Device and method for interferometrically measuring a wavefront of a test object; method for producing an optical element; lithography system

Abstract

The invention relates to a device (1) for interferometrically measuring a wavefront of a test object (2), in particular an optical element and / or the precursors thereof, comprising: - an illumination device (3) having at least one radiation source (7), which is configured and designed to emit radiation (8) having at least two different wavelengths L1 and L2 which generate a synthetic wavelength L when superimposed; - a delay device (4), which has a radiation splitter (9) and at least two reflectors (10a, 10b), wherein: the radiation splitter (9) divides the radiation (8); the radiation splitter (9) and in each case one of the reflectors (10a, 10b) form a first beam path having a first optical path length (11a) and a second beam path having a second optical path length (11 b); at least one of the reflectors (10a, 10b) is displaceable such that a distance between the radiation splitter (9) and the displaceable reflector (10a) can be adjusted in order to set an optical path difference (11) between the first beam path and the second beam path; and...

Description

[0001] Device and method for interferometric measurement of a wavefront of a test specimen; method for manufacturing an optical element; lithography system

[0002] The present application claims priority from German patent application No. 102025 100672.5, the contents of which are incorporated herein in full by reference.

[0003] The invention relates to a device for interferometric measurement of a wavefront of a test object, in particular an optical element and / or its precursors.

[0004] The invention also relates to a method for interferometric measurement of a wavefront of a test object, in particular an optical element and / or its precursors.

[0005] The invention further relates to a method for manufacturing an optical element, in particular an optical element for a lithography system.

[0006] The invention further relates to a lithography system, in particular a projection exposure system for semiconductor lithography.

[0007] Wavefront testing and analysis methods are particularly important for optical applications. For example, semiconductor lithography requires high-precision optics or optical elements that, to ensure maximum resolution and avoid imaging errors, should exhibit minimal wavefront aberrations. Wavefront aberrations can be caused by reflection from uneven surfaces or transmission through inhomogeneous materials, especially with regard to the coefficient of thermal expansion (CTE) and / or the refractive index. Such inhomogeneities, as well as schlieren (striae), can be periodic and occur at low, medium, or high frequencies. Inhomogeneities can also be due to the thermal sensitivity of the material.

[0008] Besides Raman spectroscopy and ultrasound, methods of optical interferometry are particularly suitable for measuring the wavefront and analyzing wavefront errors. In this method, the constructive and destructive superposition of coherent light waves with differences in transit time or path length creates a characteristic interference pattern with a modulated amplitude, which allows conclusions to be drawn about the wavefront emanating from the test object, i.e., for example, from the optical element, via the interference phase.

[0009] One advantage of interferometric methods is that they are non-destructive and pose little risk of damaging the test specimen. Furthermore, such methods can, in principle, achieve very high measurement accuracy, allow for the examination of larger specimens, and require comparatively little time for data acquisition. Among the known methods is the Fizeau interferometer. In this method, the surface of the specimen under test is compared to a defined surface or reference surface, also known as a Fizeau surface, by measuring the interference of reflected radiation or light.

[0010] WO 2006 / 102997 A1 discloses an interferometer system that combines a Fizeau interferometer with an optical delay device or "delay line" to reduce the disruptive influence of multiple interferences or stray interferences using optical methods. These can occur particularly with optical elements polished on both sides, such as planar plates, and cannot be separated from one another using classical interferometers.

[0011] Compared to other methods for reducing interference, such as Fourier Transform Phase Shifting Interferometry (FTPSI), data quality can be significantly improved using the methods according to WO 2006 / 102997 A1. However, the wavefront errors and local gradients measurable in this way, as well as the achievable dynamic range, are often insufficient in practice. This can be particularly true when the samples under test exhibit comparatively strong local gradients and / or when there is double transmission through the sample. While the limitations of the methods according to WO 2006 / 102997 A1 can potentially be circumvented by increasing the camera resolution and / or by combining many individual measurements for large-format samples, this involves considerable costs and time.Therefore, there is a need for simpler alternatives to increase measurement accuracy, preferably for measuring local gradients of more than 15 mrad.

[0012] The present invention is therefore based on the objective of creating a device for the interferometric measurement of a wavefront of a test object which is improved compared to the prior art, so that in particular the highest possible dynamic range can be measured.

[0013] According to the invention, this problem is solved by a device having the features mentioned in claim 1.

[0014] The present invention also aims to create a method for the interferometric measurement of a wavefront of a test object which is improved compared to the prior art, so that in particular the highest possible dynamic range can be measured.

[0015] According to the invention, this problem is solved by a method with the features mentioned in claim 13.

[0016] The present invention further aims to provide a method for manufacturing an optical element that is improved compared to the prior art, such that the optical element is analyzed and optimized, in particular, for wavefront errors. According to the invention, this objective is achieved by a method with the features specified in claim 29.

[0017] The present invention further aims to create a lithography system which is improved compared to the prior art, so that exposure with reliably shaped wavefronts can be carried out.

[0018] According to the invention, this problem is solved by a lithography system with the features mentioned in claim 30.

[0019] Advantageous embodiments of the device according to the invention, the two methods according to the invention and the lithography system according to the invention are shown, among other things, in the respective dependent claims and in the following description.

[0020] The device according to the invention for interferometric measurement of a wavefront of a test object, in particular an optical element and / or its precursors, comprises

[0021] - a lighting device with at least one radiation source, preferably with at least two radiation sources, which is configured and designed to emit radiation with at least two different wavelengths L1 and L2, which, when superimposed, produce a synthetic wavelength L,

[0022] - a delay device comprising a beam splitter and at least two reflectors, wherein the beam splitter divides the radiation, wherein the beam splitter and each of the reflectors form a first beam path with a first optical path length and a second beam path with a second optical path length, wherein at least one of the reflectors is movable so that a distance between the beam splitter and the movable reflector is adjustable in order to set an optical path difference between the first beam path and the second beam path, and

[0023] - an interferometry device comprising at least a Fizeau plate with a Fizeau reference surface and a mirror with a mirror reference surface, between which the test specimen can be arranged, preferably at least approximately plane-parallel.

[0024] According to the invention, when passing through the interferometry device, a portion of the radiation is reflectable from the Fizeau reference surface, a further portion of the radiation is reflectable from a first surface and / or a second surface of the test object, a further portion of the radiation is reflectable from the mirror reference surface, and interference patterns can be generated by at least partial interference of these portions.

[0025] The device according to the invention enables, in a particularly advantageous manner, the measurement of the wavefront or the analysis of wavefront defects of a test specimen, in particular an optical element, and furthermore, in particular an optical element for a lithography system. By combining radiation of two wavelengths, a higher dynamic range can be measured compared to the prior art, where only one wavelength is used, and in particular, test specimens with comparatively strong, high, or steep local gradients can also be measured or analyzed.

[0026] Furthermore, the delay mechanism of the device according to the invention solves the additional problem of separating multiple interferences from one another and minimizing or even eliminating interfering interference in the interference patterns. This allows, for example, better measurement of test specimens polished on both sides.

[0027] The simultaneous elimination of interference and the achievement of such a dynamic range as is provided by the device according to the invention was not possible with previous methods according to the prior art.

[0028] Furthermore, the measurement wavelength can be chosen much more freely.

[0029] Furthermore, the device according to the invention also enables the measurement of comparatively large test specimens with diameters of up to several decimeters, and potentially up to one meter, with high measurement accuracy, without the need to spatially juxtapose or "stitch" multiple data sets. For measuring test specimens with diameters of approximately 1 m, the use of a suitable beam expander may be provided.

[0030] Furthermore, the device according to the invention also offers the possibility of an advantageous combination of a number of available camera pixels of the detection device and an object size of the test object to be measured, even in the case where several data sets are spatially arranged next to each other.

[0031] Further advantages of the device according to the invention result from the use of interferometric methods, so that, among other things, an essentially contactless measurement of the test object is possible and possible damage to the test object is avoided.

[0032] For the reasons stated above, the device according to the invention can significantly contribute to improving quality assurance, for example of optical elements. This can be used during the manufacture of the optical element for optimization, in particular for eliminating identified wavefront defects, and / or for checking the wavefront before the optical element is installed in the intended optical system, for example a lithography system.

[0033] In comparison to the solution according to WO 2006 / 102997 A1, essentially a modification of the lighting device or lighting module is necessary to realize the advantages of the present invention. The advantages of the invention can thus be achieved efficiently. The device according to the invention can also be understood or described as an interferometer system.

[0034] The use of the device according to the invention is not limited to optical elements. The device according to the invention can also be used for other test objects. A wavefront can emanate from any object that interacts with light or radiation, for example, by reflecting radiation. However, measuring the wavefront of a test object can be particularly important when the test object is intended for optical applications.

[0035] An optical element or optics system can be a ready-to-use optical element for shaping and directing radiation and / or preliminary stages of optical elements or optical elements or components thereof that are still in the manufacturing process.

[0036] The optical element can be, for example, a mirror or a lens, or it can be, for example, a substrate for a mirror or a glass block as the starting material for a lens. Furthermore, the measurement can also concern an optical coating, such as an antireflective coating, on a substrate or an optical element. If the test specimen is an optical element, it may preferably be intended and / or configured for use in a lithography system or in a projection exposure system for semiconductor lithography.

[0037] Optical elements often consist at least partially of glass, preferably glass with negligible thermal expansion or a very low coefficient of thermal expansion, such as titanium-doped fused silica, SiO2-TiO2 glass, or "ultra-low expansion glass" (ULE). Optical material itself can also serve as a test specimen.

[0038] Test specimens can include, for example, test strips of optical material, "boules," flat aspheres or micrometer aspheres, flat plates, or polished optical or technical surfaces. This list is not exhaustive.

[0039] It may be provided that the device according to the invention is used for surface characterization of ground optical bodies, in particular EUV mirrors.

[0040] The lighting device can have one, two, or more than two radiation sources, wherein the lighting device can emit or does emit radiation or light with at least two different wavelengths or frequencies. This radiation need not be in the visible wavelength or frequency range. The two wavelengths can, but need not, be emitted simultaneously; they can also be emitted sequentially and / or alternately. The increased measurement accuracy of the device according to the invention, compared to the prior art, is based in particular on the fact that interference patterns or interferograms created with two wavelengths L1 and L2 are not only composed of the interference patterns of the individual wavelengths L1 and L2, but also contain a beat frequency with a synthetic wavelength L, which results from the wavelengths L1 and L2.The interferogram with both wavelengths therefore corresponds to the incoherent sum of the interference patterns of the individual wavelengths L1 and L2 with the synthetic wavelength L as a beat frequency.

[0041] The wavefront can be derived from the interference phase, but with standard methods using a wavelength A, this is only known modulo A. Furthermore, the interference patterns of individual wavelengths, especially in test specimens with strong local gradients, high roughness, or a peak-to-valley distribution with large difference values, usually contain moiré patterns, making it impossible to extract phase information associated with the wavefront of the incoming radiation.

[0042] The beat frequency in the interferograms acquired according to the invention, consisting of two wavelengths, enables phase reconstruction and thus allows conclusions to be drawn about the wavefront emanating from the test object. The inventive concept of intentionally generating a beat frequency in order to algorithmically remove moiré patterns from the interference patterns and to obtain the phase information represents a significant improvement over the prior art. Moiré patterns contained in a superposition of the interference patterns can be algorithmically removed by appropriate evaluation, in particular by a process described later in a Fourier space.

[0043] Furthermore, by utilizing the beat frequency, wavelength or frequency ranges can be accessed without significant additional effort, which would not be possible or only possible with considerable effort, for example through special radiation sources and special optics, by an existing, single radiation source.

[0044] The wavelengths, in particular the synthetic wavelength L, should preferably be matched to the test specimen, especially its surface roughness, material and / or coating. It is particularly important to ensure that the wavelengths are not absorbed in the beam path, especially not by the test specimen or any coating it may have.

[0045] The delay line serves primarily to minimize or eliminate interference in the beam path of the device or interferometry system. Specifically, interference can be completely eliminated if the optical path difference between the measurement cavity and a pre-cavity of the delay line is greater than the coherence length of the radiation used. Only those components of the radiation from the measurement cavity whose optical path difference exactly matches, or falls within the temporal coherence length of, the optical path difference set in the delay line are capable of causing interference and thus contributing to the detected interference pattern.

[0046] It is preferably provided that the first beam path and the second beam path are aligned at right angles to each other. The beam splitter is preferably aligned and arranged such that it splits the radiation emanating from the lighting device, directing one portion towards the first beam path and another portion towards the second beam path. Preferably, one of the reflectors or mirrors is arranged at each halfway point of the first optical path length of the first beam path and the second optical path length of the second beam path to redirect the radiation, in particular to reflect it back to the beam splitter. The respective optical path length can, in particular, comprise the sum of the outbound and return paths of the radiation along the beam path from the beam splitter to the corresponding reflector and back.The radiation splitter is preferably also aligned and arranged in such a way that the radiation splitter directs radiation reflected back from the reflectors onto a common beam path, in particular in the direction of the interferometry device.

[0047] The radiation splitter can be designed in particular as a beam splitter, preferably as a beam splitter cube.

[0048] At least one of the two reflectors on the first or second beam path is movable or positionable, thus allowing the optical path length of the respective beam path to be adjusted. Changing the optical path length, or the length of the beam path, results in a change in the travel time of the radiation along the beam path, assuming a constant speed of light. If the first and second optical path lengths are different, this results in an optical path difference between the first and second beam paths. The radiation on the beam path with the longer optical path length is delayed compared to the radiation on the beam path with the shorter optical path length.

[0049] The movable reflector can also be used to adjust the phase of the radiation.

[0050] The interferometry device can preferably be designed as a Fizeau interferometer.

[0051] The Fizeau plate with the Fizeau reference surface, the mirror with the mirror reference surface, and the test specimen, which typically has two opposing surfaces or a first and a second surface, are preferably arranged in a so-called Fizeau configuration. The Fizeau plate, the test specimen, and the mirror are preferably arranged at least approximately parallel to each other, with the test specimen positioned between the Fizeau plate and the mirror when the device is used as intended. Furthermore, the Fizeau plate with the Fizeau reference surface and the mirror with the mirror reference surface are preferably oriented towards the test specimen.

[0052] As the radiation passes through such a Fizeau arrangement, a multitude of reflections occur at the various surfaces, namely at the Fizeau reference surface, at the first and / or second surface of the test specimen, and at the mirror reference surface. This can lead to multiple or multiple interferences, which, depending on the chosen measurement cavity, can represent unwanted interference. The purpose of the delay device is to preferably eliminate these interferences.

[0053] The Fizeau plate typically has two plane-parallel boundary surfaces with high optical quality. The boundary surface designated as the Fizeau reference surface is of particularly exceptional quality, preferably being a so-called A / 20 surface. The Fizeau plate can, in particular, be made of glass.

[0054] The term "plane-parallel arrangement of the test specimen with respect to the Fizeau plate and the mirror" can be understood to mean, in particular, an arrangement of the first surface and / or the second surface and / or a median plane of the test specimen parallel to the Fizeau reference surface and / or the mirror reference surface. It should be noted that the first and second surfaces of the test specimen need not be parallel to each other, although they may be. Furthermore, the first and second surfaces of the test specimen need not be planar.

[0055] It is possible for the mirror of the interferometric setup or the Fizeau arrangement to be formed by the test specimen itself. In this case, the arrangement of the test specimen between the Fizeau plate and the mirror means that the Fizeau plate and the test specimen, which simultaneously forms the mirror, are positioned opposite each other. This variant can be particularly suitable if the test specimen is non-transmissive or has a low transmittance and a comparatively high reflectivity, so that a separate mirror for the interferometric measurement is not required.

[0056] Measurement of the test specimen in immersion, particularly with oil, may also be provided.

[0057] When the device according to the invention is used as intended, the illumination device emits radiation which first passes through the delay device, after which the radiation is directed into the interferometry device, where interference occurs due to the radiation reflected or partially reflected from different surfaces of the Fizeau array. Preferably, interference in the interferometry device is avoided by means of a suitably adjusted optical path difference in the delay device. The interference in the Fizeau array modulates the radiation, resulting in an interference pattern.It may be provided that the illumination device emits radiation of both wavelengths simultaneously and a combined interferogram with both wavelengths is recorded, or that the illumination device selectively emits radiation of one wavelength or the other and an interferogram with each wavelength is recorded sequentially. In the latter case, it may be particularly possible to subsequently calculate a combined interferogram with both wavelengths from the two interferograms with the individual wavelengths.

[0058] A preferred use of the device according to the invention results, among other things, from the methods according to the invention which will be described in more detail later.

[0059] It can be advantageous if the device according to the invention is set up and designed so that measurements can be carried out either with two wavelengths or standard measurements with only one wavelength.

[0060] The device according to the invention may, in addition to the lighting device, the delay device and the interferometry device, optionally include further devices or elements.

[0061] It is particularly advantageous if a detection device for recording two-dimensional image data, especially interferograms, is provided, i.e., if the device has a detection device for recording two-dimensional image data, especially interferograms.

[0062] The detection device can be configured as an optoelectronic detector or as a camera, in particular as a CCD camera and / or CMOS camera. The detection device is preferably designed and configured for acquiring two-dimensional image data or images, in particular for acquiring interference patterns or interferograms with wavelengths L1, L2, and L3.

[0063] Preferably, the detection device has the highest possible resolution and / or pixel density. With respect to the pixels, the local gradients of the test specimens are determined in particular by the angle between pixels, which can be significantly influenced by the roughness of the test specimen.

[0064] The detection device can form a measuring plane or recording plane.

[0065] Preferably, the illumination device, the delay device, the interferometry device and the detection device are arranged such that the radiation emitted by the illumination device can be detected by the detection device after passing through the delay device and the interferometry device.

[0066] The detection device is preferably arranged and aligned such that the radiation, after passing through the delay device and the interferometry device, reaches the recording plane of the detection device. The radiation is generally modulated into an interference pattern, which can be recorded by the detection device in the form of two-dimensional image data.

[0067] As an alternative to recording the measurement using the detection device, the interference pattern can also be displayed and viewed on a simple observation plane, for example a screen.

[0068] A mixed interferogram of two wavelengths, regardless of whether it was recorded directly as a single interferogram or calculated from two separate interferograms, can be represented as an incoherent sum of the interference patterns for the individual wavelengths. Let x and y be the coordinates in the observation plane and measurement plane, respectively, a(x,y) the mean brightness and the constant light component, bi(x,y) the intensity modulation for a first wavelength Ai, b2(x,y) the intensity modulation for a second wavelength A2, 1(x,y) the phase for Ai, O2(x,y) the phase for A2, öin the phase shift of an nth interference pattern for Ai, and O2n the phase shift of an nth interference pattern for A2. Then the mixed interferogram can be represented, in particular, according to the following formula (1):

[0069]

[0070] >

[0071] This can be transformed into the following expression according to formula (2), now omitting the coordinate dependency for the sake of simplicity:

[0072]

[0073] This results in two mixed terms, <Di - O2 und <Di + $2, welche den Wellenlängen (Ai x A2) / (Ai - A2) und (Ai x A2) / (Ai + A2) zugeordnet werden können.

[0074] In the context of the present invention, the wavelengths Ai and A2 can in particular be understood as the wavelengths L1 and L2 that can be emitted or generated by the lighting device.

[0075] The synthetic wavelength L can preferably be given as the quotient of the product and the difference of the two wavelengths L1 and L2, such that L = (L1 x |_2) / (L1 - L2).

[0076] This definition also includes in particular any transformations of the equation L = (L1 x |_2) / (L1 -L2), as well as similar definitions that differ from the above definition of the synthetic wavelength L by only a constant factor.

[0077] The synthetic wavelength or effective wavelength or beat wavelength L or A s This corresponds to a long-wavelength or low-frequency signal component. The further mixing term of wavelengths L1 and L2, given by (L1 x |_2) / (L1 + L2), represents a short-wavelength or high-frequency signal component. The signal of the individual wavelengths L1 and L2 is superimposed with these further signal components in the mixed interferogram. The high-frequency signal component can be considered the frequency of the superposition oscillation, which is modulated by the low-frequency signal component as the envelope.

[0078] The interferogram resulting from the measurement with two different wavelengths therefore exhibits a beat frequency, whereby the low-frequency signal component with the synthetic wavelength L is particularly important for reconstructing the wavefront emanating from the test object. Within the scope of the invention, beat frequency refers specifically to the signal component with the synthetic wavelength L.

[0079] It can preferably be provided that the lighting device generates coherent, in particular short-coherent, radiation with a coherence length, wherein the respective coherence length for the wavelengths L1 and L2 is in particular at least as large, preferably 10 times as large, further preferably 20 times as large, as the synthetic wavelength L.

[0080] The use of coherent radiation, and in particular the coherence of the synthetic wavelength, ensures the interference capability of the radiation.

[0081] The factor of 10 or 20 merely represents an additional safety factor that can be omitted if the contrast of the interference patterns is sufficient. This safety factor can be particularly relevant for comparatively long synthetic wavelengths L.

[0082] The further advantage of coherence is that no interference originates from cavities that differ by more than one coherence length from the optical path difference set in the delay device.

[0083] It may be provided that the radiation emitted by the radiation source(s) of the lighting device is pulsed.

[0084] However, it can also be intended that the radiation emitted by the radiation source(s) is continuous.

[0085] Furthermore, continuous radiation emitted by the radiation source(s) can be modulated, if necessary, by means of an optical chopper to obtain pulsed radiation for measurement.

[0086] It has proven particularly suitable if at least one radiation source is designed as a superluminescent diode and / or at least one radiation source is designed as an ultrashort pulse laser and / or at least one radiation source is designed as a supercontinuum source. This includes, among others, the following possibilities: Two radiation sources can be provided or present, which are designed as superluminescent diodes and which emit radiation of different wavelengths. Alternatively, two radiation sources can be provided, wherein one of the two radiation sources is designed as a superluminescent diode and the other radiation source is designed as a supercontinuum source, preferably with an adjustable or selectable wavelength and / or with an adjustable bandwidth.It is also possible that only one radiation source is provided, wherein the radiation source is configured as a supercontinuum source, and wherein radiation of two different wavelengths can be emitted or selected. Furthermore, one or two radiation sources can be provided, which are configured as laser radiation sources, preferably as ultrashort pulse lasers, and more preferably as femtosecond lasers, and which emit radiation of one adjustable or different wavelengths. This exemplary list of possible combinations of radiation sources is not exhaustive.

[0087] The radiation source(s) could be, for example, radiation sources based on helium-neon (HeNe) or neodymium-doped yttrium aluminum garnet (Nd:YAG).

[0088] Using a single radiation source can be advantageous because, for example, fluctuations in power, light intensity, or center-of-mass wavelength can be clearly assigned and therefore relatively easily compensated, and because they are the same for the radiation of both wavelengths L1 and L2. Furthermore, other radiation parameters are also initially the same for the radiation of both wavelengths.

[0089] To adjust the wavelength of a predefined radiation source, even if it is designed to emit only a single wavelength, various optical components and methods can be used so that the lighting device as a whole can still generate and emit at least two different wavelengths. For example, optical filters, nonlinear crystals (especially for frequency doubling), an optical parametric oscillator or "Optical Parametric Amplifier" (OPA), and / or a "Wavelength Selection Box" can be used to influence, select, or adjust the wavelength.

[0090] It is particularly advantageous if the wavelength of the radiation source or one of the radiation sources is adjustable. This allows the synthetic wavelength L to be freely selected.

[0091] Preferably, one or both of the reflectors of the delay device can be designed as angled mirrors, preferably as 90° angled mirrors.

[0092] The angled mirror can be, in particular, a retroreflector or a triple mirror. It is particularly advantageous if the optical path difference can be adjusted by means of the delay device such that the optical path difference corresponds at least approximately, preferably exactly, to the optical path in a measuring cavity, in particular the optical path between the Fizeau reference surface and / or the first surface of the test specimen facing the Fizeau plate and / or the second surface of the test specimen facing the mirror and / or the mirror reference surface, in order to minimize, preferably eliminate, interference from the measuring cavity in the interference pattern.

[0093] Measuring cavities within the Fizeau arrangement can be located, in particular between the Fizeau reference surface and the mirror reference surface, directly (Wkai) or indirectly (w). a b) with transmission through the intermediate test specimen, between the Fizeau reference surface and the first surface of the test specimen (w a i) exist between the Fizeau reference surface and the second surface of the test specimen (Wa2), the first surface of the test specimen and the mirror reference surface (WM), the second surface of the test specimen and the mirror reference surface (Wb2), and between the first and second surfaces of the test specimen (W12), where the corresponding symbols for the following equations are given in parentheses.

[0094] Multiple reflections of the radiation can occur between the aforementioned surfaces. If left uncompensated, this leads to multiple interferences or stray interferences. By matching the optical path difference of the delay device to the optical path of one of the measurement cavities, stray interferences from this measurement cavity are eliminated from the interference pattern. This eliminates the interference effect of optical path differences in the measurement cavity, as the relevant optical path differences in the measurement cavity do not correspond to the optical path difference in the delay device, even within a "tolerance range" defined by the coherence length of the radiation used. Thus, a disturbance-free interference pattern can be recorded from the selected measurement cavity.

[0095] By performing multiple measurements or several interferograms with a delay device adapted to different measurement cavities, an overall interference-free interferogram can be reconstructed.

[0096] The measured wavefronts can be represented according to formula set (3), where hi (x,y) is the fit or fit error or form deviation of the first surface of the test specimen facing the Fizeau plate and h2 (x,y) is the fit of the second surface of the test specimen facing the mirror, w a the pass of the Fizeau reference surface, Wb the pass of the mirror reference surface, i-12 the refractive index inhomogeneity as a wavefront with double passage through the test specimen, n = n (x,y,z) the nominal or mean refractive index of the test specimen at the wavelength of the radiation used for measurement and WD = Wab - Wkai is:

[0097] Wal = Wa + 2 hlWb2 = Wb + 2 112

[0098] wi2 = - 2n hi - 2n h2 + I12

[0099] Wab = Wa+ Wb - 2(nl) hi- 2(nl) h2 + ii2

[0100] (3) Wa2 = w a - 2(nl) hi-2nhz + ii2

[0101] Wbi = Wb - 2n hi- 2(nl) h2 + I12

[0102] Wkal = Wa + Wb

[0103] WD = Wab - Wkai = - 2(nl) hi - 2(nl) h2 + 112

[0104] All parameters depend on the coordinates x and y, which have been omitted for the sake of clarity. By rearranging and combining the formulas of the preceding system of equations (3), the following alternative methods for reconstructing the refractive index inhomogeneity i-12 are obtained:

[0105]

[0106] 112 = (nl)*(Wal +Wb2) - n*Wcal + Wab

[0107] If the average thickness T of the test specimen is known, the axially integrated distribution of the refractive index An over the thickness can also be determined from this:

[0108]

[0109] The refractive index inhomogeneity i-12 can be understood in particular as the relative deviation of the local refractive index from An.

[0110] Further preferred configurations and uses of the delay device are set out in WO 2006 / 102997 A1 of the applicant.

[0111] It may be provided that the test object can be illuminated simultaneously or sequentially with radiation of the two wavelengths L1 and L2.

[0112] Simultaneous illumination of the test object with both wavelengths enables a fast and direct measurement of the mixed interferogram and does not cause any additional computational or data processing effort.

[0113] Separate illumination of the test specimen with the individual wavelengths reduces the risk of crosstalk or other interfering effects on the at least partially identical beam path of the two wavelengths. Furthermore, considerable flexibility remains in data processing, for example, to combine different pairs of wavelengths. Moreover, the device can thus be used not only for the recording of interferograms with two wavelengths as provided for in the invention, but also for recording classical interferograms with only one wavelength.

[0114] The device can preferably be configured and designed to allow selection between simultaneous and separate illumination of the test specimen. However, the device can also be designed for only one of these two variants.

[0115] A beam blocker or "shutter" can preferably be used to block the radiation from one of the radiation sources and / or the radiation from one of the two wavelengths L1 and L2.

[0116] It has proven to be particularly advantageous if the phase of the radiation is adjustable and / or a phase shifter is provided or available, which is preferably arranged in the delay device, more preferably on the movable reflector, and / or in the interferometry device, more preferably on the Fizeau plate.

[0117] It can be provided that the phase shifter is preferably formed by the delay device, and further preferably by the movable reflector.

[0118] This allows the radiation to be subjected to a phase step between successive measurements. This is advantageous for carrying out the inventive method described in more detail below and, in particular, enables phase reconstruction. Reference is made in particular to the following description of step (d) of the inventive method and its preferred embodiments.

[0119] It can preferably be provided that the phase as a whole can be moved by a multiple of the synthetic wavelength L, preferably 5 times to 10 times as far as the synthetic wavelength L is.

[0120] This ensures that N interferograms can be created at intervals of a suitable phase step.

[0121] For a synthetic wavelength L of 45 pm, for example, a travel range of 225 pm to 450 pm may be suitable.

[0122] The phase shifter on the movable reflector of the delay device can be configured as a displacement mechanism for adjusting the optical path difference. This has the advantage that only one actuating element is required for the simultaneous adjustment of the radiation phase and the optical path difference. Preferably, the phase shifter and / or the displacement mechanism of the movable reflector can be a piezoelectric actuator, a piezoelectric drive, or a piezoelectric motor.

[0123] The illumination device, the delay device and / or the interferometry device may include at least one beam splitter, at least one lens, at least one collimator, at least one aperture, at least one deflecting mirror, at least one beam blocker, at least one polarizer, at least one optical filter, at least one wavelength selection module and / or at least one nonlinear or photonic crystal.

[0124] Similarly, further optical components or elements may be provided or present in the illumination device and / or the delay device and / or the interferometry device. Depending on their position in the beam path, the optical components can influence or modify the radiation from one or more radiation sources of the illumination device or the radiation on one or both of the beam paths of the delay device, for example, with regard to wavelength, bandwidth, pulse duration, beam profile, and / or beam path. The precise design of the beam path is left to the expert's discretion.

[0125] Beam splitters can be used to split a beam into two beam paths, as well as to recombine them.

[0126] At least two lenses can together form a telescope or a beam expander.

[0127] With multiple radiation sources, a beam blocker or "shutter" can be used, for example, to selectively perform a measurement with one or more wavelengths. When the beam blocker is inserted into a beam path, the radiation is blocked by it.

[0128] A wavelength selection module is an optical module designed and configured to select or adjust one or more wavelengths from incoming radiation of any wavelength. Examples include a "Wavelength Selection Box" or an "Optical Parametric Amplifier" (OPA).

[0129] Nonlinear crystals can be used, for example, to adjust or influence the wavelength of radiation. Photonic crystals also include, in particular, photonic crystal fibers or "photonic crystal fiber" (PCF).

[0130] An optical fiber may preferably be provided or present to supply the radiation to the interferometry device after it has passed through the delay device, wherein the optical fiber is preferably polarization-preserving and / or designed as a single-mode fiber and / or as a multi-mode fiber.

[0131] The optical fiber should preferably be designed for the wavelengths used, in particular L1, L2 and L.

[0132] One of the advantages of using an optical fiber or light guide to direct the radiation from the delay device to the interferometer is the ability to position the illumination and delay devices in a location that is more easily accessible for maintenance compared to the location of the interferometer and detection devices. Generally, the use of an optical fiber allows for a non-linear beam path. Furthermore, optical fibers can enhance laser safety when used with laser radiation sources.

[0133] Similarly, an optical fiber can be provided or present to supply the radiation emitted by the lighting device to the delay device.

[0134] A positioning device for positioning the test specimen may preferably be provided or already present.

[0135] Positioning can involve, among other things, moving and / or tilting the test specimen. Moving the specimen allows for the measurement of different areas and the recording of interferograms. This can be used for stitching measurement data to analyze larger specimens. Tilting the specimen allows it to be examined from different directions, enabling analysis not only in a two-dimensional plane but also in three spatial directions. Combining these methods maximizes the measurement range and potentially achieves 3D qualification.

[0136] The invention also relates to a method for interferometric measurement of a wavefront of a test object, in particular an optical element and / or its precursors, with a first surface and an opposing second surface using an interferometer system, preferably using a device according to the invention as described above, characterized by at least the following steps:

[0137] (a) Creating N interferograms with radiation of two different wavelengths L1 and L2 which, when superimposed, produce a synthetic wavelength L, wherein the N interferograms differ from each other by a phase step;

[0138] (b) If necessary, processing the interferograms to improve the signal-to-noise ratio and / or the frequency separation during subsequent evaluation;

[0139] (c) Separating low-frequency and high-frequency components of the interferograms by frequency analysis, in particular extracting the component of a beat frequency corresponding to the synthetic wavelength L; (d) Evaluating the phases by phase reconstruction to reconstruct the wavefront emanating from the test object;

[0140] (e) Eliminate any jumps, if necessary.

[0141] The advantages of the inventive method for interferometric measurement of a wavefront of a test object result analogously from the advantages of the inventive device already described above.

[0142] The method according to the invention can be carried out particularly advantageously with the device according to the invention. However, the method according to the invention can also be implemented independently of the device according to the invention with another interferometer system.

[0143] The creation of the N interferograms in step (a) can be carried out by recording a mixed interferogram with both wavelengths simultaneously or by recording two separate interferograms, each with one of the two wavelengths, which are then computationally combined to form a mixed interferogram.

[0144] The fact that the N interferograms differ from each other by a phase step according to step (a) means in particular that the phase of both wavelengths L1 and L2 is shifted by this phase step.

[0145] Step (b) can be used to prepare the interferograms created in step (a) for step (c), for example by increasing the signal-to-noise ratio through data processing or signal conditioning methods, in order to better separate the different frequencies in the subsequent step (c).

[0146] Separating different frequency components of the possibly already processed interferograms in step (c) is particularly aimed at extracting the component with the synthetic wavelength or the corresponding beat frequency.

[0147] In step (d) the phase can then be reconstructed based on the signal component of the synthetic wavelength in order to obtain the original wavefront.

[0148] Step (e) can be used to eliminate any jumps in the phase reconstructed after step (d) in order to obtain a final result that is as free of artifacts as possible.

[0149] It should be noted that discontinuities or phase jumps are preferably avoided or reduced from the outset by the method according to the invention. Steps (b) and (e) are to be understood as being to be carried out if necessary. These steps can be omitted if the test specimen and the measurement data or the N interferograms are of exceptional quality or if the data quality requirements are less stringent. In such a case, steps (b) and (e) are optional.

[0150] Preferably, steps (a) to (e) are carried out in the order shown. However, some steps can also be carried out in a different order.

[0151] It may be provided that suitable wavelengths L1 and L2 are selected in an optional preparatory step.

[0152] The choice of wavelengths L1 and L2 can preferably be based on the synthetic wavelength L. The synthetic wavelength L defines the uniqueness range or the maximum measurable local gradients. The Nyquist criterion or the Nyquist limit must be taken into account in this context.

[0153] The measurement result from the inventive method for measuring a wavefront of a test object can preferably be used to analyze the reconstructed wavefront, in particular by comparison with a reference wavefront, in order to identify any wavefront errors of the test object.

[0154] It may be provided that in step (a) each of the N interferograms is recorded by simultaneously illuminating the test object with the two wavelengths L1 and L2.

[0155] It can also be provided that in step (a) each of the N interferograms is calculated from two individual interference patterns, which are recorded successively with each of the two individual wavelengths L1 and L2.

[0156] The N interferograms can preferably each correspond to the incoherent sum of interference patterns of the individual wavelengths L1 and L2 with the synthetic wavelength L as the beat frequency.

[0157] Furthermore, the synthetic wavelength L can preferably be given as the quotient of the product and the difference of the two wavelengths L1 and L2, such that L = (L1 x [_2) / (L1 - L2).

[0158] Reference is also made to the preceding statements in connection with the device according to the invention.

[0159] For selecting suitable wavelengths L1 and L2 based on the effective or synthetic wavelength L, particularly in the optional preparation step, this means that the synthetic wavelength L is larger the smaller the difference between wavelengths L1 and L2. The synthetic wavelength L can differ from the individual wavelengths L1 and L2 by several orders of magnitude. For example, choosing L1 = 680 nm and L2 = 670 nm results in a synthetic wavelength of L = 45.56 pm. This corresponds to a factor of approximately 67 compared to the individual wavelengths L1 and L2.

[0160] By applying the Nyquist criterion, local gradients of more than 1000 mrad can be measured by selecting wavelengths L1 and L2 appropriately with respect to the synthetic wavelength L. This is significantly higher than is possible with the individual wavelengths.

[0161] Preferably, the synthetic wavelength L is chosen to be larger than the surface roughness or the local gradients of the test specimen.

[0162] Furthermore, the selected wavelengths are preferably matched to the test object in such a way that high-contrast measurement data can be recorded. Factors to be considered include, for example, the optical properties of the test object and any coating that may be present.

[0163] It has proven particularly suitable when the phase step in step (a) corresponds to a multiple of the synthetic wavelength L, where, for example, N = 5 interferograms are recorded in phase steps of L / 4. This allows for a relatively simple and robust phase evaluation algorithm.

[0164] It can be advantageous if the interferometer system used has an illumination device with at least one radiation source, preferably with at least two radiation sources, to emit radiation of wavelengths L1 and L2.

[0165] It can also be advantageous if the interferometer system used has a detection device to record two-dimensional image data, especially the interferograms.

[0166] With regard to a detection device, which may in particular be designed as a camera, it follows from the Nyquist criterion that only such local differences between neighboring pixels can be measured or resolved which are no larger than half the wavelength.

[0167] Since the synthetic wavelength L is generally larger than the individual wavelengths L1 and L2, the solution according to the invention allows for the measurement of test specimens with significantly stronger local gradients than is possible according to the prior art. In particular, using two wavelengths L1 and L2, or the resulting synthetic wavelength L, measurements beyond the Nyquist limit for the individual wavelengths L1 and L2 can also be carried out.

[0168] It is particularly advantageous if the interferometer system used has a delay device comprising a beam splitter and at least two reflectors, wherein the beam splitter divides the radiation, wherein the beam splitter and each of the reflectors form a first beam path with a first optical path length and a second beam path with a second optical path length, wherein at least one of the reflectors is movable, so that a distance between the beam splitter and the movable reflector can be adjusted in order to set an optical path difference between the first beam path and the second beam path.

[0169] It has proven particularly suitable if the interferometer system used has an interferometry device which includes at least one Fizeau plate with a Fizeau reference surface and a mirror with a mirror reference surface, between which the test specimen is arranged, preferably at least approximately plane-parallel.

[0170] For preferred embodiments of the lighting device, the delay device, the interferometry device and the detection device, reference is also made to the preceding description of the device according to the invention.

[0171] In particular, if the interferometer system used has at least a delay device and an interferometry device according to the above description, at least the following sub-steps may preferably be provided or carried out in step (a) for creating the interferograms:

[0172] (a1) If necessary, arrange the test specimen between the Fizeau reference surface and the mirror reference surface of the interferometry device, such that the first surface of the test specimen is aligned towards the Fizeau reference surface and the second surface of the test specimen is aligned towards the mirror reference surface and preferably each is plane-parallel to it;

[0173] (a2) Adjusting the optical path difference of the delay device so that, within a coherence length of the radiation, it corresponds only to a part of the optical paths of a measuring cavity, in particular the optical path between the Fizeau reference surface and / or the first surface of the test specimen facing the Fizeau plate and / or the second surface of the test specimen facing the mirror and / or the mirror reference surface;

[0174] (a3) Detection of at least one interference pattern generated by superposition of radiation reflected from the different surfaces, wherein interference from the measurement cavity underlying step (a2) is minimized, preferably eliminated.

[0175] Steps (a2) and (a3) ​​bring about the same advantages as those already described above in connection with the delay device of the device according to the invention.

[0176] In substep (a2), it can be ensured in particular that at least one optical path in the measurement cavity, especially the optical path between the Fizeau reference surface and / or the first surface of the test specimen facing the Fizeau plate and / or the second surface of the test specimen facing the mirror and / or the mirror reference surface, is excluded from interference capability, since it deviates from the path difference of the delay device by more than one coherence length. Thus, the optical path difference of the delay device is adjusted so that, within the coherence length of the radiation or radiation source, it corresponds only to a subset of the optical paths in the measurement cavity, i.e., one or more, but not all, paths. Those paths to which it does not correspond within the limits of the coherence length are then excluded from interference.

[0177] Particular reference is made to the explanations concerning the adjustment of the optical path difference and the measuring cavities in connection with the device according to the invention, and additionally to WO 2006 / 102997 A1 of the applicant by analogy.

[0178] Preferably, several interferograms are recorded, with the optical path difference of the delay device being adjusted to different measuring cavities in order to eliminate the interfering interference of several, preferably all, measuring cavities not to be measured, in such a way that only contributions from the cavity to be measured or the useful cavity remain.

[0179] This can be done, for example, in the following steps:

[0180] Adjusting the optical path difference of the delay device so that it is essentially equal to an optical path between the Fizeau reference surface and the first surface of the test specimen, and

[0181] Capturing at least one first interference pattern generated by superimposing radiation reflected by the Fizeau reference surface and radiation reflected by the first surface of the test object, wherein interfering interferences of the measurement cavity between the Fizeau reference surface and the first surface of the test object are eliminated;

[0182] Adjusting the optical path difference of the delay device so that it is essentially equal to an optical path between the second surface of the test specimen and the mirror reference surface, and

[0183] Capturing at least one second interference pattern generated by superimposing radiation reflected by the second surface of the test object and radiation reflected by the mirror reference surface, whereby interfering interferences of the measuring cavity between the second surface of the test object and the mirror reference surface are eliminated.

[0184] Further steps for the other measuring cavities can be added analogously.

[0185] The following step can also be taken:

[0186] Removing the test specimen from the measuring cavity, and

[0187] Adjusting the optical path difference of the delay device so that it is substantially equal to an optical path between the Fizeau reference surface and the mirror reference surface, and detecting at least one third interference pattern produced by superimposing radiation reflected by the Fizeau reference surface and radiation reflected by the mirror reference surface, wherein measuring cavity interference between the Fizeau reference surface and the mirror reference surface is eliminated.

[0188] This is a variation of sub-steps (a1) to (a3), whereby sub-step (a1) is essentially omitted or modified.

[0189] Based on the majority of interferograms for different measurement cavities, an overall interference-free interferogram can then be reconstructed, which represents one of the N interferograms according to step (a).

[0190] The procedure described above can also be referred to as the reconstruction of the refractive index inhomogeneity.

[0191] The majority of interferograms recorded according to substeps (a1) to (a3) ​​are recorded in accordance with step (a) using radiation of the two wavelengths L1 and L2.

[0192] It may be possible to use a compensation system in step (a) to create the interferograms in order to adapt the radiation to the test object and / or to generate a test wave suitable for the test object.

[0193] In this way, shape deviations of the test specimen can be at least partially compensated for. This may make it possible to measure test specimens with even stronger local gradients if the advantages of the invention are insufficient. However, as a rule, the solution according to the invention should eliminate the need for an additional compensation system.

[0194] It may preferably be provided that in step (a) the phase step is adjusted at least partially with a phase shifter of the delay device, in particular on the movable reflector, and / or at least partially with a phase shifter of the interferometry device, in particular on the Fizeau plate.

[0195] It may be advantageous if, in step (a) or (a1), the test specimen is positioned in the beam path of the interferometer system at a distance of more than half a coherence length, with respect to the radiation of wavelengths L1 and L2 and / or to the synthetic wavelength L from the Fizeau reference surface.

[0196] It has proven particularly suitable if, in step (b), the processing of the interferograms is carried out computationally by quadrature and / or by impingement with a carrier frequency. Based on the equations of formula (1) and formula (2) already described above in the context of the device according to the invention, quadrature after subtracting the background intensity and omitting the coordinate dependence yields the following intermediate result of formula (6), wherein <Ds die Phase der synthetischen Wellenlänge (Ai x A2) / (Ai - A2) und <Davg die Phase der mittleren Wellenlänge (Al x A2) / (Al + A2) ist, Und Wobei Ös,n = öl.n - Ö2,n Und öavg.n = öl.n + Ö2,n:

[0197]

[0198] Preferably, the squared intensity signal according to equation (6) can additionally be applied a carrier frequency.

[0199] It can be particularly advantageous if, in step (c), the separation of low-frequency and high-frequency components of the interferograms is carried out computationally by using a low-pass filter and / or Fourier transform, in particular FFT and / or inverse FFT.

[0200] It can preferably be provided that the signal is first transformed into the frequency domain using a Fourier transform, in particular a Fast Fourier Transform (FFT), the transformed signal is then filtered with a low-pass filter to eliminate high-frequency signals and to extract the signal of the synthetic wavelength, and the extracted signal is finally transformed back into the phase domain using an inverse Fourier transform. This preserves the desired phase, particularly in units of the synthetic wavelength.

[0201] Alternatively or in addition to a pure low-pass filter, other filter functions can also be used.

[0202] The filter function can, for example, be based on a Hann or Hamming function or a Hann or Hamming window. Similarly, many other filter functions known to experts may be suitable.

[0203] Preferably, in step (d) the evaluation of the phases can be carried out computationally by an N-step phase evaluation algorithm, for example a 5-step phase evaluation algorithm.

[0204] If at least 5 interferograms me were recorded in phase steps of L / 4, the phase reconstruction can be carried out in particular with a 5-step phase evaluation algorithm or 5-step algorithm, according to which the phase of the synthetic wavelength is determined from the interferograms In with ne {1 ,2, 3, 4, 5} according to formula (7):

[0205] <>

[0206]

[0207] It has proven particularly suitable if, in step (e), the elimination of discontinuities is carried out computationally using a discontinuity method.

[0208] A variety of methods known to experts are suitable for eliminating jumps or for "phase unwrapping".

[0209] The invention further relates to a method for manufacturing an optical element, in particular an optical element for a lithography system, wherein an inventive method for interferometric measurement of a wavefront of a test specimen according to the preceding description is carried out with the optical element and / or its precursors as the test specimen, and wherein the following steps are additionally provided or carried out:

[0210] (f) Analyzing the reconstructed wavefront, in particular comparing it with a reference wavefront, to identify any wavefront errors of the test specimen or optical element; (g) Machining at least one of the first and second surfaces of the test specimen or optical element based on the reconstructed wavefront to eliminate the identified wavefront errors.

[0211] The advantages of the inventive method for producing an optical element result analogously from the advantages of the inventive device and the inventive method for interferometric measurement of a wavefront of a test object already described above.

[0212] Step (f) allows conclusions to be drawn from the measurement data or the reconstructed wavefront for the test object designed as an optical element, in order to assess its quality based on its wavefront errors, for example.

[0213] Furthermore, the information obtained from the measurement of the wavefront or the analysis of the wavefront errors of the test specimen or the optical element can be used by step (g) to improve and / or optimize the optical element.

[0214] A reference wavefront can be understood in particular as an ideal, theoretically calculated, predicted and / or desired wavefront.

[0215] It may be necessary to iteratively repeat the measurement, analysis and processing of the test specimen or optical element according to steps (a) to (g) in order to gradually approximate the desired optical properties.

[0216] This document discloses an optical element manufactured using the aforementioned method for producing an optical element. Due to the precise manufacturing process, the disclosed optical element has an advantageous, particularly microscopic, surface and / or solid material structure, and thus particularly good optical properties.

[0217] In particular, inhomogeneities and striae in a titanium content of a titanium-doped quartz glass can be advantageously compensated.

[0218] The invention further relates to a lithography system, in particular a projection exposure system for semiconductor lithography, comprising an illumination system with a radiation source and optics, which has at least one optical element, wherein the optical element is measured by an inventive device and / or an inventive method for interferometric measurement of a wavefront of a test object, each according to the preceding description, and / or is produced by an inventive method for producing an optical element according to the preceding description.

[0219] The advantages of the lithography system according to the invention result analogously from the advantages of the device and the two methods according to the invention already described above.

[0220] In the field of semiconductor lithography, the advantages of the invention are of particular importance in order to ensure the best possible image quality and high resolution, so that even the smallest structures for microchips can be produced. This can be achieved primarily through carefully selected and optimized optical elements.

[0221] By measuring and / or analyzing the wavefront of the optical element, and preferably by appropriately processing the optical element or its precursors to eliminate any identified wavefront defects, it can be ensured that only high-quality optical elements are incorporated into the lithography system. The optical elements can be selected, in particular, with regard to their material homogeneity and optimized with regard to their surface quality.

[0222] The lithography system according to the invention is therefore characterized in particular by the optical elements measured and / or manufactured according to the invention, which have significantly lower wavefront errors compared to the prior art and thus contribute considerably to a higher image quality and a better resolution of the lithography system.

[0223] The lithography system can preferably be an EUV or DUV projection exposure system for semiconductor lithography or microlithography.

[0224] Analogous to the lithography system according to the invention, optical elements that have been measured using the device and / or method according to the invention for measuring the wavefront of a test specimen and, if necessary, analyzed for wavefront errors and / or manufactured using the method according to the invention for producing an optical element, can also be used for other optical systems. For example, their use in mask inspection systems for semiconductor lithography is conceivable, which also place high demands on the optical elements contained therein.

[0225] However, the application of the invention is not limited to the field of semiconductor lithography. The invention can be useful in all fields that require high-quality optical elements or other components with a tested and, if necessary, optimized wavefront.

[0226] Features described in connection with one of the subject matter of the invention, in particular those provided by the device according to the invention, the two methods according to the invention, or the lithography system according to the invention, can also be advantageously implemented for the other subject matter of the invention. Likewise, advantages mentioned in connection with one of the subject matter of the invention can also be understood to relate to the other subject matter of the invention.

[0227] It should also be noted that terms such as "comprehensive," "exhibiting," or "with" do not exclude other characteristics or steps. Furthermore, terms such as "a" or "the," which indicate a singular number of steps or characteristics, do not exclude a plurality of characteristics or steps—and vice versa.

[0228] In a purist embodiment of the invention, however, it may also be provided that the features introduced in the invention with the terms "comprising," "comprising," or "with" are exhaustively listed. Accordingly, one or more lists of features within the scope of the invention may be considered complete, for example, for each claim. The invention may, for instance, consist exclusively of the features mentioned in claim 1.

[0229] It should be noted that designations such as "first" or "second" etc. are primarily used for the purpose of distinguishing between the respective device or process features and are not necessarily intended to indicate that features are mutually dependent or related to each other.

[0230] Exemplary embodiments of the invention are described in more detail below with reference to the drawing.

[0231] The figures each show preferred embodiments in which individual features of the present invention are combined with one another. Features of an embodiment can also be implemented independently of the other features of the same embodiment and can therefore be readily combined by a person skilled in the art to form further meaningful combinations and subcombinations with features of other embodiments. In the figures, functionally identical elements are provided with the same reference numerals.

[0232] They show:

[0233] Figure 1 shows an EUV projection exposure system in meridional section;

[0234] Figure 2 shows a DUV projection exposure system;

[0235] Figure 3 shows a schematic view of an embodiment of a device according to the invention;

[0236] Figure 4 shows a schematic view of measuring cavities within a Fizeau arrangement;

[0237] Figure 5 shows a schematic view of an embodiment of a lighting device of the device according to the invention with two radiation sources;

[0238] Figure 6 shows a schematic view of an embodiment of a lighting device of the device according to the invention with a radiation source;

[0239] Figure 7 shows a schematic view of a further embodiment according to Figure 5;

[0240] Figure 8 shows a schematic view of a further embodiment according to Figure 5;

[0241] Figure 9 shows a schematic view of a further embodiment according to Figure 6;

[0242] Figure 10 shows a basic flowchart of an embodiment of a method according to the invention for the interferometric measurement of a wavefront of a test object designed as an optical element and its manufacture; and

[0243] Figure 11 shows a schematic representation of the composition of one of the N interferograms from exemplary interference patterns of wavelengths L1 and L2 according to step (a) of the method according to the invention.

[0244] The following section describes, with reference to Figure 1, the essential components of an EUV projection exposure system 100 for microlithography as an example of a lithography system. The description of the basic structure of the EUV projection exposure system 100 and its components is not intended to be restrictive.

[0245] An illumination system 101 of the EUV projection exposure system 100 comprises, in addition to a radiation source 102, an illumination optic 103 for illuminating an object field 104 in an object plane 105. A reticule 106 arranged in the object field 104 is exposed. The reticule 106 is held by a reticule holder 107. The reticule holder 107 can be moved, particularly in a scanning direction, by means of a reticule displacement drive 108.

[0246] Figure 1 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. In Figure 1, the scan direction runs along the y-direction. The z-direction runs perpendicular to the object plane 105.

[0247] The EUV projection exposure system 100 comprises a projection optic 109. The projection optic 109 serves to image the object field 104 onto an image field 110 in an image plane 111. The image plane 111 runs parallel to the object plane 105. Alternatively, an angle other than 0° between the object plane 105 and the image plane 111 is also possible.

[0248] A structure on the reticulum 106 is imaged onto a photosensitive layer of a wafer 112 located in the image plane 111 within the image field 110. The wafer 112 is held by a wafer holder 113. The wafer holder 113 can be displaced, particularly along the y-direction, via a wafer transfer drive 114. The displacement of the reticulum 106 via the reticulum transfer drive 108 and of the wafer 112 via the wafer transfer drive 114 can be synchronized.

[0249] Radiation source 102 is an EUV radiation source. Specifically, radiation source 102 emits EUV radiation 115, which is also referred to as useful radiation or illumination radiation. The useful radiation 115 has a wavelength in the range between 5 nm and 30 nm. Radiation source 102 can be a plasma source, for example, an LPP source (laser-produced plasma) or a DPP source (gas-discharged produced plasma). It can also be a synchrotron-based radiation source. Radiation source 102 can be a free-electron laser (FEL).

[0250] The illumination radiation 115, emanating from the radiation source 102, is focused by a collector 116. The collector 116 can be a collector with one or more ellipsoidal and / or hyperboloid reflective surfaces. The at least one reflective surface of the collector 116 can be illuminated with the illumination radiation 115 at grazing incidence (Gl), i.e., with angles of incidence greater than 45°, or at normal incidence (NI), i.e., with angles of incidence less than 45°. The collector 116 can be structured and / or coated, both to optimize its reflectivity for the useful radiation 115 and to suppress stray light.

[0251] After the collector 116, the illumination radiation 115 propagates through an intermediate focus in an intermediate focal plane 117. The intermediate focal plane 117 can represent a separation between a radiation source module, comprising the radiation source 102 and the collector 116, and the illumination optics 103.

[0252] The illumination optics 103 comprise a deflecting mirror 118 and, downstream in the beam path, a first faceted mirror 119. The deflecting mirror 118 can be a planar deflecting mirror or, alternatively, a mirror with an effect that influences the beam beyond the mere deflection effect. Alternatively or additionally, the deflecting mirror 118 can be designed as a spectral filter that separates a useful wavelength of the illumination radiation 115 from stray light of a different wavelength. If the first faceted mirror 119 is arranged in a plane of the illumination optics 103 that is optically conjugate to the object plane 105 as a field plane, it is also referred to as a field faceted mirror. The first faceted mirror 119 comprises a plurality of individual first facets 120, which are hereinafter also referred to as field facets. Only a few of these facets 120 are shown in Figure 1 as examples.

[0253] The first facets 120 can be designed as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or semicircular border contour. The first facets 120 can be designed as planar facets or alternatively as convexly or concavely curved facets.

[0254] As is known, for example, from DE 10 2008 009 600 A1, the first facets 120 themselves can each be composed of a plurality of individual mirrors, in particular a plurality of micromirrors. The first facet mirror 119 can in particular be designed as a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 A1.

[0255] Between the collector 116 and the deflecting mirror 118, the illumination radiation 115 runs horizontally, i.e. along the y-direction.

[0256] In the beam path of the illumination optics 103, a second faceted mirror 121 is arranged downstream of the first faceted mirror 119. If the second faceted mirror 121 is arranged in a pupil plane of the illumination optics 103, it is also referred to as a pupil faceted mirror. The second faceted mirror 121 can also be arranged at a distance from a pupil plane of the illumination optics 103. In this case, the combination of the first faceted mirror 119 and the second faceted mirror 121 is also referred to as a specular reflector. Specular reflectors are known from US 2006 / 0132747 A1, EP 1 614008 B1, and US 6,573,978.

[0257] The second faceted mirror 121 comprises a plurality of second facets 122. In the case of a pupil faceted mirror, the second facets 122 are also referred to as pupil facets. The second facets 122 can also be macroscopic facets, which may, for example, have round, rectangular, or hexagonal edges, or alternatively, facets composed of micromirrors. Reference is also made to DE 102008 009600 A1 in this regard.

[0258] The second facets 122 can have planar or alternatively convex or concave curved reflective surfaces.

[0259] The illumination optics 103 thus form a double-faceted system. This basic principle is also known as a fly's eye integrator.

[0260] It may be advantageous not to arrange the second faceted mirror 121 exactly in a plane which is optically conjugate to a pupil plane of the projection optics 109.

[0261] With the aid of the second faceted mirror 121, the individual first facets 120 are imaged into the object field 104. The second faceted mirror 121 is the last beam-shaping, or indeed the last, mirror for the illumination radiation 115 in the beam path before the object field 104.

[0262] In another embodiment of the illumination optics 103, not shown, a transmission optic can be arranged in the beam path between the second facet mirror 121 and the object field 104, which contributes in particular to imaging the first facets 120 into the object field 104. The transmission optic can have exactly one mirror, or alternatively two or more mirrors, which are arranged one behind the other in the beam path of the illumination optics 103. The transmission optic can in particular comprise one or two mirrors for normal incidence (Nl mirrors, "normal incidence" mirrors) and / or one or two mirrors for grazing incidence (Gl mirrors, "grazing incidence" mirrors).

[0263] In the embodiment shown in Figure 1, the lighting optics 103 has exactly three mirrors after the collector 116, namely the deflecting mirror 118, the field facet mirror 119 and the pupil facet mirror 121.

[0264] In a further embodiment of the lighting optics 103, the deflecting mirror 118 can also be omitted, so that the lighting optics 103 can then have exactly two mirrors after the collector 116, namely the first faceted mirror 119 and the second faceted mirror 121.

[0265] The imaging of the first facets 120 by means of the second facets 122 or with the second facets 122 and a transmission optic into the object plane 105 is regularly only an approximate imaging.

[0266] The projection optics 109 comprise a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the EUV projection exposure system 100. In the example shown in Figure 1, the projection optics 109 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 115. The projection optics 109 is a double-obscured optic. The projection optics 109 has an image-side numerical aperture that is greater than 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0267] The reflective surfaces of the mirrors Mi can be designed as freeform surfaces without an axis of rotational symmetry. Alternatively, the reflective surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflective surface shape. The mirrors Mi, like the mirrors of the illumination optics 103, can have highly reflective coatings for the illumination radiation 115. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0268] The projection optics 109 has a large object-image offset in the y-direction between a y-coordinate of a center of the object field 104 and a y-coordinate of the center of the image field 110. This object-image offset in the y-direction can be approximately as large as a z-distance between the object plane 105 and the image plane 111.

[0269] The number of intermediate image planes in the x- and y-directions in the beam path between the object field 104 and the image field 110 can be the same or, depending on the design of the projection optics 109, can differ. Examples of projection optics with different numbers of such intermediate images in the x- and y-directions are known from US 2018 / 0074303 A1.

[0270] Each of the pupil facets 122 is assigned to exactly one of the field facets 120 to form an illumination channel for illuminating the object field 104. This can result, in particular, in illumination according to Köhler's principle. The far field is divided into a multitude of object fields 104 by means of the field facets 120. The field facets 120 generate a plurality of images of the intermediate focus on the pupil facets 122 assigned to each of them.

[0271] The field facets 120 are each superimposed on the reticulum 106 by an associated pupil facet 122 to illuminate the object field 104. The illumination of the object field 104 is particularly homogeneous. It preferably exhibits a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

[0272] The illumination of the entrance pupil of the projection optics 109 can be geometrically defined by the arrangement of the pupil facets. By selecting the illumination channels, in particular the subset of pupil facets that guide light, the intensity distribution in the entrance pupil of the projection optics 109 can be adjusted. This intensity distribution is also referred to as the illumination setting.

[0273] Another preferred pupil uniformity in the area of ​​defined illuminated sections of an illumination pupil of the illumination optics 103 can be achieved by a redistribution of the illumination channels.

[0274] Further aspects and details of the illumination of the object field 104 and, in particular, the entrance pupil of the projection optics 109 are described below.

[0275] The projection optic 109 can, in particular, have a homocentric entrance pupil. This may be accessible. It may also be inaccessible.

[0276] The entrance pupil of the projection optics 109 cannot be precisely illuminated by the pupil facet mirror 121. When the projection optics 109 image the center of the pupil facet mirror 121 telecentrically onto the wafer 112, the aperture rays often do not intersect at a single point. However, a surface can be found where the pairwise determined separation of the aperture rays is minimized. This surface represents the entrance pupil or a surface conjugate to it in real space. In particular, this surface exhibits a finite curvature.

[0277] The projection optics 109 may have different entrance pupil positions for the tangential and sagittal beam paths. In this case, an imaging element, in particular an optical component of the transmission optics, should be provided between the second faceted mirror 121 and the reticle 106. This optical component will allow the different positions of the tangential and sagittal entrance pupils to be taken into account.

[0278] In the arrangement of the components of the illumination optics 103 shown in Figure 1, the pupil facet mirror 121 is arranged in a plane conjugate to the entrance pupil of the projection optics 109. The first field facet mirror 119 is arranged tilted relative to the object plane 105. The first facet mirror 119 is arranged tilted relative to an arrangement plane defined by the deflecting mirror 118.

[0279] The first faceted mirror 119 is arranged at an angle to an arrangement plane defined by the second faceted mirror 121.

[0280] Figure 2 shows an exemplary DUV projection exposure system 200. The DUV projection exposure system 200 comprises an illumination system 201, a device called a reticule stage 202 for receiving and precisely positioning a reticule 203, by which the subsequent structures on a wafer 204 are determined, a wafer holder 205 for holding, moving and precisely positioning the wafer 204 and an imaging device, namely a projection optic 206, with several optical elements, in particular lenses 207, which are held in a lens housing 209 of the projection optic 206 via mounts 208.

[0281] Alternatively or in addition to the lenses 207 shown, various refractive, diffractive and / or reflective optical elements, including mirrors, prisms, end plates and the like, may be provided.

[0282] The basic operating principle of the DUV projection exposure system 200 provides that the structures introduced into the reticulum 203 are imaged onto the wafer 204.

[0283] The illumination system 201 provides a projection beam 210 in the form of electromagnetic radiation, which is required for imaging the reticulum 203 onto the wafer 204. A laser, a plasma source, or the like can be used as the source of this radiation. In the illumination system 201, the radiation is shaped by optical elements such that the projection beam 210, upon striking the reticulum 203, exhibits the desired properties with regard to diameter, polarization, wavefront shape, and the like.

[0284] Using the projection beam 210, an image of the reticulum 203 is generated and transferred, appropriately reduced in size, to the wafer 204 by the projection optics 206. The reticulum 203 and the wafer 204 can be moved synchronously, so that areas of the reticulum 203 are mapped onto corresponding areas of the wafer 204 practically continuously during a so-called scan process.

[0285] Optionally, the air gap between the last lens 207 and the wafer 204 can be replaced by a liquid medium with a refractive index greater than 1.0. This liquid medium could, for example, be highly purified water. Such a setup is also known as immersion lithography and offers increased photolithographic resolution.

[0286] The use of the invention is not limited to use in projection exposure systems 100, 200, and in particular not limited to systems with the described configuration. The invention is suitable for any lithography or microlithography systems, but especially for projection exposure systems with the described configuration. The invention is also suitable for EUV projection exposure systems that have a lower image-side numerical aperture than that described in connection with Figure 1 and that do not have an obscured mirror M5 and / or M6. In particular, the invention is also suitable for EUV projection exposure systems that have an image-side numerical aperture of 0.25 to 0.5, preferably 0.3 to 0.4, and most preferably 0.33. Furthermore, the invention and the following embodiments are not to be understood as being limited to a specific design.

[0287] The following figures illustrate the invention only by way of example and in a highly schematic form. Figure 3 shows a schematic view of an embodiment of a device 1 for the interferometric measurement of a wavefront of a test object 2, in particular an optical element and / or its precursors. The device 1 comprises at least one illumination device 3, a delay device 4, and an interferometry device 5, and preferably a detection device 6.

[0288] The lighting device 3 comprises at least one radiation source 7, preferably at least two radiation sources 7. The lighting device 3 is configured and designed to emit radiation 8 with at least two different wavelengths L1 and L2, which, when superimposed, produce a synthetic wavelength L.

[0289] In Figure 3, as well as in the following Figures 5 to 9, the path of the radiation 8 is only simplified and highly schematically indicated by lines. In some sections, the radiation 8 of the two wavelengths L1 and L2 follows two separate paths, particularly within the illumination device 3; however, over long distances, the radiation 8 of the two wavelengths L1 and L2 travels in a collimate path, especially on the first beam path and on the second beam path of the delay device 4.

[0290] The delay device 4 comprises a beam splitter 9 and at least two reflectors 10a, 10b, wherein the beam splitter 9 divides the radiation 8, and wherein the beam splitter 9 and each of the reflectors 10a, 10b form a first beam path with a first optical path length 11a and a second beam path with a second optical path length 11b. At least one of the reflectors 10a, 10b is displaceable, so that a distance between the beam splitter 9 and the displaceable reflector 10a can be adjusted to set an optical path difference 11 between the first beam path and the second beam path.

[0291] In the following, reflector 10a is described as movable without loss of generality.

[0292] The interferometry device 5 comprises at least a Fizeau plate 12 with a Fizeau reference surface 12a and a mirror 13 with a mirror reference surface 13a, between which the test specimen 2 can be arranged, preferably at least approximately plane-parallel.

[0293] When the device 1 is used as intended, a portion of the radiation 8 is reflected by the Fizeau reference surface 12a as it passes through the interferometry device 5, a further portion of the radiation 8 is reflected by a first surface 2a and / or a second surface 2b (see Figure 4) of the test specimen 2, and yet another portion of the radiation 8 is reflected by the mirror reference surface 13a, so that interference patterns are generated by at least partial interference of these portions. The optional detection device 6 serves to record two-dimensional image data of the interference patterns, in particular interferograms 14. The interferograms 14 will be discussed in more detail elsewhere.

[0294] The synthetic wavelength L can in particular be given as the quotient of the product and the difference of the two wavelengths L1 and L2, such that L = (L1 x |_2) / (L1 - L2).

[0295] It has proven advantageous if the lighting device 3 generates coherent, in particular short-coherent, radiation 8 with a coherence length, wherein the respective coherence length for the wavelengths L1 and L2 is in particular at least as large, preferably 10 times as large, further preferably 20 times as large, as the synthetic wavelength L.

[0296] Preferably, the radiation 8 emitted by the radiation source(s) 7 of the lighting device 3 is pulsed.

[0297] It has proven particularly suitable if at least one radiation source 7 is configured as a superluminescent diode 7a and / or at least one radiation source 7 is configured as an ultrashort pulse laser and / or at least one radiation source 7 is configured as a supercontinuum source 7b. Reference is also made to Figures 7 to 9.

[0298] Preferably one or both of the reflectors 10a, 10b of the delay device 4 are designed as angled mirrors, preferably as 90° angled mirrors.

[0299] A preferred arrangement of the radiation splitter 9 and the reflectors 10a, 10b of the delay device 4 is shown in Figure 3.

[0300] It can preferably be provided that the optical path difference 11 can be adjusted by means of the delay device 4 such that the optical path difference 11 corresponds at least approximately, preferably exactly, to the optical path in a measuring cavity 15 in order to minimize, preferably eliminate, interference from the measuring cavity 15 in the interference pattern.

[0301] The optical path of the measuring cavity 15 can in particular be given by an optical path between the Fizeau reference surface 12a and / or the first surface 2a of the test object 2 facing the Fizeau plate 12 and / or the second surface 2b of the test object 2 facing the mirror 13 and / or the mirror reference surface 13a.

[0302] Figure 3 shows only an exemplary measuring cavity 15 between the Fizeau reference surface 12a and the mirror reference surface 13a, symbolized by a double arrow. The optical path of this cavity corresponds to the optical path difference 11 set at the delay device 4 according to Figure 3. For further explanations of possible measuring cavities 15, please refer to Figure 4, which describes below and contains a more detailed representation of the so-called Fizeau arrangement 27 consisting of the Fizeau plate 12, the mirror 13, and the test specimen 2.

[0303] The test object 2 can be illuminated simultaneously or sequentially with radiation 8 of the two wavelengths L1 and L2.

[0304] It can be particularly advantageous if the phase of the radiation 8 is adjustable and / or if a phase shifter is provided, which is preferably arranged in the delay device 4, further preferably on the movable reflector 10a, 10b, and / or in the interferometry device 5, further preferably on the Fizeau plate 12.

[0305] In the embodiment shown in Figure 3, the phase shifter is formed in particular by the same actuating element that also serves to displace the reflector 10a.

[0306] Alternatively or additionally, it may be provided that one of the preferably triple mirrors 10a, 10b is used to set a static cavity of the delay device, while the other of the preferably triple mirrors 10a, 10b is used for phase shifting during the measurement.

[0307] Optionally, the illumination device 3, the delay device 4 and / or the interferometry device 5 may include at least one beam splitter 16, at least one lens 17, at least one collimator 18, at least one aperture 19, at least one deflecting mirror 20, at least one beam blocker 21, at least one polarizer 22, at least one optical filter 23, at least one wavelength selection module 24 and / or at least one nonlinear or photonic crystal 25 (see, for example, Figure 8).

[0308] Furthermore, an optical fiber 26 can preferably be provided to supply the radiation 8 to the interferometry device 5 after it has passed through the delay device 4. The optical fiber 26 can preferably be polarization-preserving and / or configured as a single-mode fiber or as a multi-mode fiber.

[0309] In the embodiment shown in Figure 3, the illumination device 3 and the interferometry device 5 each have a beam splitter 16 to deflect the radiation 8 and, within the illumination device 3, to combine the radiation 8 emitted by the two radiation sources 7, which preferably has two different wavelengths L1 and L2, into a single beam. The delay device 4 has a lens 17 and / or an objective for focusing the radiation 8 into the optical fiber 26. Furthermore, the interferometry device 5 has a collimator 18 before and after the Fizeau arrangement 27, as well as an aperture 19 and a lens 17 before the detection device 6, in order to image the interference pattern generated in the Fizeau arrangement 27 onto a recording plane of the detection device 6.Figure 4 illustrates, by means of a schematic diagram, possible measuring cavities 15 within an exemplary Fizeau arrangement 27, which consists of a Fizeau plate 12 and a mirror 13, between which the test specimen 2, which may be designed in particular as an optical element and / or as a precursor to an optical element, can be arranged with its first surface 2a and second surface 2b. For an illustration of how the Fizeau arrangement 27 can be arranged within the device 1, reference is made to Figure 3 described above.

[0310] The double arrows in Figure 4 symbolize possible multiple reflections in the measuring cavities 15, which (in order from left to right) correspond to the wavefronts Wkai, w a b, w a i, w a2, Wbi, Wb2 and W12 can be assigned according to the system of equations (3) in the general description. To eliminate interference originating from one of the measuring cavities 15, the delay device 4 according to Figure 3 can be adjusted such that the optical path difference 11 between the first beam path with the first optical path length 11a and the second beam path with the second optical path length 11b corresponds to the optical path in the selected measuring cavity 15. This leads to the cancellation of the interference.

[0311] Figure 5 shows a schematic view of an embodiment of a lighting device 3 of the apparatus 1 with two radiation sources 7, while Figure 6 shows an embodiment with only one radiation source 7. The delay device 3 is shown symbolically and in a highly simplified manner in both figures. For further details of the delay device 3 and the entire apparatus 1, please refer to the description of Figure 3.

[0312] The radiation sources 7 can be designed, for example, as a superluminescent diode 7a, as an ultrashort pulse laser or as a supercontinuum source 7b, which will be explained in more detail in connection with Figures 7 to 9.

[0313] According to Figure 5, the radiation 8 emitted by the two radiation sources 7 is directed into a common beam path towards the delay device 4 by means of a beam splitter 16. It is preferably provided that the two radiation sources 7 emit radiation 8 of two different wavelengths L1 and L2. For an implementation example, reference is made in particular to Figures 7 and 8.

[0314] According to Figure 6, the radiation 8 from one radiation source 7 is first split using a beam splitter 16 and, after deflection by deflecting mirrors 20, recombined into a single beam analogous to Figure 5. Here, it can be provided either that the radiation source 7 is switchable between at least two wavelengths L1 and L2, which are used successively for two separate measurements, or that the radiation source 7 emits a constant wavelength which is modified differently on the split section of the beam path, so that two different wavelengths L1 and L2 now arrive at the second beam splitter 16. This variant with a common radiation source 7 for both wavelengths L1 and L2 is particularly advantageous, since they are thus subject to the same fluctuations.For an example of the implementation of this basic principle, particular reference should be made to Figure 9, according to which the two-part section of the beam path is used for the modification of the radiation 8 or the adjustment of the wavelengths L1 and L2.

[0315] Figures 7 and 8 show schematic views of two further embodiments of Figure 5 with two radiation sources 7. In the embodiment shown in Figure 7, the two radiation sources 7 each have a constant wavelength and are preferably configured as superluminescent diodes 7a. In the embodiment shown in Figure 8, the wavelength of one of the two radiation sources 7 is adjustable or selectable and can preferably be configured as a supercontinuum source 7b. The ability to adjust the wavelength of at least one of the two radiation sources 7 according to Figure 8 offers the advantage that the synthetic wavelength L can also be freely selected.

[0316] In both embodiments shown in Figures 7 and 8, beam blockers 21 and polarizers 22 are located on the two-part section of the beam path.

[0317] In Figure 8, on the section of the radiation source 7, 7b designed as a supercontinuum source, there is also a wavelength selection module 24, in particular a “wavelength selection box”, for selecting a specific wavelength from the emitted spectrum, a pair of lenses 17 which form a “beam expander” for beam expansion, and an optical filter 23, in particular a bandpass filter, as well as preferably a heat-absorbing IR absorption filter 23a.

[0318] Figure 9 shows a schematic view of a further embodiment according to Figure 6 with a radiation source 7, which is preferably configured as a supercontinuum source 7b. The radiation 8 emitted by the radiation source 7, 7b is first introduced into the beam path by a photonic crystal 25, in particular an optical fiber 26 configured as a photonic crystal fiber (PCF). There, the radiation 8 passes through a heat-absorbing IR absorption filter 23a, an optical filter 23, in particular a bandpass filter, a pair of lenses 17 or a beam expander, and a polarizer 22 before the radiation 8 is split by a beam splitter 16. On the two-part section of the beam path, there is also a wavelength selection module 24, in particular a wavelength selection box, a beam blocker 21, and another polarizer 22.Here, both wavelengths L1 and L2, as well as the synthetic wavelength L, can be freely adjusted within the limitations of the measurement setup. This can sometimes generate particularly high-contrast measurement data.

[0319] The beam splitters 16 according to Figures 5 to 9 can preferably be polarizing. Alternatively or additionally, polarizing optical elements, for example the polarizers 22, can preferably be positioned in the beam path such that the maximum power of the radiation 8 emitted by the radiation sources 7 is directed towards the delay device 4.

[0320] The beam blockers 21 shown in Figures 7 to 9 allow the associated device 1 to be used for measurement with either one or two wavelengths. Preferably, the device 1 is operated with the two wavelengths L1 and L2.

[0321] The “Photonic Crystal Fiber” (PCF) 26 or the photonic crystal 25 according to Figures 8 and 9 can, in particular, be used to generate a broadband spectrum. This spectrum is filtered and further modified as required by the subsequent elements in the beam path, for example by the optical filter 23, which can be used, for instance, to filter out a certain spectral range, especially as a bandpass filter and / or as a band-blocking filter.

[0322] The aspects not described in detail with respect to Figures 5 to 9, for example regarding elements 16-25 in the beam path, are analogous to the other figures. The respective arrangement of elements 16-25 in the beam path can be flexibly adapted at the discretion of a person skilled in the art.

[0323] The embodiments shown in Figures 5 to 9 can be combined as desired. For example, the embodiment shown in Figure 9 can be modified analogously to Figure 7 such that both wavelengths L1 and L2 are set by a fixed optical filter 23. The embodiment shown in Figure 9 can also be modified analogously to Figure 8 such that one of the wavelengths L1 or L2 is adjustable by a fixed optical filter 23 and the other wavelength is freely adjustable by a wavelength selection module 24.

[0324] Figures 3 to 9 also serve to disclose a method for the interferometric measurement of a wavefront of a test object 2, in particular an optical element and / or its precursors, with a first surface 2a and an opposing second surface 2b using an interferometer system, preferably using the device 1 according to the preceding description, characterized by at least the following steps (see also Figure 10):

[0325] (a) Creating N interferograms 14 with radiation 8 of two different wavelengths L1 and L2 which, when superimposed, produce a synthetic wavelength L, wherein the N interferograms 14 each differ from each other by a phase step;

[0326] (b) If necessary, processing the interferograms 14 to improve the signal-to-noise ratio and / or the frequency separation during subsequent evaluation;

[0327] (c) Separating low-frequency and high-frequency components of the interferograms 14 by frequency analysis, in particular extracting the component of a beat frequency which corresponds to the synthetic wavelength L;

[0328] (d) Evaluate the phases by phase reconstruction to reconstruct the wavefront emanating from the test specimen 2;

[0329] (e) Eliminating discontinuities, if necessary. It may be provided that in step (a) each of the N interferograms 14 is recorded by simultaneously illuminating the test specimen 2 with the two wavelengths L1 and L2. It may also be provided that in step (a) each of the N interferograms 14 is calculated from two individual interference patterns, which are recorded successively with each of the two individual wavelengths L1 and L2.

[0330] The N interferograms 14 preferably correspond to the incoherent sum of interference patterns of the individual wavelengths L1 and L2 with the synthetic wavelength L as the beat frequency. Reference is also made to Figure 11, which is described below.

[0331] The synthetic wavelength L can in particular be given as the quotient of the product and the difference of the two wavelengths L1 and L2, such that L = (L1 x [_2) / (L1 - L2).

[0332] It has proven to be particularly suitable if the phase step in step (a) corresponds to a multiple of the synthetic wavelength L, wherein preferably N = 5 interferograms 14 are recorded in phase steps of L / 4.

[0333] It is advantageous if the interferometer system used has an illumination device 3 with at least one radiation source 7, preferably with at least two radiation sources 7, to emit the radiation 8 of wavelengths L1 and L2.

[0334] It is also advantageous if the interferometer system used has a detection device 6 to record two-dimensional image data, in particular the interferograms 14.

[0335] Preferably, the interferometer system used comprises a delay device 4, which has a beam splitter 9 and at least two reflectors 10a, 10b. The beam splitter 9 divides the radiation 8, with the beam splitter 9 and each of the reflectors 10a, 10b forming a first beam path with a first optical path length 11a and a second beam path with a second optical path length 11b. It is preferably provided that at least one of the reflectors 10a, 10b is displaceable, so that a distance between the beam splitter 9 and the displaceable reflector 10a can be adjusted to set an optical path difference 11 between the first beam path and the second beam path.

[0336] It can be particularly advantageous if the interferometer system used has an interferometry device 5, which comprises at least one Fizeau plate 12 with a Fizeau reference surface 12a and a mirror 13 with a mirror reference surface 13a, between which the test specimen 2 is arranged, preferably at least approximately plane-parallel. The illumination device 3, the delay device 4, the interferometry device 5 and / or the detection device 6 of the interferometer system can preferably be the corresponding devices of the apparatus 1 as described above with reference to Figures 3 to 9.

[0337] According to one embodiment of the method for interferometric measurement of a wavefront, at least the following sub-steps may be provided in step (a) for creating the interferograms (14):

[0338] (a1) If necessary, arrange the test specimen 2 between the Fizeau reference surface 12a and the mirror reference surface 13a of the interferometry device 5, such that the first surface 2a of the test specimen 2 is aligned in the direction of the Fizeau reference surface 12a and the second surface 2b of the test specimen 2 is aligned in the direction of the mirror reference surface 13a and preferably in a plane parallel thereto;

[0339] (a2) Adjusting the optical path difference 11 of the delay device 4 so that, within a coherence length of the radiation 8, it corresponds only to a part of the optical paths of a measuring cavity 15, in particular the optical path between the Fizeau reference surface 12a and / or the first surface 2a of the test specimen 2 facing the Fizeau plate 12 and / or the second surface 2b of the test specimen 2 facing the mirror 13 and / or the mirror reference surface 13a;

[0340] (a3) Detection of at least one interference pattern generated by superposition of radiation 8 reflected from the different surfaces 2a, 2b, 12a, 13a, wherein interference from the measurement cavity 15 underlying step a2 is minimized, preferably eliminated.

[0341] It may preferably be provided that in step (a) the phase step is set at least partially with a phase shifter of the delay device 4, in particular on the movable reflector 10a, 10b, and / or is set at least partially with a phase shifter of the interferometry device 5, in particular on the Fizeau plate 12.

[0342] It has proven particularly suitable in step (a) or (a1) to arrange the test specimen 2 at a distance of more than half a coherence length, with respect to the radiation 8 of wavelengths L1 and L2 and / or to the synthetic wavelength L from the Fizeau reference surface 12a, in the beam path of the interferometer system.

[0343] In step (b) the processing of the interferograms 14 can preferably be carried out computationally by quadrature and / or by applying a carrier frequency.

[0344] In step (c), the separation of low-frequency and high-frequency components of the interferograms 14 can preferably be carried out computationally by using a low-pass filter and / or Fourier transform, in particular FFT and / or inverse FFT. In step (d), the evaluation of the phases can preferably be carried out computationally by an N-step phase evaluation algorithm, for example a 5-step phase evaluation algorithm.

[0345] In step (e) the elimination of discontinuities can preferably be carried out computationally using a discontinuity method.

[0346] Figures 3 to 9 further disclose a method for manufacturing an optical element, in particular an optical element 116, 118, 119, 120, 121, 122, Mi, 207 for a lithography system, wherein a method for interferometric measurement of a wavefront of a test specimen 2 according to the preceding description is carried out with the optical element and / or its precursors as the test specimen 2, and wherein the following additional steps are provided:

[0347] (f) Analyzing the reconstructed wavefront, in particular comparing it with a reference wavefront, in order to identify any wavefront errors of the optical element or of the test specimen 2;

[0348] (g) Machining at least one of the first and second surfaces 2a, 2b of the optical element or of the test specimen 2 based on the reconstructed wavefront to eliminate the identified wavefront errors.

[0349] Figure 10 shows a basic flowchart of a variant of the method for interferometric measurement of a wavefront of a test specimen, including the optional steps (a1) to (a3), (b) and (e), as well as the additional steps (f) and (g) according to the previously described method for manufacturing an optical element. The flowchart represents the process in a highly abstract manner.

[0350] For steps (a), (c), and (d), an exemplary data set is shown in Figure 10. In the case of step (a), this is specifically one of the N interferograms me 14 (see also Figure 11), from which the signal component of the synthetic wavelength L is extracted by frequency analysis in step (c), and from which the wavefront is finally reconstructed in step (d). The reconstructed wavefront shown could, for example, be due to horizontal schlieren on or in the test specimen.

[0351] In practice, for example, it might be possible to combine wavelengths L1 = 680 nm and L2 = 670 nm, which, according to the equation L = (L1 x L2) / (L1 - L2), yields a synthetic wavelength L of 45.56 pm. This corresponds to a factor of 67 compared to the individual wavelengths L1 and L2. With these wavelengths, N = 5 interferograms can then be recorded, each shifted by a phase step of L / 4 = 11.39 pm to the next interferogram. The aforementioned combination of values ​​is merely one of many possible combinations. The embodiment shown in Figure 10, as well as the preceding embodiments shown in Figures 3 to 9, are not limited to this. Figure 11 illustrates, in principle, the composition of one of the N interferograms 14 from exemplary interference patterns of wavelengths L1 and L2 according to step (a) of the method.The two interference patterns shown on the left for wavelengths L1 and L2 contain distinct moiré patterns, which do not allow for sufficient phase reconstruction. The long-wavelength beat visible in interferogram 14 shown on the right, or in the incoherent sum of the individual interference patterns, corresponds to the synthetic wavelength L, which is central for further analysis and ultimately for the reconstruction of the wavefront.

[0352] It should be noted again that the interferogram 14 with wavelengths L1 and L2 can be created in two different ways, by illuminating the test specimen 2 either simultaneously or sequentially with the two wavelengths L1 and L2. Reference is also made to the preceding explanations.

[0353] Figures 10 and 11 also supplement the disclosure of the device 1, as already described above with reference to Figures 3 to 9. The method can be carried out particularly advantageously using a device 1, and analogously, the device 1 is also particularly suitable for carrying out the method. However, its applicability is not limited to this.

[0354] Furthermore, Figures 3 to 11 disclose a lithography system, in particular a projection exposure system 100, 200 for semiconductor lithography, comprising an illumination system 101, 201 with a radiation source 102 and optics 103, 109, 206, which includes at least one optical element 116, 118, 119, 120, 121, 122, Mi, 207. The optical element 116, 118, 119, 120, 121, 122, Mi, 207 is measured by the aforementioned device 1 and / or the aforementioned method for interferometric measurement of a wavefront of a test specimen 2 and / or is manufactured by the aforementioned method for producing an optical element, each according to the preceding description. The optical element 116, 118, 119, 120, 121, 122, Mi, 207 corresponds to the test specimen 2 according to the preceding description.

[0355] The lithography system can be, in particular, an EUV projection exposure system 100 according to Figure 1 or a DUV projection exposure system 200 according to Figure 2. Reference is made to the preceding description. Reference numeral list

[0356] 1 Device

[0357] 2 candidates

[0358] 2a first surface (of test specimen 2)

[0359] 2b second surface (of test specimen 2)

[0360] 3 Lighting equipment

[0361] 4 Delay device

[0362] 5 Interferometry device

[0363] 6 Detection device

[0364] 7 Radiation source

[0365] 7a Superluminescent diode

[0366] 7b Supercontinuum source

[0367] 8 Radiation

[0368] 9 beam splitters

[0369] 10a Reflector, adjustable

[0370] 10b Reflector

[0371] 11 optical path difference

[0372] 11 a first optical path length

[0373] 11 b second optical path length

[0374] 12 Fizeau plate

[0375] 12a Fizeau reference surface

[0376] 13 mirrors

[0377] 13a Mirror reference surface

[0378] 14 Interferogram

[0379] 15 Measuring cavity

[0380] 16 beam splitters

[0381] 17 lens

[0382] 18 Collimator

[0383] 19 aperture

[0384] 20 deflecting mirrors

[0385] 21 jet blockers

[0386] 22 Polarizer

[0387] 23 Optical filter, in particular bandpass filter 23a Heat-absorbing IR absorption filter 24 Wavelength selection module

[0388] 25 photonic crystal

[0389] 26 optical fibers

[0390] 27 Fizeau arrangement 100 EUV projection exposure system

[0391] 101 Lighting system

[0392] 102 Radiation source

[0393] 103 Lighting optics

[0394] 104 object field

[0395] 105 Object level

[0396] 106 reticles

[0397] 107 label holders

[0398] 108 Reticle displacement drive

[0399] 109 Projection optics

[0400] 110 image field

[0401] 111 Image plane

[0402] 112 wafers

[0403] 113 wafer holders

[0404] 114 Wafer transfer drive

[0405] 115 EUV / Useful / Illumination radiation

[0406] 116 Collector

[0407] 117 Intermediate focus plane

[0408] 118 Deflection mirrors

[0409] 119 first faceted mirror / field faceted mirror 120 first facets / field facets

[0410] 121 Second faceted mirror / Pupil faceted mirror 122 Second faceted mirrors / Pupil faceted mirrors

[0411] 200 DUV projection exposure system

[0412] 201 Lighting system

[0413] 202 reticulation days

[0414] 203 reticles

[0415] 204 wafers

[0416] 205 wafer holders

[0417] 206 Projection optics

[0418] 207 lens

[0419] Version 208

[0420] 209 lens bodies

[0421] 210 Projection beam

[0422] Mi Mirror

Claims

47 Patent claims:

1. Device (1) for interferometric measurement of a wavefront of a test object (2), in particular an optical element and / or its precursors, comprising - a lighting device (3) with at least one radiation source (7), preferably with at least two radiation sources (7), which is configured and designed to emit radiation (8) with at least two different wavelengths L1 and L2, which, when superimposed, produce a synthetic wavelength L, - a delay device (4) comprising a beam splitter (9) and at least two reflectors (10a, 10b), wherein the beam splitter (9) splits the radiation (8), wherein the beam splitter (9) and each of the reflectors (10a, 10b) form a first beam path with a first optical path length (11a) and a second beam path with a second optical path length (11b), wherein at least one of the reflectors (10a, 10b) is displaceable such that a distance between the beam splitter (9) and the displaceable reflector (10a) can be adjusted in order to set an optical path difference (11) between the first beam path and the second beam path, and - an interferometry device (5) comprising at least one Fizeau plate (12) with a Fizeau reference surface (12a) and a mirror (13) with a mirror reference surface (13a) comprises, between which the test subject (2) can be arranged, preferably at least approximately parallel to the plane, wherein, when passing through the interferometry device (5), a portion of the radiation (8) is reflectable from the Fizeau reference surface (12a), wherein a further portion of the radiation (8) is reflectable from a first surface (2a) and / or a second surface (2b) of the test specimen (2), wherein yet another portion of the radiation (8) is reflectable from the mirror reference surface (13a), and wherein interference patterns can be generated by at least partial interference of these portions.

2. Device (1) according to claim 1 , wherein a detection device (6) is provided for recording two-dimensional image data, in particular interferograms (14).

3. Device (1) according to claim 1 or 2, wherein The synthetic wavelength L is given as the quotient of the product and the difference of the two wavelengths L1 and L2, such that L = (L1 x |_2) / (L1 - L2).

4. Device (1) according to claim 1, 2 or 3, wherein The illumination device (3) generates coherent, in particular short-coherent, radiation (8) with a coherence length, wherein the respective coherence length for the wavelengths L1 and L2 is in particular at least as large, preferably 10 times as large, further preferably 20 times as large, as the synthetic wavelength L.48 5. Device (1) according to any one of claims 1 to 4, wherein the radiation (8) emitted by the radiation source(s) (7) of the lighting device (3) is pulsed.

6. Device (1) according to any one of claims 1 to 5, wherein at least one radiation source (7) is designed as a superluminescent diode (7a) and / or at least one radiation source (7) is designed as an ultrashort pulse laser and / or at least one radiation source (7) is designed as a supercontinuum source (7b).

7. Device (1) according to any one of claims 1 to 6, wherein one or both of the reflectors (10a, 10b) of the delay device (4) are designed as angled mirrors, preferably as 90° angled mirrors.

8. Device (1) according to any one of claims 1 to 7, wherein The optical path difference (11) can be adjusted by means of the delay device (4) such that the optical path difference (11) corresponds at least approximately, preferably exactly, to the optical path in a measuring cavity (15), in particular to the optical path between the Fizeau reference surface (12a) and / or the first surface (2a) of the test object (2) facing the Fizeau plate (12) and / or the second surface (2b) of the test object (2) facing the mirror (13) and / or the mirror reference surface (13a), in order to minimize, preferably eliminate, interference from the measuring cavity (15) in the interference pattern.

9. Device (1) according to any one of claims 1 to 8, wherein the test object (2) can be illuminated simultaneously or successively with radiation (8) of the two wavelengths L1 and L2.

10. Device (1) according to any one of claims 1 to 9, wherein the phase of the radiation (8) is adjustable and / or a phase shifter is provided, which is preferably arranged in the delay device (4), further preferably on the movable reflector (10a), and / or in the interferometry device (5), further preferably on the Fizeau plate (12).

11. Device (1) according to any one of claims 1 to 10, wherein the illumination device (3), the delay device (4) and / or the interferometry device (5) comprises at least one beam splitter (16), at least one lens (17), at least one collimator (18), at least one aperture (19), at least one deflecting mirror (20), at least one beam blocker (21), at least one polarizer (22), at least one optical filter (23), at least one wavelength selection module (24) and / or at least one nonlinear or photonic crystal (25).49 12. Device (1) according to any one of claims 1 to 11, wherein an optical fiber (26) is provided to supply the radiation (8) to the interferometry device (5) after it has passed through the delay device (4), wherein the optical fiber (26) is preferably polarization-preserving and / or designed as a single-mode fiber or as a multi-mode fiber.

13. Method for interferometric measurement of a wavefront of a test object (2), in particular an optical element and / or its precursors, with a first surface (2a) and an opposing second surface (2b) using an interferometer system, preferably using a device (1) according to one of claims 1 to 12, characterized by at least the following steps: (a) Creating N interferograms (14) using radiation (8) of two different wavelengths L1 and L2 which, when superimposed, produce a synthetic wavelength L, wherein the N interferograms (14) differ from each other by a phase step; (b) If necessary, processing the interferograms (14) to improve the signal-to-noise ratio and / or the frequency separation during subsequent evaluation; (c) Separating low-frequency and high-frequency components of the interferograms (14) by frequency analysis, in particular extracting the component of a beat frequency which corresponds to the synthetic wavelength L; (d) Evaluating the phases by phase reconstruction in order to reconstruct the wavefront emanating from the test specimen (2); (e) Eliminate any jumps, if necessary.

14. Method according to claim 13, wherein in step (a) each of the N interferograms (14) is recorded by simultaneously illuminating the test object (2) with the two wavelengths L1 and L2.

15. Method according to claim 13 or 14, wherein in step (a) each of the N interferograms (14) is calculated from two individual interference patterns, which are recorded successively with each of the two individual wavelengths L1 and L2.

16. Method according to claim 13, 14 or 15, wherein the N interferograms (14) each correspond to the incoherent sum of interference patterns of the individual wavelengths L1 and L2 with the synthetic wavelength L as the beat frequency.50 17. Method according to any one of claims 13 to 16, wherein The synthetic wavelength L is given as the quotient of the product and the difference of the two wavelengths L1 and L2, such that L = (L1 x |_2) / (L1 - L2).

18. Method according to any one of claims 13 to 17, wherein the phase step in step (a) corresponds to a multiple of the synthetic wavelength L, wherein preferably N = 5 interferograms me (14) are recorded in phase steps of L / 4.

19. Method according to any one of claims 13 to 18, wherein The interferometer system used comprises an illumination device (3) with at least one radiation source (7), preferably with at least two radiation sources (7), to emit the radiation (8) of wavelengths L1 and L2, and / or a detection device (6) to record two-dimensional image data, in particular the interferograms (14).

20. Method according to any one of claims 13 to 19, wherein The interferometer system used comprises a delay device (4) which includes a beam splitter (9) and at least two reflectors (10a, 10b), wherein the beam splitter (9) splits the radiation (8), wherein the beam splitter (9) and each of the reflectors (10a, 10b) form a first beam path with a first optical path length (11a) and a second beam path with a second optical path length (11b), wherein at least one of the reflectors (10a, 10b) is movable, so that a distance between the beam splitter (9) and the movable reflector (10a) can be adjusted in order to set an optical path difference (11) between the first beam path and the second beam path.

21. Method according to any one of claims 13 to 20, wherein The interferometer system used comprises an interferometry device (5) which includes at least one Fizeau plate (12) with a Fizeau reference surface (12a) and a mirror (13) with a mirror reference surface (13a), between which the test specimen (2) is arranged, preferably at least approximately plane-parallel.

22. Method according to claims 20 and 21, wherein In step (a) for creating the interferograms (14) at least the following sub-steps are provided: (a1) Optionally, arrange the test specimen (2) between the Fizeau reference surface (12a) and the mirror reference surface (13a) of the interferometry device (5) such that the first surface (2a) of the test specimen (2) is aligned in the direction of the Fizeau reference surface (12a) and the second surface (2b) of the test specimen (2) is aligned in the direction of the mirror reference surface (13a) and preferably in a plane parallel thereto; (a2) Adjusting the optical path difference (11) of the delay device (4) so ​​that within a coherence length of the radiation (8) it corresponds only to a part of the optical paths of a measuring cavity (15), in particular the optical path between the Fizeau reference surface (12a) and / or the first surface (2a) of the test specimen (2) facing the Fizeau plate (12) and / or the second surface (2b) of the test specimen (2) facing the mirror (13) and / or the mirror reference surface (13a); (a3) Capturing at least one interference pattern generated by superimposing radiation (8) reflected from the different surfaces (2a, 2b, 12a, 13a), wherein interference from the measurement cavity (15) underlying step (a2) is minimized, preferably eliminated.

23. Method according to claim 20, 21 or 22, wherein in step (a) the phase step is adjusted at least partially with a phase shifter of the delay device (4), in particular on the at least one movable reflector (10a, 10b), and / or at least partially with a phase shifter of the interferometry device (5), in particular on the Fizeau plate (12).

24. Method according to any one of claims 13 to 23, wherein in step (a) or (a1) the test specimen (2) is positioned in the beam path of the interferometer system at a distance of more than half a coherence length, with respect to the radiation (8) of wavelengths L1 and L2 and / or to the synthetic wavelength L from the Fizeau reference surface (12a).

25. Method according to any one of claims 13 to 24, wherein in step (b) the processing of the interferogram me (14) is carried out computationally by quadrature and / or by applying a carrier frequency.

26. Method according to any one of claims 13 to 25, wherein In step (c) the separation of low-frequency and high-frequency components of the interferograms (14) is carried out computationally by using a low-pass filter and / or Fourier transform, in particular FFT and / or inverse FFT.

27. Method according to any one of claims 13 to 26, wherein In step (d) the phases are evaluated computationally by an N-step phase evaluation algorithm, preferably a 5-step phase evaluation algorithm.

28. Method according to any one of claims 13 to 27, wherein in step (e) the elimination of discontinuities is carried out computationally by means of a discontinuity method.

29. Method for manufacturing an optical element, in particular an optical element (116, 118, 119, 120, 121, 122, Mi, 207) for a lithography system, wherein a method for interferometric measurement of a wavefront of a test specimen (2) according to one of claims 13 to 28 is carried out with the optical element and / or its precursors as the test specimen (2), and wherein the following additional steps are provided: (f) Analyzing the reconstructed wavefront, in particular comparing it with a reference wavefront to identify any wavefront defects of the test specimen (2); (g) Machining at least one of the first and second surfaces (2a, 2b) of the test specimen (2) based on the reconstructed wavefront to eliminate the identified wavefront defects.

30. Lithography system, in particular projection exposure system (100, 200) for semiconductor lithography, with an illumination system (101, 201) with a radiation source (102) and optics (103, 109, 206) which has at least one optical element (116, 118, 119, 120, 121, 122, Mi, 207), characterized by the fact that the optical element (116, 118, 119, 120, 121, 122, Mi, 207) is measured by a device (1) according to one of claims 1 to 12 and / or a method according to one of claims 13 to 28 and / or is manufactured by a method according to claim 29.

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