Apparatus and method for measuring surface profiles and thickness profiles of immersed workpieces

Abstract

The invention relates to an apparatus (10) for measuring a surface profile or thickness profile of a workpiece (12) immersed in a liquid (20), the apparatus comprising: a telecentric focusing optical system (38); a scanning device (26) which causes a beam to be deflected in at least one spatial direction; and an optical coherence tomograph (22) which generates a measuring light beam (24) and directs it onto the workpiece (10) via the scanning device (26) and the optical system (38). A control unit (36) controls the scanning device (26) such that the measuring light beam (24) scans the surface (39) of the workpiece (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 tomograph (22).

Description

[0001] Device and method for measuring surface and thickness profiles of immersed workpieces

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a device and a method for measuring surface profiles or thicknesses of immersed glass panes or other workpieces.

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

[0006] Devices based on the principle of optical coherence tomography are known for measuring surface profiles and surface and thickness profiles of workpieces. These devices measure distances to or between scattering or reflecting structures located on or - with sufficient transparency - within the workpiece. If these structures lie on the surface, a surface profile is obtained. If interfaces or other structures lie at different depths in the workpiece, as is the case with transparent coatings or glass panes, the distances between the structures can be directly determined from the reflections on the structures, for example the thickness of a coating or the thickness of a glass pane. A thickness profile of the workpiece can be derived from the correlation of the measured thicknesses with the measurement locations.

[0007] The first devices of this type could only determine the distance for a single measuring point. To scan a surface, the measuring head had to be moved relative to the workpiece (or vice versa). Devices are now used in which a scanning device guides the measuring beam over the workpiece. The applicant markets such devices under the name "flying spot scanners."

[0008] Occasionally, the workpiece needs to be immersed in water or another liquid during measurement. This is particularly useful when the workpiece is being processed in the liquid and the measurement is being performed for the purpose of process monitoring. It would be very cumbersome to remove the workpiece from the liquid after a processing step, dry it before measuring it, and then, if necessary, reinsert it into the liquid for further processing. Wafers for semiconductor production, for example, are often immersed in water while being ground or polished. They should be measured continuously or intermittently to optimally control the grinding or polishing process.

[0009] If one uses the known devices for this purpose, one finds that the measured profile is spatially distorted, the measurements become inaccurate or cannot be carried out at all.

[0010] SUMMARY OF THE INVENTION

[0011] The object of the invention is to provide a device and a method with which surface or thickness profiles of immersed glass panes, wafers and other workpieces can be reliably measured.

[0012] With regard to the device, this object is achieved by a device for measuring a surface or thickness profile of a workpiece immersed in a liquid, which device has a focusing optical system, a scanning device configured to deflect the beam in at least one spatial direction, and an optical coherence tomograph. The coherence tomograph is configured to generate a measuring light beam and to direct it onto the workpiece via the scanning device and the optical system. A control unit is configured to control the scanning device such that the measuring light beam scans the surface of the workpiece successively at several measuring points. An evaluation unit is configured to calculate distance and / or thickness values ​​from interference signals provided by the optical coherence tomograph. According to the invention, the optical system is telecentric.

[0013] The inventors realized that spatial distortions in the measured profiles can be avoided if the optical system is telecentric. The principal rays of the beams converging on the measurement points then all enter the liquid parallel to each other, thus avoiding distortions due to different refraction at the interface with the liquid.

[0014] Generally, the optical system is aligned to the interface in question so that the principal rays enter the liquid perpendicularly. In this case, the liquid merely causes a shift in the focal plane, which is uncritical with a sufficiently large depth of field and can be easily corrected if necessary.

[0015] It has been shown that insufficient intensities of the reflected measuring light can be caused by the absorption of the measuring light in the liquid. When selecting the wavelength of the measuring light, the spectral absorption curve of the liquid should therefore be taken into account. In water and many other liquids, the absorption minimum is at wavelengths around 400 nm, i.e. at the short-wave edge of the visible spectrum, which is why coherence tomographs with white light sources (i.e. wavelengths between 400 and 700 nm) are preferred. However, depending on the liquid and workpiece, the wavelengths can also be longer (up to 1100 nm). For measuring wafers, center wavelengths between 800 nm and 950 nm or between 950 and 1100 nm are more favorable, depending on the material, because shorter-wavelength light can be absorbed too strongly by thicker wafers. The center wavelength is determined here, as usual, using a power-weighted method.

[0016] Due to its significantly higher refractive index compared to air or other gases, the liquid has a significant influence on the optical path length traveled by the measuring light beam in the measuring arm on its way to the workpiece and back to the coherence tomograph. This is irrelevant for thickness measurements, because the thickness is determined from the interference of components reflected at different interfaces of the workpiece. If both components have to travel a longer optical path length due to the liquid, this does not affect the interference.

[0017] When measuring surface profiles where reflection or scattering occurs only at one interface of the workpiece, the measuring light beam must interfere with a reference beam guided in a reference arm of the coherence tomograph. In this case, a different optical path length in the measuring arm, such as that caused by a liquid, affects the measurement because the optical paths traveled in the measuring and reference arms must be similar. If the optical path length in the measuring arm is significantly increased due to the liquid, the optical path length in the reference arm is too short to generate the necessary interference.

[0018] In one embodiment, the optical coherence tomograph therefore has a first reference arm with a first optical path length and a second reference arm with a second optical path length that differs from the first optical path length. The first reference arm and the second reference arm can be interchangeably fastened to a housing of the coherence arm, such that either the first reference arm or the second reference arm is located in the beam path of the coherence tomograph at any one time. In this way, when measuring surface profiles, it is possible to use the device both for measurements without liquid and for measurements with liquid, and / or to vary the amount of liquid significantly when measuring with liquid, as is sometimes necessary for some measuring tasks.

[0019] It's even simpler if the optical coherence tomograph has a reference arm with a variably adjustable optical path length. In this case, the coherence tomograph doesn't need to be reconfigured by exchanging different reference arms; instead, only the existing reference arm needs to be adjusted to adapt the coherence tomograph to different measurement tasks.

[0020] Fiber expansion coils, through which the measurement light is guided, allow for a certain degree of adjustability. Expanding the coil lengthens the optical fiber wound on it, thereby increasing the optical path length. However, only small changes in the optical path length can be achieved this way.

[0021] To achieve larger changes, it is more advantageous if the reference arm comprises a first sub-arm with a first optical path length and a second sub-arm with a second optical path length that differs from the first optical path length. The reference arm can then have, for example, a switch that is designed to guide reference light entering the reference arm either into the first sub-arm or into the second sub-arm. With a switch, the reference light can be directed into one of the two sub-arms without loss. Such a switch can, for example, be a fiber switch or a free-space switch that comprises a moving reflective optical element.

[0022] If losses of reference light are acceptable, this can be distributed between the two sub-arms of the reference arm, e.g., using a fiber coupler or a beam splitter. If a dimming device is then provided in each of the first and second sub-arms, each designed to prevent the propagation of reference light in the respective sub-arm, the required optical path length in the reference arm can be selected by simply controlling the dimming devices.

[0023] In one embodiment, the device has a container in which a holder for the workpiece is accommodated. The container is either made of glass or another transparent material, or it has an opening on its top that allows the measuring light beam to pass through. The container must be liquid-tight to prevent uncontrolled leakage of the liquid. The optical system is arranged so that the measuring light beam can penetrate the container from above through the opening. In the simplest case, the container is a tub with the workpiece mounted at its bottom. The measurement is taken vertically from above into the tub.

[0024] To protect the optical system from the liquid, a transparent disc can be placed in the light path between the optical system and the liquid. This is particularly relevant when the liquid is an etching agent used to process the workpiece, e.g., to thin it, as can be the case with display glass.

[0025] Preferably, the disk is positioned directly adjacent to the liquid. This prevents waves from forming on the liquid, which could disrupt the propagation of the measuring light beam through refraction and possibly diffuse reflection. This is particularly relevant when the numerical aperture of the optical system is small, because then no or very little measuring light may enter the liquid perpendicularly.

[0026] The disc can, for example, be formed directly on the container or on the optical system or attached to it. It is understood that the disc does not have to cover the entire liquid level, but can be provided only in an area through which the measuring light beam passes.

[0027] If the disc is arranged at an angle to a main beam direction of the measuring light beam, it is possible to prevent any significant amount of measuring light from being reflected at the interface of the disc facing the optical system. This prevents the light from being reflected back into the optical coherence tomograph and generating an interfering measurement signal, which, depending on whether distances or thicknesses are being measured, produces an undesirable distance or thickness value. To avoid such interference, angles of a few degrees between the main beam direction and the surface normal of the disc are sufficient, e.g., from 2° to 5°. Even an angle of 10° does not necessarily lead to a noticeable deterioration in telecentricity.

[0028] Instead of performing the measurement from above, the container can have a section on one side wall that is transparent to the measuring light beam. The optical system is then arranged so that the measuring light beam can penetrate the container through the transparent section. In this case, the aforementioned disc is an integral part of the container and automatically adjoins the liquid without having to be positioned at a specific height.

[0029] In another embodiment, the device comprises a protective container in which at least the optical system and the scanning device are accommodated. The protective container has at least one region in which it is transparent to the measuring light beam and is at least partially arranged or can be arranged in the container in which the workpiece is located. Such an arrangement is advantageous, for example, when the immersed workpiece is to be measured from the side and the container does not have a transparent section on a side wall. In this case, the optical system and the scanning device must be immersed in the container in which the workpiece is located. The protective container protects the optical system and the scanning device from the liquid.

[0030] Optimum protection of the optical system and the scanning device is achieved if the protective container is liquid-tightly closed at the top or can be sealed liquid-tight and has a container wall that is provided with a liquid-tight cable entry.

[0031] With regard to the method, the above-mentioned object is achieved by a method for measuring a surface or thickness profile of a workpiece immersed in a liquid, the method comprising the following steps: a) a container is provided which is filled with a liquid which at least partially surrounds a workpiece; b) an optical coherence tomograph generates a measuring light beam and directs it onto the workpiece via a scanning device and an optical system which is focusing and telecentric, such that the measuring light beam passes through part of the liquid while scanning the surface of the workpiece successively at a plurality of measuring points; c) an evaluation unit calculates distance and / or thickness values ​​for the measuring points from interference signals provided by the optical coherence tomograph.

[0032] The advantages and preferred embodiments mentioned above for the device apply accordingly to the method.

[0033] On its way to the workpiece, the measuring light beam preferably passes through a disc that is transparent to the measuring light beam and is preferably directly adjacent to the liquid.

[0034] In particular, the optical coherence tomograph can have a reference arm whose optical path length is adjusted to the optical path length traveled by the measuring light beam in the liquid before a measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0036] Figure 1 shows schematically a measuring device according to a first embodiment of the invention;

[0037] Figure 2 shows a variant of a coherence tomograph for the measuring device shown in Figure 1, in which the reference light is partially guided in an optical fiber;

[0038] Figure 3 shows an embodiment of a coherence tomograph in which the reference arm comprises two partial arms which can be dimmed by dimming devices;

[0039] Figure 4 shows an embodiment of a coherence tomograph in which two partial arms are connected to a fiber optic switch;

[0040] Figures 5a and 5b show an embodiment of a coherence tomograph in which two partial arms are connected to a free-beam switch, in two different positions of the free-beam switch; and

[0041] Figure 6 shows an embodiment in which a measuring head of the coherence tomograph is located in a waterproof protective container.

[0042] DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] 1. First embodiment

[0044] Figure 1 shows a schematic representation of a measuring device according to the invention, designated overall by 10. The measuring device 10 is used to measure a disk-shaped workpiece 12 that is supported by a holder 14. The holder 14 can, for example, be designed as a simple three-point bearing, as indicated in Figure 1 by supports 16. In the illustrated embodiment, the holder 14 is in turn supported on the bottom of a trough-shaped container 18 filled with water 20. A protective glass pane 21 floats on the water 20 and ensures that no waves form when the container 18 moves. In the present embodiment, the holder 14, the container 18, and the protective glass pane 21 belong to the measuring device 10. However, the container 18 can also be part of a conveyor system that successively feeds containers with different workpieces 12 to the measuring device 10.In other embodiments, the container 18 is part of a machine with which the workpiece 12 is ground, polished, etched or otherwise processed.

[0045] The measuring device 10 comprises an optical coherence tomograph 22, which generates a measuring light beam 24 and whose structure is explained in more detail below.

[0046] 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.

[0047] The measuring device 10 also includes an optical system 38, indicated in Figure 2 by three lenses L1, L2, and L3, which has the task of focusing the measuring light beam 24. The optical system 38 is designed telecentrically. As a result, the measuring light beam 24 deflected by the scanning device 26 always exits the optical system 38 parallel to its optical axis OA. Since the optical axis OA is arranged perpendicular to the protective glass pane 21, the measuring light beam 24 always enters the protective glass pane 21 and the water 20 located beneath it perpendicularly. However, a slightly oblique entry into the protective glass pane 21 can be more advantageous, as this prevents or at least reduces unwanted back reflections. Such an oblique entry of the measuring light beam 24 can be achieved by an inclined arrangement of the protective glass pane 21', as indicated by a dashed line in Figure 1.The inclined protective glass pane 2T has a surface normal that is tilted by 3° relative to the main beam direction of the measuring light beam 24. The optical path lengths traveled by the measuring light beam 24 in the protective glass pane 21 and in the water 20 are therefore the same for all scanning directions. Furthermore, the measuring light beam 24 is not deflected due to 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, into which the measuring light beam 24 is focused by the optical system 38.

[0048] The optical coherence tomograph 22 contains a broadband light source 42 with a center wavelength between 800 nm and 950 nm. The coherence tomograph further comprises a first beam splitter 44, which splits 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 the reference light beam 46, and an object arm 50, which comprises the optical system 38 and the scanning device 26 and in which the measurement light beam 24 is guided.

[0049] During a measurement, the measuring light beam 24 propagating in the object arm 50 is focused onto the surface 40 of the workpiece 10 facing the optical system 38, is partially reflected there, 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.

[0050] 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 10 (for example, to the lens L3) at the point of incidence of the measuring light beam 24. If it is to be ensured that the focus 38 is always positioned exactly on the surface 40 of the workpiece 12, regardless of its dimensions, the optical system 38 can contain one or more movable lenses with which the axial position of the focal point 39 can be changed. Details of this and other aspects of the optical coherence tomograph 22 can be found in DE 10

[0051] 2017 128 158 A1.

[0052] The center wavelength of the measuring light beam 24 can be selected such that the measuring light beam is minimally attenuated as it passes twice through the water 20 and into the workpiece 12. A reflection then also occurs on the lower surface 58 of the workpiece 12 facing away from the measuring device 10, 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 workpiece 12 and thus its thickness can be deduced.

[0053] When selecting the thickness measurement mode described above, 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 switching from distance mode to 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 switching back to distance mode, the switchable dimming device 60 clears the path for the reference light beam 46 again.

[0054] 2. Second embodiment - Reference arm with optical fiber

[0055] In the embodiment shown in Figure 1, the measuring light beam 24 propagates entirely in free space. In other embodiments, the light is partially guided in optical fibers.

[0056] Figure 2 shows a section of a coherence tomograph 22, which can be part of the measuring device 10 shown in Figure 1. In this embodiment, the measurement light propagating as a free beam in the reference arm 48 is focused by a first lens 70 onto the end 71 of an optical fiber 72. The measurement light emerging from the other end 74 is collimated by a second lens 76 and directed onto the mirror 52. Since the optical fiber 72 can be wound up, very long optical path lengths can be realized in a small space in the reference arm 48.

[0057] 3. Third embodiment - partial arms with dimming device

[0058] If the measuring device 10 is to be used for surface profile measurements with and without liquid 20, the optical path length in the reference arm 48 must be able to be changed by a larger amount.

[0059] For this purpose, a reference arm 48 can be used, as shown schematically in Figure 3. In this coherence tomograph 22, the light generated by the light source 42 is guided via fiber optics where it propagates as a free beam in the embodiment shown in Figure 2, and vice versa. Above all, the reference arm 48 contains a fiber optic 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 both sub-arms 82a, 82b, the light is partially guided again as a free beam, with lenses 84, 86 each creating a section in which the reference light is collimated.In the upper arm 82a, in the relevant section, there is a glass body 85, through which the reference light travels an optical path length that approximately corresponds to the optical path length traveled by the measuring light in the water 20. Instead of the glass body 85, another medium can of course also be used, e.g., water located in a transparent container.

[0060] Furthermore, a dimming device 88 is arranged in each sub-arm 82a, 82b, which can be used to prevent the propagation of the reference light in the sub-arms 82a, 82b. By appropriately controlling the dimming devices 88, it can be ensured that the reference light can propagate either only in the first sub-arm 82a or only in the second sub-arm 82b. In this way, unwanted interference of the measuring light with light guided in the "wrong" sub-arm is avoided. If the measuring device 10 is used to measure a surface profile without water, the dimming devices 88 are controlled so that the reference light propagates only in the second sub-arm 82b.If a measurement is to be carried out with a workpiece 12 immersed in water 20, the two dimming devices 88 exchange their roles so that the reference light propagates only in the first partial arm 82a, whose optical path length is significantly longer and is adapted to the effect of the water 20.

[0061] 4. Fourth embodiment - partial arms with switch

[0062] The embodiment shown in Figure 4 largely corresponds to that of Figure 3. In Figure 4, however, the reference light is not distributed evenly between the two partial arms 82a, 82b by a passive fiber optic beam splitter 80, but by an active fiber optic switch 90. In this way, no reference light is lost, which enters the wrong partial arm 82a or 82b and has to be dimmed there.

[0063] Instead of the fiber optic switch 90, a free-beam switch 100 can also be used, as illustrated in Figures 5a and 5b. The free-beam switch 100 comprises a moving reflective optical element, which in the illustrated embodiment is formed by a deflection prism 102 and can be moved back and forth with the aid of an actuator 104 selectively between a passive position outside the beam path (see Figure 5a) and an active position within the beam path (see Figure 5b).

[0064] If the deflection prism 102 is in its passive position outside the beam path, the reference light enters the second partial arm 82b. If the deflection prism 102 is moved to the active position with the aid of the actuator 104, the reference light is deflected by 90° and enters the first partial arm 82a, where it passes through the glass body 85 (see Figure 5b). 5. Fifth Embodiment - Protective Container

[0065] In the embodiment shown in Figure 1, the container 18 has an opening on its top side, which is covered by the protective glass pane 21. The optical system 38 of the measuring device 10 is arranged such that the measuring light beam 24 can penetrate into the container 18 from above through the opening.

[0066] For certain measuring tasks, the container 18 and the location where the workpiece 12 is held cannot be freely selected. For example, if the workpiece 12 is attached to a side wall of the container 18 and the container is made of a material that is opaque to the measuring light beam 24, as illustrated in Figure 6, the measurement cannot be performed from above.

[0067] In such configurations, it is more advantageous if the measuring device 10 comprises a protective container 110 in which at least the optical system 38 and the scanning device 26 are accommodated. The protective container 110 has a side wall 112 with a glass insert 114 that is transparent to the measuring light beam 24. The optical system 38 and the scanning device 26 can be combined to form a measuring head with its own housing 116. Such a protective container 110 can be immersed in the larger container 18, so that a measurement can also be performed with a horizontal optical axis OA.

[0068] The protective container 110 can also be completely immersed in the water 20 if, as shown in Figure 6, it has a lid 118 with which it can be closed liquid-tight at the top. The wall of the protective container 110 then only needs to have a sealed cable duct 120 to route a cable 122 for the measuring light and control signals to a control unit 124, which contains, among other things, the reference arm 48, the light source 42, and the detector 56.

Claims

PATENT CLAIMS 1. A device (10) for measuring a surface or thickness profile of a workpiece (12) immersed in a liquid (20), comprising a focusing optical system (38), a scanning device (26) configured to deflect the beam in at least one spatial direction, an optical coherence tomograph (22) configured to generate a measuring light beam (24) and to direct it onto the workpiece (10) via the scanning device (26) and the optical system (38), a control unit (36) configured to control the scanning device (26) such that the measuring light beam (24) successively scans the surface (39) of the workpiece (10) at a plurality of measuring points, and an evaluation unit (57) configured to calculate distance and / or thickness values from interference signals provided by the optical coherence tomograph (22), characterized in that the optical system (38) is telecentric.

2. Device according to one of the preceding claims, characterized by a container (18) in which a holder (14) for the workpiece (12) is accommodated.

3. Device according to claim 2, characterized in that the container (18) has an opening on its upper side and the optical system (38) is arranged so that the measuring light beam (24) can penetrate into the container (18) from above through the opening.

4. Device according to claim 3, characterized in that a disc (21) transparent to the measuring light beam (24) is arranged in the light path between the optical system (38) and the liquid (20).

5. Device according to claim 4, characterized in that the disc (21) is directly adjacent to the liquid (20).

6. Device according to claim 4 or 5, characterized in that the disc (21) is arranged inclined to a main beam direction of the measuring light beam.

7. Device according to claim 2, characterized in that the container (18) has an area on a side wall in which it is transparent to the measuring light beam (24), and that the optical system (38) is arranged so that the measuring light beam (24) can penetrate into the container (18) through the transparent area.

8. Device according to claim 2, characterized by a protective container (110) in which at least the optical system (38) and the scanning device (26) are accommodated, wherein the protective container (110) has at least one region (114) in which it is transparent to the measuring light beam (24), and wherein the protective container (110) is at least partially arranged in the container (18) or can be arranged therein.

9. Device according to claim 8, characterized in that the protective container (110) is closed or can be closed in a liquid-tight manner at the top and has a container wall which is provided with a liquid-tight cable leadthrough (120).

10. Device according to one of the preceding claims, characterized in that the optical coherence tomograph (22) has a reference arm (48) whose optical path length is variably adjustable.

11. Device according to claim 10, characterized in that the reference arm (48) comprises a first partial arm (82a) with a first optical path length and a second partial arm (82b) with a second optical path length which differs from the first optical path length.

12. A method for measuring a surface or thickness profile of a workpiece (12) immersed in a liquid (20), comprising the following steps: a) a container (18) is provided which is filled with a liquid (20) which at least partially surrounds a workpiece (12); b) an optical coherence tomograph (22) generates a measuring light beam (24) and directs it onto the workpiece (10) via a scanning device (26) and an optical system (38) which is focusing and telecentric, such that the measuring light beam (24) passes through part of the liquid (20) while scanning the surface of the workpiece (10) successively at a plurality of measuring points; c) an evaluation unit (57) calculates distance and / or thickness values from interference signals provided by the optical coherence tomograph (22).

13. Method according to claim 12, wherein the measuring light beam (24) passes through a disc (21) transparent to the measuring light beam (24) on its way to the workpiece (10).

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

15. Method according to one of claims 12 to 14, wherein the optical coherence tomograph (22) has a reference arm (48) whose optical path length is adapted, before a measurement, to the optical path length traveled by the measuring light beam (24) in the liquid (20).

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