodiment according to the invention. The y-axis representing the intensity of the distributions shown is scaled differently for each of the distributions for better representation. Both scattering angle distributions g ₁ and g ₂ approximately have a tophat distribution in the angle space, ie an angular distribution that is as homogeneous as possible up to a critical angle, with the critical angle of the scattering angle distribution g ₁ being considerably smaller than the critical angle of the scattering angle distribution g ₂ . The scattering angle distribution thus generated in the input wave 18, which corresponds to the convolution of g ₁ and g ₂ , thus also has a top hat distribution. This ensures that the test object 14 has a constant brightness. Expressed in general terms, a tophat distribution then occurs when the critical angles of the scattering angle distributions g ₁ and g ₂ differ from one another as much as possible.

In 3 a further embodiment of the lighting device 16 is illustrated. The associated embodiment of the measuring device 10 results from replacing the lighting device 16 according to FIG 1 by the lighting device 16 according to 3 . The lighting device 16 according to 3 differs from the lighting device 16 according to 1 by the configuration of the modulation module and the optics 32. The modulation module 30-2 according to 3 comprises a spatial light modulator operated in reflection, which is also referred to as SLM (Spatial Light Modulator). The SLM is configured to bring about a temporal modulation of the radiation distribution impinging on the diffuser 28 . For this purpose, the SLM imprints an intensity and/or phase pattern on the irradiated measurement radiation 22 in reflection. Over time, the SLM is switched between a variety of intensity and/or phase patterns. This takes place with simultaneous control of the modulation module 30-2 and the rotating lens 28 by the control module 34.

The result of this modulation of the radiation distribution impinging on the diffusing disk 28, which is matched to the rotation of the diffusing disk 28, is analogous to the embodiment according to FIG 1 an increase in the number n of speckle pattern configurations mediated during the exposure time and thus a reduction in the measurement error passeRMS.

According to one embodiment variant, the spatial light modulator of the modulation module 30 - 2 has a transmission of more than twice the pupil filling of the input wave 18 generated by the illumination device 16 in relation to the pupil 51 of the camera optics 52 . The pupil filling is to be understood as meaning the degree of surface filling of a radiation distribution in the pupil 51 of the camera optics 52 . The radiation distribution in the pupil 51 of the camera optics 52 corresponds to the angle-resolved intensity distribution on the detection surface of the detector 54. The pupil filling is preferably less than 30% of the maximum pupil filling, for example due to the above-mentioned annular illumination of the diffuser 28.

The optics 32 of the illumination device 16 according to FIG 3 is configured such that the far field of the spatial light modulator is projected onto the lens 28 . This is done, for example, by designing the optics 32 as a 2f configuration.

In 4 a further embodiment of the lighting device 16 is illustrated. The associated embodiment of the measuring device 10 results from replacing the lighting device 16 according to FIG 1 by the lighting device 16 according to 4 . The lighting device according to 4 differs according to the lighting device 3 only by the configuration of the modulation module. The modulation module 30-3 according to 4 comprises at least one beam splitter 60, a deflection mirror 62, at least two electro-optical phase modulators 56, also referred to as EOM, and a curved mirror 58. Further beam splitters in the beam path are possible.

The electro-optical phase modulators 56 are each configured to bring about a rapid, spatially homogeneous phase modulation of incident light. That is, the phase is over the beam cross-section on the relevant Pha senmodulator 56 irradiated measurement radiation 22 uniformly modulated over time. The modulation frequency of the electro-optical phase modulators 54 is at least 100 kHz, in particular at least 200 kHz.

The measurement radiation 22 provided by the waveguide 24 is split by the beam splitter 60 and the deflection mirror 62 into two beams, which are each directed at one of the electro-optical phase modulators 56 and each contain a portion, for example half, of the measurement radiation 22 provided. The respective beam passes through the relevant phase modulator 56 with the application of an individual phase modulation specified by the control module 34 for the respective phase modulator 52 . This means that the phase modulations specified for the two phase modulators generally differ from one another. The differently phase-modulated beams are then brought together by reflection on the curved mirror 58 on the diffusing screen 28; in particular, the beams on the diffusing screen 28 are superimposed.

in the in 4 Illustrated arrangement, the electro-optical phase modulators 56 are operated in parallel, so that the two-dimensional phase pattern of the incident on the rotating lens 28 radiation can be varied with a high temporal frequency. In particular, the modulation frequency of the modulation module 30-3 exceeds the rotational frequency of the diffuser 28 by at least one order of magnitude.

The result of a modulation of the radiation distribution impinging on the diffusing disk 28, which is coordinated by the control module 34 to the rotation of the diffusing disk 28, is analogous to the embodiment according to FIG 1 an increase in the number n of speckle pattern configurations mediated during the exposure time and thus a reduction in the measurement error passeRMS.

In 5 A further embodiment according to the invention of a measuring device, denoted here by reference number 110, for measuring a shape of an optical surface 12 of a test specimen 14 is illustrated. The measuring device 110 according to FIG 5 differs from the measuring device 10 according to FIG 1 initially in the configuration of the lighting device 16 in that the modulation module 30-1 and the optics 32 are missing. The measurement radiation 22 provided by the waveguide 24 is radiated directly onto the rotating diffuser disk 28 by means of the lens 26 after the beam has been widened to the diameter 2R. Furthermore, the interferometer 20 differs according to the measuring device 110 5 from that according to 1 to the extent that a diffractive optical element 64 is provided for splitting the measurement radiation 22 into the incoming test wave 46i and the reference wave 44 .

The test wave 46i and the reference wave 44 then run in different beam paths of the interferometer 110, with the test wave 46i being reflected back on the surface 12 of the test object 14 and the reference wave 44 on a reflection surface 68 of a reference mirror 66 to the diffractive optical element 64. The lengths L2 and L1 of the beam paths, i.e. the path length L2 of the test wave 46i between the diffractive optical element 64 and the test object 14 and the path length L1 of the reference wave 44 between the diffractive optical element 64 and the reference mirror 66, differ by less than 10 cm, for example (i.e. |L1-L2| < 10 cm), in particular by less than 1 cm from each other.

The diffractive optical element 64 is also configured to at least partially adapt the wave front of the test wave 46 to a target shape of the optical surface 12 . The waves 46r and 44 that are reflected back impinge on the diffractive optical element 64 again and are deflected back into the beam path of the incoming measurement radiation 22 in the opposite direction, overlapping one another. The detection of the interferograms by means of the detector 54 of the detection device takes place analogously to the above with reference to FIG 1 description made. in 5 illustrated embodiment is analogous to 1 an image position 70 of the test object 14 on the detection surface of the detector 70. An image position 72 of the reference surface 68 of the reference mirror 66, however, is located between the camera optics 52 and the detector 70.

By limiting the length difference |L1-L2| as defined above of radiation arms is referred to 1 mentioned effective path length difference ΔOPD in the interferometer is kept so small that the measurement error generated by the speckle pattern generated on the diffusing disk 28 turns out to be small. The speckle patterns of the test wave 46r and of the reference wave 44 superimposed on the detector 54 differ only slightly due to the small path length difference, which is why the measurement error caused by the superimposition is small.

Der Pupillenausleuchtungsparameter β wurde prinzipiell bereits vorstehend erläutert und ist bei einer kreisförmigen Pupillenausleuchtung das sogenannte Setting σ, das die kreisförmige Ausleuchtung in radialer Richtung kennzeichnet.According to an embodiment of the measuring device 110 according to FIG 5 an image position 70 of the specimen 14 is spaced from an image position 72 of the reference mirror 66 within the detection device 48 by less than the depth of field DOF of the camera optics 52 . The depth of field DOF is defined by the quotient of the wavelength λ of the input wave 18 and the product of one pupil nillumination parameter β of the input shaft 18 and the square of the numerical aperture (NA) of the camera optics 52: $DOf=\frac{\mathrm{\lambda }}{{\mathrm{\beta }}^2\cdot \mathrm{<figure-callout\; id="2"\; label="N\; A"\; filenames="DE102022201462A1\_0015.png"\; state="\{\{state\}\}">N</figure-callout>}{A}^{}{}^2}.$

The pupil illumination parameter β has already been explained in principle above and is the so-called setting σ in the case of a circular pupil illumination, which characterizes the circular illumination in the radial direction.

In the case of a pupil 51 illuminated in an annular manner, the square of the pupil illumination parameter is defined as follows: β ² =σ _max ² −σ _min ² , where σ _max denotes the outer boundary and σ _min denotes the inner boundary in the radial direction of the annular illumination. The depth of focus with a ring-shaped pupil illumination is thus defined as follows: $DOf=\frac{\mathrm{\lambda }}{\left({\sigma }_{max}{}^2\cdot {\sigma }_{min}{}^2\right)\cdot \mathrm{<figure-callout\; id="2"\; label="N\; A"\; filenames="DE102022201462A1\_0015.png"\; state="\{\{state\}\}">N</figure-callout>}{A}^{}{}^2}.$

The above description of exemplary embodiments, embodiments or embodiment variants is to be understood as an example. The disclosure thus made will enable those skilled in the art to understand the present invention and the advantages attendant thereto, while also encompassing variations and modifications to the described structures and methods that would become apparent to those skilled in the art. Therefore, all such alterations and modifications insofar as they come within the scope of the invention as defined in the appended claims, and equivalents, are intended to be covered by the protection of the claims.

Reference List

10

measuring device

12

optical surface

14

examinee

16

lighting device

18

input shaft

20

interferometer

22

measuring radiation

24

waveguide

26

lens

28

rotating diffuser

30-1

modulation module

30-2

modulation module

30-3

modulation module

32

optics

33

illumination-side pupil plane

34

control module

36

beam splitter

38

collimator

40

reference element

42

Fizeau surface

44

reference wave

46i

incoming test wave

46r

reflected test wave

48

detection device

50

cover

51

pupil

51b

illumination pupil

52

camera optics

54

detector

56

electro-optical phase modulator

58

curved mirror

60

beam splitter

62

deflection mirror

64

diffractive optical element

66

reference mirror

68

reflection surface

70

Image position of the optical surface of the specimen

72

Image position of the reflection surface of the reference surface

110

measuring device

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2006/077145 A2 [0002]

Claims (17)

Measuring device (10) for determining a shape of an optical surface (12) of a test piece (14) with: - an illumination device (16) for generating an input wave (18), - an interferometer (20) for splitting the input wave into a test wave (46i) directed onto the optical surface and a reference wave (44), the illumination device comprising a diffuser disk (28) rotating during measurement operation, a modulation module (30-1, 30-2 , 30-3) and a control module (34), and wherein the control module is configured to control the modulation module in such a way that it causes a temporal modulation of a radiation distribution (22) directed onto the lens. measuring device claim 1 , in which the modulation module is configured for temporal modulation of a phase distribution of the radiation directed onto the lens. measuring device claim 1 or 2 , in which the modulation module is configured for temporal modulation of an intensity distribution of the radiation directed onto the diffuser. Measuring device according to one of the preceding claims, in which the modulation module comprises at least one further diffusing screen (30-1). measuring device claim 4 wherein the control module is configured to cause the lenses to rotate at different speeds. measuring device claim 4 or 5 , in which the two diffusers have different scattering angle distributions. Measuring device according to one of the preceding claims, in which the modulation module comprises a spatial light modulator (30-2). measuring device claim 7 , which is configured such that the far field of the spatial light modulator is projected onto the lens. Measuring device according to one of the preceding claims, in which the modulation module (30-3) comprises at least two electro-optical phase modulators (56). measuring device claim 9 , in which the electro-optical phase modulators are arranged in such a way that the diffusing screen is irradiated by the phase modulators with different irradiation angle ranges. Measuring device according to one of the preceding claims, in which the interferometer has a Fizeau element (40) for splitting the input wave into the test wave and the reference wave and the input wave has an illumination at the location of the rotating diffusing disk with a maximum extension of 2R with: $R>\sqrt{0.3\mu m\cdot \frac{f^2}{L}},$

where f is the focal length of a collimator (38) of the interferometer and L is the distance between the Fizeau element and the specimen, and the shape measurement accuracy of the measuring device is better than 1 nm in the root mean square. Measuring device according to one of the preceding claims, in which the interferometer (20) has a beam-splitting element (40, 64) for splitting the input wave into the test wave and the reference wave and a detection device (48) for detecting a superposition of the test wave after interaction with the test object has an interferogram formed with the reference wave after reflection on a reference surface (42, 68), in which the input wave at the location of the rotating diffuser has an illumination with a maximum expansion of 2R with $R>\sqrt{0.3\mu m\cdot \frac{f_{Ok\mathrm{and}}{}^2}{l_{CCD}}},$

where f _oku is the focal length of a camera optics (52) of the detection device and I _CCD is the distance between the image positions (70, 72) of the test object and a reference surface, and where the form measurement accuracy of the measuring device is better than 1 nm in the root mean square. Measuring device (110) for determining the shape of an optical surface (12) of a test specimen (14), with: - an illumination device (16) with a diffuser (28) rotating during measurement operation for generating an input wave (18), - a diffractive optical element ( 64), which is configured to produce, by diffraction from the input wave, on the one hand a test wave (46i) directed at the optical surface with a wave front that is at least partially adapted to a target shape of the optical surface and on the other hand a reference wave (44) directed at a reference mirror (66) and - a detection device (48) for detecting an interferogram which is generated in a detection plane by superimposition of the test wave after interaction with the test object and the reference wave reflected back by the reference mirror, the path length of the test wave increasing between the diffractive optical element and the test object differs from the path length of the reference wave between the diffractive optical element and the reference mirror by less than 10 cm. Measuring device (110) for determining the shape of an optical surface (12) of a test piece (14) with: - an illumination device (16) with a diffusing disk (28) rotating during measurement operation for generating an input wave (18), - a diffractive optical element (64), which is configured to generate, by diffraction from the input wave, on the one hand a test wave (46i) directed onto the optical surface with a wavefront that is at least partially adapted to a desired shape of the optical surface and on the other hand a reference mirror (66 ) directed reference wave (44) to generate, as well as - a detection device (48) with camera optics (52) characterized by a depth of field for detecting an interferogram, which is generated by superimposing the test wave after interaction with the test object and the reference wave reflected back by the reference mirror, with an image position of the test object being different from an image position of the reference mirror is spaced within the detection device by less than the depth of field of the camera optics. Measuring device according to one of the preceding claims, which is configured to specifically modify speckle patterns generated by the temporal modulation of the radiation distribution directed onto the diffusing disk by means of the diffusing disk in the input shaft in such a way that by mediating the speckle pattern during the rotation of the diffusing disk, a Form measurement error of the measuring device is reduced to less than 1 nm in the root mean square. Measuring device according to one of the preceding claims, which is configured to produce a speckle contrast in the intensity distribution on the optical surface (12) of the specimen (14) of less than 1%. Measuring device (10) for determining a shape of an optical surface (12) of a test piece (14) with: - an illumination device (16) which illuminates an illumination pupil (51b) with an extension of at least 10 µm and is configured to generate an input wave (18), - an interferometer (20) for splitting the input wave (18) into a test wave (46i) directed at the optical surface (12) and a reference wave (44), wherein the measuring device (10) is configured to measure a speckle contrast in one of the To generate test wave (46i) on the optical surface (12) of the specimen (14) generated intensity distribution of less than 1%.

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