Apparatus and method for wafer measurement - Patents.com
Patent Information
- Application Number
- JP2024549448
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-02-24
- Filing Date
- 2023-01-25
- Publication Date
- 2025-12-02
Smart Images
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Abstract
Description
[Technical field]
[0001] The present invention relates to an apparatus and method for non-contact measurement of wafer geometric dimensions, such as TTV, bend, and warp. [Background technology]
[0002] Wafers are circular or square disks with a thickness of about 1 mm, used as substrates for integrated circuits, micromechanical components and photoelectric coatings. Wafers are produced from monocrystalline or polycrystalline blanks (so-called ingots). The blank is cut perpendicular to its longitudinal axis to give individual wafers. Typically, silicon wafers are used, but other materials may also be used, such as glass wafers, for example for applications in the manufacture of macrolens arrays or in the field of augmented reality.
[0003] The shape of the wafer must meet strict geometric specifications. These specifications include the Total Thickness Variation (TTV), which is the maximum difference between the thickest and thinnest points of the wafer. Bow is defined as the maximum deviation of the central surface of the wafer from a reference plane. Warp is also understood by those skilled in the art as the deviation of the central surface of the wafer from a reference plane after the entire wafer surface has been compensated for bow.
[0004] In addition to common definitions such as TTV, bending and warpage, wafer characterization sometimes uses similar but slightly different definitions to evaluate the deviation of a wafer from its ideal shape.
[0005] To meet specifications, these geometric dimensions must be measured at least randomly, and most preferably during normal production runs.
[0006] US 2012 / 0257207 A1 describes an apparatus for measuring wafers that combines a Michelson interferometer for distance measurement and a reflectometer for thickness measurement. Measurements are performed at individual points. To obtain measurements over the entire surface, the wafer must be moved relative to the fixed measuring device.
[0007] However, in such known measurement methods, the wafer cannot be moved as quickly as desired due to its inertia, and therefore the speed is slow. In order to ensure stability and accuracy of the movement, high quality is required for the actuator for moving the wafer, and the corresponding measurement device is also expensive.
[0008] The present invention aims to provide an apparatus and method for measuring wafers very quickly and at low cost. Summary of the Invention
[0009] Regarding the apparatus, this object is achieved by an apparatus for wafer measurement, said apparatus comprising: an optical coherence tomograph configured to generate a measurement light beam and direct the measurement light beam through an optical system to the wafer; a scanning device having two scanning mirrors each mounted rotatably about exactly one axis and configured to deflect the measurement light beam in two spatial directions; a control unit configured to control the scanning device such that the measurement light beam sequentially scans the wafer surface at a plurality of measurement points; an evaluation unit configured to calculate a distance value and / or a thickness value based on an interference signal provided by the optical coherence tomography device, At least one of the two scanning mirrors is assigned a collision avoidance device that is configured to limit the rotation angle of the at least one scanning mirror.
[0010] The invention is based on the realization that on the one hand a larger measurement field can be obtained with a larger optical system, in particular by solving the problem of field curvature, which allows wafer measurements with known measuring devices comprising optical coherence tomography and scanning devices. The inventors have already developed a number of measures to solve this problem.
[0011] DE 10 2017 128 158 A1 discloses such a known measuring device. In one embodiment described therein, a measuring light beam is generated by an optical coherence tomography (OCT) and deflected by a mirror scanner, one scanning mirror being provided for each deflection direction. After passing through a planar field objective, the measuring light beam is incident on the surface to be measured approximately parallel to the axis, resulting in a telecentric beam path. During surface scanning, only the extremely lightweight scanning mirror moves, while the measurement object remains stationary. In this way, a rapid scanning of the surface of the measurement object is possible.
[0012] Another advantage of such a measuring device is that the mirror deflection can be read out in real time and adjusted by the control circuit based on the set deflection, allowing extremely accurate positioning of the measuring light beam on the measurement object, even in the presence of parasitic residual vibrations due to the movement of the scanning mirror.
[0013] The inventors have found that in this known measurement device, when the wafer surface is scanned two-dimensionally with the measurement light beam, field curvature occurs: simply put, this means that the focus of the measurement light beam does not move on a plane during the scan, but on a curved surface, so that the wafer surface which is actually flat appears curved.
[0014] For very small wafers with diameters of about 80 mm or less, the field curvature is still tolerable, but for larger wafers, the field curvature increases quadratically with the diameter of the wafer being measured, making it unacceptable. Currently, the distance d between two measurement points that a scanning device can reach is max140mm≦d max ≦600mm, preferably 280mm≦d max A wafer that is ≦450 mm is considered a large wafer. max = 300 mm, the measurement error due to the field curvature is typically already of the order of 1 mm. Therefore, without additional measures, the above-mentioned measuring device is unsuitable for measuring large wafers, where distance measurements require an accuracy of 1 μm or less, even when using objectives with a large planar field. Therefore, for large wafers, the measurement error of the known measuring device is three orders of magnitude larger.
[0015] The inventors further realized that the extra optical path length difference that causes the field curvature is primarily due to the scanning mirrors. To be able to pivot without colliding, the two scanning mirrors must be separated by a large distance. The spatial distance between the rotation axes of the scanning mirrors was found to have the greatest effect on the field curvature.
[0016] So the easiest way to reduce the field curvature is to use one scanning mirror rotatably mounted around two axes, instead of two scanning mirrors, each rotatably mounted around exactly one axis. However, a conventional two-axis scanning mirror cannot be pivoted as quickly and precisely as two one-axis scanning mirrors. However, the required measurement time is significantly shorter than known methods that require a moving wafer.
[0017] Another way to reduce the field curvature is to use two scanning mirrors, each mounted rotatably around exactly one axis, as before, but the distance between the scanning mirrors must be significantly reduced. However, in this case, there is a risk of collision when the scanning mirrors are rotated at large angles. In order to avoid such a situation, in the present invention, at least one of the two scanning mirrors is assigned a collision prevention device that is set to limit the rotation angle of at least one scanning mirror. This is based on the consideration that conventional scanning mirrors usually have a rotation angle range that is partially useful for the application of the present application. If the collision prevention device initially limits the possible rotation angle (usually ±20°) to the actually required rotation angle (for example ±10°, preferably ±5°), then a collision of the scanning mirrors can be reliably avoided even if the distance between the scanning mirrors is very small.
[0018] The collision avoidance device may have at least one mechanical stop for limiting the rotation angle, which may be, for example, a plastic or rubber damper, which the at least one scanning mirror hits if it is deflected, for example, by more than ±10° or ±5°.
[0019] According to a preferred embodiment of the invention, at least one scanning mirror is rotatably mounted on a scanning mirror holder, and the collision prevention device is fixed to the scanning mirror holder by means of insertion, clamping, screwing, gluing or other methods. Fixing the collision prevention device by insertion, clamping or gluing has the advantage that the collision prevention device is attached to the scanning mirror holder without interfering with the mechanical structure of the scanning mirror holder and the scanning mirror itself. The function and adjustment of the scanning mirror are not affected. The collision prevention device is preferably shaped so that it is non-rotatably supported on at least one surface of the scanning mirror holder. This surface is formed with a counter bearing for absorbing the forces generated when the mirror collides.
[0020] Optionally, the collision avoidance device is integral with the surrounding member or fixed to the surrounding member, for example a pin in the body of the device.
[0021] Alternatively, the stop may act directly on the axis of rotation of the at least one scanning mirror or on a protrusion formed on the axis of rotation.
[0022] Optionally, a damper may be attached to the reflective or underside of at least one of the scanning mirrors. The damper is preferably attached at a location where the scanning mirror would collide to dampen the collision. It is also preferred that the damper be attached to the mirror and abut against a separate attached mechanical stop.
[0023] Alternatively or additionally, the collision avoidance device may be an electronic limiting device arranged to electronically prevent the application of control signals to at least one of the scanning mirrors that would exceed a predefined rotation angle range. For example, a choke coil is very reliable for limiting the control current of the scanning mirror.
[0024] Alternatively or additionally, the control software for the scanning mirror may be configured to prevent the generation of control signals that would cause excessive deflection of the scanning mirror, in which case one of the above measures may be additionally employed as a safeguard to avoid a software malfunction generating an erroneous signal that would result in excessive deflection of one of the two scanning mirrors.
[0025] Another possible alternative or additional means for reducing the field curvature is to use an optical system that includes at least one, and preferably two, anamorphic optical elements for correcting the field curvature. These anamorphic elements are preferably cylindrical lenses. The orientation of their symmetry axes coincides with the orientation of the scanning mirror rotation axis.
[0026] This alternative or additional measure does not reduce the field curvature, but eliminates its influence by calibration. For this, the evaluation unit should be set to compensate for the curvature of the field in which the measuring light beam is focused by the optical system by performing a calculated correction on the measured distance values. This field usually overlaps with the focal plane of the optical system. During the calibration, for example, a highly precisely manufactured flat glass is measured. Deviations from the measured flatness result in correction values, which are subtracted or added to the measured values during the subsequent measurement, depending on the different sign.
[0027] When carrying out the calculated corrections, the correction values may be read from a correction table stored in the evaluation unit, which preferably stores correction values for different operating wavelength ranges. The correction values for these different operating wavelength ranges do not have to be obtained by a single calibration measurement, but may be derived from the correction values obtained for a particular operating wavelength range on the basis of theoretical considerations, in which case Zernike polynomials may in particular be used.
[0028] Instead of reading the correction values from a correction table, they may be calculated by a formula, which may be derived, for example, from a polynomial fit, for example a Zernike polynomial.
[0029] Of course, calibration may be performed in addition to the above means.
[0030] Specifically, an optical coherence tomography device that can be used in the present invention is A light source; a beam splitter configured to split light generated by the light source into a measurement light beam and a reference light beam; a reference arm for guiding a reference light beam; an object arm for guiding the measurement light beam through an optical system and a scanning device; and a detector configured to generate an interference signal from a superposition of the reference light beam guided in the reference arm and a portion of the measurement light beam reflected on the wafer.
[0031] In one embodiment, the reference arm is provided with a switchable blocking device that is set to block the propagation of the reference light in the reference arm during thickness measurement. The switchable blocking device allows switching between a "distance mode" for evaluating the interference of the measurement light beam with the reference light beam and a "thickness mode". In the thickness mode, the interference of the measurement light beam reflected at two plane-parallel interfaces of the wafer is used to infer the distance between the interfaces (i.e. the thickness of the wafer). In this way, the detector can generate an interference signal from the superposition of parts of the measurement light beam reflected at two different interfaces of the wafer. In the thickness mode, the blocking device blocks the propagation of the reference light beam in the reference arm to prevent interference of the unwanted reference light beam from the reference arm with the wanted signal. The blocking device is preferably switched automatically each time the measurement mode is changed.
[0032] In another embodiment, the reference light beam is not guided as a free beam, at least entirely as a free beam, in the reference arm, but at least partially in an optical fiber. In a coiled optical fiber, the light can be guided over a long optical path length in a small space. If the dispersions of the object arm and the reference arm are different, it is preferable to homogenize the dispersion.
[0033] Regarding the method, the object mentioned at the outset is achieved by a method for measuring a wafer, which method comprises the steps of: a) generating a measurement light beam by an optical coherence tomography; b) directing a measurement light beam onto the wafer by an optical system; c) deflecting the measurement light beam in two spatial directions by a scanning device (26) having two scanning mirrors each mounted rotatably around exactly one axis and controlled such that the measurement light beam scans the wafer surface sequentially at a plurality of measurement points, the rotation angle of at least one of the two scanning mirrors being limited by a collision avoidance device; d) calculating distance and / or thickness values based on the interference signal provided by the optical coherence tomography.
[0034] Distance between two measurement points d max 140mm≦d max ≦600mm, preferably 280mm≦d max The scanning device may be controlled so that the angle is ≦450.
[0035] In one embodiment, the optical coherence tomography system comprises: generating light by a light source; splitting the light generated by the light source station into a measurement light beam and a reference light beam by a beam splitter; the reference light beam is guided in a reference arm; the measurement light beam is guided in the object arm; The detector generates an interference signal from a superposition of a reference light beam guided in the reference arm and a portion of the measurement light beam reflected from the wafer.
[0036] During thickness measurements, a light blocking device may temporarily block the reference light beam from propagating in the reference arm.
[0037] If the optical system has a field in which the measurement light beam is focused, the field curvature can be compensated for by performing a calculation correction on the measured distance values, which may be read out from a correction table stored in the evaluation unit, and which may store correction values for different operating wavelengths. [Brief description of the drawings]
[0038] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. [Figure 1] FIG. 2 is a perspective view of a test wafer, not drawn to scale. [Diagram 2] FIG. 1 is a schematic diagram of a measuring device according to the present invention. [Diagram 3] 1 is a simplified perspective view of a portion of a scanning device, including a mechanical stop for a scanning mirror, which is part of a measurement device according to the present invention; [Figure 4] FIG. 2 is a simplified perspective view of a portion of a scanning device including only one scanning mirror, which is part of a measurement device according to another embodiment. [Figure 5a] 13 is a diagram showing two orthogonal meridian sections of an optical system having a cylindrical lens, which is a part of a measurement device according to another embodiment. FIG. [Figure 5b] 13 is a diagram showing two orthogonal meridian sections of an optical system having a cylindrical lens, which is a part of a measurement device according to another embodiment. FIG. [Figure 6] FIG. 2 illustrates a reference arm of an optical coherence tomography system according to an embodiment, in which a reference light beam traverses a portion of its optical path in an optical fiber. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] 1. Wafer measurement 1 is a perspective view of a wafer 10, not drawn to scale. In the embodiment shown, the wafer 10 is in the shape of a right circular cylinder, with the thickness obviously exaggerated. In practice, the wafer 10 may have a diameter of, for example, 300 mm, and a thickness of only about 1 mm. Wafers 10 having square rather than circular surfaces may also be used.
[0040] The ideal cylindrical shape of wafer 10 is indicated by dashed line 12. Due to manufacturing tolerances, deviations from the ideal shape may occur, but such deviations are exaggerated in Figure 1. To identify such deviations, wafer 10 must be measured. By measuring the topography of the two wafer surfaces, all of the typical geometric specifications of the wafer can be obtained, such as TTV, bow, warpage, etc.
[0041] The distribution of the measurement points used for the topography measurement is adapted to the specific measurement requirements. In the embodiment shown in Fig. 1, the measurement points are aligned along two lines 11, 13 that are perpendicular to each other, meet in the centre of the wafer 10 and each extend to the periphery of the wafer 10. Of course, other measurement patterns are also possible, e.g. spiral or grid patterns. The measurement points may be very closely spaced, e.g. on the order of a few micrometers. For other measurement patterns, the pitch may be 1 mm or less.
[0042] 2. Structure and function of the measuring device 2 is a schematic diagram showing a measurement apparatus, generally designated 14. Measurement apparatus 14 is for measuring a wafer 10 mounted on a holder 18. Holder 18 may be, for example, a simple three-point support, as indicated by support 20 in FIG. 2. In the embodiment shown, holder 18 is itself supported by a base 16. Holder 18 and base 16 are not typically part of measurement apparatus 14.
[0043] When measurements are performed during production, the wafer 10 may be provided to the measurement device 14 by, for example, a transport device.
[0044] The measurement device 14 includes an optical coherence tomograph 22 for generating a measurement light beam 24, the structure of which will be described in detail below.
[0045] The scanning device 26, designated by the reference number 26, variably deflects the measurement light beam 24 in two spatial directions. To this end, the scanning device 26 comprises a first scanning mirror 28 rotatably mounted about a first rotation axis 30. A second scanning mirror 32 is rotatably mounted about a second rotation axis 34 perpendicular to the first rotation axis 30. The scanning mirrors 28, 32 are driven by galvanometer drives (not shown) which are controlled by a control unit 36.
[0046] The measurement device 14 includes an optical system 38, represented in Figure 2 by three lenses L1, L2 and L3. In the illustrated embodiment, the optical system 38 focuses the measurement light beam 24 deflected by the scanning device 26 for near normal incidence on a surface 40 of the wafer 10 that faces the optical system.
[0047] The optical coherence tomograph 22 includes, in a known manner, a light source 42, a first beam splitter 44 which splits the light generated by the light source into a measurement 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 utilizes the optical system 38 and the scanning device 26. The measurement light beam 24 is guided in the object arm.
[0048] During measurement, the measurement light beam 24 propagating in the object arm 50 is focused on the surface 40 of the wafer 10, where it is partially reflected and returns along the same optical path through the object arm 50 to the first beam splitter 44. There, the reflected part of the measurement light beam 24 is superimposed with the reference light beam 46 guided in the reference arm 48 and reflected by a mirror 52. A second beam splitter 54 directs both light parts to a detector 56, which converts the optical reference signal into an electrical signal.
[0049] In the illustrated embodiment, the optical coherence tomography 22 is FD-OCT (FD stands for Fourier Domain). The detector 56 therefore comprises a spectrometer for detecting the spectral intensity distribution. An evaluation unit 57 connected to the detector 56 can thereby determine the distance from the surface 40 at the point of incidence of the measurement light beam 24 to the measuring device 14 (e.g. lens L3) in a known manner. For further details of the optical coherence tomography, reference is made to DE 10 2017 128 158 A1 mentioned at the beginning.
[0050] The wavelength range of the light generated by the light source 42 may be selected such that at least a part of the measurement light beam 24 is transmitted through the wafer 10. In this case, a reflection occurs at the lower surface 58 of the wafer 10, remote from the measuring device 14, which can also be detected by the optical coherence tomography 22. In this case, the two measurement light beams 24 that have traveled different optical path lengths are superimposed. The detector 56 then detects the difference in optical path lengths in a known manner based on the periodicity of the interference, and thereby infers the distance between the two surfaces 40, 58 of the wafer 10 and, further, the thickness of the wafer.
[0051] When the thickness mode is selected, it is necessary to block the propagation of light in the reference arm 48, since such light would also cause interference, resulting in the generation of unwanted interference signals. For this purpose, the reference arm 48 includes a switchable blocking device, indicated at 60, which may be, for example, a central or focal plane shutter. When switching from the distance mode to the thickness mode, the switchable blocking device 60 automatically closes, thereby preventing the light from the reference arm 48 from interfering with the detector 56. When switching back to the distance mode, the switchable blocking device 60 again opens the passage for the reference light beam 46.
[0052] Optionally, the reference arm 48 comprises a monitoring mechanism for the shading device 60, in particular a sensor capable of ascertaining the state of the shading device 60. For example, the monitoring mechanism can be used to ascertain the current state after a power interruption.
[0053] In the embodiment shown in Fig. 2, the measurement light 24 propagates completely in free space. In another embodiment, the light is partially guided by optical fibers, which will be described below with reference to Fig. 6. If the OCT 22 is arranged in its own housing, it is preferred to also guide the light by optical fibers between the OCT 22 and the scanning device 26. In this case, the scanning device 26 and the optical system 38 can be mounted in a compact and lightweight measuring head, which can be conveniently mounted in different positions.
[0054] 3. Field Curvature Although the optical system 38 is a flat field optical system that focuses incident parallel light onto a flat focal plane, it has been found that the focal point of the measurement light beam 24 is not perfectly located on a single plane.
[0055] This unwanted field curvature is due to the spatial arrangement of the two scanning mirrors 28, 32, which causes subtle changes in path length that increase quadratically with increasing rotation angle of the scanning mirrors 28, 32 and with increasing distance from the measurement point to the optical axis OA of the optical system 38.
[0056] Typically, the optical path length in the reference arm 48 is selected to match the optical path length to the focal point of the measurement light beam 24 in the object arm 50. If a reflecting surface is located outside the focal point, the coherence tomography 22 will understand this reflecting surface as being further or closer than the optical path length defined by the reference arm 48. However, the fact that the focal point of the measurement light beam 24 is not perfectly located in one plane leads to an actually flat surface appearing curved. At 150 mm from the optical axis OA, the total measurement error is already of the order of 1 mm, which is three orders of magnitude larger than the required measurement accuracy of 1 μm.
[0057] To solve this problem, the measuring device 14 must be calibrated and measured before delivery. For this purpose, a calibration standard in the form of a flat glass plate manufactured with high precision must be measured. The flatness deviations measured as a result of the field curvature are converted into correction values and stored in a table in the evaluation unit 57. A correction value is obtained for each accessible corner measurement point on the surface of the calibration standard by the scanning device 26 and assigned to this measurement point. The correspondence between the measurement points and the correction values may be stored in a table in the evaluation unit 57.
[0058] Alternatively, the correction values may be calculated by a formula obtained from a calibration measurement. A combined solution may be used in which only the correction values for some of the interpolation points are stored in a table, and the correction values for positions between these interpolation points are obtained by interpolation.
[0059] When measuring the actual wafer 10 after such calibration, depending on the different signs, the corresponding correction value is added or subtracted from the obtained measurement value, thereby obtaining a corrected measurement value, which may be output 5 to a user of the measurement device 14 for further processing.
[0060] 4. Field curvature reduction measures As an alternative or additional means of calibration, any means capable of reducing the field curvature to a maximum extent may be used.
[0061] For example, one such measure is to reduce the distance between the two scanning mirrors 28, 32. Usually, the distance is selected so that the scanning mirrors 28, 32 do not collide with each other in any case, including large rotation angles. However, the measurement of the wafer 10 usually requires only a small rotation angle of ±10°, or even ±5°. The smaller the rotation angle, the closer the two scanning mirrors 28, 32 can be. However, since the scanning mirrors 28, 32 usually have a large rotation angle range, it is necessary to provide a collision prevention device to ensure that the scanning mirrors 28, 32 do not come into contact with each other during operation. In the present invention, in the embodiment shown in FIG. 3, the collision prevention device has a mechanical stop 64, which can be made of rubber, for example. The mechanical stop 64 is positioned so as to limit the settable rotation angle of the scanning mirrors 28, 32 to the required rotation angle.
[0062] Depending on the distance and size of the scan mirrors, it may be necessary to place a collision avoidance device on only one of the two scan mirrors 28, 32.
[0063] The field curvature is even less if the scanning device 26 has only one scanning mirror 28' mounted preferably about two orthogonal axes of rotation 30, 34 (as shown in schematic perspective view in FIG. 4), rather than two scanning mirrors 28, 32 rotatably mounted about one axis of rotation 30 or 34, respectively.
[0064] 5a and 5b show two orthogonal meridian sections of the optical system 38 of the measuring device 14 according to an alternative embodiment, i.e. in the XZ plane in FIG. 5a and in the YZ plane in FIG. 5b. The measuring device 14 is likewise provided with two scanning mirrors, where the field curvature is reduced by a suitably set system 38. To this end, the optical system 38 comprises a first cylindrical lens 66 and a second cylindrical lens 68, the symmetry axes of the cylindrical lenses 66, 68 being perpendicular to each other, as shown in FIGS. 5a and 5b. The two cylindrical lenses 66, 68 each correspond to one of the two scanning mirrors 28, 32. The orientation of the symmetry axes of the cylindrical lenses 66, 68 therefore coincides with the orientation of the rotation axes 30, 34 of the scanning mirrors 28 or 32.
[0065] 6 shows a portion of the optical coherence tomography system 22. The optical system 38 must have a relatively large focal length to allow the measurement light beam 24 to scan to the edge of the large wafer 10, which increases both the geometric path length and the optical path length in the object arm 50. The optical path length in the reference arm 48 must also be correspondingly long.
[0066] Due to the complex light beam folding required for long distance propagation of the free beam, it may be more advantageous if the measurement light beam 24 is not guided as a free beam, or at least entirely as a free beam, in the reference arm 48, but at least partially by an optical fiber. In Fig. 6, the measurement light beam is coupled into an end 71 of a coiled optical fiber 72 by a coupling lens 70 arranged in the optical path after the blocking device 60. A second coupling lens 76 is provided between the end 74 of the optical fiber 72 and the mirror 52 for coupling the reference light beam reflected by the mirror 52 back into the optical fiber 72.
[0067] However, the division of the reference arm 48 into an optical fiber guide section and a free beam guide section is not limited to the embodiment shown in Fig. 6. In particular, in an alternative implementation, the beam splitter 44 may be an optical fiber coupler.
Claims
1. an optical coherence tomography (22) configured to generate a measurement light beam (24) and direct the measurement light beam to the wafer (10) via an optical system (38); a scanning device (26) having two scanning mirrors (28, 32) mounted rotatably about exactly one axis (30, 34) each, and configured to deflect the measurement light beam (24) in two spatial directions; a control unit (36) configured to control the scanning device (26) so that the measurement light beam (24) sequentially scans the surface of the wafer (10) at a plurality of measurement points; an evaluation unit (57) configured to calculate distance and / or thickness values based on the interference signal provided by the optical coherence tomography (22), at least one of the two scanning mirrors (28, 32) is assigned a collision prevention device (64) configured to limit the rotation angle of the at least one scanning mirror (28, 32); An apparatus (14) for measuring said wafer (10).
2. 2. The device according to claim 1, wherein the anti-collision device comprises at least one mechanical stop (64).
3. 3. The device of claim 2, wherein the mechanical stop (64) is made of plastic or rubber.
4. 4. Apparatus according to claim 2 or 3, characterized in that the at least one scanning mirror (28, 32) is rotatably mounted on a scanning mirror holder, and the anti-collision device is fixed to the scanning mirror holder.
5. 4. Device according to claim 2 or 3, characterized in that the mechanical stop (64) acts on the axis of rotation of the at least one scanning mirror (28, 32) or on a protrusion formed on the axis of rotation.
6. 4. The apparatus of claim 2 or 3, wherein the at least one scanning mirror (28, 32) has a reflective surface or a back surface, and the mechanical stop is attached to the reflective surface or the back surface.
7. 4. The apparatus according to claim 1, wherein the collision avoidance device comprises an electronic limiting device configured to electronically prevent the provision of a control signal to the at least one scanning mirror (28, 32) that would exceed a predetermined rotation angle range.
8. Distance d between two measurement points ｍａｘ is 140mm≦d ｍａｘ 4. The device according to claim 1, wherein the thickness is ≦600 mm.
9. The optical coherence tomography (22) a light source (42); a beam splitter (44) configured to split the light generated by the light source into the measurement light beam (24) and a reference light beam (46); a reference arm (48) for guiding said reference light beam (46); an object arm (50) for guiding the measurement light beam (24) through the optical system (38) and the scanning device (26); and a detector (56) configured to generate the interference signal from a superposition of a reference light beam (46) guided in the reference arm (48) and a portion of the measurement light beam (24) reflected on the wafer (10).
10. 10. The apparatus of claim 9, wherein the reference arm (48) is provided with a switchable blocking device (60) configured to block propagation of the reference light beam (46) in the reference arm (48) during thickness measurements.
11. 4. The device according to claim 1, wherein the optical system (38) has a field on which the measurement light beam (24) is focused, and the evaluation unit (57) is configured to compensate for the field curvature by performing a calculated correction on the measured distance values.
12. 12. The device according to claim 11, characterized in that, when calculating the correction, correction values can be read out from a correction table stored in the evaluation unit (57).
13. 13. The apparatus of claim 12, wherein the correction table stores correction values for different operating wavelength ranges.
14. Apparatus according to any one of claims 1 to 3, characterized in that the optical system includes at least one anamorphic optical element (66, 68) for correcting field curvature.
15. a) generating a measurement light beam (24) by an optical coherence tomography (22); b) directing said measurement light beam (24) onto the wafer (10) via an optical system (38); c) deflecting the measurement light beam (24) in two spatial directions by a scanning device (26) having two scanning mirrors (28, 32) mounted rotatably about exactly one axis (30, 34) each, the scanning device (26) being controlled so that the measurement light beam scans the surface (40) of the wafer (10) sequentially at a plurality of measurement points, the rotation angle of at least one of the two scanning mirrors (28, 32) being limited by a collision prevention device (64); d) calculating distance and / or thickness values based on the interference signals provided by the optical coherence tomography device (22); A method for measuring the wafer (10), comprising:
16. 16. The method of claim 15, wherein the collision avoidance device limits the rotation angle by electronically preventing the provision of a control signal to the at least one scanning mirror (28, 32) that would exceed a predetermined rotation angle range.