Interferometric device

Abstract

A measuring device (10; 110; 210, 210; 310); 410, 410; 510, 510; 610, 610; 710) for interferometrically determining a characteristic (50; 710) of a shape (50) of a test surface (12) of an object to be tested (14), 52) comprising an illumination device (22) for generating an input wave (24); a separation module (18; 24) configured to generate two plane waves (32, 34) from the input wave; 118); 318, 318; 418, 418; 518), the two plane waves having parallel propagation directions and being offset from each other across the propagation directions; the wavefront adaptation module (20; 720) for generating two measurement waves (44, 46) by adapting respective wavefronts of plane waves offset from each other to a target shape of the optical test surface; and a detector (56) for capturing at least one interferogram (64) generated by superimposing the measurement waves (44r, 46r) after the measurement waves (44r, 46r) interact with the test surface.

Description

[0001] This application claims the priority of German Patent Application No. 10 2023 206874.5 filed on July 20, 2023. The entire disclosure content of this patent application is incorporated into the present application by reference. Technical Field

[0002] The invention relates to a measuring device and a method for interferometrically determining the shape properties of a test surface of a measured object. For example, optical elements used in microlithography can be measured as measured objects. In particular, the determined shape properties can be the shape of the test surface itself. Due to the need for smaller structures, higher requirements are placed on the optical properties of the optical elements used in microlithography. Therefore, the shape of the optical surfaces of these optical elements must be determined with the highest possible accuracy. Background Art

[0003] Interferometric measurement devices and methods in which a diffractive optical element generates a test wave and a reference wave from an input wave are known for high-precision interferometric measurements of optical surfaces down to the sub-nanometer range. A diffractive optical element can be used to adapt the wavefront of the test wave to the target surface of the object under test so that it will be perpendicularly incident on the target shape at each position and will be reflected back upon itself. Deviations from the target shape can then be determined with the aid of an interference pattern formed by the superposition of the reflected test wave and reference wave.

[0004] For example, 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 and a reference wave, the test wave having a wavefront at least partially adapted to the target shape of a test surface directed toward the test surface to be measured. The reference wave is reflected back to the CGH by a reflective optical element. Reference is also made to DE 10 2011004 376 B3.

[0005] A problem with such known measuring devices is that measurement errors may occur, for example, due to air schlieren or grating structure defects in the CGH. The measuring device can be operated in a high vacuum in order to minimize measurement errors that can be traced back to air schlieren. Calibration methods can be used to avoid measurement errors that can be traced back to grating structure defects. However, all these measures increase the outlay required for the interferometric measurement or the complexity of the measuring device. Summary of the invention

[0006] It is an object of the present invention to provide a measuring device and a method with which the above-mentioned problems are solved and in particular properties of the shape of a test surface can be determined with very high accuracy and at reasonable cost.

[0007] Solution according to the invention

[0008] According to the present invention, the above-mentioned purpose can be achieved, for example, by a measuring device for interferometrically determining the properties of the shape of a test surface of a measured object. The measuring device includes an irradiation device for generating an input wave and a separation module configured to generate two plane waves from the input wave, the two plane waves having parallel propagation directions and being offset from each other across the propagation directions. In addition, the measuring device includes a wavefront adaptation module and a detector, the wavefront adaptation module being used to generate two measurement waves by adapting the respective wavefronts of the plane waves offset from each other to the target shape of the optical test surface, and the detector being used to capture at least one interference pattern generated by superimposing the measurement waves after the measurement waves interact with the test surface.

[0009] For example, the property of the shape of the test surface that can be determined by the measuring device can be the test surface shape or a construction of the test surface shape or a derivative of the shape of the test surface itself, as will be explained in detail below. Plane waves with parallel propagation directions that are offset from each other can also be referred to as sheared plane waves. The offset is achieved in the transverse direction, i.e. across the propagation direction of the input wave. In this case, the extent of the offset is at least one wavelength of the input wave, and is advantageously at least a multiple of the wavelength of the input wave, i.e. at least about 0.5 μm, in particular at least about 10 μm in the case of wavelengths in the visible range. The wavefront adaptive module can be configured as a diffractive optical element, for example as a CGH or as a lens element, in particular as an aspherical lens element.

[0010] By generating two measurement waves using sheared plane waves, these measurement waves, which realize the functions of test wave and reference wave, propagate along almost the same beam path through the cavity between the wavefront adaptation module and the test surface. Therefore, possible air schlieren in the cavity have essentially the same influence on the two measurement waves, whereby the interference pattern is only slightly modified (if at all). Therefore, the construction according to the invention allows the measurement device to be operated at normal pressure or only slightly reduced ambient pressure, for example in a rough vacuum or a fine vacuum. In any case, operation in a high vacuum can be avoided, thereby reducing the costs for operating the measurement device. According to one embodiment, a laminar, homogeneous, uniform and stable flow suitable for passing through the cavity at normal pressure can also be ensured with the help of a specially designed flow box. Using calibration associated with a so-called gold mirror, the undesirable effects caused by this reproducible laminar flow can be almost completely eliminated.

[0011] Furthermore, defects or deviations in the wavefront adaptive module have almost the same effect on the sheared plane waves in the configuration of the measuring device according to the invention. Thus, possible grating structure defects, if the wavefront adaptive module is configured as a diffractive optical element, or possible lens element defects, if the wavefront adaptive module is configured as an aspherical lens element, have almost the same effect on both measured waves. Thus, a large amount of management can be performed without complex wavefront adaptive module calibration methods.

[0012] The generation of parallel plane waves according to the invention by means of a separation module is advantageous because, apart from a lateral offset, the two measurement waves are identical copies of one another, i.e. both measurement waves have a wavefront completely adapted to the surface to be measured. The only difference between the two measurement waves is the fact that, in the case of decomposing the measurement waves into local waves and marking the origin in the light source, local waves having the same origin in the light source or having the same coherence are laterally offset in the two measurement waves.

[0013] Thus, the two measuring waves propagate back on themselves exactly self-collimatingly, so that even if the (ideal or nominal) shape of the test surface deviates significantly from the shape of the flat plate, the measuring waves meet each other on the detector with exact precision. Thus, measurement artifacts are very reliably avoided and a high measurement accuracy can be obtained for measured objects with any desired surface shape, in particular with surface shapes that have significant deviations from a plane. Thus, the method already ensures that, already in the idealized case, the two measuring waves propagate back on themselves exactly and interfere on the detector in an ideal manner, since the waves arrive at the detector at the same point or nearly the same point.

[0014] In contrast, radiating waves (especially plane waves or extended waves) with non-parallel propagation directions at the wavefront adaptation module and / or using measuring waves with non-parallel propagation directions will not result in exact self-collimation of the measuring waves and will therefore lead to increased measurement artifacts.

[0015] Furthermore, the wavefront adaptation module can be certified relative to the plane mirror, for example with regard to material inhomogeneities of the wavefront adaptation module, ie the plane mirror is arranged instead of the measured object during the certification, which is also called calibration.

[0016] According to various embodiments, the separation module may be configured to generate more than two plane waves from the input wave, for example at least three or at least five plane waves having parallel propagation directions and laterally offset from each other. In this case, the wavefront adaptation module may be correspondingly configured to generate more than two measurement waves, for example at least three or at least five measurement waves, by adapting the respective wavefronts of the plane waves offset from each other to the target shape of the optical test surface. According to various further embodiments, the measuring device may also include more than one separation module, for example at least two, at least three or at least five separation modules, each separation module being configured to generate at least two plane waves having parallel propagation directions and offset from each other across the propagation directions from the input wave. According to various further embodiments, the measuring device may also include more than one wavefront adaptation module, for example at least two, at least three or at least five wavefront adaptation modules, so as to generate at least two, at least three or at least five measurement waves in each case by adapting the respective wavefronts of the plane waves offset from each other to the target shape of the optical test surface.

[0017] According to one embodiment, the measuring device is configured such that the measuring waves, after their interaction with the test surface and before being incident on the detector, pass through the separation module in a direction opposite to the radiation of the input waves, and the measuring waves are shifted towards each other in the process. In this context, "shifted towards each other" is understood to mean that the measuring waves are closer to each other after the shift than before the shift. In particular, the shift is achieved in such a way that the measuring waves are located exactly on top of each other.

[0018] According to a further embodiment, the measuring apparatus comprises an evaluation device which is configured to determine at least one derivative of the shape of the test surface using the at least one captured interference pattern.

[0019] According to another embodiment, the measuring device is configured to change the separation direction of the two plane waves. This can be achieved, for example, by rotating the separation module. Advantageously, the derivatives of the test surface are determined in different directions.

[0020] According to another embodiment, the measuring device is configured to change the lateral offset of the two plane waves and thus the size of the derivative difference or the spatial resolution. The two directions of the derivative and the size of the derivative differential and thus the spatial resolution can be modified, wherein the reflection condition of the self-collimation of the measuring wave at the reflector is always satisfied.

[0021] According to another embodiment, the evaluation device is configured to determine derivatives of the shape of the test surface at a plurality of positions on the test surface and to determine the shape of the test surface by integrating the determined derivatives. Advantageously, the integration utilizes derivatives in different directions at the respective positions on the test surface. The separation direction is understood to mean a direction in which the propagation directions of the two measurement waves are offset from each other.

[0022] Additionally, measurements with different sizes of separation differences can also be exploited by different sizes of beam offsets and can additionally be used in combination by calculation. In addition to considering derivatives in different directions, it is also advantageous to include derivative directions rotated 180° and so that the derivative differences have opposite signs, such that both left-handed and right-handed derivatives in a particular direction are captured at the surface.

[0023] According to another embodiment, the separation module includes at least one beam direction separation element, at least two, at least three or at least five beam direction separation elements according to various embodiment variations, and at least one direction matching element, at least two, at least three or at least five direction matching elements according to various embodiment variations, at least one beam direction separation element is configured to generate at least two intermediate waves, at least three or at least five intermediate waves according to different embodiment variations from the input wave, whose propagation directions are different, and the direction matching element is configured to generate the at least two mutually offset plane waves by matching the propagation directions of the at least two intermediate waves to each other.

[0024] According to a variant embodiment, the distance between the beam direction separation element and the direction matching element is of the order of magnitude of the distance between the wavefront adaptive module and the test surface. The same applies to the distance between the direction matching element and the wavefront adaptive module. The spatial resolution can be set by selecting the distance between the beam direction separation element and the direction matching element. On the one hand, changing the distance between the beam direction separation element and the direction matching element can cause a change in the offset of the measurement wave, and therefore a change in the size of the derivative difference, and therefore a change in the spatial resolution. On the other hand, the distance change can be used simultaneously for phase shifting, wherein the distance change is generally smaller than the distance change in the case of setting the spatial resolution.

[0025] According to a further embodiment, the beam direction separation element and / or the direction matching element is configured as a diffractive optical element, in particular a CGH. In order to prevent reflections, according to an embodiment variant, the CGHs are tilted relative to each other.

[0026] According to another embodiment, the beam splitting element and / or the direction matching element are respectively configured to diffract the incident radiation only into the zeroth diffraction order and, in absolute terms, into the first diffraction order, eg into the zeroth diffraction order and the +1st diffraction order.

[0027] According to a further embodiment, the beam splitting element and / or the direction matching element are configured to diffract the incident radiation only into +1 and -1 orders of diffraction, respectively.

[0028] According to a further embodiment, the beam direction separating element and the direction matching element are diffractive optical elements having an inverted configuration with respect to each other.

[0029] According to a further embodiment, the beam direction separation element and / or the direction matching element is configured as a shearing prism. In this respect, for example a Nomarski prism, a Wollaston prism or a Roger prism may come into consideration.

[0030] According to another embodiment, the measuring device further comprises a variable phase delay element (also called phase retarder) for phase shifting purposes. The latter can, for example, contain liquid crystals. The phase delay element brings different phase delays to wavefronts of different polarizations and thus, depending on the arrangement of the phase delay element in the beam path, brings different phase delays to the two intermediate waves already for the two measurement waves.

[0031] According to a further embodiment, the direction matching element is configured as a shearing prism which is arranged in an opposite orientation with respect to a shearing prism of the same type acting as the beam direction separating element.

[0032] According to another embodiment, the beam direction separation element is configured to separate a second intermediate wave from the input wave, wherein the first intermediate wave is a portion of the input wave that passes through the beam direction separation element without deflection. To this end, the beam direction separation element can be configured, for example, as a Roger prism or a CGH.

[0033] According to a further embodiment, the propagation direction of the intermediate wave is oriented symmetrically with respect to the propagation direction of the input wave.

[0034] According to another embodiment, the measuring device is configured to modify the distance between the beam direction separating element and the direction matching element. Modifying the distance can be used to modify the relative phase angle between two plane waves that are offset from each other. This provides a so-called phase shift. In diffractive optical elements, in particular CGHs, phase shifts are also possible by lateral displacements of the diffractive optical elements. Therefore, when the beam direction separating element and the direction matching element are each implemented as a diffractive optical element, the phase shift can also be achieved by lateral displacements of the beam direction separating element or the direction matching element. Modifying the distance between the beam direction separating element and the direction matching element can also be used for the purpose of changing the size of the derivative difference and thus changing the spatial resolution.

[0035] Embodiments of beam direction separation elements and / or direction matching elements in the form of CGHs are advantageous since additional diffraction auxiliary structures (e.g. cat's eyes or Littrow structures) can be present on the CGH and these structures facilitate adjustment of the two elements of the separation module relative to each other or relative to the interferometer (the entire optical unit upstream of the separation module, i.e. the illumination and imaging unit) and also facilitate adjustment of the wavefront adaptation module relative to the separation module or the entire interferometer.

[0036] According to another embodiment, the separation module comprises a shear plate. The latter preferably has parallel sides, but can also be implemented in the form of a wedge. In any case, the shear plate is configured such that a first output wave is generated from the light of the input wave and leaves the shear plate at an exit side opposite to the entry side without further reflection; a second output wave is generated by the light of the input wave which is initially reflected back and forth at the exit side and the entry side of the shear plate before it leaves at the exit side. The phase shift can be achieved, for example, by tilting the shear plate. Tilting the shear plate results in a modification of the path length difference of the two light beams in the plate.

[0037] According to another embodiment, the measuring apparatus is configured to measure a mirror for EUV microlithography as an object to be measured, in particular a projection lens or a mirror of an illumination system of an EUV projection exposure apparatus.

[0038] Furthermore, the above-mentioned objects can be achieved, for example, by a method for interferometrically determining the properties of the shape of a test surface of a measured object. The method comprises generating two plane waves with two parallel propagation directions by separating an input wave, the two plane waves being offset from each other across the propagation directions. Furthermore, two measurement waves are generated from the plane waves by adapting the respective wavefronts of the plane waves offset from each other to the target shape of the optical test surface, and at least one interference pattern is generated by superimposing the measurement waves after the measurement waves interact with the test surface and capturing the interference pattern.

[0039] The features specified in the aforementioned embodiments, exemplary embodiments or embodiment variants of the measuring device according to the invention may be applied accordingly to the method according to the invention, and vice versa. These and other features according to the embodiments of the invention will be explained in the description of the drawings and in the claims. The individual features may be implemented as embodiments of the invention individually or in combination. In addition, they may describe advantageous embodiments that can be protected independently and, depending on the circumstances, only claim these advantageous embodiments during or after the pendency of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The aforementioned features and further advantageous features of the invention will be illustrated in the following detailed description of an exemplary embodiment or an embodiment or an embodiment variant according to the invention with reference to the attached schematic drawings, in which:

[0041] Figure 1 An exemplary embodiment of a measuring device for the interferometric determination of the shape of a test surface of a measured object is shown, the measuring device having a separation module made of two diffractive optical elements for generating two parallel plane waves and a wavefront adaptation module for generating two measurement waves from the plane waves,

[0042] Figure 2 shows the certification or calibration according to Figure 1 An exemplary embodiment of a device of a wavefront adaptation module,

[0043] Figure 3 shows the certification or calibration including the Figure 1 An exemplary embodiment of an interferometer device of a separate module of a measuring device,

[0044] Figure 4 A further exemplary embodiment of a measuring device for interferometric determination of the shape of a test surface of a measured object is shown, which has a design and Figure 1 The diffractive optical elements of different separation modules are designed,

[0045] Figure 5 A further exemplary embodiment of a measuring device for the interferometric determination of the shape of a test surface of a measured object is shown, wherein the diffractive optical elements of the separation module have a tilted arrangement,

[0046] Figure 6 A further exemplary embodiment of a measuring device for the interferometric determination of the shape of a test surface of a measured object is shown, wherein one of the two diffractive optical elements of the separation module is formed by a shear prism,

[0047] Figure 7 A further exemplary embodiment of a measuring device for the interferometric determination of the shape of a test surface of a measured object is shown, wherein the two diffractive optical elements of the separation module are formed by shear prisms,

[0048] Figure 8 A further exemplary embodiment of a measuring device for the interferometric determination of the shape of a test surface of a measured object is shown, wherein one of the two diffractive optical elements of the separation module is configured as a Roger prism,

[0049] Fig. 9 A further exemplary embodiment of a measuring device for interferometrically determining the shape of a test surface of a measured object is shown, wherein the separation module is formed by a shear plate, and

[0050] Fig.10 A further exemplary embodiment of a measuring device for the interferometric determination of the shape of a test surface of an object to be measured is shown, wherein the wavefront adaptation module is formed by a lens element. DETAILED DESCRIPTION

[0051] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS ACCORDING TO THE PRESENT INVENTION

[0052] In the exemplary embodiments or embodiments or embodiment variants described below, elements that are similar to each other in function or structure have the same or similar reference numerals as much as possible. 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 present invention.

[0053] For ease of description, a Cartesian xyz coordinate system is shown in the drawings, from which the corresponding positional relationships of the components shown in the drawings are apparent. Figure 1 , the y direction extends vertically out of the plane of the drawing, the x direction extends toward the right, and the z direction extends downward.

[0054] Figure 1 An exemplary embodiment 10 of a measuring device for interferometrically determining properties of a shape z(x, y) (reference numeral 50) of a test surface 12 of a measured object 14 is schematically depicted. The determined properties may be the shape z(x, y) of the test surface 12 or one or more derivatives dz / dx and / or dz / dy (reference numeral 52) of the shape 50 of the test surface 12 itself. The shape of the test surface 12 or in particular the target shape of the test surface 12 may be non-spherical; in particular, it may be designed as an aspherical surface or as a free-form surface. An aspherical surface is understood to be a rotationally symmetric surface which deviates from any sphere by at least 0.05 mm, in particular by at least 0.1 mm, at least 1 mm or at least 5 mm. In the present context, such an aspherical surface is also referred to as a rotationally symmetric aspherical surface or simply as a sphere. A free-form surface is understood to be a shape having a deviation of at least 5 μm, in particular at least 10 μm, from any rotationally symmetric aspherical surface. Furthermore, the free-form surface deviates from any sphere by at least 0.05 mm, in particular at least 0.1 mm, at least 1 mm or at least 5 mm.

[0055] The measuring device 10 is particularly suitable for very precisely measuring optical surfaces of aspherical mirrors or freeform mirrors for microlithography using exposure radiation in the extreme ultraviolet (EUV) spectral range. The EUV wavelength range extends to wavelengths below 100 nm and particularly relates to wavelengths of about 13.5 nm or about 6.8 nm. In order to reduce or correct aberrations, such optical surfaces require shape determination down to the sub-nanometer range. However, the measuring device 10 is also suitable for measuring the surfaces of many other objects.

[0056] The measuring device 10 comprises a beam generation and evaluation device 16, a separation module 18 and a wavefront adaptation module configured as a diffractive optical element 20. The beam generation and evaluation device 16 comprises an illumination device 22 for generating an input wave 24 and comprising a radiation source 26 and a collimating lens element 28.

[0057] The radiation source 26 generates measurement radiation 30 in the form of an extended wave that is sufficiently coherent for interference. According to an exemplary embodiment, the radiation source 26 is a helium-neon laser with a wavelength of about 633 nm. The measurement radiation 30 can also have different wavelengths in the visible or invisible wavelength range of electromagnetic radiation. The measurement radiation 30 generated by the radiation source 26 initially passes through a beam splitter 31 and then through a collimating lens element 28.

[0058] The input wave 24 generated by the illumination device 22 is a plane wave, i.e. it has a plane wavefront 25. The input wave 24 is incident on the separation module 18, which is configured to separate the input wave 24 into two plane waves 32 and 34, i.e. waves having plane wavefronts 33 and 35 with parallel propagation directions. In other words, the beam paths of the plane waves 32 and 34 are offset from each other across their propagation directions in the separation direction. Figure 1 In the exemplary embodiment of , separation is achieved by separating plane wave 34 from input wave 24 , while plane wave 32 is formed by the remaining radiated portion of input wave 24 . Figure 1 The separation direction depicted in is the x-direction.

[0059] In accordance with Figure 1 In an exemplary embodiment of the present invention, the separation module 18 comprises a beam direction separation element in the form of a diffractive optical element 36 designed as a CGH and a direction matching element in the form of a diffractive optical element 38 also designed as a CGH. The beam direction separation element is configured to generate two intermediate waves 40 and 42 with different propagation directions from the input wave 24. The intermediate waves 40 and 42 have plane wavefronts 41 and 43 that are tilted to each other. In accordance with Figure 1 In an exemplary embodiment of the present invention, intermediate waves 40 and 42 are generated by separating an intermediate wave 42 having a slightly inclined propagation direction from input wave 24, while intermediate wave 40 is formed by the remaining radiation portion of input wave 24. To this end, diffractive optical element 36 is configured to generate 0th and 1st order diffraction of input wave 24 in transmission, wherein the 0th order diffraction is used as intermediate wave 40, and the 1st order diffraction is used as intermediate wave 42.

[0060] The direction matching element 38 is configured to match the propagation directions of the two intermediate waves 40 and 42 to each other, thereby generating two plane waves 32 and 34 offset from each other. Figure 1In the exemplary embodiment of , this is achieved by slightly deflecting the intermediate wave 24, specifically so that the intermediate wave 24, now in the form of the plane wave 32, receives the same propagation direction as the intermediate wave 40. To this end, the diffractive optical element 38 is configured to generate in each case in transmission a 0th and a 1st diffraction order of the intermediate waves 40 and 42, wherein the 0th diffraction order of the intermediate wave 40 acts as the plane wave 32 and the 1st diffraction order of the intermediate wave 42 acts as the plane wave 34. In other words, the two diffractive optical elements 36 and 38 have configurations that are inverted with respect to one another.

[0061] For example, the wavefront adaptation module in the form of a diffractive optical element 20 designed as a CGH is configured to adapt the corresponding wavefronts of the plane waves 32 and 34 to the target shape of the test surface 12 by diffraction into the first order of diffraction, so that for example two measurement waves 44 and 46 are generated, each with an aspherical wavefront 45 and 47. As a result of adapting the wavefronts 45 and 47 to the target shape, the measurement waves 44 and 46 will be incident in a perpendicular manner at every position on the test surface 12, and if the test surface corresponds exactly to the target shape, the measurement waves 44 and 46 will be reflected back exactly on themselves. Deviations from the target shape lead to interference patterns, which allow the real surface shape to be determined very accurately in the superposition of the reflected measurement waves 44r and 46r (i.e., the measurement waves 44 and 46 after the interaction of the measurement waves 44 and 46 with the test surface 12), which superposition is described in more detail below.

[0062] The reflected measuring waves 44r and 46r pass through the diffractive optical element 20 acting as a wavefront adaptation module and, in doing so, are converted back into mutually offset waves running in parallel, which differ from the plane waves 32 and 34 only in the deviation caused by the deviation of the test surface 12 from its target shape. The reflected measuring waves 44r and 46r thus travel in the beam path of the intermediate waves 40 and 42 and of the input wave 24, albeit in a direction opposite to the direction of radiation of the input wave 24. When passing through the separation module 18 in opposite directions, the measuring waves 44r and 46r are offset towards each other, precisely so that the measuring waves 44r and 46r are located exactly on each other.

[0063] When the measuring wave 44r passes through the diffractive optical element 38, the wave generated by the 0th order diffraction of the measuring wave 44r is still referred to as the measuring wave 44r on another route of the beam path, and the 1st order diffraction of the measuring wave 44r represents the unused remaining wave 72. Similarly, when the measuring wave 46r passes through the diffractive optical element 38, the wave generated by the 1st order diffraction of the measuring wave 46r is still referred to as the measuring wave 46r on another route of the beam path, and the 0th order diffraction of the measuring wave 46r also represents the unused remaining wave 72. Similarly, the unused remaining wave 72 is generated at the diffractive optical element 36.

[0064] Such residual waves may also appear in the outward branch when the input wave 24 passes through the diffractive optical element 36 and the diffractive optical element 38. According to one embodiment, the diffractive optical elements 36 and 38 may be embodied as blazed gratings such that unwanted diffraction orders are substantially suppressed in their efficiency.

[0065] Thus, after passing through the separation module 18 in the reverse direction, the reflected measurement waves 44r and 46r follow the same beam path, i.e. they are no longer offset from each other, and have wavefronts 60 and 62 that differ from the plane wavefront only by the deviations caused by the deviations of the test surface 12 from its target shape. Since the measurement waves 44 and 46 are incident on the test surface 12 with a slight offset from each other, these deviations of the test surface 12 become apparent through phase deviations in the wavefronts 60 and 62.

[0066] In the beam generation and evaluation device 16, the reflected measuring waves 44r and 46r are deflected by the beam splitter 31 in the direction of the camera 58 after they have passed through the collimating lens element 28. Both the reflected measuring waves 44r and 46r pass through the eyepiece 54 and are incident on the detector 56 of the camera 58. The detector 56 comprises, for example, a CCD sensor and captures a plurality of interference patterns 64 generated by the superposition of the reflected measuring waves 44r and 46r. The above-mentioned residual waves 72 have such a beam path that their radiation does not contribute or only contributes insignificantly to the interference pattern 64, because these residual waves are no longer incident on the camera 58 at the same position as the used waves and therefore no longer meet the conditions for constructive interference. The local residual waves and the local used waves of the same source point arrive at the camera at different positions, or in other words, the residual waves and the used waves at a point on the camera 58 are no longer completely coherent with each other because they originate from different source positions.

[0067] The captured interference pattern 64 is generated by modifying the relative phase angle between the two mutually offset plane waves 32 and 34 or the two measurement waves 44 and 46; this is called phase shift. Figure 1 In an embodiment of the present invention, this is achieved by modifying the distance between a diffractive optical element 38 serving as a direction matching element and a diffractive optical element 36 serving as a beam direction separating element. In this case, the distance is modified by an axial displacement of the diffractive optical element 38. For this purpose, a displacement device is provided, which is indicated by a double-headed arrow 39. Alternatively, the diffractive optical element 36 can also be displaced axially. According to other embodiments, the phase shift can also be achieved by changing the wavelength λ of the measuring radiation 30 or by other measures. The phase shift can also be achieved by a lateral displacement of a beam direction separating element designed as a diffractive optical element 36 or a direction matching element designed as a diffractive optical element 38.

[0068] Furthermore, the measuring device 10 is configured to change the separation direction of the two plane waves 32 and 34. To this end, according to Figure 1 The separation module 18 in the embodiment of the present invention is rotatably mounted in the axial direction, ie rotatably mounted relative to the incident radiation direction of the input wave 24. According to a variant embodiment, the separation module 18 is thus rotatably mounted in the axial direction. Figure 1 The interference pattern 64 (hereinafter also referred to as the first set of interference patterns) is captured in the position shown, after which it is rotated 90°, in which the separation direction is oriented in the x-direction. For this purpose, a rotation device 65 shown by a double-headed arrow is provided. This arranges the separation direction in the y-direction. In this arrangement, the second set of interference patterns 64 is determined with an appropriate phase shift.

[0069] Both sets of captured interference patterns 64 are transmitted to an evaluation device 66, which comprises a first evaluation module 68 and a second evaluation module 70. In the first evaluation module 68, the derivatives dz / dx and dz / dy (reference numeral 52) of the shape 50 of the test surface 12 are determined from the interference patterns 64. In this case, the first set of interference patterns 64 determined for the separation in the x-direction is used to determine the derivative dz / dx, and the second set of interference patterns 64 determined for the separation in the y-direction is used to determine the derivative dz / dy.

[0070] The derivatives 52 are determined at the individual coordinate points (x, y) of the test surface 12, i.e. at a plurality of positions on the test surface 12. By integrating the determined derivatives dz / dx and dz / dy, the shape 50 of the test surface 12 is determined in the second evaluation module 70. The measuring device 10 is operated in air at normal pressure. This is possible because the interference waves, in particular the measuring waves 44r and 46r, pass through almost the same beam path, so that air turbulence or air pressure changes have hardly any influence on the measurement accuracy of the measuring device 10. Therefore, it can be carried out without the measuring device 10 being arranged in a vacuum, in particular in a high vacuum.

[0071] The size of the beam separated between the intermediate waves 40 and 42 and thus the spatial resolution of the shape measurement can be changed by changing the distance between the diffractive optical elements 36 and 38.

[0072] according to Figure 1 Another advantage of the measuring device 10 is that the test surface 12 to be checked is also used as a reference surface, so that no additional optical element is required as a reference reflector. This eliminates the expense of producing and checking such a reference reflector. This also eliminates possible influences of errors on the measurement. In other test optical units with reference or Fizeau elements, the test wave and the reference wave take very different optical paths and therefore also experience errors of different magnitudes. Furthermore, the use of the test surface 12 as a reference surface enables a contrast ratio of similar size to be achieved, regardless of the reflectivity of the test reflector.

[0073] according to Figure 1A further advantage of the measuring device 10 is that the contrast is virtually independent of the reflectivity of the measured mirror, in particular virtually independent of whether the mirror is coated or not.

[0074] according to Figure 1 A further advantage of the measuring device 10 is that, in particular, in addition to a coherent light source, an incoherent light source can also be used, with the advantage that annoying reflections in the interference pattern are significantly suppressed.

[0075] It should also be mentioned that if the wavefront adaptive module 20 is designed as a diffractive optical element or CGH, it is a diffractive optical element with only one encoding function and the test wave and the reference wave see the module almost identically, with the result that the diffractive optical element typically has fewer production errors and can be managed without special additional external qualifications and without special calibration measurements of the module performed with additional mirrors in the test setup.

[0076] Figure 2 The certification or calibration of a diffractive optical element 20 used as a wavefront adaptation module is shown. For this purpose, a plane calibration mirror 74 is arranged instead of the measured object 14. The interference pattern generated by the superposition of plane waves 32 and 34 (here referred to as calibration waves 76 and 78) that pass through the diffractive optical element 20 with the 0th order of diffraction is evaluated on the detector 56. Thereby, material inhomogeneities in the substrate of the diffractive optical element 20, for example, can be determined.

[0077] Figure 3 The certification or calibration of an interferometer comprising a separation module 18 is shown. For this purpose, a plane calibration mirror 74 is arranged instead of the diffractive optical element 20. The interference pattern resulting from the superposition of the plane waves 32 and 34 reflected at the calibration mirror 74 on the detector 16 is evaluated. From this, material inhomogeneities in the substrate of the diffractive optical elements 36 and 38 or other deviations in the beam path of the interferometer can be determined, for example. Taking into account the Figure 3 Certification, according to Figure 2 The certification of can be used to separate the impact of the errors of the wavefront adaptation module from almost all other error contributions in the interferogram.

[0078] Figure 4 A further exemplary embodiment 110 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. The measuring device 110 is used in conjunction with a method according to Figure 1 The measuring device 10 differs only in the configuration of the separation module 18, which is Figure 4 The light beam generating and evaluating device 16 is only in the phase shift method. Figure 4 Schematically depicted in .

[0079] The separation module 118 further includes diffractive optical elements configured as a beam direction separation element and a direction matching element, which are indicated by reference numerals 136 and 138, respectively. Figure 4 Diffractive optical elements 136 and 138 of FIG. 1 differ from diffractive optical elements 36 and 38 in being configured to operate in the 1st diffraction order rather than in the 0th / 1st diffraction order. That is, intermediate waves 140 and 142 are generated in transmission at diffractive optical element 136 in the −1st and +1st diffraction orders of input wave 24. As a result, the propagation directions of intermediate waves 140 and 142 are symmetrically oriented with respect to the propagation direction of input wave 24. In addition, the first diffraction order of intermediate wave 140 is then used as plane wave 32, and the first diffraction order of intermediate wave 142 is used as plane wave 34.

[0080] Figure 5 A further exemplary embodiment 210 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. The measuring device 210 is used in conjunction with a method according to Figure 4 The measuring device 110 differs only in that the diffractive optical elements 136 and 138 are advantageously tilted by at least 1°, in particular by at least 10°, relative to a perpendicular arrangement relative to the beam direction of the input wave 24. By displacing the diffractive optical element 138, which serves as a direction matching element, the tilting can cause a phase shift, as in the case of Figure 1 For this purpose, a displacement device indicated by a double-headed arrow 39 is provided in the measuring device 210 .

[0081] Another important advantage of this tilted version of the separation module is the suppression of reflected light reflected at the unstructured surfaces of the beam direction separation element and the direction matching element in the non-tilted case, in which case the reflected light returns back upon itself in the direction of the input wave 24 .

[0082] Figure 6 A further exemplary embodiment 310 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. The measuring device 310 is used in conjunction with a method according to Figure 5 The measurement device 210 of FIG. 1 is different only in that the element used as a beam splitting element is not formed by the diffractive optical element 136 but by a shearing prism 336. The shearing prism 336 is configured to split the input wave 24 (including unpolarized light or diagonally polarized light in this exemplary embodiment) into two beams that are tilted relative to each other and have different polarizations, which are then used as intermediate waves 140 and 142.

[0083] Exemplary embodiments of the shear prism 336 may be designed as a Nomarski prism 336a or a Wollaston prism 336b. Figure 6As shown, in the case of the Nomarski prism 336a and the Wollaston prism 336b, the non-polarized or diagonally polarized input wave 24 is split into two linearly polarized beams at right angles to each other, which are deflected in different directions relative to the input wave 24 and serve as intermediate waves 140 and 142. In this case, the intermediate wave 140 can be symmetrically oriented relative to the propagation direction of the input wave 24.

[0084] According to Figure 5 Similar to the measuring device 210 of FIG. 3 , the phase shift in the measuring device 310 may be caused by the axial displacement of the diffractive optical element 138 used as the direction matching element. In addition, the phase shift may also be caused by the lateral displacement of the shearing prism 336.

[0085] Alternatively, a variable phase delay element 339 can be used for phase shifting purposes. The variable phase delay element 339 can be arranged between the diffractive optical element 118 of the separation module 318 and the diffractive optical element 20 used as the wavefront adaptive module 20. The variable phase delay element 339 (also called a phase retarder) can contain, for example, liquid crystal and bring different phase delays to wavefronts of different polarizations. Therefore, this results in different phase delays of the two plane waves 32 and 34 or the two return measurement waves 44r and 46r.

[0086] Figure 7 A further exemplary embodiment 410 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. The measuring device 410 is used in conjunction with a method according to Figure 6 The measuring device 310 of FIG. 4 differs only in that the direction matching element is also formed by a shearing prism 438. In the resulting separation module 418, the function of the shearing prism 438 corresponds to the function of the shearing prism 336 in the opposite direction. That is, the shearing prism 438 is arranged in the opposite orientation relative to the shearing prism 336 of the same type. According to an exemplary embodiment not shown in the drawings, the separation module 18 can also include a diffractive optical element as a beam direction separation element and a shearing prism as a direction matching element.

[0087] Figure 8 A further exemplary embodiment 510 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. The measuring device 510 is used in conjunction with a method according to Figure 6 The measuring device 310 of FIG. 1 is different only in that the shear prism 336 used as the beam direction separation element is not configured as a Nomarski prism or a Wollaston prism, but as a Roger prism 336c. Figure 6 The embodiments are different. Figure 6The intermediate waves 140 and 142 formed by the shear prism 336 are oriented symmetrically with respect to the direction of propagation of the input wave 24, according to Figure 8 The intermediate wave 40 in the measuring device 510 has the same propagation direction as the input wave 24 due to the use of the Roger prism 336 c, while the intermediate wave 42 is inclined relative to the input wave 24 .

[0088] The resulting beam guidance in the separation module 518 thus corresponds substantially to the beam guidance also present according to Figure 1 The beam guide in Example 10. Therefore, according to Figure 1 A diffractive optical element 38 or a diffractive optical element having a similar configuration may be used as a direction matching element. Alternatively, a Roger prism having a reverse orientation may be used instead of the diffractive optical element 38 as the direction matching element. According to another exemplary embodiment not shown in the drawings, the diffractive optical element 36 is combined with a Roger prism used as a direction matching element.

[0089] like Figure 8 As shown, an unpolarized or diagonally polarized input wave 24 is converted into an ordinary beam (intermediate wave 40) that passes through the prism without deflection and an extraordinary beam (intermediate wave 42) that is deflected in the case of the Roger prism 336c. The two beams are polarized at right angles to each other. The size of the beam separation and therefore the spatial resolution of the shape measurement can be changed by shifting the two partial elements of the Roger prism or by changing the distance between the Roger prism (i.e., the shear prism 336) and the diffractive optical element 38.

[0090] Fig. 9 A further exemplary embodiment 610 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. The measuring device 610 is used in conjunction with a method according to Figure 1 The measuring device 10 of FIG. 6 differs only in that the separation module is not formed by the two diffractive optical elements 36 and 38 , but by a shear plate 618 .

[0091] The shear plate 618 has parallel side faces, i.e. it has the shape of a cuboid with parallel entry face 619 and exit face 620 for measuring radiation 30. The shear plate is arranged obliquely with respect to the input wave 24, i.e. the normals of the entry face 619 and the exit face 620 are inclined with respect to the propagation direction of the input wave 24, advantageously precisely by at least 1°, in particular by at least 10° or at least 20°. Some radiation of the input wave 24 enters the shear plate 618 at face 619 and is separated at the opposite face 620.

[0092] Some of the radiation incident at face 620 again directly exits shear plate 618 and forms first plane wave 32. The remainder of the radiation is reflected at face 620. Due to the tilt of shear plate 618, the reflected beam is tilted relative to the beam incident at face 620. Some of the reflected radiation is reflected again at face 619 and then emerges at face 620 as second plane wave 34. Second plane wave 34 is offset relative to first plane wave 32.

[0093] The phase shift can be achieved by tilting the shear plate 618. For this purpose, the measuring device 610 has a tilting device 639. Other phase shifting options can also be applied here, for example changing the wavelength of the measuring radiation 30 as described above.

[0094] Fig.10 A further exemplary embodiment 710 of a measuring device for interferometric determination of the shape of a test surface 12 of a measured object 14 is schematically depicted. Figure 1 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Fig. 9 The measuring device 10 , 110 , 210 , 310 , 410 , 510 or 610 differs only in that the wavefront adaptation module is not configured as a diffractive optical element but as a lens element 720 , in particular as an aspherical lens element.

[0095] The above description of exemplary embodiments, embodiments or embodiment variations should be understood as by way of example. The disclosure thus achieved first enables those skilled in the art to understand the present invention and the advantages associated therewith, and secondly includes changes and modifications to the described structures and methods, which are also apparent in the understanding of those skilled in the art. Therefore, all such changes and modifications, as long as they fall within the scope of the present invention as defined in the appended claims, and equivalents are intended to be covered by the protection of the claims.

[0096] Reference numerals list

[0097] 10. Measuring equipment

[0098] 12 Test Surface

[0099] 14 Object to be measured

[0100] 16 Beam generation and evaluation device

[0101] 18 Separation Module

[0102] 20Diffractive optical elements as wavefront adaptive modules

[0103] 22 Irradiation device

[0104] 24 Input Wave

[0105] 25 Plane wavefront

[0106] 26 Radiation Source

[0107] 28 Collimating lens elements

[0108] 30 Measuring radiation

[0109] 31 beam splitter

[0110] 32 First plane wave

[0111] 33 Plane wavefront

[0112] 34 Second plane wave

[0113] 35 Plane wavefront

[0114] 36Diffractive optical element as a beam direction separation element

[0115] 38Diffractive optical elements as direction matching elements

[0116] 39 displacement device

[0117] 40 First intermediate wave

[0118] 41 Plane wavefront

[0119] 42 Second intermediate wave

[0120] 43 Plane wavefront

[0121] 44 First measurement wave

[0122] 44r The first reflected measurement wave

[0123] 45 Wavefront of the first measurement wave

[0124] 46 Second measurement wave

[0125] The second measurement wave reflected by 46r

[0126] 47 Wavefront of the second measurement wave

[0127] 50Test surface shapes

[0128] 52 Derivative of the shape of the test surface

[0129] 54 eyepiece

[0130] 56 detectors

[0131] 58 Camera

[0132] 60 Wavefront of the first reflected measured wave

[0133] 62 Wavefront of the second reflected measured wave

[0134] 64 Interference Pattern

[0135] 65 Rotating device

[0136] 66 Evaluation Device

[0137] 68 First Assessment Module

[0138] 70 Second Assessment Module

[0139] 72 Remaining Waves

[0140] 74 plane calibration mirror

[0141] 76 Calibration Wave

[0142] 78 calibration wave

[0143] 110 Measuring equipment

[0144] 118 Separation Module

[0145] 136 Diffractive optical element as a beam direction separation element

[0146] 138 Diffractive optical elements as direction matching elements

[0147] 139 Wavelength Manipulator

[0148] 210 Measuring equipment

[0149] 310 Measuring equipment

[0150] 318 Separation Module

[0151] 336 Shear Prism

[0152] 336a Nomarski prism

[0153] 336b Wollaston Prism

[0154] 336c Roger Prism

[0155] 339 Variable Phase Delay Element

[0156] 410 Measuring equipment

[0157] 418 Separation Module

[0158] 438 Shear Prism

[0159] 510 Measuring equipment

[0160] 518 Separation Module

[0161] 610 Measuring equipment

[0162] 618 Clipboard

[0163] 619 Inlet or outlet face

[0164] 620 Inlet or outlet face

[0165] 639 Tilt Device

[0166] 720 as a lens for the wavefront adaptive module

Claims

1. A measuring device (10; 110; 210; 310; 410; 510; 610; 710) for interferometric determination of a property (50; 52) of a shape (50) of a test surface (12) of a measured object (14), comprising: - an illumination device (22) for generating an input wave (24), - separation module (18; 118; 318; 418; 518), configured to generate two plane waves (32, 34) from the input wave, the two plane waves (32, 34) having parallel propagation directions and offset from each other across the propagation directions, a wavefront adaptation module (20; 720) for generating two measurement waves (44, 46) by adapting the respective wavefronts of mutually offset plane waves to a target shape of the optical test surface, and - a detector (56) for capturing at least one interference pattern (64) generated by superimposing the measurement waves (44r, 46r) after their interaction with the test surface.

2. The measuring device according to claim 1, The arrangement is such that the measurement waves (44r, 46r) pass through the separation module (18) in a direction opposite to the radiation of the input wave (24) after they interact with the test surface and before they are incident on the detector (56), and the measurement waves are deflected towards each other in the process.

3. The measuring device according to claim 1 or 2, An evaluation device (66) is included, which is configured to use the at least one captured interference pattern (64) to determine at least a derivative (52) of the shape of the test surface (50).

4. The measuring device according to any one of the preceding claims, The device is configured to change the separation direction of the two plane waves (32, 34).

5. The measuring device according to claim 3 or 4, in, The evaluation device (66) is configured to determine derivatives (52) of the shape of the test surface at a plurality of locations on the test surface and to determine the shape (50) of the test surface by integrating the determined derivatives.

6. The measuring device according to any one of the preceding claims, in, The separation module (18; 118; 318; 418; 518) comprises a beam direction separation element (36; 136; 336) and a direction matching element (38; 138; 438), wherein the beam direction separation element is configured to generate two intermediate waves (40, 42; 140, 142) having a propagation direction different from that of the input wave (24), and the direction matching element is configured to generate the two mutually offset plane waves (32, 34) by matching the propagation directions of the two intermediate waves with each other.

7. The measuring device according to claim 6, in, The beam direction separation element and / or the direction matching element is configured as a diffractive optical element (36; 136, 38; 138).

8. The measuring device according to claim 7, in, The beam splitting element (36) and / or the direction matching element (38) are respectively configured to separate the incident radiation (24; 40, 42) diffracts only into the zeroth order and, in absolute terms, into the first order.

9. The measuring device according to claim 7, in, The beam splitting element (136) and / or the direction matching element (138) are configured to diffract incident radiation into only +1st and -1st diffraction orders, respectively.

10. The measuring device according to any one of claims 6 to 9, in, The beam direction separation element (36; 136) and the direction matching element (38; 138) are diffractive optical elements having inverted configurations with respect to each other.

11. The measuring device according to claim 6, in, The beam direction separation element (336) and / or the direction matching element (438) are configured as shear prisms.

12. The measuring device according to claim 6 or 11, in, The direction matching element (438) is configured as a shearing prism which is arranged in an opposite orientation relative to a shearing prism of the same type used as the beam direction separating element (336).

13. The measuring device according to any one of claims 6 to 12, in, The beam direction separation element (36; 336) is configured to separate the second intermediate wave (42) from the input wave (24), wherein the first intermediate wave (40) is a portion of the input wave that passes through the beam direction separation element without deflection.

14. The measuring device according to any one of claims 6 to 13, in, The propagation direction of the intermediate wave (140, 142) is oriented symmetrically with respect to the propagation direction of the input wave (24).

15. The measuring device according to any one of claims 6 to 14, The device is configured to modify the distance between the beam direction separating element (36) and the direction matching element (38).

16. The measuring device according to any one of claims 1 to 5, in, The separation module (618) includes a shear plate.

17. The measuring device according to any one of the preceding claims, The invention is configured to measure a mirror for EUV microlithography as a measured object (14).

18. A method for interferometric determination of a property (50; 52) of the shape of a test surface (12) of a measured object (14), comprising the following steps: - generating two plane waves (32, 34) having two parallel propagation directions by splitting the input wave, said two plane waves being offset from each other across said propagation directions, - generating two measurement waves (44, 46) from the plane wave by adapting the respective wavefronts of the plane waves offset from one another to a target shape of the optical test surface, and - generating at least one interference pattern (64) by superimposing the measurement waves (44r, 46r) after their interaction with the test surface, and capturing the interference pattern.

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