Device and method for measuring wafers

Abstract

The invention relates to a device (14) for measuring wafers (10) comprising an optical coherence tomography device (22) which generates a measuring light beam (24) and directs it towards the wafer (10) via an optical system (38), and a scanning unit (26) which deflects the measuring light beam (24) in two spatial directions and has exactly two scanning mirrors (28, 32) that are each rotatably mounted about exactly one axis (30, 34). A control unit (36) controls the scanning unit (26) such that the measuring light beam (24) scans the surface of the wafer (10) consecutively at multiple measuring points. An evaluation unit (57) calculates spacing values and / or thickness values from interference signals provided by the optical coherence tomography device (22). A collision protection device (64) limits the rotational angle of at least one of the two scanning mirrors (28, 32).

Description

[0001] Device and method for measuring wafers

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a device and a method for contactless measurement of geometrical parameters of wafers such as TTV, bending or warping.

[0005] 2. Description of the state of the art

[0006] Wafers are circular or square discs approximately one millimeter thick that serve as substrates for integrated circuits, micromechanical components, or photoelectric coatings. Wafers are manufactured from monocrystalline or polycrystalline ingots, which are sawn into individual wafers perpendicular to their longitudinal axis. Silicon wafers are most commonly used, but other materials are also employed, such as glass wafers for the production of microlens arrays or for augmented reality applications.

[0007] The shape of the wafers must meet strict geometric specifications. These specifications include the TTV (Total Thickness Variation), which is the maximum difference between the thickest and thinnest points of a wafer. Bow is defined as the maximum deviation of the wafer's median surface from a reference plane. Warp is defined by those skilled in the art as the deviation of the wafer's median surface from the reference plane, at which the bowing is already corrected across the entire wafer surface.

[0008] In addition to these common definitions of TTV, bow and warpage, similar, slightly different definitions are sometimes used in wafer characterization to characterize wafer deviations from the ideal shape.

[0009] To comply with specifications, these geometric dimensions must be measured at least randomly and preferably during regular production. US 2012 / 0257207 A1 discloses a wafer measuring device that combines a Michelson interferometer for distance measurements with a reflectometer used for thickness measurements. Measurements are taken at individual points. To obtain measurements across the entire surface, the wafer is moved relative to the stationary measuring device.

[0010] However, this well-known measurement method is slow, as the wafer's inertia prevents it from being moved at any desired speed. The corresponding measuring devices are also expensive, as the actuators for the wafer's movement must be of particularly high quality to ensure smooth and precise movements.

[0011] The object of the invention is to provide a device and a method with which wafers can be measured very quickly and yet cost-effectively.

[0012] SUMMARY OF THE INVENTION

[0013] With regard to the device, this object is achieved by a device for measuring wafers with an optical coherence tomograph which is configured to generate a measuring light beam and to direct it onto the wafer via an optical system, a scanning device which has two scanning mirrors which are each mounted so as to be rotatable about exactly one axis and which is configured to deflect the measuring light beam in two spatial directions, a control unit which is configured to control the scanning device such that the measuring light beam scans the surface of the wafer successively at a plurality of measuring points, an evaluation unit which is configured to calculate distance values ​​and / or thickness values ​​from interference signals provided by the optical coherence tomograph, wherein at least one of the two scanning mirrors is assigned a collision protection device which is configured to limit the angle of rotation of the at least one scanning mirror.

[0014] The invention is based on the discovery that a conventional measuring device comprising an optical coherence tomograph and a scanning device can be used for wafer measurement if, on the one hand, a larger optical system is used, which allows for a larger measurement field and, above all, solves the problem of field curvature. The inventors have developed several approaches to solving this problem.

[0015] Such a known measuring device is known from DE 10 2017 128 158 A1. In one embodiment described therein, an optical coherence tomograph (OCT) generates a measuring light beam that is deflected by a mirror scanner, with a scanning mirror provided for each deflection direction. After passing through a flat-field objective, the measuring light beam impinges on the surface to be measured essentially parallel to its axis, resulting in a telecentric beam path. When scanning the surface, only the very light scanning mirror moves, while the measurement object remains stationary. This allows its surface to be scanned very quickly.

[0016] A further advantage of such a measuring device is that the deflection of the mirrors can be read in real time and adjusted to the desired deflection using a control loop. Even if the scanning mirrors are subject to parasitic residual oscillations due to their movement, this allows for very precise positioning of the measuring light beam on the measurement object.

[0017] The inventors have determined that with this known measuring device, a field curvature occurs when the measuring light beam scans the surface of a wafer two-dimensionally. In other words, this means that the focus of the measuring light beam does not sweep across a plane during scanning, but rather a curved surface. A wafer surface that is actually flat therefore appears curved. While the field curvature could still be tolerated for very small wafers up to approximately 80 mm in diameter, this is not possible for larger wafers, as the field curvature increases quadratically with the diameter of the wafer to be measured. Larger wafers are considered to be those where two measuring points accessible by the scanning device are separated by a distance d ma x with 140 mm < d ma x < 600 mm and preferably with 280 mm < d max < 450 mm. For a distance d max= 300 mm, the measurement error due to field curvature is typically already around 1 mm. Without additional measures, the measuring device described above is therefore unsuitable for measuring larger wafers, even when using a larger flat-field lens, since the required accuracy for distance measurement is less than 1 pm. The measurement error of the known measuring device is therefore three orders of magnitude too large for larger wafers.

[0018] The inventors further recognized that the additional optical path length differences that lead to the field curvature are primarily caused by the scanning mirrors. To pivot without collision, the two scanning mirrors must be relatively far apart. It has been found that the spatial separation of the scanning mirrors' rotation axes makes the dominant contribution to the field curvature.

[0019] The simplest measure to reduce field curvature is therefore to use a single scanning mirror that can rotate around two axes instead of two scanning mirrors, each of which is mounted for rotation around a single axis. However, the currently available two-axis scanning mirrors cannot be swiveled as quickly and precisely as two single-axis scanning mirrors. Compared to the conventional method, which requires moving the wafer, this still results in a significant reduction in the required measurement time.

[0020] Another approach to reducing the field curvature is to use two scanning mirrors, each mounted so as to rotate about exactly one axis, as before, but to significantly shorten the distance between the scanning mirrors. However, this entails the risk of the scanning mirrors colliding at larger angles of rotation. To prevent this, according to the invention, at least one of the two scanning mirrors is assigned a collision protection device which is designed to limit the angle of rotation of the at least one scanning mirror. The idea behind this is that the available scanning mirrors usually have a range of angles of rotation that is only partially required for the application at hand. If the collision protection device can reduce the initially possible angles of rotation (typically ±20°) to the actually required angles of rotation (e.g.± 10° and preferably ± 5°), collisions between the scanning mirrors can be reliably excluded even with very small distances between the scanning mirrors.

[0021] The collision protection device can have at least one mechanical stop to limit the angle of rotation. This stop can be, for example, a plastic or rubber damper, which the at least one scanning mirror hits if it is deflected by more than, for example, ± 10° or ± 5°.

[0022] According to a preferred embodiment of the invention, the at least one scanning mirror is rotatably mounted on a scanning mirror holder, and the collision protection device is plugged, clipped, screwed, glued, or secured to this scanning mirror holder in some other way. Attachment by plugging, clipping, or gluing has the advantage that the collision protection device can also be subsequently applied to a scanning mirror holder without having to intervene in the scanning mirror holder and, if applicable, the mechanics of the scanning mirror itself. The function and adjustment of the scanning mirror remain unaffected. It is expedient if the collision protection device is shaped such that it is supported against twisting on at least one surface of the scanning mirror holder. This surface forms a counterbearing for the force applied by a stop of the mirror.

[0023] Alternatively, the collision device is integrated into or attached to a surrounding component, for example to a pin in a base body of the device.

[0024] Alternatively, the stop can act directly on the rotational axis of the at least one scanning mirror or on a projection formed on the rotational axis. Alternatively, the damper can be mounted on the reflective surface or the back of the at least one scanning mirror. It is expediently applied at the points where the mirrors would collide with each other and dampens the collision. It is also possible for a damper to be mounted on the mirror, which abuts against a mechanical stop mounted elsewhere.

[0025] Alternatively or additionally, it is possible to implement the collision device as an electronic limiting device configured to electronically prevent the supply of control signals to the at least one scanning mirror that would result in exceeding a predetermined rotation angle range. Choke coils, for example, are very reliable for limiting the control current for the scanning mirrors.

[0026] Alternatively or additionally, the control software for the scanning mirrors can be designed to prevent the generation of control signals that would cause excessive deflection of the scanning mirrors. In this case, the additional use of one of the above-mentioned measures is also useful as a safeguard in case a software malfunction occurs and an erroneous signal is generated that would cause excessive deflection of one of the two scanning mirrors.

[0027] Another alternative or additional measure for reducing field curvature involves using an optical system that contains at least one, and preferably two, anamorphic optical elements to correct the field curvature. The anamorphic elements are preferably cylindrical lenses. Their symmetry axes are aligned with the rotational axes of the scanning mirrors.

[0028] Another alternative or additional measure is not to reduce the field curvature, but to eliminate its effects through calibration. For this purpose, the evaluation unit should be configured to mathematically correct the measured distance values ​​to compensate for any curvature of the field into which the optical system focuses the measuring light rays. This field usually coincides with the focal plane of the optical system. During calibration, for example, a highly precisely manufactured flat glass is measured. The measured deviations from planarity represent correction values, which are subtracted or added to the measured values ​​in subsequent measurements, depending on their sign.

[0029] For computational correction, the correction values ​​can be read from a correction table stored in the evaluation unit. Preferably, the correction table contains correction values ​​for different operating wavelength ranges. These correction values ​​for different operating wavelength ranges do not have to be determined through individual calibration measurements, but can also be derived based on theoretical considerations from correction values ​​determined for a specific operating wavelength range. Zernike polynomials can also be used in this case.

[0030] As an alternative to reading from a correction table, the correction values ​​can also be calculated from a mathematical formula. This formula can be derived, for example, from a polynomial fit. Zernike polynomials, for example, can be used as polynomials.

[0031] Of course, calibration can also be carried out in addition to the measures explained above.

[0032] An optical coherence tomograph that can be used for the invention can in particular comprise: a light source, a beam splitter that is configured to split light generated by the light source into the measuring light beam and a reference light beam, a reference arm for guiding the reference light beam, an object arm that uses the optical system and the scanning device to guide the measuring light beam, and a detector that is configured to generate the interference signals from a superposition of the reference light beam guided in the reference arm with a portion of the measuring light beam reflected by the wafer.

[0033] In one embodiment, a switchable dimming device is arranged in the reference arm, which is designed to prevent the propagation of the reference light in the reference arm when performing thickness measurements. The switchable dimming device makes it possible to switch between a "distance mode", in which the interference between the measuring light beam and the reference light beam is evaluated, and between a

[0034] "Thickness mode". In thickness mode, the distance between the interfaces (i.e., the thickness of the wafer) is determined from the interference between the reflections of the measuring light beam at the two plane-parallel interfaces of the wafer. The detector thus generates the interference signals from a superposition of portions of the measuring light beam reflected at two different interfaces of the wafer. To prevent the unnecessary reference light beam from the reference arm from interfering with the desired interference signal in thickness mode, the propagation of the reference light beam in the reference arm is prevented by the dimming device. The dimming device is preferably switched automatically each time the measuring mode is changed.

[0035] In another embodiment, the reference light beam in the reference arm is not guided as a free beam, or at least not entirely, but at least partially in an optical fiber. Coiled optical fibers allow light to be guided over long optical paths in a small space. If the object arm and the reference arm have different dispersions, measures to compensate for the dispersion are appropriate.

[0036] With regard to the method, the object posed at the outset is achieved by a method for measuring wafers with the following steps: a) generating a measuring light beam with an optical coherence tomograph; b) directing the measuring light beam onto the wafer with an optical system; c) deflecting the measuring light beam in two spatial directions with a scanning device which is controlled such that the measuring beam scans the surface of the wafer successively at a plurality of measuring points, wherein the scanning device (26) has two scanning mirrors which are each mounted so as to be rotatable about exactly one axis, wherein angles of rotation of at least one of the two scanning mirrors are limited by a collision protection device; d) calculating distance values ​​and / or thickness values ​​on the basis of interference signals provided by the optical coherence tomograph.

[0037] The scanning device can be controlled so that two measuring points have a distance d max with 140 mm < d max < 600 mm and preferably with 280 mm < d max < 450 mm.

[0038] In one embodiment, the optical coherence tomograph is characterized in that a light source generates light; a beam splitter splits the light generated by the light source into the measuring light beam and a reference light beam; the reference light beam is guided in a reference arm; the measuring light beam is guided in an object arm; a detector generates the interference signals from a superposition of the reference light beam guided in the reference arm with a portion of the measuring light beam that is reflected by the wafer.

[0039] When performing thickness measurements, the propagation of the reference light beam can be temporarily blocked by a dimming device in the reference arm. If the optical system has a field into which the measuring light beams are focused, the measured distance values ​​can be mathematically corrected to compensate for any curvature of the field. For mathematical correction, correction values ​​can be read from a correction table stored in the evaluation unit. The correction table can contain correction values ​​for different operating wavelengths.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0042] Figure 1 shows a wafer to be measured in a perspective and not to scale view;

[0043] Figure 2 shows a measuring device according to the invention in a schematic representation;

[0044] Figure 3 shows important parts of a scanning device which is part of a measuring device according to the invention and contains mechanical stops for the scanning mirrors, in a simplified perspective view;

[0045] Figure 4 shows important parts of a scanning device which is part of a measuring apparatus according to a further embodiment and contains only a single scanning mirror, in a simplified perspective view;

[0046] Figures 5a and 5b show an optical system with cylindrical lenses, which is part of a measuring device according to a further embodiment, in two orthogonal meridional sections;

[0047] Figure 6 shows a reference arm of an optical coherence tomograph according to an embodiment in which the reference light beam travels part of the optical path in an optical fiber. DESCRIPTION OF PREFERRED EMBODIMENTS

[0048] 1. Wafer measurement

[0049] Figure 1 shows a wafer 10 in a perspective, but not definitive, view. In the illustrated embodiment, the wafer 10 has the shape of a right circular cylinder, with the thickness significantly exaggerated. For example, real wafers 10 have a diameter of 300 mm, while the thickness is only approximately 1 mm. Occasionally, wafers 10 are used whose surface is not circular, but square.

[0050] The ideal circular-cylindrical shape of wafer 10 is indicated by dashed lines 12. Due to manufacturing tolerances, deviations from this ideal shape may occur, which are exaggerated in Figure 1. To determine these deviations, wafer 10 must be measured. By measuring the topography of both wafer surfaces, all common geometric specifications of the wafer, such as TTV, warpage, and warping, can be derived.

[0051] The distribution of the measuring points at which the topography is measured is adapted to the respective measuring task. In the embodiment shown in Figure 1, it is assumed that the measuring points are arranged along two lines 11, 13 that are arranged perpendicular to one another, intersect in the center of the wafer 10, and each extend to the circumferential edge of the wafer 10. Other measuring patterns, e.g., spirals or grid patterns, are of course also conceivable. The measuring points can be very closely spaced, e.g., have a spacing of just a few micrometers. For other measuring patterns, the spacing is in the range of 1 mm.

[0052] 2. Structure and function of the measuring device

[0053] Figure 2 shows a schematic representation of a measuring device, designated overall by 14. The measuring device 14 is used to measure a wafer 10, which is supported here by a holder 18. The holder 18 can, for example, be designed as a simple three-point support, as indicated in Figure 2 by supports 20. In the illustrated embodiment, the holder 18 is in turn supported on a base 16. As a rule, the holder 18 and the base 16 are not part of the measuring device 14.

[0054] When measuring within the production process, the wafer 10 can also be fed to the measuring device 14, for example, by a conveyor device.

[0055] The measuring device 14 comprises an optical coherence tomograph 22, which generates a measuring light beam 24 and whose structure is explained in more detail below.

[0056] A scanning device 26, indicated overall by 26, variably deflects the measuring light beam 24 into two spatial apertures. For this purpose, the scanning device 26 has a first scanning mirror 28, which is mounted for rotation about a first axis of rotation 30. A second scanning mirror 32 is mounted for rotation about a second axis of rotation 34, which is oriented perpendicular to the first axis of rotation 30. The scanning mirrors 28, 32 are driven by galvanometer drives (not shown), which are controlled by a control unit 36.

[0057] The measuring device 14 also includes an optical system 38, which is indicated in Figure 2 by three lenses L1, L2, and L3. In the illustrated embodiment, the optical system 38 focuses the measuring light beam 24 deflected by the scanning device 26 such that it always strikes the surface 40 of the wafer 10 facing the optical system approximately perpendicularly.

[0058] The optical coherence tomograph 22 contains, in a manner known per se, a light source 42, a first beam splitter 44 which splits the light generated by the light source into the measuring light beam 24 and a reference light beam 46, a reference arm 48 for guiding the reference light beam 46 and an object arm 50 which uses the optical system 38 and the scanning device 26 and in which the measuring light beam 24 is guided.

[0059] During a measurement, the measuring light beam 24 propagating in the object arm 50 is focused onto the surface 40 of the wafer 10, where it is partially reflected and returns via the object arm 50 to the first beam splitter 44 on the same light path. There, the reflected portion of the measuring light beam 24 is superimposed on the reference light beam 46 guided in the reference arm 48 and reflected there by a mirror 52. Both light components are directed by a second beam splitter 54 onto a detector 56, which converts the optical reference signal into an electrical signal.

[0060] In the illustrated embodiment, the optical coherence tomograph 22 is designed as an FD-OCT (FD stands for Fourier Domain). The detector 56 therefore contains a spectrometer that records the spectral intensity distribution. From this, an evaluation unit 57 connected to the detector 56 can calculate, in a conventional manner, the distance of the surface 40 to the measuring device 14 (for example, the lens L3) at the point of incidence of the measuring light beam 24. For further details on the optical coherence tomograph, reference is made to DE 10 2017 128 158 A1, already mentioned above.

[0061] The wavelength range of the light generated by the light source 42 can be selected such that the measuring light beam 24 can at least partially penetrate the wafer 10. A reflection then also occurs on the lower surface 58 of the wafer 10 facing away from the measuring device 14, which reflection is detected by the optical coherence tomograph 22. In this case, two measuring light beams 24 that have traveled different optical path lengths are superimposed. The detector 56 then detects the difference in the optical path lengths in a conventional manner via the periodicity of the interference, from which the distance between the two surfaces 40, 58 of the wafer 10 and thus its thickness can be deduced.

[0062] If the thickness mode described above is selected, light should be prevented from propagating in the reference arm 48, as this would also contribute to interference and thus generate unwanted noise. For this purpose, the reference arm 48 contains a switchable dimming device indicated at 60, which can be designed, for example, as a central or slotted shutter. When changing from the distance mode to the thickness mode, the switchable dimming device 60 is automatically closed, whereby no light from the reference arm 48 can contribute to the interference on the detector 56. When changing back to the distance mode, the switchable dimming device 60 clears the path for the reference light beam 46 again. The reference arm 48 optionally has a control mechanism for the dimming device 60, in particular a sensor which can determine the state of the dimming device 60.With the help of the control mechanism, the current status can be checked, for example, after a power interruption.

[0063] In the embodiment shown in Figure 2, the measurement light 24 propagates entirely in free space. In other embodiments, the light is partially guided in optical fibers, as explained further below with reference to Figure 6. If the optical coherence tomograph 22 is housed in its own housing, it is advisable to also guide the light between the optical coherence tomograph 22 and the scanning device 26 in an optical fiber. The scanning device 26 and the optical system 38 can then be housed in a spatially compact and lightweight measuring head that can be mounted at different locations with little effort.

[0064] 3. Field curvature

[0065] Although the optical system 38 is designed as a flat-field optic, so that collimated light entering the optical system 38 is focused into a flat focal plane, it was determined that the focal points of the measuring light beam 24 are not exactly in one plane.

[0066] This undesirable field curvature is due to the fact that the two scanning mirrors 28, 32 are arranged spatially one behind the other, resulting in path length changes that are not easily comprehensible. These path length changes increase quadratically with increasing rotation angles of the scanning mirrors 28, 32 and thus with increasing distance of the measurement points from the optical axis OA of the optical system 38.

[0067] Typically, the optical path length in the reference arm 48 is selected such that it matches the optical path length in the object arm 50 up to the focal point of the measuring light beam 24. If a reflective surface is located outside the focal point, the coherence tomograph 22 interprets this as meaning that the surface in question is further or closer than the optical path length specified by the reference arm 48. However, if the focal points of the measuring light beam 24 are not exactly in one plane, this results in an otherwise flat surface appearing curved. At a distance of 150 mm from the optical axis OA, the resulting measurement error is already around 1 mm, which is three orders of magnitude more than the required measurement accuracy of 1 pm.

[0068] To solve this problem, a calibration measurement is performed before delivery of the measuring device 14. For this purpose, a high-precision calibration standard in the form of a flat glass plate is measured. The deviations from planarity measured due to the field curvature are translated into correction values, which are stored in a table in the evaluation unit 57. For each measurement point that can be approached by the scanning device 26 on the surface of the calibration standard, a correction value is determined and assigned to this measurement point. The assignment of measurement points and correction values ​​can be stored in a table in the evaluation unit 57.

[0069] Alternatively, the correction values ​​can be calculated using a formula derived from the calibration measurement. Combined solutions are also conceivable, in which correction values ​​are stored in tables for individual sampling points only, and the correction values ​​for locations between the sampling points are obtained by interpolation.

[0070] When real wafers 10 are measured after such a calibration, the corresponding correction values ​​are added or subtracted from the obtained measured values, depending on their sign. This results in a corrected set of measured values, which can be output to the user of the measuring device 14 for further processing.

[0071] 4. Measures to reduce field curvature

[0072] Alternatively or in addition to calibration, measures can be taken to reduce the field curvature as much as possible.

[0073] One such measure can, for example, consist of reducing the distance between the two scanning mirrors 28, 32. This distance is normally selected such that the scanning mirrors 28, 32 cannot collide under any circumstances, even at larger angles of rotation. For the measurement of wafer 10 described above, however, only relatively small angles of rotation in the order of ± 10° or even only ± 5° are generally required. The smaller angles of rotation make it possible to arrange the two scanning mirrors 28, 32 closer to one another. Since the scanning mirrors 28, 32 generally have a larger range of angles of rotation, a collision protection device must be provided to reliably prevent the scanning mirrors 28, 32 from touching each other during operation. In the exemplary embodiment according to the invention shown in Figure 3, the collision protection device has mechanical stops 64, which can be formed, for example, by rubber lips.The mechanical stops 64 are positioned in such a way that they limit the structurally possible rotation angles of the scanning mirrors 28, 32 to the required rotation angles.

[0074] Depending on the distance and size of the scanning mirrors, it may be sufficient to provide only one of the two scanning mirrors 28, 32 with a collision protection device.

[0075] The field curvature becomes even smaller if the scanning device 26 does not have two scanning mirrors 28, 32, which are each mounted rotatably about a single axis of rotation 30 or 34, but only a single scanning mirror 28', which is mounted about two preferably orthogonal axes of rotation 30, 34, as illustrated in Figure 4 in a perspective schematic representation.

[0076] Figures 5a and 5b show the optical system 38 of the measuring device 14 according to an alternative embodiment in two orthogonal meridional sections, namely in the XZ plane in Figure 5a and in the YZ plane in Figure 5b. This measuring device 14 also features two scanning mirrors. The field curvature is reduced here by a suitably designed system 38. For this purpose, the optical system 38 has a first cylindrical lens 66 and a second cylindrical lens 68, wherein the axes of symmetry of the cylindrical lenses 66, 68 run perpendicular to one another, as can be seen in Figures 5a and 5b. The two cylindrical lenses 66, 68 are each assigned to one of the two scanning mirrors 28, 32. The orientation of the axes of symmetry of the cylindrical lenses 66, 68 is therefore coordinated with the orientation of the axes of rotation 30, 34 of the scanning mirrors 28 and 32, respectively. Figure 6 shows a section of the optical coherence tomograph 22.To ensure that the measuring light beam 24 can also scan large wafers 10 right up to their edges, the optical system 38 must have a large focal length, which results in a large geometric and optical path length in the object arm 50. The optical path length in the reference arm 48 must also be correspondingly long.

[0077] Since free-beam propagation over long distances requires complex beam folding, it may be more advantageous if the measuring light beam 24 is not guided, or at least not entirely, as a free beam in the reference arm 48, but at least partially in an optical fiber. In Figure 6, a coupling lens 70 arranged in the light path behind the screening device 60 couples the measuring light beam into one end 71 of a rolled-up optical fiber 72. A second coupling lens 76 is arranged between the end 74 of the optical fiber 72 and the mirror 52 to couple the reference light beam reflected by the mirror 52 back into the fiber 72.

[0078] The splitting of the reference arm 48 into a fiber-guided and a free-beam-guided part is not limited to the embodiment shown in Fig. 6. In particular, the beam splitter 44 can also be implemented as a fiber coupler in an alternative embodiment.

Claims

PATENT CLAIMS Device (14) for measuring wafers (10), comprising an optical coherence tomograph (22) configured to generate a measuring light beam (24) and direct it onto the wafer (10) via an optical system (38), a scanning device (26) having two scan mirrors (28, 32) each rotatably mounted about exactly one axis (30, 34) and configured to deflect the measuring light beam (24) in two spatial directions, a control unit (36) configured to control the scanning device (26) such that the measuring light beam (24) successively scans the surface of the wafer (10) at several measuring points, and an evaluation unit (57) configured to calculate distance values ​​and / or thickness values ​​from interference signals provided by the optical coherence tomograph (22), wherein at least one of the two scan mirrors (28, 32) has a Collision protection device (64) is assigned, which is designed toto limit the rotation angle of the at least one scan mirror (28, 32). Device according to claim 1, characterized in that the collision protection device has at least one mechanical stop (64). Device according to claim 2, characterized in that the mechanical stop (64) is made of plastic or rubber. Device according to claim 2 or 3, characterized in that the at least one scan mirror (28, 32) is rotatably mounted on a scan mirror holder and the collision protection device is attached to the scan mirror holder. Device according to claim 2 or 3, characterized in that the mechanical stop (64) acts on an axis of rotation of the at least one scan mirror (28, 32) or on a projection formed on the axis of rotation. Device according to claim 2 or 3, characterized in that the at least one scan mirror (28, 32) has a reflective surface and a back side, and that the mechanical stop is mounted on the reflective surface or the back side. Device according to any one of the preceding claims, characterized in that the collision device has an electronic limiting device configured to electronically prevent the supply of control signals to the at least one scan mirror (28, 32) that would lead to an exceedance of a predetermined rotation angle range. Device according to any one of the preceding claims, characterized in that two measuring points are a distance d m a X with 140 mm < d max < 600 mm. Device according to one of the preceding claims, characterized in that the optical coherence tomograph (22) comprises: a light source (42), a beam splitter (44) configured to split light generated by the light source into the measurement light beam (24) and a reference light beam (46), a reference arm (48) for guiding the reference light beam (46), an object arm (50) utilizing the optical system (38) and the scanning device (26) for guiding the measurement light beam (24), a detector (56) configured to generate interference signals from a superposition of the reference light beam (46) guided in the reference arm (48) with a portion of the measurement light beam (24) reflected at the wafer (10).Device according to claim 9, characterized in that a switchable dimming device (60) is arranged in the reference arm (48), which is configured to prevent the propagation of the reference light beam (46) in the reference arm (48) when thickness measurements are being carried out. Device according to any one of the preceding claims, characterized in that the optical system (38) has a field into which the measuring light beams (24) are focused, and that the evaluation unit (57) is configured to computationally correct the measured distance values ​​to compensate for a curvature of the field. Device according to claim 11, characterized in that correction values ​​can be read from a correction table stored in the evaluation unit (57) for computational correction. Device according to claim 12, characterized in that correction values ​​for different operating wavelength ranges are stored in the correction table. Device according to any one of the preceding claims, characterized in that the optical system for correcting a field curvature includes at least one anamorphic optical element (66, 68).Method for measuring wafers (10), comprising the following steps: a) generating a measuring light beam (24) with an optical coherence tomograph (22); b) directing the measuring light beam (24) onto the wafer (10) with an optical system (38); c) deflecting the measuring light beam (24) in two spatial directions with a scanning device (26) which has two scan mirrors (28, 32) each rotatably mounted about exactly one axis (30, 34) and is controlled such that the measuring beam scans the surface (40) of the wafer (10) successively at several measuring points, wherein the rotation angles of at least one of the two scan mirrors (28, 32) are limited by a collision protection device (64); d) Calculating distance values ​​and / or thickness values ​​based on interference signals provided by the optical coherence tomograph (22). Method according to claim 15, characterized in that the collision protection device for limiting the rotation angles electronically prevents the supply of control signals to the at least one scan mirror (28, 32) that would lead to an exceedance of a predetermined rotation angle range.

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