DE102021214364A1 - Measuring device and measuring method for interferometric measurement of a surface shape - Google Patents

Abstract

A method for interferometric measurement of a shape of a test surface comprises the steps: splitting a measurement radiation into a measurement wave directed onto the test surface and a reference wave; generating different phase differences between the measurement wave and the reference wave at the location of a detector; Recording a first set of interference patterns and at least a second set of interference patterns, each of which is generated by superimposing the reference wave with the measurement wave after it has been reflected on the test surface, each set having a plurality of interference patterns generated with different phase differences and the second set compared to the first set is phase-shifted by a respective static offset. Furthermore, a third set and at least a fourth set are recorded, with the fourth set being phase-shifted relative to the third set by the respective static offset, and with the first set and the second set forming a first set pair and the third set and the fourth set, a second set pair is formed, the first set pair and the second set pair being phase-shifted relative to one another by a predeterminable first dynamic offset, and the shape of the test surface being determined by calculating the at least four sets.

Description

FIELD OF APPLICATION AND PRIOR ART

The invention relates to a measuring device and a measuring method for interferometric measurement of the shape of a test surface.

In particular, the invention relates to an interferometric measuring device with a phase shifting device for generating different phase differences between a measuring wave and a reference wave and a corresponding interferometric measuring method.

Known examples of such measuring devices are white light interferometers (WLI) or phase-shifting interferometers (PSI). These interferometers are used, among other things, for a pass or roughness measurement of optical surfaces. For this purpose, a measuring wave reflected by the surface to be examined is generally superimposed with a reference wave and the interference pattern generated in the process is recorded. After a first interference pattern has been detected, the phase of the reference wave is shifted in relation to the measuring wave. The second interference pattern generated afterwards is also recorded, and so on. With a complex mathematical modeling and iterative calculation, the surface topography of the test surface can be reconstructed from the recorded interference patterns. Due to the phase shift, surface structures can also be determined that are significantly smaller than the wavelength λ of the measuring wave, ie smaller than the measuring wavelength λ.

A problem with phase-shifting interferometry is vibration and other disturbances caused by the phase-shifting. As a rule, these lead to interference strips in the evaluated results of the surface topographies. The interfering fringes appear with a dominant double frequency to the interference fringes occurring due to the height difference. A height difference of λ/2 corresponds to an interference fringes in the camera image. The greater the gradient of the area to be examined, the more interference fringes appear per unit area. The interference strips are often partially eliminated in roughness measurements by fitting polynomials such as Zernike, Legendre or Chebyshew polynomials. However, this usually also leads to a partial loss of measured surface information, so that reducing the interference strips with the help of an adjustment induces its own measurement errors.

In view of these problems, the DE 10 2019 208 028 A1 Improvements in measuring devices and measuring methods. The measuring device comprises a beam splitting element for splitting a measuring radiation into a measuring wave directed onto the test surface and a reference wave, a detector for recording an interference pattern generated by superimposing the reference wave with the measuring wave after it has been reflected on the test surface, a phase shifting device for generating different phase differences between the measuring wave and the reference wave at the location of the detector, and an evaluation device for determining the shape of the test surface by calculating a first set of interference patterns with at least a second set of interference patterns, each set having at least eight interference patterns generated with different phase differences and the phase differences of the second set have a respective offset with respect to the phase differences of the first set.

As a result, improvements in the clutter problem can be achieved. However, according to the observations of the inventors, it is not always possible to completely suppress noise stripes without losing useful information about the surface shape.

TASK AND SOLUTION

Against this background, the object of the invention is to provide a measuring device and a measuring method for interferometric measurement of the shape of a test surface, which have a measurement accuracy that is improved once again compared to the prior art.

In order to solve this problem, the invention provides a measuring device with the features of claim 1 and a measuring method with the features of claim 2 . Preferred developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference.

According to one formulation of the invention, a measuring device for interferometric measurement of a shape of a test surface is provided, which has a beam splitting element for splitting a measuring radiation into a measuring wave directed at the test surface and a reference wave, a detector for recording a signal by superimposing the reference wave with the measuring wave after its reflection interference pattern generated at the test surface, and phase shifting means for generating different phase differences between the measurement wave and the reference wave at the location of the detector. Furthermore, the measuring device comprises an evaluation device for determining the shape of the test surface by calculating a first set of interference patterns with at least a second set of interference patterns, each set having a large number of interference patterns generated with different phase differences and the phase differences of the second set compared to the phase differences of the first set have a respective static offset. The phase differences are to be understood in each case as the phase differences between the measuring wave and the reference wave at the location of the detector. The different phase differences of the respective set are generated by so-called "phase shifting" by means of the phase shifting device.

A set is a section from a larger total of recorded interference images. In this application, the entirety is also referred to as an interference image stack or as an “overall stack”. A set corresponds to a subset of the total stack and is also referred to here as a "substack". Sets or sub-stacks are calculated by the evaluation device for the reconstruction of the test surface. A set can, for example, have eight or more interference patterns generated with different phase differences, possibly also at least sixteen or also at least twenty-four interference patterns generated with different phase differences. The number of fringe patterns in each set need not be an integer multiple of four or eight.

The measuring radiation can, for example, be provided by means of an irradiation device in a coherence length or spectral width sufficient for an interferometric measurement and has one, several or a continuum of wavelengths. For example, a laser or a light-emitting diode can be used as a light source for this purpose. According to one embodiment, the measurement radiation has a spectrum of wavelengths with a Gaussian curve as the intensity envelope, a so-called Gaussian spectrum. Such a measurement radiation is used in particular in white-light interferometry.

The phase shifting device can, for example, spatially shift the beam splitting element or another optical element of the measuring device in such a way that the path length of the measuring wave to the detector changes compared to the path length of the reference wave to the detector. This results in a phase shift between the measuring wave and the reference wave. Preferably, the phase shifter is configured to shift the phase in eight steps or a greater number of steps, such as sixteen or sixty-four steps, for each set of interference patterns.

The static offset can also be referred to as static displacement. The static offset is the offset between two sub-stacks from the same overall stack of a measurement.

According to the claimed invention, the entire measurement consists of several partial measurements. An interference image stack with the same initial phase results from each partial measurement. Two sub-stacks, which have a static offset, are cut out of the interference image stack of each partial measurement. This can be described in such a way that, in addition to a first set and a second set, a third set of interference patterns and at least a fourth set of interference patterns are recorded or determined, with the phase differences of the fourth set compared to the phase differences of the third set having the respective static offset . A first pair of sets is formed by the first set and the second set from a first partial measurement, and a second pair of sets is formed by the third set and the fourth set from a second partial measurement, with the first pair of sets and the second pair of sets having a specifiable dynamic offset, i.e have a certain phase difference to each other. In other words: the second pair of sentences is phase-shifted by a predefinable dynamic offset compared to the first pair of sentences.

A sentence pair contains several sentences, in particular exactly two different sentences. A sentence pair can also contain more than two sentences.

In the case of the measuring device, the evaluation device is designed to determine the shape of the test surface by calculating the first set of interference patterns with each additional one of the recorded sets, ie to determine the second set, the third set, the fourth set, and optionally further sets of interference patterns.

In the measurement method, the shape of the test surface is accordingly determined by offsetting the first set of interference patterns with each additional set of interference patterns recorded.

As mentioned, a measurement can be described in such a way that it comprises several partial measurements. For example, a first partial measurement is based on the first set and the second set. To carry out a measurement, several partial measurements are carried out. Different partial measurements are based on different set pairs. For example, a second partial measurement can be based on the third and fourth sets. Preferably, consecutive or adjacent partial measurements have a dynamic offset that is dependent on the partial measurement.

The dynamic offset is called "dynamic" because this offset can change from partial measurement to partial measurement, e.g. in equidistant steps. However, it is also possible for other specifications to be made for the dynamic offset.

Using the method described in the invention, the errors can be separated from the real topography and thus almost completely eliminated without falsifying the actual topography. The interference strips usually have different, in particular reversed phase positions, so that they mutually balance out when the evaluation results from each individual set are added up, while the useful information, in this case the actual topography, only adds up. After averaging all evaluation results, essentially only the actual topography without the interference information is obtained.

Preferred embodiments are characterized in that the second pair of sets is shifted by the predeterminable dynamic (phase) offset compared to the first pair of sets in such a way that the third set has the predeterminable dynamic offset compared to the first set and the fourth set has the predeterminable dynamic offset compared to the second set have offset. In other words: the first pair of sets and the second pair of sets can have the specifiable first dynamic offset to one another in such a way that the phase differences of the third set compared to the phase differences of the first set each have the specifiable first dynamic offset and the phase differences of the fourth set compared to the phase differences of the second Set each have the predeterminable first dynamic offset.

Among other things, this means that dynamic offsets between odd-numbered records (record 1 and record 3) and between even-numbered records (record 2 and record 4) are "symmetrical", i.e. the same. This enables a particularly efficient evaluation and minimization of interference strips. This results in interference stripe suppression for a partial measurement by the set pair from this partial measurement, in that the interference strips from the respective evaluations of sets with a static offset mutually compensate at least in part. In addition, further interference stripe suppression between the partial measurements results from the set pairs offset with dynamic offsets.

In some embodiments it is provided that at least a third pair of sentences is formed, the third pair of sentences having a recorded fifth set of interference patterns and a recorded sixth set of interference patterns, the first pair of sentences and the third pair of sentences having a specifiable second dynamic offset, i.e. a specific have phase difference to each other. A more reliable measurement can be achieved by increasing the number of sets that are offset against one another.

Although the first and the second dynamic offset can be of the same size, it is preferably provided that the predeterminable first dynamic offset and the predeterminable second dynamic offset differ from one another.

According to a development, it is provided that the dynamic offset for the partial measurements is symmetrical about a reference position, with the reference position preferably being a focus position of the measuring device. In this way, artefacts due to different defocus values in the partial measurements can be avoided.

In preferred embodiments it is provided that an increment of the dynamic offset δ _dyn depends on the wavelength λ of the measuring wave and the number N of _{partial measurements} of the partial measurements measurement is determined. This allows the effect of noise stripe suppression to be optimized. It has proven to be particularly favorable if the relationship step size=λ/4*N _{partial measurement} applies to the step size.

This results in pairs of sets whose offset is λ/8, as a result of which the interference strips form an inverted phase from the partial measurements and can therefore compensate one another particularly well. For example, if the measurement consists of two partial measurements, the set pair from the first partial measurement has an offset λ / 8 to the set pair from the second partial measurement. If, on the other hand, the measurement consists of four partial measurements, the first pair of sets has an offset of λ/8 relative to the third pair of sets, while there is also an offset of λ/8 between the second pair of sets and the fourth pair of sets. If the measurement consists of eight partial measurements, the first pair of sets and the fifth pair of sets, the second pair of sets and the sixth pair of sets, the third pair of sets and the seventh pair of sets as well as the fourth pair of sets and the eighth pair of sets each have an offset of λ / 8 to one another.

The features specified with regard to the embodiments listed above can be transferred accordingly to the method according to the invention.

character list

• 1 zeigt schematisch ein Ausführungsbeispiel einer Messvorrichtung zur interferometrischen Vermessung einer Form einer Testoberfläche eines Objekts;
• 2 zeigt ein Diagramm, in welchem die Intensität des Interferenzsignals bei einem exemplarischen Pixel des Detektors für verschiedene Phasenverschiebungen zusammen mit Informationen zu ausgewählten Sätzen von Interferenzbilder bei einer Teilmessung dargestellt ist;
• 3A, 3B und 3C zeigen schematisch drei aufeinander folgende Teilmessungen mit schrittweise wachsendem dynamischen Offset;
• 4 zeigt schematisch acht Teilmessungen;
• 5 zeigt eine ausgewertete Oberfläche ohne Unterdrückung der Störstreifen.
• 6 zeigt eine ausgewertete Oberfläche mit Unterdrückung der Störstreifen nach dem Verfahren aus der DE 10 2019 208 028 A1 .
• 7 zeigt die ausgewertete Oberfläche mit Unterdrückung der Störstreifen gemäß einer Ausführungsform des Verfahrens gemäß der Erfindung.

Further advantages and aspects of the invention result from the claims and from the following description of preferred exemplary embodiments of the invention, which are explained below with reference to the figures.

• 1 shows schematically an embodiment of a measuring device for interferometric measurement of a shape of a test surface of an object;
• 2 Figure 12 shows a diagram showing the intensity of the interference signal at an exemplary pixel of the detector for different phase shifts together with information on selected sets of interference images in a partial measurement;
• 3A , 3B and 3C show schematically three consecutive partial measurements with stepwise increasing dynamic offset;
• 4 shows schematically eight partial measurements;
• 5 shows an evaluated surface without suppression of the interference stripes.
• 6 shows an evaluated surface with suppression of the interference stripes according to the method from DE 10 2019 208 028 A1 .
• 7 shows the evaluated surface with suppression of the clutter strips according to an embodiment of the method according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

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.

For the practical use of the method, the hardware of a measuring device can be used, as for example from the DE 10 2019 208 028 A1 is known. Differences from the exemplary embodiments there are primarily in the process control and in the evaluation of the measurements.

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

1 shows schematically a measuring device 10 for interferometric measurement of a form of a test surface 12 of an object 14. The measuring device 10 is particularly suitable for measuring flat, spherical or free-form surfaces of optical elements, such as mirrors for microlithography in the extreme ultraviolet (EUV ) spectral range. The EUV wavelength range extends to wavelengths below 100 nm and relates in particular to wavelengths of approximately 13.5 nm or 6.8 nm. However, the surface of many other objects can also be measured with the measuring device 10 .

The measuring device 10 comprises an illumination device 16 for providing a measurement radiation 18, a beam splitter 20 for deflecting the measurement radiation 18 in the direction of the test surface 12, an interference lens 22 for focusing the measurement radiation 18 onto the test surface 12 and a detection device 24 for recording and processing interference patterns.

In this exemplary embodiment, the lighting device 16 contains a light-emitting diode 26 as a light source and a ground-glass screen 28 for homogenizing the radiation emitted by the light-emitting diode 26 . The light-emitting diode 26 is designed in such a way that the measurement radiation 18 has a spectrum of frequencies with a coherence length or spectral width suitable for interferometry. For example, the spectrum corresponds to a Gaussian spectrum with a maximum intensity at a wavelength λ and adjacent frequencies or wavelengths with decreasing intensity according to a Gaussian curve. The wavelength λ with maximum intensity is also referred to below as the wavelength λ of the measurement radiation. With such a measuring radiation 18, the measuring device 10 is configured in particular for white-light interferometry. In other versions, however, a monochromatic light source, e.g. a laser, can also be used for phase-shifting interferometry.

Furthermore, the illumination device 16 includes a condenser lens 30, with which as much measurement radiation 18 as possible is coupled as uniformly as possible into the beam path of the interference lens 22. For this purpose, the condenser lens 30 preferably images the light source of the light-emitting diode 26 onto the opening of the interference lens 22 . In addition or as an alternative, the measuring device 12 can contain further lenses, not shown in FIG.

The beam splitter 20 is configured and arranged in the measuring device 10 in such a way that at least a proportion of the measuring radiation 18 coming from the illumination device 16 is deflected in the direction of the test surface 12 . Furthermore, at least a portion of the measurement radiation reflected by the test surface 12 passes through the beam splitter 20 without changing direction to the detection device 24 . The beam paths between the illumination device 16 and the beam splitter 20 and between the test surface 12 and the detection device 24 are in 1 each marked with an optical axis 32 .

The interference objective 22 focuses the measurement radiation 18 coming from the beam splitter 20 onto the test surface 12. For this purpose, the interference objective 22 comprises one or more optical elements, of which 1 an objective lens 34 is shown symbolically. Furthermore, the interference objective 22 includes a beam splitting element 36 which allows a portion of the measurement radiation 18 to pass as a measurement wave 38 to the test surface 12 , while another portion of the measurement radiation 18 is reflected as a reference wave 40 . A semi-transparent mirror, for example, can be used as the beam splitting element 36 .

The reference wave 40 impinges on a reference mirror 42 of the interference lens 22 and is reflected back by this to the beam splitting element 36 . The size of the reference mirror 42 preferably corresponds approximately to the area of the test surface 12 illuminated by the measuring shaft 38 . The measuring wave 38 is reflected by the test surface 12 and then passes through the interference lens 22 again, also in the direction of the beam splitter 20.

The reflected measurement wave 38 and the reference wave 40 pass the beam splitter 20 at least partially without a change in direction and run to the detection device 24. A lens 44 of the detection device 24 focuses the measurement wave 38 and the reference wave 40 onto a detection plane 46 of a detector 48. The detector 48 is in this Embodiment designed as a CCD camera or as a CMOS camera. In the detection plane 46, an interference signal is generated for each location of the illuminated area of the test surface 12 at a corresponding pixel of the detector 48 by superimposing the measuring wave 38 with the reference wave 40, which is combined to form an interference pattern for the entire illuminated area.

The respective interference signal depends on the difference in path length between the measuring wave 38 and the reference wave 40 or their phase difference. While the path length of the reference wave 40 to the point at which the reference wave 40 leaves the interference objective 22 is predetermined, the path length of the measuring wave 38 to this point is determined by the height of the test surface 12 in the z-direction and the distance from the interference objective 22 determined by the test surface 12. The phase difference between the measuring shaft 38 and the reference shaft 40 can be changed by a displacement 50 of the interference lens 22 in the z-direction.

For such a phase shift, the measuring device 10 includes a phase shifting device 52 for changing the phase difference between the measuring shaft 38 and the reference wave 40. In this exemplary embodiment, the phase shifting device 52 contains a piezo system 54 for shifting the interference lens 22 along the optical axis 32 and thus according to 1 in the z-direction . With such a displacement 50, the distance between the beam splitting element 36 and the test surface 12 and thus the path length of the measuring shaft 38 changes, while the path length of the reference wave 40 between the beam splitting element 36 and the reference mirror 42 remains constant.

The piezo system 54 includes a controller 56 and in 1 piezo elements, not shown, and is configured in such a way that the interference lens 22 can be displaced in steps by a fraction of the wavelength λ of the measurement radiation 18 in each case. For example, the piezo system 54 can be used to carry out a stepwise shift of λ/64 over more than one, preferably more than 2, wavelengths for measuring the test surface 12 . Because of the return path of the measurement shaft 38 between the beam splitting element 36 and the test surface 12, a shift of λ/64 corresponds to a phase shift of π/16. With each set phase shift, the detector 48 can detect an interference pattern generated in the detection plane 46 by superimposing the measuring wave 38 with the reference wave 40 .

Furthermore, the measuring device 10 includes an evaluation device 58 for determining the shape of the test surface 12 by means of the interference pattern recorded by the detector 48 at different phase shifts. For this purpose, the evaluation device 58 comprises a suitably configured data processing unit and a data memory. Alternatively or additionally, the measuring device 10 can contain an interface to a network or a data memory in order to determine the shape of the test surface 12 with the aid of an external evaluation unit and interference patterns transmitted or stored via the network.

During phase shifting, an equidistant movement takes place through the piezo system, during which the optical path length in the object arm changes. This creates an intensity signal that is sinusoidal in phase in each pixel on the camera plane. 2 1 shows, by way of example, in a diagram, the intensity I (in arbitrary units AU) of the interference signal at an exemplary pixel of the detector 48 for different phase shifts.

At the in 2 In the example shown, the measuring device is configured to record an overall stack GS of interference patterns by rectified stepwise changing of the phase difference and to assemble or select the two sets of interference patterns from the overall stack. In other words: the overall stack GS is recorded by successively changing the phase difference either by means of increasing steps or by means of decreasing steps and from this sub-stacks SS1 and SS2 are assembled to generate the first and second set. In this case, interference patterns with respect to phase differences which occur in both sub-stacks are recorded only once and then assigned to the respective sub-stacks.

A substack contains a subset of interferograms from a larger set of interferograms, the overall stack.

In the example, a specified number of initially recorded interference patterns are assigned only to the first sub-stack SS1 (filled black dots) and a corresponding number of later recorded interference patterns are only assigned to the second sub-stack SS2 (grey circles). All intervening interference patterns are allocated to both sub-stacks or both sets.

The first sub-stack SS1 corresponds to the first record, the second sub-stack SS2 to the second record, etc., so that reference sign SS1 can also be used for the first record SS1 and reference sign SS2 for the second record SS2.

Recorded interference patterns from the overall stack GS for the sub-stacks are selected, among other things, in such a way that between the interference patterns of the first sub-stack SS1 and the second sub-stack SS2 (i.e. between the first and the second set) the above-described static offset or the static offset δ _stat in the phase difference.

The static offset δ _stat corresponds to the predefinable distance between sub-stack SS1 and sub-stack SS2, in this case δ _stat =λ/8. For the 2 this means that the first interferogram 11-1 of the sub-stack SS1 (or the first set SS1) and the first interferogram 11-2 of the sub-stack SS2 (or the second set SS2) have a distance of λ/8 from one another or that the first Interferogram 11-2 of the sub-stack SS2 is only recorded or detected at a phase distance of λ/8 from the first interferogram of the sub-stack SS1.

In the present exemplary embodiment, each set or each sub-stack has sixty-four interference patterns generated at different phase differences. According to other exemplary embodiments, the sets can also, for example, each comprise only thirty-two, sixteen or eight interference patterns. For each partial measurement, the phase differences of the second sub-stack SS2 compared to the phase differences of the first sub-stack SS1 have a static offset with a predetermined phase difference, in particular a phase offset of π/2.

In this application, the term “static offset” refers to a previously specified offset in the phase shift between, for example, the first interference pattern 11-1 of the first substack SS1 and the first interference pattern 11-2 of the second substack SS2 and that remains constant during the measurement.

The static offset is therefore present between the sub-stacks or sets of a partial measurement. This partial measurement is based on the first and second sets.

In the method described for the first time in this application, the dynamic offset δ _dyn between adjacent partial measurements is also introduced during the evaluation.

Im the from the DE 10 2019 208 028 A1 known methods, only a static offset is used, which is λ/8 in the example there. In one variant, a measurement is evaluated twice with different sub-stacks (first sub-stack SS1 and second sub-stack SS2). A sub-stack is a section of the entire image stack (overall stack GS) of a measurement. The two substacks have a static or constant offset of λ/8.

The method explained in the present application differs from this conventional method, among other things, in that not only a static offset δ _stat between sub-stacks, but also a dynamic offset δ _dyn between two sets of pairs of different partial measurements is used.

The dynamic offset is a predeterminable phase shift between a first pair of sets, formed from the first and the second set, and at least one second pair of sets, formed from a third and a fourth set, as will be described in more detail below.

Calculation examples and algorithms are explained below. The following measurement parameters and evaluation parameters are used in the example. The measurement parameters are: N _{partial measurement} , N _{sampling_per_period} ; _scan N . The evaluation parameters are: N _select ; N _offset ; δ _stat = N _{sample_per_period} /4; Δ=N _{sampling_per_period} /(2*N _{partial measurement} ); λ. The meaning of these parameters is as follows: N _{partial measurement} Number of partial measurements for a measurement N _{samples_per_period} Number of interference patterns per period (one period corresponds to half an effective wavelength) _scan N Number of interference patterns for a partial measurement N _select Number of phase steps to be evaluated in a partial measurement _Noffset Offset of the first interference pattern for the evaluation (to avoid interference from possibly disturbed phase steps at the beginning of the phase shift) δ _stat Static offset δ _dynamic Dynamic offset Δ Increment of the dynamic offset λ Effective wavelength considering the NA

$${\text{Satz\_2: N}}_{\text{offset}}+1+{\mathrm{\delta }}_{\text{stat}}:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{ausw}}+{\mathrm{\delta }}_{\text{stat}}=25:88$$

$${\text{Satz\_3: N}}_{\text{offset}}+1+\mathrm{\Delta }\text{*}1:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{ausw}}+\mathrm{\Delta }*1=13:76$$

$${\text{Satz\_4: N}}_{\text{offset}}+1+{\mathrm{\delta }}_{\text{stat}}\mathrm{+\Delta }\text{*}1:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{ausw}}+{\mathrm{\delta }}_{\text{stat}}+\mathrm{\Delta }*1=29:92$$

n = 1 bis 8 ... $$\text{Satz\_2n}-1{\text{: N}}_{\text{offset}}+1+\mathrm{\Delta }\text{*}\left(\text{n}-1\right):{\text{N}}_{\text{offset}}+{\text{N}}_{\text{ausw}}+\mathrm{\Delta }*\left(\text{n}-\text{1}\right)=5+4\text{n:68}+4\text{n}$$

$$\text{PSn\_2n}-2:{\text{N}}_{\text{offset}}+1+{\mathrm{\delta }}_{\text{stat}}+\mathrm{\Delta }*\left(\text{n}-1\right):{\text{N}}_{\text{offset}}+{\text{N}}_{\text{ausw}}+{\mathrm{\delta }}_{\text{stat}}+\mathrm{\Delta }*\left(\text{n}-1\right)=21+4\text{n:4}+\text{4n}$$

In one embodiment, the following parameters were used: N _{partial measurement} = 8; N _{sample_per_period} = 64; N _sample = 128; N _offset = 8; N _select = 64. This results in the following values for the static and the dynamic offset: δ _stat = 16; Δ = 4; δ _dyn= (n-1)Δ, where n represents the numbering of the partial measurements. In this example, n runs from 1 to 8. An exemplary calculation can then proceed as follows. $${\text{Set\_1:N}}_{\text{offset}}+1:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{excl}}=9:72$$

$${\text{Set\_2: N}}_{\text{offset}}+1+{\mathrm{\delta }}_{\text{stat}}:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{excl}}+{\mathrm{\delta }}_{\text{stat}}=25:88$$

$${\text{Set\_3: N}}_{\text{offset}}+1+\mathrm{\Delta }\text{*}1:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{excl}}+\mathrm{\Delta }*1=13:76$$

$${\text{Sentence\_4: N}}_{\text{offset}}+1+{\mathrm{\delta }}_{\text{stat}}\mathrm{+\Delta }\text{*}1:{\text{N}}_{\text{offset}}+{\text{N}}_{\text{excl}}+{\mathrm{\delta }}_{\text{stat}}+\mathrm{\Delta }*1=29:92$$

n = 1 to 8 ... $$\text{Set\_2n}-1{\text{:N}}_{\text{offset}}+1+\mathrm{\Delta }\text{*}\left(\text{n}-1\right):{\text{N}}_{\text{offset}}+{\text{N}}_{\text{excl}}+\mathrm{\Delta }*\left(\text{n}-\text{1}\right)=5+4\text{n:68}+4\text{n}$$

$$\text{PSn\_2n}-2:{\text{N}}_{\text{offset}}+1+{\mathrm{\delta }}_{\text{stat}}+\mathrm{\Delta }*\left(\text{n}-1\right):{\text{N}}_{\text{offset}}+{\text{N}}_{\text{excl}}+{\mathrm{\delta }}_{\text{stat}}+\mathrm{\Delta }*\left(\text{n}-1\right)=21+4\text{n:4}+\text{4n}$$

• Parameter Satz_1 beschreibt den ersten Satz von Interferenzmustern, also im Beispiel den ersten Substapel SS1 der ersten Teilmessung. Das erste Interferenzmuster dieses Substapels ist N_offset+1, das letzte Interferenzmuster des Substapels entspricht N_offset+N_ausw. Dieser erste Substapel umfasst somit insgesamt 64 Interferenzmuster bzw. Bilder, und zwar die Interferenzmuster mit den Nummern 9 bis 72. Die Notation 9:72 bedeutet also „9 bis 72“.
• Parameter Satz_2 beschreibt den zweiten Satz von Interferenzmustern, also im Beispiel den zweiten Substapel SS2 der ersten Teiilmessung. Die Nummerierung der Bilder bzw. Interferenzmuster unterscheidet sich von der Nummerierung des ersten Satzes um den statischen Offset δ_stat , der im Beispielsfall 16 Bilder beträgt. Dieser zweite Substapel umfasst somit insgesamt 64 Interferenzmuster bzw. Bilder, und zwar die Interferenzmuster mit den Nummern 25 bis 88. Der erste Satz SS1 und der zweite Satz SS2 bilden ein erstes Satzpaar und werden in einer ersten Teilmessung verrechnet.

This notation is to be understood as follows:

• Parameter Set_1 describes the first set of interference patterns, ie in the example the first substack SS1 of the first partial measurement. The first fringe pattern of this substack is N _offset +1, the last fringe pattern of the substack is N _offset +N _select . This first sub-stack thus includes a total of 64 interference patterns or images, namely the interference patterns with the numbers 9 to 72. The notation 9:72 therefore means “9 to 72”.
• Parameter Set_2 describes the second set of interference patterns, ie in the example the second substack SS2 of the first partial measurement. The numbering of the images or interference patterns differs from the numbering of the first set by the static offset δ _stat , which is 16 images in the example. This second sub-stack thus includes a total of 64 interference patterns or images, specifically the interference patterns numbered 25 to 88. The first set SS1 and the second set SS2 form a first set pair and are calculated in a first partial measurement.

The subsequent third set and the fourth set, i.e. set_3 and set_4, result from the sub-stack of the second partial measurement. These include the interference patterns numbered 13 to 76 (third set) and numbered 29 to 92 (fourth set). This is where the dynamic offset δ _dyn = (n-1)Δ comes into play for the first time, with the variable n increasing step by step from partial measurement to partial measurement, specifically with a number N _{partial measurement} of partial measurements from the value 1 to N _{partial measurement} . The third set SS3 and the fourth set SS4 form a second set pair and are calculated in a second partial measurement.

The calculation for the other partial measurements of the measurement is carried out analogously.

In order to further explain the principle, an example with 8 (eight) partial measurements (8-fold partial measurements) is used again below. Table 1 shows the respective offsets of the partial measurements with number n: Table 1 n 1 2 3 4 5 6 7 8th δ _stat λ/8 λ/8 λ/8 λ/8 λ/8 λ/8 λ/8 λ/8 δ _dynamic 0 λ/32 2λ/32 3λ/32 4λ/32 5λ/32 6λ/32 7λ/32

While the static offset remains constant at λ/8, the dynamic offset varies from 0 to 7λ/32 with an equidistant step of λ/32. This means that the starting point (i.e. the image number at which the first sub-stack SS1 begins) shifts by λ/32 with each additional partial measurement.

Two evaluations result from each individual partial measurement. Therefore there are a total of 16 evaluations with 8 partial measurements. The results of all 16 evaluations are averaged. When averaging, the interference strips from 16 evaluations cancel each other out.

For further illustration, the 3A , 3B and 3C and 4 in connection with Table 1. These illustrate a first partial measurement ("No.1", 3A ), a second partial measurement (“No.2”, 3B ) and a third partial measurement (“No.3”) 3C ) and an overview with eight partial measurements ( 4 ) .

For simplicity, imagine that at No. 1 _δdyn = 0, at No. 2 _δdyn = λ/32, at No. 3 _δdyn = 2λ/32. This means that the starting point of each first sub-stack (SS1, SS3, SS5 etc.) is “somewhere else” for each subsequent partial measurement, i.e. always offset by λ/32. For the sake of clarity, λ/32 should correspond to “the distance between two interferograms or images” (see the dots in the lines of the respective sub-stacks).

First partial measurement or partial measurement 1 (No. 1, 3A ): For partial measurement 1 with δ _dyn =0, sub-stack SS1 and sub-stack SS2 (i.e. the first set and the second set of the first pair of sets) only have the static offset δ _stat to one another, as shown above. In other words: the first interferogram 11-2 of the second sub-stack is phase-shifted by the static offset relative to the first interferogram 11-1 of the first sub-stack.

Second partial measurement or partial measurement 2 (No. 2, 3B ): Other sub-stacks or sets are taken as a basis for carrying out the second partial measurement, namely a third sub-stack or a third set SS3 and a fourth sub-stack or a fourth set SS4. These belong to a second set pair. The sets of the second pair of sets have the same number of interferograms as the first and second sets, but a proportion of the interferograms of these third and fourth sets differ from those of the first and second sets.

The third sub-stack SS3 and the fourth sub-stack SS4 (i.e. the sets of the second pair of sets) continue to have the static offset to one another, i.e. the first interferogram 11-3 of the third sub-stack SS3 and the first interferogram 11-4 of the fourth sub-stack SS4 still have the distance have λ/8 to each other. In contrast to partial measurement 1, however, in partial measurement 2 the first interferogram 11-3 of the third sub-stack SS3 is offset in phase by δ _dyn = λ/32 (= “distance between two interferograms”) compared to the first interferogram 11-1 of the first sub-stack SS1 in partial measurement 1. In contrast to partial measurement 1, in partial measurement 2 the first interferogram 11-4 of the fourth sub-stack SS4 is offset in phase by δ _dyn = λ/32 (= “distance between two interferograms”) compared to the first interferogram 11 -2 of the second sub-stack SS2 in partial measurement 1.

Third partial measurement or partial measurement 3 (No. 3, FIG. 3C): Other sub-stacks or sets are taken as a basis for carrying out the third partial measurement, namely a fifth sub-stack or a fifth set SS5 and a sixth sub-stack or a sixth set SS6. These belong to a third set pair. In partial measurement 3, sub-stack SS5 and sub-stack SS6 still have the static offset to one another, ie first interferogram 11-5 of sub-stack SS5 and first interferogram 11-6 of sub-stack SS6 continue to be at a distance of λ/8 from one another. In contrast to partial measurement 1, however, in partial measurement 3 the first interferogram 11-5 of the sub-stack SS5 is offset in phase by _δdyn = 2λ/32 (=“distance of two times two interferograms”) compared to the first interferogram 11-1 of the sub-stack SS1 in partial measurement 1. Likewise, in contrast to partial measurement 1, in partial measurement 3 the phase of the first interferogram of the sub-stack SS6 is offset by δ _dyn= 2λ/32 (=“distance of two times two interferograms”) compared to the first interferogram of the sub-stack SS2 at partial measurement 1.

The explanations above apply analogously to the further partial measurements.

The dynamic offsets of the table are preferably chosen “symmetrically” with respect to a reference point or a reference position, in particular with respect to a focal point. To illustrate this, shows 4 below each other the partial measurements of a measurement with the respective offsets from Table 1 and the reference position RE, which corresponds to the focus position here.

The first fringe of the first set and the last fringe of the last set should be the same distance from the focus position. In 4 an example is shown. Given only a partial measurement (ie, a pair of sets), the distance from the first fringe to the focus position should be half the length of the phase shift. The length of the phase shift of the first pair of sets is λ/2 plus the static offset λ/8, i.e. 5λ/8. The first interference pattern should therefore be at a distance of 5λ/16 from the focus position. If there are a total of eight partial measurements, the last pair of sets should be shifted by 7A/32 compared to the first pair of sets. Half of 7λ/32 is 7λ/64. Therefore, the first fringe pattern of the first pair of sets is at 5λ16 plus 7λ/64, which is 27λ/64. In this way, artefacts due to different defocus values in the partial measurements can be avoided

An advantage of the new procedure over the procedure from DE 10 2019 208 028 A1 lies in the fact that the interference stripes are not only suppressed in each individual partial measurement by the first sub-stack SS1 and second sub-stack SS2, but can be further reduced by the further partial measurements, so that the interference stripes are even better suppressed after the overall evaluations.

The dynamic offset can be determined individually. The increment of the dynamic offset for two adjacent partial measurements can be determined, for example, as follows: λ / (4* number of partial measurements). The step size for the example is λ / (4* 8) = λ / 32. Other values are possible.

Based on 5 until 7 example results of vibration noise stripe suppression are explained. 5 shows an evaluated surface without suppression of the interference stripes. 6 shows an evaluated surface with suppression of the interference stripes according to the method from DE 10 2019 208 028 A1 . the 7 shows the evaluated surface with suppression of the clutter according to an embodiment of the method according to the invention (using static and dynamic offset)

QUOTES INCLUDED IN DESCRIPTION

This list of the 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

• DE 102019208028 A1 [0005, 0029, 0031, 0056, 0076, 0078]

Claims (7)

Measuring device (10) for interferometric measurement of a shape of a test surface (12), comprising: a beam splitting element (36) for splitting a measuring radiation (18) into a measuring wave (38) directed onto the test surface (12) and a reference wave (40), a Detector (48) for recording an interference pattern (62, 66) generated by superimposition of the reference wave (40) with the measurement wave (38) after its reflection on the test surface (12), a phase shifting device (52) for generating different phase differences between the measurement wave ( 38) and the reference wave (40) at the location of the detector (48), and an evaluation device (58) for determining the shape of the test surface (12) by calculating a first set (SS1) of interference patterns with at least one second set (SS1). Interference patterns, each set (SS1, SS2) having a plurality of interference patterns generated with different phase differences, and the phase differences of the second S sets (SS2) compared to the phase differences of the first set (SS1) have a respective static offset, characterized by a third set (SS3) of interference patterns and at least a fourth set (SS4) of interference patterns, the phase differences of the fourth set (SS4) opposite the phase differences of the third set (SS3) are phase-shifted by the respective static offset, and wherein a first set pair is formed by the first set (SS1) and the second set (SS2) and by the third set (SS3) and the fourth set ( SS4) a second pair of sets is formed, the first pair of sets and the second pair of sets being phase-shifted relative to one another by a predefinable dynamic offset, and the evaluation device (58) being designed to determine the shape of the test surface (12) by calculating the at least four sets (SS1 , SS2, SS3, SS4). Method for interferometric measurement of a form of a test surface (12) with the steps: splitting a measurement beam (18) into a measurement wave (38) directed at the test surface (12) and a reference wave (40), generating different phase differences between the measurement wave (38) and the reference wave (40) at the location of a detector (48), recording a first set (SS1) of interference patterns and at least a second set (SS2) of interference patterns, each of which is determined by superimposing the reference wave (40) with the measurement wave (38). the reflection of which is generated on the test surface (12), each set (SS1, SS2) having a plurality of interference patterns generated with different phase differences in each case, and the second set (SS2) being phase-shifted relative to the first set (SS2) by a respective static offset, characterized in that a third sentence (SS3) and at least a fourth sentence (SS4) is recorded, the vie The first set (SS4) is phase-shifted relative to the third set (SS3) by the respective static offset, and a first set pair is formed by the first set (SS1) and the second set (SS2) and by the third set (SS3) and the fourth set (SS4) forms a second pair of sets, the first pair of sets and the second pair of sets being phase-shifted relative to one another by a predeterminable first dynamic offset, and the shape of the test surface (12) being calculated by calculating the at least four sets (SS1, SS2, SS3, SS4) is determined. procedure after claim 2 , characterized in that a calculation of the first set (SS1) with the second set (SS2) forms a partial measurement; to carry out a measurement, several partial measurements are carried out based on different pairs of sets, and adjacent partial measurements have a dynamic offset dependent on the partial measurement. procedure after claim 2 or 3 , characterized in that the second pair of sets is shifted by the predeterminable first dynamic offset compared to the first pair of sets, that the third set is shifted by the predeterminable first dynamic offset compared to the first set and the fourth set is shifted by the predeterminable first dynamic offset compared to the second set Offset is shifted. Procedure according to one of claims 2 until 4 , characterized in that at least a third pair of sentences is formed, the third pair of sentences being formed by a recorded fifth sentence (SS5) and a recorded sixth sentence (SS6), the first pair of sentences and the third pair of sentences being offset by a predefinable second dynamic offset to one another are out of phase. procedure after claim 5 , characterized in that the predeterminable first dynamic offset and the predeterminable second dynamic offset differ from one another. Procedure according to one of claims 2 until 6 , characterized in that the dynamic offset for the partial measurements is symmetrical about a reference position (RE), the reference position preferably being a focus position of the measuring device.

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