Interferometry method and interferometry apparatus

Abstract

To enable precise measurement within a short measurement time by a relatively simple optical setup in interference measurement of a shape of a surface.SOLUTION: A light source 116 supplies two input waves having different wave lengths to generate a test wave (125-1 and 125-2) for each wave length and a reference wave (128-1 and 128-2) for each wave length through a diffractive element (124). The test wave (125-1 and 125-2) reflected at a test object 114 and the reference wave (128-1 and 128-2) reflected at a reference optical element are superimposed to be diffracted again the diffractive element (124). An interferogram produced in a capture plane (148-1 and 148-2) is captured by superposition between the test wave (125-1 and 125-2) and the reference wave (128-1 and 128-2) for each wave length. The interferograms acquired according to wavelength are jointly evaluated considering difference between wavelengths.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a measurement method and a measuring apparatus for interferometric measurement of the surface shape of an object under test. [Background technology]

[0002] A preferred application area is the measurement of the surface shape of mirrors used in optical systems for EUV microlithography, i.e., microlithography that utilizes extreme ultraviolet (EUV) radiation. Such mirrors often have free-form surfaces, i.e., surface shapes that deviate significantly from rotationally symmetric spherical or aspherical shapes.

[0003] For high-precision interferometric measurements of the surface shape of an object under test, such as the optical element for the projection lens of a microlithography projection exposure apparatus, a diffractive optical system is often used as a so-called zero-optical system. In this case, the wavefront of the wave under test is adapted to the target shape by the diffractive optical element so that the wave under test is incident perpendicularly to the target shape at all points on the surface, where it is reflected and travels in the opposite direction. The deviation from the target shape can be determined by superimposing the wave under test, which is a reference wave, with the reference wave. The diffractive optical element used may be, for example, a computer hologram (CGH).

[0004] Patent Document 1 describes an interferometric measurement method and measuring apparatus that enables high-precision measurement of the surface shape of large objects and / or objects that are hotter than the surrounding area by using complex coding CGH.

[0005] The term "complex coding CGH" generally refers to a CGH that has different diffraction characteristics, performs two or more different functions due to those different diffraction characteristics, and has two or more different structural patterns superimposed on each other in a common plane.

[0006] The measuring device of Patent Document 1 comprises a light source that supplies an input wave, a diffractive optical element arranged in the beam path of the input wave, which is appropriately configured to generate a wave under test from the input wave by diffraction, having a wavefront directed toward the object under test and at least partially conforming to the target shape of the optical surface, and also to generate a reference wave having a propagation direction shifted from the propagation direction of the wave under test, a reflective optical element arranged in the beam path of the reference wave and designed for back reflection of the reference wave, and an acquisition device that acquires an interferogram generated on the acquisition plane by the superposition of the wave under test and the back-reflected reference wave after they have been further diffracted by the diffractive optical element after interaction with the object under test and back-reflected reference wave, respectively.

[0007] One advantage of using diffractive optical elements in measuring instruments is that, if necessary, additional functions for calibration can be given to the diffractive optical elements. The purpose of calibration may be, for example, to enable on-site measurement of manufacturing-related geometric optical errors (also referred to as writing errors or placement errors) on the CGH.

[0008] Patent Document 2 describes a measuring device that includes a complex coding phase grating having at least three different phase functions for a diffractive optical element to generate at least one calibration wave. For the generation of three calibration waves, a complex coding phase grating having five different phase functions, i.e., a quintuple coding diffractive optical element, is used.

[0009] In reality, complex coding diffractive optical elements have drawbacks in addition to their advantages. The greater the number of different codings, the higher the risk of undesirable diffraction orders that can result in undesirable reflection paths that reduce measurement accuracy. Calibration measurements are time-consuming and require additional components in the measuring instrument, such as calibration mirrors. Furthermore, to minimize errors due to drift, the measuring instrument must be kept as stable as possible throughout the entire measurement time (calibration measurements and surface measurements of the object under test). Finally, interferometric wavefronts include exact effects. These exact effects arise because, while calculations of CGH structural patterns assume structures lie in a common (two-dimensional) plane, actual CGHs have three-dimensional structures after fabrication. These effects are corrected using model-based methods. The inaccuracies and complexity of models for calculating exact effects increase exponentially with the number of codings on the CGH. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] German Patent Application Publication No. 10 2015 209 490 (US Patent Application Publication No. 2018 / 106591) [Patent Document 2] German Patent Application Publication No. 10, 2017, 217, 369 Specification [Overview of the project] [Problems that the invention aims to solve]

[0011] The present invention aims to provide a measurement method and apparatus for interferometric measurement of the surface shape of an object under test, which operates using a diffractive optical element and enables precise measurement within a short measurement time with a relatively simple optical setup of the measurement apparatus. [Means for solving the problem]

[0012] To achieve this objective, the present invention provides a measurement method having the features of claim 1 and a measurement device having the features of claim 9. Preferred variations are specified in the independent claims. The language of all claims is incorporated herein by reference.

[0013] This measurement method is useful for interferometric measurement of the surface shape of an object under test, such as an optical element in an optical system. The object under test may be, for example, a planar mirror or a concave or convex mirror. A first input wave having a first wavelength λ1 and a second input wave having a second wavelength λ2 that is different from the first wavelength, i.e., shorter or longer than the first wavelength, are used for measurement. In other words, at least two different wavelengths are used for measurement, which means that the measurement method is a multicolor measurement method. A first test wave, directed towards the object under test and having a wavefront that at least partially matches the target shape of the optical surface, and a first reference wave, directed towards a first reflective optical element and having a propagation direction different from the propagation direction of the first test wave, are generated from the first input wave. The first test wave and the first reference wave are generated from the first input wave by diffraction at a diffractive optical element. Similarly, a second test wave and a second reference wave are generated from the second input wave by diffraction at the same diffractive optical element. The second detected wave also has a wavefront that at least partially fits the target shape of the optical surface, whereas the second reference wave is directed towards the second reflective optical element and has a propagation direction different from that of the second detected wave. After interaction with the object under test, the first detected wave is superimposed with the first reference wave after back reflection at the first reflective optical element. After interaction with the object under test, the second detected wave is superimposed correspondingly with the second reference wave after back reflection at the second reflective optical element, and the two detected waves (first and second detected waves) and the two reference waves (first and second reference waves) are diffracted again at the same diffractive optical element for superimposition. The first interferogram generated in the first acquisition plane by the superimposition of the first detected wave and the first reference wave, and the second interferogram generated in the second acquisition plane by the superimposition of the second detected wave and the second reference wave, are acquired and then jointly evaluated considering the difference between the first and second wavelengths.

[0014] Therefore, a measurement device suitable for performing this measurement method has a light source suitable for supplying first and second input waves having different wavelengths (first and second wavelengths). This measurement device further includes a diffractive optical element that generates the above-mentioned test wave and reference wave from input waves of different wavelengths. Furthermore, a reflective optical element is provided for each wavelength, which is positioned in the beam path of each reference wave and designed for back reflection of each reference wave. The acquisition device works to acquire the first and second interferograms generated in the acquisition plane by wavelength-appropriate superposition of the first and second test waves after interaction with the test object and the back-reflected first and second reference waves, respectively, after they have been further diffracted by the diffractive optical element. Wavelength-appropriate superposition here means that the interferogram used for evaluation is obtained only from the test wave and reference wave of the same wavelength. When the first and second interferograms are jointly evaluated in the evaluation device, the difference between the first and second wavelengths is taken into consideration. In other words, the difference between wavelengths (wavelength difference Δλ = λ1 - λ2) or the wavelength ratio α = λ1 / λ2, or terms corresponding to variables derived therefrom, are considered in the evaluation, for example. Put another way, the evaluation is not performed separately for each wavelength; rather, additional information that would not be available without a combined evaluation of wavelengths can be obtained from the difference between wavelengths.

[0015] A key advantage of multicolor measurement is the elimination of the need to perform separate calibration measurements, which allow for the confirmation and consideration of geometric optical errors in diffractive optical elements during evaluation. Therefore, measurement time can be saved compared to conventional methods that include separate calibration measurements. Furthermore, this measurement device can have a simpler setup because it eliminates the need for separate devices for calibration measurements, such as calibration mirrors.

[0016] Furthermore, relatively simple diffractive optical elements can be used. In particular, the use of quadruple-encoded or triple-encoded CGHs is often sufficient, making quintuple-encoded or higher CGHs unnecessary. These CGHs tend to produce less interference reflection than quintuple-encoded or higher CGHs.

[0017] Finally, in some cases, it is also possible to simultaneously measure the placement error and the shape, that is, the surface shape.

[0018] In a preferred embodiment, the wavelength ratio α = λ1 / λ2, that is, the quotient of the wavelengths used, is less than 2 when λ1 > λ2, particularly in the range of 1.2 to 1.5. As a result, the available fringe density of the structure on the computer hologram is neither too large nor too small, and yet it can provide the required diffraction angle and diffraction intensity. The quotient or wavelength ratio preferably approximately corresponds to an irrational number, for example, approximately corresponds to the square root of 2. As a result, the reflection can be further suppressed, and the measurement accuracy can be improved. When the total value of the wavelength ratio before the decimal point is one digit (that is, less than 10), in this context, "approximately" means that the wavelength ratio has at least three significant digits, that is, is valid up to at least the second decimal place and coincides with the nearest irrational number.

[0019] In some method variants, the first measurement at the first wavelength and the second measurement at the second wavelength are carried out simultaneously. By simultaneous measurement, since the drift effect does not affect this point as a result of each wavelength passing through the same measuring device at a given time, particularly high measurement accuracy can be achieved. In the corresponding measuring device, a spatial separation of the reference paths can be provided. Thus, there can be two separate reflective optical elements arranged in different downstream propagation directions with respect to the diffractive optical element.

[0020] In another embodiment, the first measurement at the first wavelength and the second measurement at the second wavelength can be carried out successively, that is, without time overlap. The measurements at different wavelengths can be carried out alternately and sequentially a plurality of times. If there is a time lag between the measurements at different wavelengths, a simpler measurement setup using only a single reflective optical element can be used, which serves as both the first reflective element (for retroreflection of the first reference wave) and the second reflective element (for retroreflection of the second reference wave). The structural dimensions for generating the first and second reference waves with the diffractive optical element must then be adapted to each other such that the first and second reference waves have the same propagation direction.

[0021] Thus, a preferred embodiment of the measuring device is characterized in that the diffractive optical element is configured to generate exactly three or exactly four different structural patterns from the first input wave and the second input wave to the first detected wave, the second detected wave, and the first reference wave and the second reference wave.

[0022] Since no separate calibration is required, in a preferred embodiment, in addition to the first detected wave, the second detected wave, the first reference wave, and the second reference wave, no further measurement wave, particularly a calibration wave directed to a calibration mirror, is generated. In that case, the measuring device can be set such that, in addition to the first reflective optical element and the second reflective optical element, no further reflective optical element for retroreflecting the waves generated by the diffractive optical element back to the diffractive optical element is provided.

[0023] Further advantages and aspects of the present invention will become apparent from the claims and from the description of exemplary embodiments of the present invention described below with reference to the drawings.

Brief Description of the Drawings

[0024] [Figure 1] A schematic diagram of a prior art reference measuring device is shown. [Figure 2] A schematic diagram of a measuring device according to a first embodiment of the present invention is shown. [Figure 3] A schematic diagram of a measuring device according to a second embodiment of the present invention is shown. [Figure 4] A schematic diagram of a measuring device according to a third embodiment of the present invention is shown.

Modes for Carrying Out the Invention

[0025] To facilitate understanding of various aspects of the measuring method and the measuring device and to explain some differences from the prior art, FIG. 1 shows a schematic diagram of a prior art measuring device from Patent Document 1 for reference.

[0026] The measuring device 10 in Figure 1 is designed to interferometrically measure the shape of the optical surface 12 of the object under test 14. Using the measuring device 10, in particular, the deviation of the actual shape of the surface 12 from the target shape can be determined. The object under test 14 provided may be, for example, a mirror of a projection lens for EUV microlithography having a surface designed as a free-form surface for reflecting EUV radiation.

[0027] The measuring device 10 includes a light source 16 that supplies a sufficiently coherent measuring radiation as the input wave 18. In an exemplary embodiment, the light source 16 includes an optical waveguide 20 having an output surface 22. The optical waveguide 22 is connected to a radiation source (not shown), for example, a laser. For example, a helium-neon laser having a wavelength of about 633 nm can be provided for this purpose.

[0028] The measuring device includes a diffractive optical element (DOE) 24 that generates a detected wave 26 and a reference wave 28 from an input wave 18, and a reflective optical element 30 that reflects the reference wave 28. The diffractive optical element 24 is configured in the form of a complex coding CGH and includes a diffractive structure 34 that forms two diffractive structure patterns 34 arranged superimposed on each other in a plane.

[0029] One of the diffraction structure patterns is configured to generate a detected wave 26 having a wavefront that at least partially conforms to the shape of the optical surface 12. The other diffraction optical structure pattern generates a reference wave 28 having a plane wavefront. In the diffraction structure, the detected wave 26 can be generated, for example, by first-order diffraction of the first structure pattern, and the reference wave 28 can be generated by first-order diffraction of the second structure pattern.

[0030] The measuring device 10 further includes an acquisition device 36 having a beam splitter 38 that derives a combination of the reflected signal wave 26 and the reflected reference wave 28 from the beampath of the input wave 18, and an interferometer camera 40 for acquiring an interferogram generated by the superposition of the reference wave 28 onto the signal wave 26.

[0031] The illumination radiation supplied by the light source 16 exits the exit surface 22 of the optical waveguide 20 in the form of an input wave 18 having a spherical wavefront, and propagates along the propagation axis 42 directed toward the diffractive optical element 24. In this process, the input wave 18 first passes through the beam splitter 38 and then through the diffractive optical element 24.

[0032] The diffractive optical elements 24 transmit a test wave 26 from the input wave 18 by diffraction at one of the diffractive structural patterns arranged in a superposition pattern, with the test wave having a wavefront that is directed towards the surface 12 of the object under test 14 and conforms to the target shape of the surface 12. During this conversion, the wavefront is adapted so that the test wave is incident perpendicularly at each location on the surface of the target shape, where it is reflected and travels in the opposite direction.

[0033] The detected wave 26 propagates in the direction of the object under test 14, and then, after being generated by the diffractive optical element 24, is incident on the optical surface of the object under test 14. The detected wave 26 is reflected by the surface 12 and returns to the diffractive optical element 24, where it is diffracted again as it passes through the diffractive structure 34. At this time, the reflected detected wave 26 is returned to a substantially spherical wave, and its wavefront has a deviation from the spherical wavefront corresponding to the deviation of the surface 12 of the detected wave 14 from the target shape.

[0034] The diffractive optical element 24 further generates a reference wave 28 directed towards the reflective optical element 30 from the input wave 18 by diffraction on the other side of its diffractive structural pattern. The reference wave 28 has a propagation direction that is shifted from the propagation direction of the wave under test 26, and a wavefront that is suited to the surface shape of the reflective optical element 30.

[0035] The reference wave 30 is generated by the diffractive optical element 24, then incident on the reflective optical element 30, where it is reflected and travels in the reverse direction. In this case, no further optical elements are located in the beam path of the reference wave 28 or the beam path of the wave under test 26. The reflected reference wave 28 passes through the diffractive optical element 24 again and is diffracted again. At that time, the reflected reference wave 28 is converted back into a spherical wave.

[0036] Therefore, the diffractive optical element 24 also functions to superimpose the reflected reference wave 28 onto the reflected test wave 26. Both waves are incident on the beam splitter 38 as a focused beam 44, where they are reflected in the direction of the interferometer camera 40. Both focused beams 44 travel through the eyepiece 46 and are finally incident on the acquisition plane 48 of the interferometer camera 40. The interferometer camera 40 can be configured, for example, in the form of a CCD sensor, to acquire the interferogram generated by the interfering waves. An aperture may be placed at the focus of the focused beam 44 as a spatial filter to reduce scattered radiation.

[0037] An evaluation device (not shown) of the measuring device 10 measures the actual shape of the optical surface 12 of the object under test 14 from the acquired interferogram.

[0038] Calibration of a diffractive optical element can be performed as part of a measurement. For this purpose, the diffractive optical element 24 may include a diffractive structure 34 that forms three or more diffractive optical patterns superimposed on a plane. Such a diffractive optical element having four superimposed diffractive structural patterns is disclosed, for example, in German Patent Application Publication No. 10 2012 217 800, where the different diffractive structural patterns are formed by individual phase functions of a complex coding phase grating. A CGH that generates two calibration waves having different propagation directions in addition to the detected wave and the reference wave is described in relation to Figure 1 of Patent Document 1. For the generation of three calibration waves, a complex coding phase grating having five different phase functions, i.e., a quintuple coding diffractive optical element, is used (see Patent Document 2).

[0039] The inventors have analyzed in detail the advantages and limitations of these conventional procedures and provide a solution that utilizes the advantages of this technique while substantially avoiding or at least reducing its disadvantages.

[0040] The cavity of the above-described conventional interferometer or measuring device consists of a complex-coded CGH 24, a planar reference mirror 30, and a test object 14. In the phase-shift method, the reference mirror 30 is moved perpendicular to the optical direction. Thus, the difference in the wavefront between the reference mirror R(x, y) and the test object P(x, y) can be spatially resolved from the interferogram measured by the detector 40. V(x,y)=P(x,y)-R(x,y)

[0041] The wavefront of the reference mirror can be measured from the outside. For this purpose, the reference mirror can be rotated or displaced, for example, within a plane. Any error with respect to the reference mirror can be determined in this way. The desired wavefront of the test object can thus be determined as follows. P(x,y)=V(x,y)+R(x,y)

[0042] This applies to a setup with an ideal CGH without errors. However, in reality, the CGH mainly causes two types of errors, specifically (i) geometric optical errors caused by inaccurate positioning (arrangement) of the structure, and (ii) exact errors resulting from the interaction of light with a 3D structure that are not considered in the design of the CGH (as a 2D structure) which cause.

[0043] Therefore, the measured signal has the following form using the geometric optical errors P GO , R GO and the exact errors P Rig , R Rig : V=(P-R)+(P GO -R GO )+(P Rig -R Rig )

[0044] The exact errors can be determined based on a model. For this purpose, the CGH can be measured from the outside. Based on these measurements, Maxwell's equations can subsequently be solved. The solution of these calculations gives the desired P Rig, R Rig That is the case.

[0045] The placement error can be determined, for example, using three reference spheres. For this purpose, quintuple coding CGH can be used, which generates five waves from the input wave: specifically, a reference wave, a tested wave, and three spherical waves. The unknown placement error P GO -R GO This is linearly interpolated from spherical measurements. This procedure has the following particular drawbacks: (i) Five-coded CGH can cause many undesirable reflections. (ii) Linear interpolation of geometric optical errors assumes a stable cavity. Drift in the interferometer setup can lead to false results. (iii) The model-based exact error increases exponentially with the number of codings on the CGH. The insufficient precision of Maxwell's equations is a limiting factor in the EUV optical unit.

[0046] Examples of solutions that at least reduce the drawbacks are described below. Common to these solutions is the use of a multicolor measurement method and measuring device characterized by measurements being performed at (at least) two different wavelengths. It is preferable to use light with exactly two different wavelengths for measurement.

[0047] Figure 2 shows an exemplary first embodiment of a measuring device 110 for interferometric measurement of the shape of the optical surface 112 of an object under test 114. The object under test may be, for example, a mirror of a projection lens for EUV microlithography operating with EUV radiation at wavelengths less than 100 nm, particularly around 13.5 nm or about 6.8 nm. The concave aspherical surfaces of the mirror may be freeform surfaces having deviations of more than 5 μm from each rotationally symmetric aspherical surface and / or deviations of more than 1 mm from each spherical surface.

[0048] The light source 116 of the measuring device 110 is designed to supply a sufficiently coherent measuring radiation having at least two different wavelengths, specifically a first wavelength λ1 and a second wavelength λ2 different from the first wavelength. The wavelength ratio α = λ1 / λ2 is in the range of, for example, 1.2 to 1.5 (when λ1 > λ2), and may be above or below this range in some cases.

[0049] The multicolor light source 116 may have, for example, a tunable solid-state laser capable of generating laser light of different wavelengths in the visible spectrum. For example, a tunable frequency-stabilized monolithic Nd:YAG laser can be used, which is frequency-stabilized to the hyperfine structure lines of molecular iodine and its frequency is doubled internally to a wavelength of 532 nm. The two wavelengths are set with high precision, and the exact frequency values ​​can be determined, for example, using a frequency comb generator (a measuring device for high-precision frequency measurement) or an atomic clock. The methods for generating two different frequencies that can be used here are described in the specialist paper "Frequency-comb-referenced two-wavelength source for absolute distance measurements" by N. Schuhler et al., Optics Letter 31, Issue 21 (2006), Pages 3101-3103. The disclosures in that specialist paper are referenced in this explanation.

[0050] In the example shown in Figure 2, the light source is configured to emit light of two wavelengths simultaneously. Two spatially superimposed input waves of different wavelengths are generated exiting from the light source's output surface 122, specifically a first input wave 118-1 having a first wavelength λ1 and a second input wave 118-2 having a different second wavelength λ2.

[0051] The measuring device 110 has a diffractive optical element 124 in the form of a complex coded computer hologram (CGH) operating in a transmission manner in this example. The diffractive optical element 124 has a substrate of high-purity fused silica on which a diffractive structure 134 is formed, and the diffractive structure 134 forms exactly four diffractive structural patterns arranged in a plane, which generate exactly four output waves with different characteristics from an input wave. The structural patterns are arranged superimposed on a common plane.

[0052] A first detection wave 125-1 (having a first wavelength) having a wavefront that at least partially matches the target shape of the optical surface 112 is generated from the first input wave. The second input wave is converted into a second detection wave 125-2 having a wavefront that similarly matches the target shape of the optical surface but with a different wavelength (second wavelength). The first and second detection waves propagate in the same direction relative to the object under test 114.

[0053] The diffractive optical element 124 is provided with another diffractive structure to form a first reference wave 128-1 from the first input wave by diffraction, and the propagation direction of the first reference wave 128-1 is different from the propagation directions of the first and second detected waves and is directed toward the first reflective optical element 130-1. Yet another diffractive structure is designed to generate a second reference wave 128-2 from the second input wave by diffraction, and the second reference wave 128-2 is directed toward the second reflective optical element 130-2. The propagation direction of the second reference wave is different from the propagation direction of the first reference wave and the propagation direction of the detected wave. In this example, the reflective optical elements 130-1 and 130-2 are positioned on both sides downstream of the diffractive optical element 124 with respect to the detected wave.

[0054] After reflection from the object under test 114, the first detected wave returns towards the diffractive optical element 124 and, after interacting with the object under test 114, is superimposed with the first reference wave after back reflection from the first reflective optical element 130-1. The same applies to the second detected wave, which is superimposed with the second reference wave after reflection from the second reflective optical element 130-2. Due to the superimposition, the first and second detected waves and the first and second reference waves are each diffracted again by the diffractive optical elements and return together towards the light source.

[0055] A beam splitter 138 is positioned in the beam path between the light source's emission surface 122 and the diffractive optical element 124, and this can be considered as an integral part of the acquisition device 136. The acquisition device 138 acquires a first interferogram generated by the superposition of the first detected wave 125-1 and the first reference wave 128-1 in the first acquisition plane 148-1, and a second interferogram generated by the superposition of the second detected wave 125-2 and the second reference wave 128-2 in the second acquisition plane 148-2. The acquisition planes correspond to the photosensitive surfaces of the interferometer cameras 140-1 and 140-2, respectively. The interferometer cameras 140-1 and 140-2 can, for example, be equipped with CCD sensors, and each can acquire interferograms generated by interference waves of the same wavelength.

[0056] For this purpose, the waves reflected by the beam splitter 138 first travel through the color correction optical system 146, which collimates the waves of both wavelengths without generating any significant chromatic aberration and guides them toward the wavelength-selective beam splitter 148, which reflects the superimposed wave of the first wavelength to the first interferometer camera 140-1, while transmitting the superimposed wave of the second wavelength to the second interferometer camera 140-2.

[0057] The phase shift of the two reference mirrors 130-1 and 130-2 allows this setup to determine the phase difference between the object under test and the two reference mirrors for two wavelengths.

[0058] The interferogram is evaluated by an evaluation device 150 connected to an interferometer camera. In this case, a combined evaluation is performed considering the wavelength difference or two different wavelengths. Specifically, terms corresponding to the difference between wavelengths (λ1-λ2) or the wavelength ratio (λ1 / λ2) or variables derived therefrom are considered in the evaluation. Based on interferograms acquired at at least two different wavelengths, the evaluation device 150 measures the actual shape of the optical surface 112 of the object under test 114.

[0059] The evaluation particularly focuses on the placement error of the CGH. P GO (λ)-RGO (λ) = const × λ The phase information of the object being tested is the same for all wavelengths, as shown by P(λ1)-R(λ1)=P(λ2)-R(λ2) in relation to the wavelength used.

[0060] From the two measurements at λ1 and λ2, both the unknown const and PR values ​​were obtained.

number

[0061] Therefore, it is possible to simultaneously measure the geometric optical error of CGH using quadruple-coded CGH.

[0062] A key advantage of multicolor measurement is that it eliminates the need for separate calibration measurements. Consequently, the necessary optical components (e.g., calibration mirrors) can be omitted, and measurement time can be shortened compared to conventional techniques.

[0063] An exemplary second embodiment of a measuring device 210 designed for multicolor measurement of the shape of an optical surface will be described with reference to Figure 3. In this example, an object 214 in the form of an EUV concave mirror will be measured. For clarity, identical, similar, or equivalent components are denoted by the same reference numerals as in Figure 2, but with 100 added.

[0064] The measuring device 210 enables alternative measurement of geometric optical errors using quadruple-coded CGH. A multicolor light source 216 capable of generating at least the first and second wavelengths of the type described above is also used here. The light source is capable of emitting the two wavelengths sequentially in time (i.e., without temporal overlap), preferably multiple times alternately and rapidly (see Figure λ(t)). It is possible to switch between the two wavelengths in cycles of, for example, seconds, which here means that the individual measurement time for each wavelength is about one second or a few seconds.

[0065] The differences from the measurement device in Figure 2 are, in particular, due to the structure of the diffractive optical element 124 being different from that of the previous exemplary embodiment. Furthermore, the measurement device 210 requires only a single reference mirror 230 acting as a first reference mirror (for the first wavelength) and a second reference mirror (for the second wavelength). Moreover, it can operate with only one interferometer camera 240 connected to the evaluation device 250 and sufficiently sensitive to both wavelengths. Thus, compared to the first embodiment, the second interferometer camera and the beam splitter upstream of the interferometer camera can be omitted.

[0066] A quadruple-coded CGH is used, just as in the exemplary embodiment described above. The diffraction structures 234 for the two detected waves may correspond to or deviate from the corresponding diffraction structures or codings in the exemplary first embodiment (Figure 2). In either case, the diffraction structures for generating the two reference waves are adapted to each other such that the diffraction orders of maximum intensity propagate in the same direction for the first and second wavelengths, so that both wavelengths can reflect and reverse the reference waves using the same refraction and optical elements 230. Thus, the diffraction structures have different line spacings for each wavelength to achieve the same diffraction in the same spatial direction.

[0067] The measuring device or interferometer is illuminated with two wavelengths sequentially in time from the light source 216. The light is collimated in either case by the color correction optical unit 246 after passing through the corresponding cavity of the beam splitter 238 of the acquisition device 236. The interferometer camera 420 records the interferogram. If a phase shift is used for measurement, after the measurement at the first phase, the reference mirror is moved parallel to the propagation direction of the reference wave. Subsequently, measurements at the first wavelength and the second wavelength (or vice versa) are performed sequentially. The steps are repeated until the desired number of interferograms are acquired at different phase positions.

[0068] The light source 216 is designed to switch quickly between two wavelengths (e.g., in cycles of seconds) (see λ(t) diagram). For this reason, measurements at different wavelengths substantially "see" the same state of the measuring instrument, meaning that drift-related measurement errors are generally negligible. The evaluation remains the same compared to the exemplary first embodiment. In this case as well, from the two measurements at λ1 and λ2,

number

[0069] Next, with reference to Figure 4, an exemplary third embodiment of the measuring device 310, specifically designed for measuring placement errors in triple-coded CGH, will be described. The measurement setup is similar to that of the exemplary first embodiment in Figure 2, so that the same reference numerals plus 200 are used for the same or corresponding or equivalent components.

[0070] Differences from the exemplary embodiment relate to the illumination beam path relating to the diffractive optical element 324, and further to the diffractive structure 334 of the diffractive optical element 324, which is simply a triple-coded computer hologram. The acquisition device 336, which has a collimation optical unit 346, a beam splitter 345, and interferometer cameras 340-1, 340-2 connected to the evaluation device 350, may correspond to that in Figure 2.

[0071] A color correction collimation optical unit 311 is inserted into the beam path between the light source 316 and the diffractive optical element 324 so that the diffractive optical element 324 is collimated with input waves of two different wavelengths. This is because the number of codings can be reduced to as few as three when the diffractive optical element 324 is collimated.

[0072] The first coding generates the first detected wave (a detected wave of the first wavelength), and the second coding generates the second detected wave (a detected wave of the second wavelength). The third coding corresponds to a linear grating and generates a plane reference wave at all wavelengths. Here, shorter wavelengths are diffracted more strongly than longer wavelengths. The first reference mirror 330-1 (reflecting optical element 330-1) for back reflection of the reference wave having the first wavelength and the second reference mirror 330-2 are oriented at different angles to the diffractive optical element 324 according to the law of diffraction, so that the first and second reference waves are incident perpendicularly to the respective first or second reference mirrors. For example, it is possible to use the first and -1 order diffractions of radiation diffracted by a linear grating. In either case, a plane wave is formed. In this case, the two plane reference mirrors can be positioned on different sides of the diffractive optical element, i.e., on different sides of the beam path extending between the diffractive optical element 324 and the object under test 314. The measurement and evaluation remain unchanged compared to the first proposal. Here too, measurements are performed simultaneously at both wavelengths, and the two unknown const and PR values ​​can be determined from the measurements according to the above formula.

[0073] The advantages of this novel method can be combined as follows: The frequency or wavelength of the radiation to be measured is (for example, 1 × 10⁻⁶). -13 or 1 × 10 -14 This relates to the variable that can be best measured (with measurement accuracy up to a certain range). Therefore, in the evaluation according to the above formula, it can be assumed that the measurement uncertainty of the denominator (wavelength difference λ1-λ2) is negligibly small. Quadruple-coded CGH or triple-coded CGH, in principle, produces less interference reflection than quintuple-coded CGH or higher, so the interference reflection produced by this measurement device is relatively small. Simultaneous measurement ensures that the measurement state of the object under test during the measurement time is identical for both measurement signals (i.e., for both wavelengths). Therefore, the drift effect does not adversely affect the ratio of the results for the two wavelengths. The measurement period can be reduced to at least 1 / 4 compared to conventional measurements that also perform calibration measurements in addition to the measurement of the object under test. Reducing the number of codes on the CGH also reduces the complexity and uncertainty of the calculation of Maxwell's equations, which means that more accurate measurement results are possible in this respect as well.

[0074] The reflectivity of the reference mirror should be fitted so that the interference contrast is maximized at each wavelength. As described above, the optical system of the acquisition device (e.g., 246) that collimates the wave under measurement before acquisition should be color-corrected, i.e., it should not produce or have chromatic aberration. In the exemplary embodiment of Figure 4, this should also apply to the collimation optical unit 311 of the beam path between the light source and the diffractive optical element. Chromatic aberration can be avoided, for example, by using a mirror optical unit for this purpose.

Claims

1. A measurement method for interferometrically measuring the shape of the optical surface (112) of an object (114) to be measured, A first input wave having a first wavelength λ1 and a second wavelength λ2 having a different wavelength from the first wavelength λ1. The steps include supplying a second input wave, Steps of generating a first detected wave (125-1) directed toward the object under test (114) and having a wavefront that at least partially matches the target shape of the optical surface (112), and a first reference wave (128-1) directed toward the first reflective optical element (130-1) and having a propagation direction shifted from the propagation direction of the first detected wave (125-1), by diffraction at the diffractive optical element (124); The steps of generating a second test wave (125-2) directed toward the object under test (114) and having a wavefront that at least partially matches the target shape of the optical surface (112), and a second reference wave (128-2) directed toward the second reflective optical element (130-2) and having a propagation direction shifted from the propagation direction of the second test wave (125-2), by diffraction at the diffractive optical element (124), The first detected wave (125-1) after interaction with the object under test (114) is superimposed with the first reference wave (128-1) after back reflection by the first reflective optical element (130-1), and the second detected wave (125-2) after interaction with the object under test (114) is superimposed with the second reference wave (128-2) after back reflection by the second reflective optical element (130-2), wherein the first and second detected waves (125-1, 125-2) and the first and second reference waves (128-1, 128-2) are respectively diffracted again by the diffractive optical element (124) for superimposition. The steps include acquiring a first interferogram generated in the first acquisition plane (148-1) by superimposing the first detected wave (125-1) and the first reference wave (128-1), and a second interferogram generated in the second acquisition plane (148-2) by superimposing the second detected wave (125-2) and the second reference wave (128-2), A step of jointly evaluating the first and second interferograms, taking into account the difference between the first and second wavelengths. Measurement methods including

2. In the measurement method according to claim 1, the wavelength ratio α = λ1 / λ2 between the first wavelength and the second wavelength satisfies the condition 1.2 ≤ α ≤ 1.5, and / or the first wavelength and the second wavelength A measurement method characterized in that the wavelength ratio α = λ1 / λ2 between two points is approximately an irrational number.

3. The measurement method according to claim 2, characterized in that a term corresponding to the difference between wavelengths (λ1 - λ2) or the wavelength ratio (λ1 / λ2) or a variable derived therefrom is taken into consideration in the joint evaluation.

4. A measurement method according to claim 1, 2, or 3, characterized in that a first measurement at the first wavelength and a second measurement at the second wavelength are performed simultaneously.

5. A measurement method according to claim 1, 2, or 3, characterized in that a first measurement at the first wavelength and a second measurement at the second wavelength are performed in succession.

6. A measurement method according to any one of claims 1 to 5, characterized in that, in addition to the first and second detected waves and the first and second reference waves, no further measurement waves, particularly calibration waves directed towards a calibration mirror, are generated.

7. A measurement method according to any one of claims 1 to 6, characterized in that it uses a diffractive optical element having exactly three or exactly four different structural patterns configured to generate the first detected wave, the second detected wave, and the first reference wave and the second reference wave from the first input wave and the second input wave.

8. A measurement method according to any one of claims 1 to 7, characterized in that a color correction collimating optical unit (311) is inserted into the beam path between a light source (316) and the diffractive optical element (324) so ​​that the diffractive optical element (324) is illuminated with collimating light.

9. A measuring device (210) for interferometrically measuring the shape of the optical surface (112) of an object (114), A light source (116) that supplies a first input wave (118-1) having a first wavelength λ1 and a second input wave (118-2) having a second wavelength λ2 different from the first wavelength λ1, A diffractive optical element (124) arranged in the beampath of the first and second input waves, configured to generate first and second detection waves (125-1, 125-2) from the first and second input waves (118-1, 118-2) for each wavelength by diffraction, having wavefronts directed toward the object under test (114) and at least partially conforming to the target shape of the optical surface (112), and to generate first and second reference waves (128-1, 128-2) having propagation directions shifted from the propagation direction of the related detection waves (125-1, 125-2), For each of the aforementioned wavelengths, a reflective optical element (130-1, 130-2) is arranged in the beam path of each of the aforementioned reference waves (128-1, 128-2) and is designed for back reflection of each of the aforementioned reference waves (128-1, 128-2), An acquisition device (136) acquires first and second interferograms generated in the acquisition plane (148-1, 148-2) by the superposition of the first and second detected waves (125-1, 125-2) after interaction with the object under test (114) and the back-reflected first and second reference waves (128-1, 128-2), respectively, after they have been further diffracted by the diffractive optical element (214), An evaluation device (150) for jointly evaluating the first and second interferograms, taking into account the difference between the first and second wavelengths, A measuring device equipped with the following features.

10. In the measuring device according to claim 9, the light source satisfies the condition 1.2 ≤ α ≤ 1.5 with respect to the wavelength ratio α = λ1 / λ2 between the first wavelength and the second wavelength, and / or the The wavelength ratio α = λ1 / λ2 between the first wavelength and the second wavelength is approximately an irrational number. A measuring device characterized by being configured to generate the second input wave.

11. In the measuring device according to claim 10, the evaluation device (150) measures the difference between wavelengths (λ1-λ2) or the wavelength ratio (λ1 / λ2) or a variable derived therefrom. A measuring device characterized by being configured to consider corresponding terms in the joint evaluation.

12. A measuring device according to claim 9, 10, or 11, wherein the light source (116) is configured to simultaneously generate the first input wave (118-1) and the second input wave, and / or separate reflective optical elements (130-1, 130-2) for back reflection of each of the reference waves (128-1, 128-2) are provided for each wavelength, and the reflective optical elements (130-1, 130-2) are arranged in different propagation directions with respect to the diffractive optical element (124).

13. A measuring device according to claim 9, 10, or 11, wherein the light source (216) is configured to sequentially generate the first input wave and the second input wave, and / or is provided with only one reflective optical element (230) that acts as a first reflective element for back reflection of the first reference wave and a second reflective element for back reflection of the second reference wave, and the structural dimensions of the diffractive optical element are mutually adapted to generate the first and second reference waves such that the first and second reference waves have the same propagation direction.

14. A measuring device according to any one of claims 9 to 13, wherein a color correction collimating optical unit (311) is inserted in the beam path between the light source (316) and the diffractive optical element (324) so ​​that the diffractive optical element (324) is illuminated with collimating light.

15. A measuring device according to any one of claims 9 to 14, characterized in that the diffractive optical elements (124, 224, 324) have exactly three or exactly four different structural patterns configured to generate the first detected wave, the second detected wave, the first detected wave, and the second detected wave from the first input wave and the second input wave.

16. A measuring device according to any one of claims 9 to 14, characterized in that, in addition to the reflective optical elements (130-1, 130-2), the measuring device does not have any further reflective optical elements, particularly a calibration mirror, for the back reflection of waves generated by the diffractive optical elements to the diffractive optical elements.

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