Apparatus and method for measuring surface profile and thickness profile of an immersed workpiece

Abstract

A device (10) for measuring the surface profile or thickness profile of a workpiece (12) immersed in a liquid (20) comprises a telecentric, focusing optical system (38), a scanning arrangement (26) for causing deflection of the beam in at least one spatial direction, and an optical coherence tomograph (22) which generates a measuring beam (24) and directs it via the scanning arrangement (26) and the optical system (38) onto the workpiece (10). A control unit (36) controls the scanning arrangement (26) such that the measuring beam (24) scans the surface (39) of the workpiece (10) successively at a number of measuring points. An evaluation unit (57) calculates distance values and / or thickness values from the interference signals provided by the optical coherence tomograph (22).

Description

Background Technology 1. Technical Field The present invention relates to an apparatus and a method for measuring the surface profile or thickness of an immersed glass plate or other workpiece.

[0002] 2. Existing Technology Known devices are used to measure the surface profile and surface and thickness profiles of a workpiece based on the principle of optical coherence tomography. These devices measure the distance at a measurement point to scattering or reflecting structures located on or (in sufficiently transparent cases) within the workpiece, or the distance between these structures. If these structures are located on a surface, a surface profile is obtained. If interfaces or other structures are located at different depths within the workpiece (e.g., in the case of transparent coatings or glass plates), the distance between the structures, i.e., the thickness of the coating or glass plate, can be directly determined based on reflections from these structures. Based on the correlation between the measured thickness and the measurement location, the thickness profile of the workpiece can be derived.

[0003] Early devices of this type could only determine the distance for a single measurement point. To scan an entire surface, the measuring head needs to move relative to the workpiece, or vice versa. Current devices guide the measuring beam across the workpiece using a scanning mechanism. The applicant markets this type of equipment under the name "Flying Spot Scanner."

[0004] During measurement, it is sometimes necessary to immerse the workpiece in water or another liquid. This is particularly advantageous when workpieces are processed in a liquid and measurements are taken for process monitoring. Removing and drying the workpiece after a processing step, measuring it, and then, if necessary, re-immersing it for further processing is tedious. For example, wafers used in semiconductor manufacturing are often immersed in water during grinding or polishing. In such cases, continuous or intermittent measurements are required to achieve optimal control of the grinding or polishing process.

[0005] If known equipment is used for this purpose, it will be found that the measured profile is spatially distorted, the measurement is inaccurate, or the measurement is simply impossible. Summary of the Invention

[0006] The object of the present invention is to provide an apparatus and a method for reliably measuring the surface profile or thickness profile of immersed glass plates, wafers and other workpieces.

[0007] Regarding the equipment, the solution for achieving the above objectives is an apparatus for measuring the surface profile or thickness profile of a workpiece immersed in a liquid, comprising a focusing optical system, a scanning device (designed to deflect the beam along at least one spatial direction), and an optical coherence tomography (OCT) scanner. The OCT scanner is designed to generate a measurement beam and direct it toward the workpiece via the scanning device and the optical system. A control unit is designed to control the scanning device to sequentially scan the surface of the workpiece at several measurement points. An evaluation unit is designed to calculate distance and / or thickness values ​​based on the interference signals provided by the OCT scanner. According to the invention, the optical system is telecentric.

[0008] The inventors realized that if the optical system is telecentric, spatial distortion of the measured profile can be avoided. In this case, the main rays of the beam converging at the measurement point all enter the liquid parallel to each other, thus avoiding distortion caused by different refractive indices at the interface with the liquid.

[0009] Generally, the optical system is aligned with the interface so that the main ray enters the liquid perpendicularly. Where the depth of field is sufficiently large, the liquid causes only a minor focal plane shift, which can be easily corrected if necessary.

[0010] It has been proven that absorption of the measurement light in a liquid can result in excessively low intensity of the reflected measurement light. Therefore, the spectral absorption curve of the liquid should be considered when selecting the wavelength of the measurement light. For water and many other liquids, the minimum absorption occurs at approximately 400 nm, which is at the short-wavelength edge of the visible spectrum. Therefore, a coherent tomography scanner with a white light source (i.e., wavelength between 400 and 700 nm) is preferred. However, depending on the liquid and the workpiece, the wavelength can be longer (up to 1100 nm). Depending on the material, a center wavelength between 800 and 950 nm or between 950 and 1100 nm is more advantageous for wafer measurements because shorter wavelengths are excessively absorbed by thicker wafers. Here, the center wavelength is typically determined using a power-weighted method.

[0011] The refractive index of liquids is significantly higher than that of air or other gases. Therefore, the liquid has a substantial influence on the optical path length of the measuring beam as it travels through the measuring arm to the workpiece and back to the coherence tomography scanner. This is not significant in thickness measurement, as thickness is determined by the interference of the components reflected at different interfaces of the workpiece. When both components must traverse longer optical path lengths due to the liquid, the interference is unaffected.

[0012] In surface profile measurements where reflection or scattering occurs only at one interface of the workpiece, the measuring beam must interfere with a reference beam guided in the reference arm of a coherence tomography scanner. In this case, the varying optical path lengths in the measuring arm due to the liquid will affect the measurement, as the optical path lengths in the measuring and reference arms must be similar. If the optical path length in the measuring arm is significantly prolonged due to the liquid, the optical path length in the reference arm will be too short to produce the necessary interference.

[0013] Therefore, in one embodiment, the optical coherence tomography (OCT) scanner includes a first reference arm having a first optical path length and a second reference arm having a second optical path length different from the first optical path length. The first and second reference arms can be interchangeably mounted to the housing of the coherence arm, such that at any given time, either the first or second reference arm is located in the beam path of the OCT scanner. This allows the device to be used for both liquid-free and liquid-containing measurements in surface profilometry, and / or, in liquid-containing measurements, the amount of liquid can be significantly varied, which is sometimes necessary in certain measurement tasks.

[0014] A simpler solution is to use an optical coherence tomography (OCT) scanner with an adjustable reference arm. In this case, there is no need to modify the OCT scanner by replacing it with a different reference arm; instead, the existing reference arm can be adjusted to adapt the OCT scanner to different measurement tasks.

[0015] The fiber optic expansion coil used to guide the measurement light achieves a certain degree of adjustability. By expanding the coil, the fiber wound around it is lengthened, thereby increasing the optical path length. However, this only allows for small variations in the optical path length.

[0016] To achieve greater flexibility, a more advantageous approach is to have the reference arm comprise a first sub-arm with a first optical path length and a second sub-arm with a second optical path length different from the first optical path length.

[0017] In this case, the reference arm may have a transducer designed to selectively direct the reference light entering the reference arm into either the first or second sub-arm. The transducer enables lossless guidance of the reference light to one of the two sub-arms. Such a transducer may be, for example, a fiber optic transducer or a free-beam transducer, which includes moving reflective optics.

[0018] If the loss of the reference light is acceptable, it can be distributed to the two sub-arms of the reference arm, for example, using an optical fiber coupler or beam splitter. If, in this case, a light-shielding device is provided in the first and second sub-arms respectively to prevent the reference light from propagating in the respective sub-arms, the desired optical path length can be selected in the reference arm by simply controlling the light-shielding device.

[0019] In one embodiment, the device has a container housing a support for the workpiece. The container is either made of glass or another transparent material, or has an opening on its top side through which a measuring beam can pass. The container must be liquid-tight enough to prevent uncontrolled leakage of liquid. The optical system is arranged to allow the measuring beam to enter the container from above through the opening. In the simplest case, the container is a tank, and the workpiece is held at the bottom of the tank. Measurement is performed by vertically entering the tank from above.

[0020] To protect the optical system from the influence of liquids, a plate transparent to the measuring beam can be placed in the optical path between the optical system and the liquid. This is especially important if the liquid is used to process the workpiece, such as an etchant used to thin it, as is the case with display glass.

[0021] Preferably, the plate is directly adjacent to the liquid. This prevents the formation of waves on the liquid that interfere with the propagation of the measurement beam due to refraction and, in some cases, diffuse reflection. This is especially important when the numerical aperture of the optical system is small, as this allows little or no measurement light to enter the liquid perpendicularly.

[0022] The plate can be formed directly on the container or optical system, or fixed thereon. Of course, the plate does not need to cover the entire liquid surface, but only needs to be placed in the area through which the beam being measured passes.

[0023] When the plate is tilted relative to the main beam direction of the measurement beam, significant reflection of the measurement light at the interface between the plate and the optical system can be prevented. This prevents the measurement light from returning to the optical coherence tomography (OCT) scanner and generating interfering measurement signals. These signals produce undesirable distance or thickness values, depending on whether the measurement is of distance or thickness. To avoid this interference, an angle of a few degrees, such as 2° to 5°, between the main beam direction and the surface normal of the plate is sufficient. Even a 10° angle does not necessarily lead to a significant deterioration in telecentricity.

[0024] As an alternative to measurements taken from above, the container may have a region on its sidewall that is transparent to the measuring beam. In this case, the optical system is arranged such that the measuring beam can pass through this transparent region into the container. In this configuration, the aforementioned plate is an integral part of the container and automatically adjoins the liquid, eliminating the need for it to be positioned at a specific height.

[0025] In another embodiment, the device has a protective container in which at least an optical system and a scanning device are housed. The protective container has at least a region transparent to the measuring beam and is at least partially arranged within or potentially arranged within the container containing the workpiece. This arrangement is advantageous, for example, when it is necessary to measure an immersed workpiece from the side and there are no transparent sections on the sidewalls of the container. In this case, the optical system and scanning device must be immersed in the container containing the workpiece. The protective container protects the optical system and scanning device from the influence of the liquid.

[0026] When the protective container is liquid-tight or liquid-sealed with its walls fitted with liquid-tight cable sheaths, optimal protection for the optical system and scanning device can be achieved.

[0027] In terms of method, the solution to achieve the above objective is a method for measuring the surface profile or thickness profile of a workpiece immersed in a liquid, wherein the method comprises the following steps: a) Provide a container filled with liquid that at least partially surrounds the workpiece; b) An optical coherence tomography scanner generates a measurement beam and directs the measurement beam toward the workpiece through a scanning device and a focusing telecentric optical system, so that the measurement beam passes through a portion of the liquid while the measurement beam scans the surface of the workpiece successively at several measurement points. c) The evaluation unit calculates the distance and / or thickness values ​​for the measurement points based on the interference signals provided by the optical coherence tomography scanner.

[0028] The advantages and preferred construction schemes mentioned above regarding the device are applicable accordingly to the method.

[0029] Preferably, the measuring beam passes through a plate that is transparent to the measuring beam and preferably directly adjacent to the liquid on its way to the workpiece.

[0030] Optical coherence tomography scanners, in particular, may have a reference arm whose optical path length is adjusted before measurement based on the optical path length of the measurement beam in the liquid. Attached Figure Description

[0031] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Wherein: Figure 1 A measuring device according to a first embodiment of the present invention is schematically shown; Figure 2 Showing for use as Figure 1 A variant of the coherence tomography scanner of the measuring device shown, wherein the reference light is partially guided in an optical fiber; Figure 3An embodiment of a coherence tomography scanner is shown, wherein the reference arm includes two sub-arms that can be shielded from light by a light-shielding device; Figure 4 An embodiment of a coherence tomography scanner is shown, in which two sub-arms are connected to an optical fiber converter; Figure 5a and Figure 5b An embodiment of a coherence tomography scanner is shown, wherein two sub-arms are connected to the free beam converter at two different locations; and Figure 6 One embodiment is shown in which the measuring head of the coherence tomography scanner is located in a waterproof protective container. Detailed Implementation

[0032] 1. First Embodiment Figure 1 This is a schematic diagram of the measuring device of the present invention, generally indicated by 10. The measuring device 10 is used to measure a plate-shaped workpiece 12 supported by a bracket 14. The bracket 14 can be constructed, for example, as a simple three-point support mechanism, such as in... Figure 1 As shown by support 16. In the illustrated embodiment, support 14 is supported on the bottom of a trough-shaped container 18 filled with water 20. A protective glass plate 21 floats on the water 20 and prevents waves from forming during the movement of the container 18. In this embodiment, support 14, container 18, and protective glass plate 21 belong to measuring device 10. However, container 18 may also be part of a conveying device that sequentially transports containers containing different workpieces 12 to measuring device 10. In other embodiments, container 18 is part of a machine for grinding, polishing, etching, or other processing of workpieces 12.

[0033] The measuring device 10 includes an optical coherence tomography scanner 22 that generates a measuring beam 24, the structure of which will be described in detail below.

[0034] The scanning device 26, generally designated 26, allows the measurement beam 24 to be variably deflected along two spatial directions. For this purpose, the scanning device 26 has a first scanning mirror 28, which is supported in a manner rotatable about a first rotation axis 30. A second scanning mirror 32 is supported in a manner rotatable about a second rotation axis 34 perpendicular to the first rotation axis 30. The scanning mirrors 28 and 32 are driven by mirror drivers (not shown), which are controlled by a control unit 36.

[0035] The measuring device 10 also includes an optical system 38, which in Figure 2Three lenses, L1, L2, and L3, are used to focus the measurement beam 24. The optical system 38 employs a telecentric design. Therefore, the measurement beam 24, deflected by the scanning device 26, always exits the optical system 38 parallel to the optical axis OA. The optical axis OA is arranged perpendicular to the protective glass plate 21, so the measurement beam 24 always enters the protective glass plate 21 and the water 20 located below it perpendicularly. However, entering the protective glass plate 21 at a slight angle would be more advantageous, as this would avoid or at least reduce undesirable back reflections. (As shown in...) Figure 1 As indicated by the dashed lines, the oblique entry of the measurement beam 24 is achieved by the inclined arrangement of the protective glass plate 21'. The surface normal of the inclined protective glass plate 21' is inclined at 3° relative to the main beam direction of the measurement beam 24. Therefore, for all scanning directions, the optical path length of the measurement beam 24 is the same in the protective glass plate 21 and in the water 20. Furthermore, the measurement beam 24 is not deflected by refraction when passing through the protective glass 21 and the water 20. The protective glass 21 and the water 20 only affect the axial position of the focal point 39, at which the optical system 38 focuses the measurement beam 24.

[0036] The optical coherence tomography scanner 22 includes a broadband light source 42 with a center wavelength between 800 nm and 950 nm. The coherence tomography scanner also includes a first beam splitter 44 that splits the light generated by the light source into a measurement beam 24 and a reference beam 46, a reference arm 48 for guiding the reference beam 46, and an object arm 50 that includes the optical system 38 and the scanning device 26, in which the measurement beam 24 is guided.

[0037] During measurement, the measurement beam 24, propagating in the object arm 50, is focused onto the surface 40 of the workpiece 10 facing the optical system 38, where it is partially reflected and returns to the first beam splitter 44 along the same optical path through the object arm 50. There, the reflected component of the measurement beam 24 is superimposed on the reference beam 46, which is guided in the reference arm 48 and reflected at the mirror 52. Both optical components are aligned by the second beam splitter 54 to the detector 56, which converts the optical reference signal into an electrical signal.

[0038] In the illustrated embodiment, the optical coherence tomography scanner 22 is configured as an FD-OCT (FD stands for Fourier Domain). Therefore, the detector 56 includes a spectrometer that detects the spectral intensity distribution. Accordingly, the evaluation unit 57, connected to the detector 56, can calculate, in a known manner, the distance between the surface 40 and the measuring device 10 (e.g., with lens L3) at the point of incidence of the measuring beam 24. If it is necessary to ensure that the focal point 38 is always precisely positioned on the surface 40 of the workpiece in a manner independent of the size of the workpiece 12, the optical system 38 may include one or more movable lenses for changing the axial position of the focal point 39. Related details, as well as other aspects of the optical coherence tomography scanner 22, are described in DE 10 2017 128 158 A1.

[0039] The center wavelength of the measuring beam 24 can be chosen such that the beam is attenuated to a minimum during its two passes through the water 20 and within the workpiece 12. Subsequently, reflection also occurs on the lower surface 58 of the workpiece 12 facing away from the measuring device 10, which is detected by the optical coherence tomography scanner 22. In this case, the two measuring beams 24 with different optical path lengths are superimposed. The detector 56 then detects the difference in optical path lengths in a known manner through the periodicity of interference, thereby calculating the distance between the two surfaces 40 and 58 of the workpiece 12, and further calculating the thickness of the workpiece.

[0040] If the aforementioned thickness measurement mode is selected, light should be prevented from propagating in the reference arm 48, as this light will also participate in interference, thus generating undesirable interference signals. To this end, the reference arm 48 includes a switchable light-shielding device, indicated by 60, which can be configured, for example, as a center shutter or a slit shutter. When switching from distance mode to thickness mode, the switchable light-shielding device 60 automatically closes, thereby preventing light from the reference arm 48 from participating in interference on the detector 56. If switching back to distance mode, the switchable light-shielding device 60 releases the path for the reference beam 46.

[0041] 2. Second embodiment - Reference arm with optical fiber In such Figure 1 In the illustrated embodiment, the measurement beam 24 propagates entirely in free space. In other embodiments, the light is partially guided within an optical fiber.

[0042] Figure 2 A portion of a coherence tomography scanner 22 is shown, which may be as follows: Figure 1This is a part of the measuring device 10 shown. In this embodiment, in the reference arm 48, the measuring light, which propagates as a free beam, is focused by the first lens 70 onto the end 71 of the optical fiber 72. The measuring light emitted from the other end 74 is collimated by the second lens 76 and directed towards the face mirror 52. Because the optical fiber 72 can be wound, a very long optical path length can be achieved in a confined space within the reference arm 48.

[0043] 3. Third embodiment - Sub-arm with light-shielding device If the measuring device 10 needs to be used to measure surface profiles in the presence and absence of liquid 20, it must be able to change the optical path length in the reference arm 48 to a large extent.

[0044] Therefore, it can be used as follows: Figure 3 The reference arm 48 is schematically shown in the diagram. Regarding this coherence tomography scanner 22, the light source 42 produces, in... Figure 2 In the illustrated embodiment, the light propagating as a free-beam is guided by an optical fiber, and vice versa. The reference arm 48 specifically includes an optical fiber beam splitter 80, which divides the reference arm 48 into a first sub-arm 82a with a first optical path length and a second sub-arm 82b with a different second optical path length. In these two sub-arms 82a, 82b, the light is again partially guided as a free-beam, wherein lenses 84, 86 each create a segment in which the reference light is collimated. In the upper sub-arm 82a, a glass body 85 is provided in the relevant segment, and the optical path length traversed by the reference light as it passes through this glass body is approximately the same as the optical path length traversed by the measuring light in the water 20. Of course, another medium can be used instead of the glass body 85, such as water in a transparent container.

[0045] A light-shielding device 88 is also provided in each sub-arm 82a, 82b to prevent reference light from propagating in the sub-arm 82a, 82b. By controlling the light-shielding device 88 accordingly, it can be ensured that the reference light can propagate either in the first sub-arm 82a or only in the second sub-arm 82b. This avoids undesirable interference between the measurement light and the light guided in the "wrong" sub-arm.

[0046] If the measuring device 10 is used to measure the surface profile in the absence of water, the light-shielding device 88 is controlled in such a way that the reference light propagates only in the second sub-arm 82b. If it is necessary to measure the workpiece 12 immersed in water 20, the two light-shielding devices 88 switch roles, so that the reference light propagates only in the first sub-arm 82a, which has a much longer optical path and is adapted to the effects of water 20.

[0047] 4. Fourth Embodiment - Sub-arm with Converter like Figure 4 The illustrated embodiments and Figure 3The implementation methods are basically the same. However, in Figure 4 In this process, the reference light is not evenly distributed to the two subarms 82a and 82b by a passive fiber beam splitter 80, but is distributed by an active fiber converter 90. This prevents the loss of reference light that enters the wrong subarm 82a or 82b and must be blocked there.

[0048] like Figure 5a and Figure 5b As shown, a free-beam converter 100 can also be used instead of the fiber optic converter 90. The free-beam converter 100 includes a moving reflective optical element, which in the illustrated embodiment is constituted by a polarizing prism 102, and can be selectively positioned in a passive position outside the beam path by means of an actuator 104 (see [link to diagram]). Figure 5a ) and active positions located within the beam path (see Figure 5b It moves back and forth between ( ).

[0049] If the polarizing prism 102 is in its passive position outside the beam path, the reference light enters the second sub-arm 82b. If the polarizing prism 102 is moved into the active position by means of actuator 104, the reference light is deflected 90° and enters the first sub-arm 82a, where it penetrates the glass body 85 (see...). Figure 5b ).

[0050] 5. Fifth Embodiment - Protective Container In such Figure 1 In the illustrated embodiment, the container 18 has an opening on its top side, which is covered by a protective glass plate 21. The optical system 38 of the measuring device 10 is arranged such that a measuring beam 24 can pass through the opening from above and enter the container 18.

[0051] In some measurement tasks, the container 18 and the position for holding the workpiece 12 cannot be freely chosen. For example, if the workpiece 12 is fixed to the side wall of the container 18, and this container is made of a material through which the measuring beam 24 cannot pass, such as... Figure 6 As shown, it is impossible to measure from above.

[0052] In such cases, it is more advantageous if the measuring device 10 includes a protective container 110, in which at least the optical system 38 and the scanning device 26 are housed. The protective container 110 has a sidewall 112 containing a glass insert 114, which is transparent to the measuring beam 24. The optical system 38 and the scanning device 26 can be combined to form a measuring head with its own housing 116. This protective container 110 can be immersed in a larger container 18, thus allowing measurements to be performed using the horizontal optical axis OA.

[0053] In the protective container 110, such as Figure 6The protective container 110, with a cover 118 for liquid-tight sealing with the container facing upwards, can also be completely immersed in water 20. In this case, the walls of the protective container 110 must only have a sealed cable sheath 120 to guide the cables 122 used for measuring light and for control signals to a control device 124, which includes a reference arm 48, a light source 42, and a detector 56.

Claims

1. A device (10) for measuring the surface profile or thickness profile of a workpiece (12) immersed in a liquid (20), comprising: Focused optical system (38). The scanning device (26) is designed to deflect the beam along at least one spatial direction. An optical coherence tomography scanner (22) is designed to generate a measurement beam (24) and direct the measurement beam toward the workpiece (10) via the scanning device (26) and the optical system (38). The control unit (36) is designed to control the scanning device (26) so that the measuring beam (24) scans the surface (39) of the workpiece (10) successively at several measuring points. The evaluation unit (57) is designed to calculate distance and / or thickness values ​​based on the interference signals provided by the optical coherence tomography scanner (22). Its features are, The optical system (38) is telecentric.

2. The device according to the preceding claims, characterized in that... A container (18) in which a support (14) for the workpiece (12) is housed.

3. The device according to claim 2, characterized in that, The container (18) has an opening on its top side, and the optical system (38) is arranged such that the measuring beam (24) can pass through the opening from above into the container (18).

4. The device according to claim 3, characterized in that, A plate (21) that is transparent to the measuring beam (24) is provided in the optical path between the optical system (38) and the liquid (20).

5. The device according to claim 4, characterized in that, The plate (21) is directly adjacent to the liquid (20).

6. The device according to claim 4 or 5, characterized in that, The plate (21) is arranged at an angle relative to the main beam direction of the measuring beam.

7. The device according to claim 2, characterized in that, The container (18) has a region on its sidewall in which it is transparent to the measuring beam (24), and the optical system (38) is arranged in such a way that the measuring beam (24) can pass through the transparent region into the container (18).

8. The device according to claim 2, characterized in that... A protective container (110) housing at least the optical system (38) and the scanning device (26), wherein the protective container (110) has at least one region (114) in which the protective container is transparent to the measuring beam (24), and wherein the protective container (110) is at least partially disposed in or may be disposed in the container (18).

9. The device according to claim 8, characterized in that, The protective container (110) is liquid-tight or liquid-sealed and has a container wall equipped with a liquid-tight cable sheath (120).

10. The device according to any one of the preceding claims, characterized in that, The optical coherence tomography scanner (22) has a reference arm (48) with adjustable optical path length.

11. The device according to claim 10, characterized in that, The reference arm (48) includes a first sub-arm (82a) having a first optical path length and a second sub-arm (82b) having a second optical path length different from the first optical path length.

12. A method for measuring the surface profile or thickness profile of a workpiece (12) immersed in a liquid (20), comprising the following steps: a) Provide a container (18) filled with liquid (20) that at least partially surrounds the workpiece (12); b) An optical coherence tomography scanner (22) generates a measurement beam (24) and directs the measurement beam toward the workpiece (10) via a scanning device (26) and a focusing and telecentric optical system (38) such that the measurement beam (24) passes through a portion of the liquid (20) while the measurement beam scans the surface of the workpiece (10) at several measurement points in succession. c) The evaluation unit (57) calculates the distance and / or thickness values ​​based on the interference signals provided by the optical coherence tomography scanner (22).

13. The method according to claim 12, wherein the measuring beam (24) passes through a plate (21) that is transparent to the measuring beam (24) on its way to the workpiece (10).

14. The method according to claim 13, wherein the plate (21) is directly adjacent to the liquid (20).

15. The method according to any one of claims 12 to 14, wherein the optical coherence tomography scanner (22) has a reference arm (48) whose optical path length is adjusted prior to measurement according to the optical path length of the measurement beam (24) in the liquid (20).

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