DE102020207946A1 - Measuring device for the interferometric determination of a surface shape - Google Patents

Abstract

The invention relates to a measuring device (10) for the interferometric determination of the shape of an optical surface (12) of a test object (14). The measuring device (10) comprises an irradiation device (16) for generating an input wave (18) and a diffractive optical element (20) which is configured to generate a first test wave (22) with an at least partially to generate a wave front adapted to the desired shape of the optical surface (12). Furthermore, the measuring device (10) is configured to direct the first test wave (22) onto the optical surface (12) and to generate a second test wave (24) which is also directed to the optical surface (12) and when it hits the optical surface (12) is tilted relative to the first test shaft (22). The invention also relates to a corresponding method for interferometric determination of the shape of an optical surface (12) of a test object (14).

Description

Background of the invention

The invention relates to a measuring device and a method for interferometric determination of a shape of an optical surface of a test object. For example, an optical element for microlithography is measured as a test object. As a result of the need for ever smaller structures, ever higher demands are placed on the optical properties of the optical elements used in microlithography. The optical surface shape of these optical elements must therefore be determined with the highest possible accuracy.

For high-precision interferometric measurement of optical surfaces down to the subnanometer range, interferometric measuring devices and methods are known in which a diffractive optical element generates a test wave and a reference wave from an input wave. The wavefront of the test wave can be adapted to a target surface of the test object by the diffractive optical element in such a way that it would impinge perpendicularly on the target shape at any location and would be reflected back by this. With the aid of the interferogram formed by superimposing the reflected test wave and the reference wave, deviations from the nominal shape can then be determined.

For example, in WO 2016/188620 A2 describes a measuring device with a complex coded computer-generated hologram (CGH) as a diffractive optical element. From an input wave, the CGH generates a test wave directed onto the surface to be measured with a wave front that is at least partially adapted to a desired shape of the optical surface and a plane reference wave. The reference wave is reflected back to the CGH by a reflective optical element. Furthermore, the CGH generates a calibration wave with a plane wave front and a calibration wave with a spherical wave front from the input wave. The calibration waves are reflected back into themselves by means of a plane and a spherical calibration mirror. The CGH is calibrated with the aid of the calibration shafts. In this way, for example, local changes in position, such as CGH deformations or CGH distortions, can be corrected and measurement errors can thus be reduced.

A problem with these known measuring devices and methods with a diffractive optical element is that undesired wavefront changes can occur due to a necessarily three-dimensional realization of calculated two-dimensional grating or hologram structures. These wavefront changes are also referred to as “rigorous effects” and cannot be completely calibrated out with the aid of the calibration waves. Further errors can occur due to the calibration mirror used. These circumstances cause measurement errors when determining a surface shape and thus reduce the measurement accuracy.

Underlying task

It is an object of the invention to provide a measuring device and a method with which the aforementioned problems are solved and, in particular, the measuring accuracy when determining the shape of an optical surface is increased.

Solution according to the invention

According to the invention, the aforementioned object can be achieved, for example, with a measuring device for interferometric determination of a shape of an optical surface of a test object, which comprises an irradiation device for generating an input wave and a diffractive optical element. The diffractive optical element is configured to generate a first test wave from the input wave with a wave front that is at least partially adapted to a nominal shape of the optical surface. The measuring device is also configured to direct the first test wave onto the optical surface and to generate a second test wave which is also directed onto the optical surface and is tilted relative to the first test wave when it hits the optical surface.

In particular, the irradiation device provides electromagnetic radiation that is sufficiently coherent for interferometric measurements in one or more different wavelengths. For this purpose, the irradiation device can comprise a suitable laser, for example. Furthermore, coherent input waves can be provided via an arrangement comprised by the measuring device with lenses, mirrors, fiber elements or a combination of these elements. To reduce speckle effects is In addition, a non-vanishing light conductance value of the interferometer is advantageous such that there is approximately an illumination by plane (i.e. coherent) waves locally on the diffractive optical element, with different illumination directions being incoherent to one another and only having small angle differences. The maximum of the angle difference of the irradiation at a point of the diffractive optical element defines the illumination NA of the system. For some embodiments the illumination NA is on the order of 1 mrad, but for others smaller illumination NAs are advantageous.

The diffractive optical element comprises, for example, a computer-generated hologram (CGH) for generating the first, the second or both test waves. A computer-generated hologram is generated by calculating a suitable line structure as a diffractive structure using a computer and suitable methods, for example ray tracing, and then writing the calculated line structure on or in the surface of a substrate.

When generated on the diffractive optical element, the first test wave can be directed directly onto the optical surface or, after it has been generated, be directed onto the optical surface by a further optical element of the measuring device. The second test wave is also referred to below as a sheared test wave and is generated, for example, with the aid of the diffractive optical element or one or more further optical elements of the measuring device. In the case of the second test shaft, too, the alignment with the optical surface to be measured can take place during generation or only afterwards by means of one or more suitable optical elements of the measuring device.

The solution according to the invention reduces the influence of rigorous effects on the measurement. Rigorous effects are due to the problem that the effect of a diffraction grating at high line densities, where a grating period is not significantly greater than the wavelength of the radiation used, is difficult to predict with a simple diffraction theory. Furthermore, production-related parameters of the grille, such as the web height, flank steepness and rounding of edges, have an influence on the effect of the grille. Such influences are not only defined by the grating period and are also referred to as rigorous effects in the technical field at hand.

According to one embodiment according to the invention, the measuring device further comprises a detection device for detecting an interferogram which is generated by superimposing the two test waves after the respective interaction with the optical surface. For example, the detection device contains a camera, for example a CCD camera, for detecting an interferogram generated in a detection plane of the measuring device by superimposing the two test waves.

According to a further embodiment according to the invention, when it hits the optical surface, the second test shaft is tilted by a maximum of 1 mrad, in particular by a maximum of 0.1 mrad, with respect to the first test shaft. In particular, according to an embodiment of the invention, the second test shaft is tilted at the optical surface by a maximum of 0.1 mrad with respect to the first test shaft.

In one embodiment according to the invention, the measuring device further comprises a control device which is configured to arrange the test object one after the other in a plurality of tilted positions while a measuring process is being carried out. For this purpose, the measuring device comprises, for example, a correspondingly tiltable holding device for holding the test object. The holding device preferably comprises suitably designed actuators, such as piezo elements, for tilting the holding device into a specific tilted position.

By arranging the test object in the different tilt positions, a so-called "phase shift" occurs. Different phase differences are generated between the test shaft and the sheared test shaft in successive phase steps. The interferograms generated in the individual phase steps are recorded by means of the detection device.

One embodiment of the invention comprises an evaluation device which calculates the derivatives of the form deviation of the test object from the nominal form along the direction of the phase shift from the interferograms generated during a phase shift in a tilting direction. By integrating the derivations by means of the evaluation device, the form deviation of the test object is then determined in the direction mentioned.

According to a further embodiment, the phase shifting takes place in two different directions, in particular in two mutually orthogonal directions, with a separate one for each of the directions diffractive optical element is arranged in the measuring device. By integrating all the resulting derivations by means of a suitably configured evaluation device, the form deviation of the test object is then determined in both directions and thus two-dimensionally.

In an embodiment of the measuring device according to the invention, the tilt positions of the test object relate to a test object tilt axis which is arranged parallel or at an acute angle to a test shaft tilt axis, to which the tilt of the two test shafts to each other when they hit the optical surface relates. In particular, the angle between the tilt axis of the test object and the tilt axis of the test shafts is less than 45 ° or less than 10 °. By tilting the test object in the shear direction, that is to say in the direction of tilting the two test shafts relative to one another, a phase shift and thus an adjustable phase offset between the interfering fields can be implemented. Phase shifting is used to convert the fringe interference images into phases or to derive them.

According to a further embodiment of the invention, the second test shaft is tilted relative to the first test shaft with respect to a first test shaft tilting axis and the measuring device is further configured to generate a third test wave which is also directed at the optical surface and when it hits the optical surface is tilted relative to the first test shaft with respect to a second test shaft tilt axis. In one embodiment variant, the second test shaft tilting axis deviates at least 45 ° from the first test shaft tilting axis. In particular, the measuring device can be configured in such a way that the two test shaft tilting axes are orthogonal to one another. A two-dimensional fit deviation can be determined by combining the measured deviations of the surface shape of the test object from the nominal shape in the two different shear directions.

An embodiment of the measuring device according to the invention furthermore comprises at least one calibration mirror and the diffractive optical element is configured to generate a calibration wave directed onto the calibration mirror from the input wave. The calibration shaft can be directed directly onto the calibration mirror or be directed onto the calibration mirror by a further optical element of the measuring device. The calibration wave is preferably used to detect distortions of the diffraction structures of the diffractive optical element with respect to a target configuration. Using this information, the errors generated by the distortions when determining the shape of the optical surface of the test object are calculated out. In particular, in one embodiment according to the invention, the measuring device comprises three calibration mirrors and the diffractive optical element is configured to generate three calibration waves from the input wave, which are each directed to one of the three calibration mirrors. According to a further embodiment, the calibration waves are each spherical waves.

In one embodiment according to the invention, the irradiation device is configured to generate the input wave with such an optical frequency distribution that the two test waves are generated from two different optical frequencies of the input wave when the input wave interacts with the diffractive optical element. In other words, the frequency-resolved intensity distribution of the input wave has a maximum of two different frequencies. The ratio between the frequency spacing of the two adjacent frequencies to the frequency is preferably less than 10 ^-4 . The different frequency leads to a split of the input wave at the diffractive optical element into a test wave with a first frequency and a test wave sheared in relation to this with a second frequency. The angle between the directions of propagation of the test wave and the sheared test wave depends on the wavelength, a lattice constant of the diffractive element and the relationship between frequency spacing and frequency. Furthermore, the input wave has sufficient coherence for an interferometric measurement at both frequencies.

For this purpose, the irradiation device of the measuring device according to an embodiment according to the invention comprises a frequency comb generator for generating the optical frequency distribution. The frequency comb generator includes, for example, a pulsed femtosecond laser. Such a laser generates light pulses with a pulse duration in the femtosecond range. Alternatively, the frequency comb generator can also contain a linear optical cavity with an optical modulator, or it can be constructed in some other way known to the person skilled in the art. A frequency comb generator is for example in the DE 10 2012 212 663 A1 described.

According to one embodiment according to the invention, the diffractive optical element comprises a complex-coded phase grating with a first phase function for generating the first test wave and a second phase function for generating the second test wave. A complex coded phase grating is for example in DE 10 2012 217 800 A1 described. Such a diffractive optical element can also be considered complex coded CGH. Diffractive structure patterns for generating the first and the second test wave are arranged superimposed on one another in one plane. The first and second test waves can thus have different wave fronts and directions of propagation. According to a further embodiment, the diffractive optical element comprises a complex-coded Quint-CGH for generating a first test wave, a sheared test wave and three spherical calibration waves with different directions of propagation.

A phase shift between the test shaft and the sheared test shaft occurs, for example, by tilting the test object by means of a suitably configured holding device. For small shearings, the derivatives of the surface shape in the shear direction are essentially determined. From this, with a suitable evaluation device, a deviation from a nominal shape of the test object in the mentioned direction is determined by integration. Compared to conventional shear interferometry, there is a shear or phase shift in the diffractive optical element and in the test object without additional error-inducing optical elements. In this embodiment, a small shear s is advantageous, so that the interfering waves do not have any high-frequency phase differences p, or in other words, the fringe density in the interferogram remains sufficiently small to resolve the interference fringes and their edges in the camera. For small shear s, p is then proportional to the directional derivative of the test object pass h in the direction of the shear and to the amount of shear, in particular the stripe density, i.e. directly proportional to s Interferogram is proportional to 1 / s. This angle limits both the illumination and imaging NA of the interferometric system. Both can, for example, be less than 0.1 or 0.01.

Another embodiment with a complex coded phase grating, which allows a larger tilt angle between the test waves and thus a larger illumination and imaging NA, uses a further phase function in the complex coded grating to convert the first and second test waves onto the same wave after reflection on the test object to be mapped after the second run of the CGH, provided the test item is free of errors. For example, the first test wave can be adapted to the yoke, the second test wave can be a further diffraction order of the first test wave, and the additional CGH phase function can be adapted to the phase difference of the second wave reflected on the test object and the illumination wave in the CGH plane. The disadvantage of this embodiment is that the CGH surface is no longer approximately conjugate to itself in the sense that beam splitting and recombination take place at approximately the same location of the CGH, so that the interference phase a priori also detects manufacturing errors of the interferometer outside the CGH, which one require separate calibration. In this embodiment it is advantageous to use the CGH in reflection in order to avoid aberrations of a coherent illumination wave as a result of the passage of the CGH rear side and the subsequent propagation in the homogeneous, unstructured medium.

According to a further embodiment according to the invention, the measuring device further comprises a beam splitter arranged in the beam path of the first test shaft, which beam splitter is configured to split off the second test shaft from the first test shaft. In particular, the diffractive optical element of the measuring device can comprise a Quad-CGH for generating the test wave and three calibration waves. The beam splitter then divides the second, sheared test wave from the test wave generated by the Quad-CGH. Furthermore, in these embodiments too, a holding device can be configured in such a way that phase shifting can be carried out by tilting the test object.

According to one embodiment of the invention, the beam splitter is configured as a diffraction grating. For example, different diffraction orders are used, such as the +1. and -1. Diffraction order of the first test wave radiating onto the diffraction grating as first and second, sheared test waves directed onto the optical surface of the test object. When using a beam splitter, rigorous errors of the diffractive optical element cancel each other out by forming a shear difference. Errors caused by the beam splitter can generally be taken into account more easily by calibration or are irrelevant.

According to one embodiment of the invention, a first diffractive optical element is configured to generate the first test wave and the second test wave from the input wave, and the measuring device comprises a second diffractive optical element which is configured to generate the first test wave and the third test wave to create. The plane spanned by the directions of propagation of the first and second test waves is preferably perpendicular to the plane spanned by the directions of propagation of the first and third test waves. In other words, the tilt axis of the second test shaft relative to the first test shaft is orthogonal to the tilt axis of the third test shaft relative to the first test shaft. In this way, derivatives of the optical surface are created in two mutually orthogonal shear directions certainly. With a suitably configured evaluation device, a two-dimensional deviation of the surface from a nominal shape can be determined from these derivations by means of integration.

Furthermore, according to one embodiment of the invention, a beam splitter is rotatably arranged in the beam path of the first test shaft in such a way that the second test shaft is split off in a first rotary position and the third test shaft is split off from the first test shaft in a second rotary position. For example, the beam splitter is designed to be rotatable by 90 °. As already stated above, the shear direction established by the first and second test shaft is orthogonal to the shear direction established by the first and third test shaft. This enables a very precise two-dimensional measurement of the optical surface.

The aforementioned object can also be achieved, for example, with a method for interferometric determination of a shape of an optical surface of a test object, the method generating an input wave, generating a first test wave with a wavefront from the input wave that is at least partially adapted to a nominal shape of the optical surface by means of a diffractive optical element, and directing the first test wave onto the optical surface. The method further comprises generating a second test wave, which is also directed at the optical surface and is tilted relative to the first test wave when it hits the optical surface.

One embodiment of the method according to the invention comprises recording an interferogram which is generated by superimposing the two test waves after each interaction with the optical surface. Furthermore, in one embodiment of the method according to the invention, the test object is arranged one after the other in a plurality of tilted positions while a measurement process is being carried out.

The features specified with regard to the above-mentioned embodiments, exemplary embodiments or design variants, etc. of the measuring device according to the invention can be transferred accordingly to the method according to the invention. 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 that can be independently protected and whose protection may only be claimed during or after the application is pending.

Figure list

• 1 ein erstes Ausführungsbeispiel einer erfindungsgemäßen Messvorrichtung zur interferometrischen Bestimmung einer Form einer optischen Oberfläche eines Testobjekts mit einem Frequenzkammgenerator in einer schematischen Veranschaulichung,
• 2 ein zweites Ausführungsbeispiel einer erfindungsgemäßen Messvorrichtung mit einem computergenerierten Hologramm zur Erzeugung einer ersten Prüfwelle, einer gescherten Prüfwelle und drei sphärischer Kalibrierwellen in einer schematischen Veranschaulichung,
• 3 ein drittes Ausführungsbeispiel einer erfindungsgemäßen Messvorrichtung mit einem computergenerierten Hologramm zur Erzeugung einer ersten Prüfwelle und von Kalibrierwellen, sowie einem Strahlenteiler zur Erzeugung einer gescherten Prüfwelle aus der ersten Prüfwelle, sowie
• 4 den Strahlengang von Spiegel-angepasster Prüfwelle und dazu verkippter Welle in einer Ausführungsform mit starker Kippung, bei der das CGH nicht näherungsweise selbstkonjugiert ist und die Beleuchtung durch zueinander inkohärente ebene Wellen erfolgt.

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

• 1 a first embodiment of a measuring device according to the invention for the interferometric determination of a shape of an optical surface of a test object with a frequency comb generator in a schematic illustration,
• 2 a second embodiment of a measuring device according to the invention with a computer-generated hologram for generating a first test wave, a sheared test wave and three spherical calibration waves in a schematic illustration,
• 3 a third embodiment of a measuring device according to the invention with a computer-generated hologram for generating a first test wave and calibration waves, and a beam splitter for generating a sheared test wave from the first test wave, and
• 4th the beam path of the mirror-adapted test wave and the wave tilted to it in an embodiment with strong tilt, in which the CGH is not approximately self-conjugate and the illumination is carried out by mutually incoherent plane waves.

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 as far as possible with the same or similar reference symbols. Therefore, in order to understand 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.

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

In 1 is schematically a measuring device 10 for the interferometric determination of a shape of an optical surface 12th of a test object 14th shown. The measuring device 10 is particularly suitable for the highly precise measurement of the optical surface of an aspherical mirror or a free-form mirror for microlithography with exposure radiation in the extreme ultraviolet (EUV) spectral range. The EUV wavelength range extends to wavelengths below 100 nm and relates in particular to wavelengths of around 13.5 nm or 6.8 nm. In order to reduce or correct errors in such optical surfaces, shape determination down to the subnano range is necessary. The measuring device 10 but is also suitable for measuring the surface of many other objects.

The measuring device 10 comprises an irradiation device 16 for providing an input shaft 18th , a diffractive optical element 20th for generating a first test wave 22nd and a second, sheared test shaft 24 , a holding device 26th for holding and tilting the test object 14th and a detector 28 for capturing interferograms.

The irradiation facility 16 comprises in this embodiment a frequency comb generator 30th . The frequency comb generator 30th represents an input shaft 18th each with an intensity maximum at two different frequencies f ₁ , f ₂ . Furthermore, the input shaft 18th at both frequencies there is sufficient coherence for interferometric measurements. The ratio between the frequency spacing Δf of the two frequencies with maximum intensity and the first frequency f is between 10 ^-4 and 10 ^-5 . Thus, in this sense it is about neighboring frequencies.

The frequency comb generator 30th includes, for example, a pulsed femtosecond laser, such as a mode-locked titanium-sapphire laser. Such a laser generates light pulses with a pulse duration in the femtosecond range. Such a frequency comb generator is for example in DE 10 2012 212 663 A1 described. Alternatively, the frequency comb generator 30th be constructed in a different manner known to the person skilled in the art, or it can be used instead of the frequency comb generator 30th another radiation source with two intensity maxima at neighboring frequencies can be used. The one from the frequency comb generator 30th outgoing input shaft 18th is on the diffractive optical element 20th directed. The measuring device 10 more, in 1 Optical elements, not shown, such as lenses, mirrors or fiber elements contain.

The diffractive optical element 20th comprises a complex coded Quad-CGH with different diffractive structure patterns 32 , which are superimposed in a plane. The diffractive structural pattern 32 are used to generate the first test wave 22nd and three calibration waves 34 , 36 , 38 each configured with a different direction of propagation and wavefront from a monochromatic input wave. The wave front of the first test wave 22nd is by bending the input wave 18th on the diffractive optical element 20th such a target shape of the optical surface 12th adapted so that it would hit vertically at every location and be reflected back in itself. Furthermore, according to the present embodiment, each of the three calibration waves 34 , 36 , 38 by transforming the input wave 18th on the diffractive structure patterns 32 a spherical wave front.

The diffractive structure patterns 32 as a quadruple complex coded phase grating, each with a phase function for generating the first test wave 22nd and the three calibration waves 34 , 36 , 38 be trained. A complex coded phase grating is shown in DE 10 2012 217 800 A1 described. Alternatively, amplitude gratings can also be used as diffractive structures. Furthermore, the complex coded CGH can be designed as an off-axis CGH or carrier frequency CGH. In the case of such a CGH known to a person skilled in the art, the angle of incidence of an incoming wave differs in the zeroth order of diffraction from the angle of the outgoing wave.

mit der Wellenlänge λ und der Frequenz f der ersten Prüfwelle, dem Frequenzabstand Δf zwischen den beiden Frequenzen und einer Gitterkonstanten d. Die gescherte Prüfwelle 24 ist somit gegenüber der ersten Prüfwelle 22 um eine Kippachse 40 um weniger als 1 mrad verkippt. Die Prüfwellen-Kippachse 40 liegt in diesem Ausführungsbeispiel parallel zur y-Achse. Für die drei Kalibrierwellen 34, 36, 38 gilt entsprechendes. Zu jeder Kalibrierwelle 34, 36, 38 mit der Frequenz f₁ wird eine gescherte Kalibrierwelle 42, 44, 46 mit der Frequenz f₂ erzeugt.The input shaft 18th is at the diffractive optical element 20th not monochromatic, but rather has a component with the first frequency f ₁ and a further component with the second frequency f ₂ . The frequency splitting leads to a beam splitting at the diffractive optical element 20th because the diffraction depends on the frequency. In addition to a first test wave 22nd a second or sheared test wave is generated with the first frequency f ₁ 24 generated with the second frequency f ₂ . The angle Δω between the directions of propagation of the first test wave 22nd and the sheared test shaft 24 is on the order of magnitude $$\Delta \mathrm{\omega }=\mathrm{\lambda }/\text{d}\cdot \Delta \text{f}/\text{f}$$

with the wavelength λ and the frequency f of the first test wave, the frequency spacing Δf between the two frequencies and a lattice constant d. The sheared test shaft 24 is therefore opposite to the first test wave 22nd around a tilt axis 40 tilted less than 1 mrad. The test shaft tilting axis 40 in this embodiment is parallel to the y-axis. For the three calibration waves 34 , 36 , 38 the same applies. For every calibration wave 34 , 36 , 38 with the frequency f ₁ becomes a sheared calibration wave 42 , 44 , 46 generated with the frequency f ₂ .

The first and the sheared test shaft 22nd , 24 are of the diffractive optical element 20th on the optical surface to be measured 12th of the test object 14th directed. From the optical surface 12th become the test waves 22nd , 24 to the diffractive optical element 20th reflected back. The test object 14th is used for this by the holding device 26th held. The holding device 26th is configured such that in addition to an adjustment of the test object 14th tilting (arrow 50 ) of the test object 14th and thus the optical surface 12th around a test object tilt axis 48 is feasible. In particular, several different tilt angles can be achieved with the holding device 26th to adjust. The test object tilted into one of several tilt positions 52 is in 1 shown in dashed lines. Here is the test object tilt axis 48 parallel to the test shaft tilting axis 40 or to the y-direction.

Tilting the test object 14th by the holding device 26th successively in different tilted positions is controlled by a suitably designed control device 54 the measuring device 10 controlled. For example, one or more in 1 actuators, not shown, of the holding device 26th with the help of the control device 54 be performed. The control device 54 can be designed for this purpose to generate suitable control signals.

By tilting the test object 14th A phase shift takes place between the first test shaft in the x-direction or the shear direction 22nd and the sheared test shaft 24 . With interferograms, described in more detail below, at different tilt angles and thus different phase offsets between the first and the sheared test wave 22nd , 24 the derivation of the phases and thus the directional derivation of the optical surface is determined 12th in the shear direction.

The one from the optical surface 12th reflected test waves 22nd , 24 run back to the diffractive optical element 20th and then towards the frequency comb generator 30th . When passing through the diffractive optical element 20th a reverse transformation of the first and the sheared test shaft takes place 22nd , 24 . On one in the beam path between the frequency comb generator 30th and the diffractive optical element 20th arranged beam splitter 56 the detection device 28 becomes at least a portion of the reflected first test wave 22nd and the reflected sheared test wave 24 towards a camera 58 diverted.

Both convergent test waves 22nd , 24 go through an eyepiece 60 and hit a detection plane 62 the camera 58 . The camera 58 includes, for example, a CCD sensor and sensed by the interfering test waves 22nd , 24 generated fringe interferograms. In the focus of the convergent test waves 22nd , 24 can an in 1 Aperture, not shown, can be arranged to reduce disruptive scattered radiation.

One in 1 evaluation device, not shown, of the measuring device 10 determines the derivation of the phases from several recorded interferograms at different tilt angles or phase offsets and from this the derivation of the direction of the optical surface in the shear direction. Based on the derivation of the direction, the evaluation device determines the deviation from a nominal shape or the shape of the optical surface with respect to the shear direction or x-direction.

For this purpose, the evaluation device comprises a suitably configured data processing unit and uses appropriate calculation methods known to those skilled in the art. Alternatively or additionally, the measuring device 10 contain an interface to a network or a data memory in order to determine the surface shape of the test object 14th with the help of an external evaluation unit and interferograms transmitted or stored via the network.

To determine the surface shape with respect to a second shear direction that is not parallel to the x-direction, the measuring device 10 an in 1 comprise not shown second diffractive optical element. Alternatively, the first diffractive optical element can also be rotated 20th be provided with the help of an appropriately configured bracket. In particular, diffractive occurs through the second optical element or the rotation of the first diffractive optical element 20th a shear of the test shaft 22nd in the direction of the y-axis and thus perpendicular to the x-axis. Next to the first test wave 22nd a third test wave sheared in the y-direction is generated in this way. The test shaft tilting axis is thus parallel to the x-axis. The test object is tilted accordingly 14th with the help of the holding device 26th and the control device 54 around a test object tilting axis parallel to the x-axis and thus at different tilting angles in the direction of the y-direction or shear direction.

Analogously to the directional derivation in the x-direction described above, the evaluation device now also determines the directional derivation of the surface shape in the y-direction from recorded interferrograms at various tilt angles. With the help of a combination of the determined directional derivatives of the optical surface in the x and y directions, the evaluation device finally determines a two-dimensional deviation of the optical surface from a desired shape and thus the shape of the surface.

According to further exemplary embodiments, an additional variation of the respective tilting direction or the frequency difference is possible in order to optimize the surface measurement. This is made possible, for example, with the aid of an appropriately configured frequency comb generator or a suitably designed holding device for the test object or the diffractive optical element.

The measuring device also comprises 10 three spherical calibration mirrors 64 , 66 , 68 . The first spherical calibration shaft 34 and the first calibration shaft sheared for this purpose 42 are of the diffractive optical element 20th on a first spherical calibration mirror 64 directed. Analogous to the test wave 22nd lies between the directions of propagation of the first calibration wave 34 and the first sheared calibration shaft 42 an angle Δω before. From the first calibration mirror 64 become the first and the sheared first calibration shaft 34 , 42 to the diffractive optical element 20th reflected back. The second spherical calibration wave becomes accordingly 36 and the sheared second calibration shaft 44 on a second calibration mirror 66 , as well as the third calibration wave 38 and the third sheared calibration shaft 76 on a third calibration mirror 68 directed and reflected back by these. The directions of propagation of the calibration waves 34 , 36 , 38 are different and in particular do not lie together on one level.

After passing the diffractive optical element again 20th hit the back transformed calibration waves 34 , 36 , 38 and the sheared calibration shafts 42 , 44 , 46 on the beam splitter 56 and are at least partially towards the camera 58 diverted. Analogous to the test waves 22nd , 24 a detection is made by superimposing a calibration wave 34 , 36 , 38 and the associated sheared calibration shaft 42 , 44 , 46 in the acquisition level 62 generated interferogram by the camera 58 . Tilting of the respective calibration mirror can also occur 64 , 66 , 68 take place in several different tilt positions for a phase shift with the help of a suitably designed bracket.

The evaluation device is for evaluating the detected interference patterns of the calibration waves 34 , 36 , 38 configured. In particular, the evaluation device is used to determine wavefront errors due to misalignments or placement errors of the diffractive optical element 20th for a calibration. Furthermore, the evaluation device is for taking into account determined calibration corrections of a calibration when determining the surface shape of the test object 14th educated.

The measuring device 10 is insensitive to wavefront changes due to rigorous effects, since these affect the first test wave in essentially the same way 22nd and the sheared test shaft 24 affect and thus cancel out when interfering. Another advantage is that instead of a reference wave, a sheared test wave is used to superimpose the first test wave. In this way, no reference mirror is required for a back reflection of a reference wave, which has to be adjusted and can cause measurement errors.

2 shows a second embodiment of a measuring device 10 for determining a shape of an optical surface 12th of a test object 14th . The measuring device 10 largely corresponds to the measuring device 1 . It is therefore only a section of the measuring device 10 with a diffractive optical element 20th , the test object 14th and three calibration mirrors 64 , 66 , 68 shown. For this and other components of the measuring device, not shown 10 will in particular also refer to the description 1 referenced.

In contrast to the measuring device after 1 represents an irradiation device of the measuring device 10 a monochromatic input wave 18th ready. The input shaft 18th thus essentially has only one frequency. For this purpose, the irradiation device can, for example, use a suitable laser Generate coherent electromagnetic radiation with a frequency in the visible or invisible spectral range include.

mit j = 1,2,3, w_j komplexe Konstanten, c_j reelle Normierungs-Konstanten bestimmt durch die Lage der Spiegel und die Wellenlänge, s der Schervektor, x ein Punkt in der x,y-Ebene.
Die Taylorentwicklung der Phase nach der Scherung s liefert die Approximation $$t\left(x\right)\approx 2w_p\text{cos}\left(c_p{s}^t.z_p^{\text{'}\text{'}}\left(x\right).x\right)\text{exp}\left(i{c}_p\nabla z_p\left(x\right).x\right)+{\displaystyle \sum _{j=1\dots 3}{w}_j\text{exp}\left(i{c}_j\nabla z_j.x\right)}$$

Furthermore, the diffractive optical element comprises 20th in this exemplary embodiment a five-fold complex coded computer-generated hologram 72 for generating a first test wave 22nd , a sheared second test shaft 24 as well as three spherical calibration waves 34 , 36 , 38 with different directions of propagation from the input wave 18th . Such a CGH is also referred to as a quint CGH. The transfer function t of the Quint-CGH is a linear combination of the test and calibration waves 22, 24 and 34, 36, 38: If z _P , z ₁ ... z _{3 are} the z coordinates of the test object surface and the three calibration spheres as a function of the x , y level so is $$\begin{array}{c}t\left(x\right)=w_p\text{exp}\left(i{c}_p\nabla z_p\left(x+\frac{s}{2}\right).x\right)+w_p\text{exp}\left(i{c}_p\nabla z_p\left(x-\frac{s}{2}\right).x\right)\\ +{\displaystyle \sum _{j=1...3}{w}_j\text{exp}\left(i{c}_j\nabla z_j.x\right)}\end{array}$$

with j = 1,2,3, w _j complex constants, c _j real normalization constants determined by the position of the mirror and the wavelength, s the shear vector, x a point in the x, y-plane.
The Taylor expansion of the phase after the shear s provides the approximation $$t\left(x\right)\approx 2w_p\text{cos}\left(c_p{s}^t.z_p^{\text{'}\text{'}}\left(x\right).x\right)\text{exp}\left(i{c}_p\nabla z_p\left(x\right).x\right)+{\displaystyle \sum _{j=1...3}{w}_j\text{exp}\left(i{c}_j\nabla z_j.x\right)}$$

The implementation of the transfer function t in the CGH can be accompanied by further approximations such as, for example, a step grating in the phase or amplitude effect.

Alternatively, the measuring device can also have two or more CGHs arranged one behind the other in the beam path for generating the test waves 22nd , 24 and the calibration waves 34 , 36 , 38 contain.

in Richtung von Δx_j realisiert. Die Messvorrichtung 10 umfasst dafür eine in 2 nicht dargestellte Halteeinrichtung zum Verkippen des Testobjekts 14. Die von der optischen Oberfläche 12 reflektierten Prüfwellen 22, 24 durchlaufen wiederum das diffraktive optische Element 20 und werden dann analog zur Messvorrichtung nach 1 von einem Strahlenteiler auf eine Kamera umgelenkt, welche entstehende Interferogramme erfasst. Bei einer Auswertung werden für kleine Scherungen Δx_j im Wesentlichen die Richtungsableitung der Oberfläche 12 in Δx_j-Richtung ermittelt. Mittels einer Integration nach einer geeigneten Filterung, wie beispielsweise einem Zernikefit, lässt sich daraus die Oberflächenform bestimmen.A phase shift Δϕ _j between j-sheared interfering test waves 22nd , 24 is also in this embodiment by tilting 50 of the test object 14th by an angle ω with $$\mathrm{\omega }=\mathrm{\lambda }/2\mathrm{\pi }\cdot \Delta {\mathrm{\varphi }}_{\text{j}}/\Delta {\text{x}}_{\text{j}}$$

realized in the direction of Δx _j . The measuring device 10 includes an in 2 Holding device, not shown, for tilting the test object 14th . The one from the optical surface 12th reflected test waves 22nd , 24 in turn pass through the diffractive optical element 20th and are then analogous to the measuring device 1 deflected by a beam splitter onto a camera, which records interferograms that arise. In an evaluation, for small shearings Δx _j are essentially the directional derivative of the surface 12th determined in the Δx _j direction. The surface shape can be determined from this by means of integration after suitable filtering, such as a Zernikefit.

In contrast to conventional shear interferometry, the measuring device takes place 10 a shear or a phase shift in the CGH 72 and by tilting the test object 14th instead of additional, error-prone optical elements, such as checkerboard grids or DIC prisms. Furthermore, measurement errors due to rigorous effects are also reduced in this measuring device and errors due to reference mirrors are avoided.

In 3 becomes a third embodiment of a measuring device 10 illustrated. The measuring device 10 corresponds essentially to the measuring device according to 2 . It is therefore again only a section of the measuring device 10 shown and otherwise refer to the description 1 and 2 referenced. Also with this measuring device 10 an irradiator provides a monochrome input wave 18th with essentially one frequency ready.

In contrast to the measuring device according to 2 comprises the diffractive optical element 20th a fourfold complex coded CGH 74 to generate the first test wave 22nd and three calibration waves 34 , 36 , 38 each with a different direction of propagation and wave front from the monochromatic input wave 18th . Such a CGH is also called a Quad-CGH 74 and has already been referred to above with reference to 1 described in more detail.

The measuring device also includes 10 a beam splitter 76 , which is in the beam path between the diffractive optical element 20th and the test object 14th is arranged. The beam splitter 76 splits from that of the diffractive optical element 20th coming first test wave 22nd a portion as a sheared test shaft 24 from. Correspondingly, the beam splitter takes place 76 a breakdown by the Quad-CGH 74 provided calibration waves 34 , 36 , 38 in the way that each to a calibration wave 34 , 36 , 38 a sheared calibration shaft 42 , 44 , 46 is produced.

The beam splitter 76 is designed, for example, as a diffraction grating and is rotatably held by a holder. By turning it by 90 °, the shear direction is also turned by 90 ° and a third sheared test shaft is generated in this way. A phase shift in successively acquired interferograms is in turn caused by tilting the test object 14th enables. Analogously to the exemplary embodiments described above, this measuring device can also be used 10 the directional derivative of the optical surface 12th in different shear directions and then determine the shape of the optical surface with high precision. Rigorous effects are reduced by forming the shear difference. Furthermore, errors can be caused by the beam splitter 76 are easier to calibrate than errors caused by reference mirrors or are irrelevant when measuring the optical surface 12th .

4th shows the beam path from a mirror-adapted first test wave 22nd and a shaft in the form of a sheared test shaft that is tilted at points 24 in a reflective embodiment with strong tilting and illumination by at least one illumination wave 80 in the form of a plane wave in which the diffractive optical element in the form of a CGH is not approximately self-conjugated. The test wave 22nd or the associated first diffractive structure of the diffractive structure pattern 32 In the complex coded CGH, the surface of the test object is adapted, ie the beam path shown as an example is perpendicular to the test object 14th , while the second diffractive structure of the diffractive structure pattern 32 in the complex coded CGHs to the second test wave 24 is designed so that after reflection on the test object 14th and diffraction at the first diffractive structure - or, alternatively, in the case of a predetermined offset dx, the diffraction at a third diffractive structure - the illumination wave 80 is reproduced except for light reversal and false light from unused, not shown diffraction orders.

The above description of exemplary embodiments, embodiments or design variants is to be understood as exemplary. The disclosure thus made enables the person skilled in the art, on the one hand, to understand the present invention and the advantages associated therewith, and, on the other hand, also includes obvious changes and modifications of the structures and methods described in the understanding of those skilled in the art. Therefore, it is intended to cover all such changes and modifications insofar as they come within the scope of the invention as defined in the appended claims, and equivalents of the scope of protection of the claims.

List of reference symbols

10

Measuring device

12

optical surface

14th

Test object

16

Irradiation facility

18th

Input shaft

20th

diffractive optical element

22nd

first test wave

24

second, sheared test shaft

26th

Holding device

28

Detection device

30th

Frequency comb generator

32

diffractive structure patterns

34

first spherical calibration shaft

36

second spherical calibration shaft

38

third spherical calibration wave

40

Test shaft tilting axis

42

first sheared calibration shaft

44

second sheared calibration shaft

46

third sheared calibration shaft

48

Test object tilt axis

50

Tilting test object

52

tilted test object

54

Control device

56

Beam splitter

58

camera

60

eyepiece

62

Acquisition level

64

first calibration mirror

66

first calibration mirror

68

first calibration mirror

70

monochrome radiation source

72

Quint-CGH

74

Quad CGH

76

Beam splitter

80

Lighting wave

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed 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 2016/188620 A2 [0003]
• DE 102012212663 A1 [0021, 0038]
• DE 102012217800 A1 [0022, 0040]

Claims (15)

Measuring device (10) for the interferometric determination of a shape of an optical surface (12) of a test object (14), comprising: - An irradiation device (16) for generating an input wave (18), and - A diffractive optical element (20) which is configured to generate a first test wave (22) with a wavefront which is at least partially adapted to a desired shape of the optical surface (12) from the input wave (18), the measuring device (10) is further configured to direct the first test wave (22) onto the optical surface (12) and to generate a second test wave (24) which is also directed onto the optical surface (12) and when it strikes the optical surface (12) is tilted relative to the first test shaft (22). Measuring device according to Claim 1 which further comprises a detection device (28) for detecting an interferogram, which is generated by superimposing the two test waves (22, 24) after each interaction with the optical surface (12). Measuring device according to Claim 1 or 2 , in which the second test shaft (24) when it hits the optical surface (12) is tilted by a maximum of 1 mrad with respect to the first test shaft (22). Measuring device according to one of the preceding claims, which further comprises a control device (54) which is configured to arrange the test object (14) one after the other in a plurality of tilted positions (52) while a measuring process is being carried out. Measuring device according to Claim 4 , wherein the tilt positions (52) of the test object (14) relate to a test object tilt axis (48) which is arranged parallel or at an acute angle to a test shaft tilt axis (40), whereupon the tilt of the two test shafts (22, 24) relates to each other when they strike the optical surface (12). Measuring device according to one of the preceding claims, wherein the second test shaft (24) is tilted with respect to the first test shaft (22) with respect to a first test shaft tilting axis (40) and the measuring device (10) is further configured to generate a third test shaft which is also directed at the optical surface (12) and when it hits the optical surface (12) is tilted relative to the first test shaft (22) with respect to a second test shaft tilting axis. Measuring device according to one of the preceding claims, which further comprises at least one calibration mirror (64, 66, 68) and wherein the diffractive optical element (20) is configured to furthermore point to the calibration mirror (64, 66, 68) from the input shaft (18) ) to generate directed calibration wave (34, 36, 38). Measuring device according to one of the preceding claims, in which the irradiation device (16) is configured to generate the input wave (18) with an optical frequency distribution such that the two test waves (22, 24) when the input wave (18) interacts with the diffractive optical element (20) can be generated from two different optical frequencies of the input shaft (18). Measuring device according to Claim 8 , in which the irradiation device (16) comprises a frequency comb generator (30) for generating the optical frequency distribution. Measuring device according to one of the Claims 1 to 7th wherein the diffractive optical element (20) comprises a complex coded phase grating (72) with a first phase function for generating the first test wave (22) and a second phase function for generating the second test wave (24). Measuring device according to one of the Claims 1 to 7th which further comprises a beam splitter (76) which is arranged in the beam path of the first test shaft (22) and which is configured to split off the second test shaft (24) from the first test shaft (22). Measuring device according to Claim 11 , in which the beam splitter (76) is configured as a diffraction grating. Measuring device according to Claim 6 as well as one of the Claims 8 to 10 , wherein the first diffractive optical element (20) is configured to the input shaft (18) the first test wave (22) and the second To generate test wave (24), and the measuring device (10) comprises a second diffractive optical element which is configured to generate the first test wave (22) and the third test wave. Measuring device according to Claim 6 as well as one of the Claims 11 and 12th , the beam splitter (76) being rotatably arranged in the beam path of the first test shaft (22) such that the second test shaft (24) is split off in a first rotary position and the third test shaft is split off from the first test shaft (22) in a second rotary position. Method for the interferometric determination of a shape of an optical surface (12) of a test object (14) with: - generating an input shaft (18), - a generation of a first test wave (22) with a wave front which is at least partially adapted to a nominal shape of the optical surface (12) from the input wave (18) by means of a diffractive optical element (20), - Directing the first test shaft (22) onto the optical surface (12), and - A generation of a second test wave (24), which is also directed onto the optical surface (12) and is tilted relative to the first test wave (22) when it hits the optical surface (12).

Images