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DE-102024132847-A1 - Method and device for measuring transparent multilayer objects with a coherence tomograph

DE102024132847A1DE 102024132847 A1DE102024132847 A1DE 102024132847A1DE-102024132847-A1

Abstract

In a method for measuring multilayer objects (30) with a coherence tomograph (11), measuring light is directed from an optical system (25) in an object arm (18) onto a surface of the object (30). The interference of measuring light (OL), which is reflected at an optical interface (40, 42, 44) of the object (30), with measuring light (RL), which is guided in the reference arm (16), is detected by a detector (36) and converted into electrical signals. A spectrum is generated from this by Fourier transformation, with each interference being represented by a distance peak (D1, D2, D3) in the spectrum. In a process automatically controlled by a control unit (39), this measurement is repeated several times, with the distance between the object (30) and a reference plane (52) being changed at each repetition. The axial position of the reference plane corresponds to an optical path length of the reference arm (18). By analyzing the distance peaks (D1, D2, D3) and preferably thickness peaks, which represent a distance between interfaces, a unique assignment between the distance peaks and the interfaces (40, 42, 44) is determined, so that distance values can be assigned to these interfaces.

Inventors

  • Lucas Bialowons
  • Simon Mieth

Assignees

  • PRECITEC OPTRONIK GMBH

Dates

Publication Date
20260513
Application Date
20241111

Claims (16)

  1. Method for measuring multilayer objects (30) with a coherence tomograph (11) having an object arm (18) and a reference arm (16), the method comprising the following steps: a) measuring light generated by the coherence tomograph (11) is directed in the object arm (18) by an optical system (25) onto a point on the surface of the object (30); b) measuring light (OL) reflected at an optical interface (40, 42, 44) of the object (30) passes through the optical system (25) back into the coherence tomograph (11) and interferes there with measuring light (RL) guided in the reference arm (16); c) the interference obtained in step b) is detected by a detector (36) of the coherence tomograph (11) and converted into electrical signals; d) A spectrum is generated from the electrical signals obtained in step c) by Fourier transformation, wherein each interference obtained in step b) is represented by a distance peak (D1, D2, D3) in the spectrum; characterized by the following further steps: e) In a process automatically controlled by a control device (39), steps a) to d) are repeated multiple times, wherein at each repetition a distance between the object (30) and a reference plane (52) is changed, which is located in an object arm (18) of the coherence tomograph (11) and whose axial position corresponds to an optical path length of the reference arm (18); f) An evaluation unit (38) determines, by analyzing the distance peaks (D1, D2, D3) obtained in step e) and preferably thickness peaks representing a distance between interfaces, a unique assignment between the distance peaks and the optical interfaces (40, 42, 44) and assigns distance values to the optical interfaces on the basis of this assignment.
  2. Procedure according to Claim 1 , characterized in that in step f) the evaluation device also determines a unique assignment between the thickness peaks corresponding to a distance between two optical interfaces (40, 42, 44) and the distances between the optical interfaces through the analysis and assigns thickness values to the distances between the interfaces on the basis of this assignment.
  3. Procedure according to Claim 1 or 2 , characterized in that in step e) for thickness peaks (T1, T2, T1 + T2) in the spectrum, which correspond to a distance between two optical interfaces (40, 42, 44), it is determined whether there is a pair of distance peaks (D1, D2, D3) which each correspond to a distance to one of the two optical interfaces.
  4. Method according to one of the preceding claims, characterized in that in step e) it is determined whether, in addition to a thickness peak (T1, T2, T1 + T2) corresponding to a distance between two optical interfaces, there is another thickness peak (2xT1) corresponding to a multiple of the distance.
  5. Method according to one of the preceding claims, characterized in that the evaluation unit (38) determines the position of the reference plane (52) relative to the object (30) in step f).
  6. Procedure according to Claim 5 , characterized in that in further measurements the object (30) is arranged relative to the reference plane (52) such that the reference plane (52) is always located outside the object (30).
  7. Method according to one of the preceding claims, characterized in that in step e) the distance between the object (30) and the reference plane (52) is changed by moving the object (30) with the aid of a lifting table (33) controlled by the control device (39).
  8. Method according to one of the preceding claims, characterized in that the measuring light (OL) is directed to a plurality of different points, and that at the majority of these points steps e) and f) are not carried out, but rather modified distance values are assigned to the optical interfaces (40, 42, 44) on the basis of a previously determined assignment, taking into account shifts of the distance peaks (D1, D2, D3).
  9. Method according to one of the preceding claims, characterized in that the optical system (25) focuses the measuring light (OL) in a focus (54) and, prior to step a), determines in an automatically controlled pre-adjustment by the control device (39) at which position of the focus (54) relative to the object (30) the distance peaks (D1, D2, D3) and/or thickness peaks, corresponding to a distance between two optical interfaces (40, 42, 44), stand out most clearly from a noise background (56).
  10. Procedure according to Claim 9 , characterized in that during pre-adjustment the position of the focus (54) relative to the object (30) changed and the intensity of at least one distance or thickness peak is measured.
  11. Procedure according to Claim 10 , characterized in that the position of the focus (54) relative to the object (30) is changed by moving the object (30) using a lifting table (33).
  12. Procedure according to one of the Claims 9 until 11 , characterized in that a camera (58) measures the position of the focus (54) relative to the object (30).
  13. Procedure according to Claim 12 , characterized in that, based on the position of the focus (54) relative to the object (30) measured by the camera (58), a coarse adjustment is carried out in which the position of the focus relative to the object is changed, and that afterwards a fine adjustment is carried out in which the position of the focus relative to the object is changed again and the height of at least one distance or thickness peak is measured.
  14. Procedure according to one of the Claims 12 or 13 , characterized in that the camera (58) is a stereo camera with a camera optic whose optical axis is arranged at an angle between 5° and 90° to an optical axis of the optical system (25).
  15. Procedure according to one of the Claims 9 until 14 , characterized in that during pre-adjustment an inclination angle of a specimen support (35) on which the object (30) is placed is changed.
  16. Device for measuring multilayer objects, comprising a) an optical system (25), b) a coherence tomograph (11) having a detector (36), an object arm (28), and a reference arm (16), and configured to generate measurement light (OL) and direct it in the object arm (18) via the optical system (25) to a point on the surface of the object (30), wherein the detector (36) is configured to detect interference between measurement light (OL) reflected at an optical interface (40, 42, 44) of the object (30) and returned to the coherence tomograph (11) via the optical system (25) and measurement light (RL) reflected in the reference arm (16), and to convert the detected interference into electrical signals, c) an evaluation unit (38) configured to derive electrical signals from the electrical signals generated by the detector (36) by Fourier transformation to generate a spectrum in which each detected interference is represented by a distance peak (D1, D2, D3) in the spectrum, d) a control device (39) configured to perform multiple measurements in which a distance between the object (30) and a reference plane (52) located in an object arm (18) of the coherence tomograph (11) is changed and whose axial position corresponds to an optical path length of the reference arm (16), wherein the evaluation unit (38) is configured to make a unique mapping between the distance peaks and the optical interfaces by analyzing the obtained distance peaks (D1, D2, D3) and to assign distance values to the optical interfaces (40, 42, 44) on the basis of this mapping.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention The invention relates to a method and a device for measuring transparent multilayer objects using SD OCT, where SD is the acronym for spectral domain and OCT is the acronym for optical coherence tomography. 2. Description of the state of the art For measuring the surface profiles of workpieces and other objects, devices based on the principle of SD OCT have been increasingly used for several years. These devices measure the distance to a scattering or at least partially reflective surface of the object without contact at individual measuring points. For this purpose, the interference of light reflected from the surface with light guided in a reference arm of the OCT is detected and spectrally analyzed. Repeating this measurement at a large number of measuring points yields a surface profile of the object. If the object is sufficiently transparent to the measuring light, the distances to internal optical interfaces can also be measured with high accuracy. Such optical interfaces typically exist in multilayer objects, e.g., bonded wafers or coated workpieces. If components of the measuring light reflected from two different interfaces interfere, the coherence tomograph measures the distance between the two interfaces, i.e., the thickness of one of several wafers or a coating. If the light propagation is temporarily blocked in the reference arm, only thicknesses, and not distances, are measured. By combining distance and thickness measurements, it is possible, for example, to check whether the surface of a coated workpiece has the desired shape and whether the thickness of the coating is constant everywhere. The first devices of this type could only determine the distance and/or thickness for a single measuring point. To scan an area, the workpiece had to be moved relative to the measuring device (or vice versa). From the DE 10 2022 104 416 A1 A device is known in which a scanning unit guides the measuring beam over the workpiece. The applicant markets such devices under the name "Flying Spot Scanner". SD OCT measuring devices generate raw measurement data in the form of interference spectra, from which the desired distance or thickness values are derived by Fourier transformation. Each distance or thickness value corresponds to a peak in the transformed spectrum, which can be displayed on a screen. For example, when using an SD OCT measuring device to measure distances and thicknesses of a glass plate, three peaks are displayed when using the reference arm: two peaks for the distances to the two interfaces of the plate and one peak for its thickness. If the glass plate is moved parallel to the direction of the measuring beam, the two distance peaks move, while the thickness peak remains stationary, since the thickness of the glass plate does not change with movement in this direction. In this way, the displayed peaks can usually be easily assigned to the quantities being measured. If the light propagation in the OCT's reference arm is interrupted, only the thickness peak is displayed, while the two distance peaks disappear. This allows the thickness peaks to be clearly identified. However, even with only two interfaces, the spectrum obtained through the Fourier transform can sometimes become unclear, namely when multiple reflections occur. Multiple reflections occur when light that has already been reflected once at a first interface is reflected again at another interface on its way back to the coherence tomograph. For example, with a glass plate with an upper and a lower interface, light reflected at the lower interface can be reflected from the inside at the upper interface, directed back towards the lower interface, and then reflected again towards the coherence tomograph. Although the light, reflected a total of three times, usually has only a low intensity, it can still, after interference with the strong signal from the reference arm, lead to a distance peak visible in the spectrum, which appears to represent an interface at a greater distance. With interfaces that strongly reflect the measurement light, multiple reflections with five or seven reflections visible in the spectrum can even result. If an object consisting of two layers is to be measured, using the reference arm, even without multiple reflections, results in six peaks that must be assigned to the quantities to be measured: three distance peaks, one thickness peak for each layer, and another thickness peak representing the total thickness of the layer system. For a layer system consisting of three layers, four distance and thickness peaks are generated. six thickness peaks, etc. If additional peaks are then added, caused by the aforementioned multiple reflections, it becomes difficult or even impossible for an operator to keep track and assign the peaks to the correct interfaces. Additional problems can arise during evaluation if thickness and distance peaks overlap completely or partial