US-20260126284-A1 - METHOD AND DEVICE FOR DETERMINING A LOCAL HEIGHT OF A SAMPLE SURFACE

US20260126284A1US 20260126284 A1US20260126284 A1US 20260126284A1US-20260126284-A1

Abstract

A method for determining a local height of a location on a sample surface. A structured light microscope for determining a local height of a location on a sample surface. A computer readable data carrier including a computer program that, when run on a processor of an structured light microscope according to the disclosure, causes the structured light microscope to perform the method according to the disclosure.

Inventors

Ruslan Akhmedovich SEPKHANOV
NITISH KUMAR

Assignees

MITUTOYO CORPORATION

Dates

Publication Date: 20260507
Application Date: 20251105
Priority Date: 20241106

Claims (15)

1 . A method for determining a local height of a location on a sample surface, wherein the method includes: projecting an optical pattern on the sample surface; moving the sample surface relative to a structured light microscope between multiple measurement positions along an optical axis of the structured light microscope; obtaining a contrast through-focus relation associated with the location of the sample surface by measuring a contrast of the optical pattern on the location at each of the multiple measurement positions; determining a local tilt angle of the sample surface at the location based on comparing the obtained contrast through-focus relation to multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the comparison, wherein each of the calibrated contrast through-focus relations is associated with a tilt angle; obtaining a height correction for the location based on the obtained local tilt angle; and obtaining the local height by determining an in-focus position of the optical pattern along the optical axis and correcting with the height correction.
2 . The method according to claim 1 , wherein the multiple calibrated contrast through-focus relations, associated tilt angles and associated height corrections are obtained by: measuring, with the structured light microscope at each of the multiple measurement positions, a contrast of an optical pattern on a flat test surface having a known height to obtain the multiple calibrated contrast through-focus relations, wherein different calibrated contrast through-focus relations are obtained by positioning the test surface at different tilt angles relative to the optical axis; obtaining, for each of the calibrated contrast through-focus relation, the associated height correction by determining the height of the flat test surface based on the associated calibrated contrast through-focus relation and calculating the difference of the known height of the flat test surface from the determined height; and storing the calibrated contrast through-focus relations, associated tilt angles and associated height corrections.
3 . The method according to claim 1 , wherein the multiple calibrated contrast through-focus relations, associated tilt angles and associated height corrections are obtained by: measuring, with the structured light microscope at each of the multiple measurement positions, a contrast of an optical pattern on a test surface having a known three-dimensional shape with a known height profile to obtain the multiple calibrated contrast through-focus relations, wherein different calibrated contrast through-focus relations are obtained by positioning the test surface at different lateral positions relative to the optical axis; obtaining, for each of the calibrated contrast through-focus relation, the associated height correction by determining the height of the flat test surface based on the associated calibrated contrast through-focus relation and calculating the difference of the height of the test surface from the determined height; and storing the calibrated contrast through-focus relations, associated tilt angles and associated height corrections.
4 . The method according to claim 1 , wherein the comparing the obtained contrast through-focus relation to multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the comparison includes determining a similarity between the obtained contrast through-focus relation and each of the multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the determined similarity.
5 . The method according to claim 4 , wherein the similarity between the obtained contrast through-focus relation and the multiple calibrated contrast through-focus relations is based on contrast through-focus curves that are associated with the respective contrast through-focus relations.
6 . The method according to claim 4 , wherein the determining the local tilt angle of the sample surface at the location includes calculating a weighted average of a first tilt angle associated with the calibrated contrast through-focus relation having the highest similarity with the obtained contrast through-focus relation and a second tilt angle, wherein the second tilt angle is neighbouring the first tilt angle and is associated with the calibrated contrast through-focus relation having the second highest similarity of calibrated contrast through-focus relations associated with tilt angles neighbouring the first tilt angle, wherein the weights of the weighted average are based on the determined similarities.
7 . The method according to claim 6 , wherein the determining the height correction includes calculating a weighted average of the height corrections associated with the first and second tilt angles, wherein the weights of the weighted average are based on the determined similarities.
8 . A structured light microscope for determining a local height of a location on a sample surface, the microscope comprising: a projector for projecting an optical pattern on the sample surface; an optical sensor for obtaining a contrast of the optical pattern projected on the sample surface; a sample holder for holding the sample surface at different measurement positions relative to the optical sensor along an optical axis of the structured light microscope; and a processor functionally connected to the optical sensor, sample holder and projector, wherein the processor is configured for: obtaining a contrast through-focus relation associated with the location of the sample surface based on a contrast, measured by the optical sensor at each of the multiple measurement positions, of the optical pattern, projected on the sample surface by the projector; determining a local tilt angle of the sample surface at the location based on comparing the obtained contrast through-focus relation to multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the comparison, wherein each of the calibrated contrast through-focus relations is associated with a tilt angle; obtaining a height correction for the location based on the obtained local tilt angle; and obtaining the local height by determining an in-focus position of the optical pattern along the optical axis and correcting with the height correction.
9 . The structured light microscope according to claim 8 , wherein the structured light microscope is configured for obtaining the multiple calibrated contrast through-focus relations, associated tilt angles, and associated height corrections by: measuring, with the with the optical sensor at each of the multiple measurement positions, a contrast of an optical pattern on a flat test surface having a known height to obtain the multiple calibrated contrast through-focus relations, wherein different calibrated contrast through-focus relations are obtained by positioning the test surface at different tilt angles relative to the optical axis with the sample holder; obtaining, by the processor and for each of the calibrated contrast through-focus relation, the associated height correction by determining the height of the flat test surface based on the associated calibrated contrast through-focus relation and calculating the difference of the known height of the flat test surface from the determined height; and storing, by the processor, the calibrated contrast through-focus relations, associated tilt angles and associated height corrections.
10 . The structured light microscope according to claim 8 , wherein the structured light microscope is configured for obtaining the multiple calibrated contrast through-focus relations, associated tilt angles, and associated height corrections by: measuring, with the optical sensor at each of the multiple measurement positions, a contrast of an optical pattern on a test surface having a known three-dimensional shape with a known height profile to obtain the multiple calibrated contrast through-focus relations, wherein different calibrated contrast through-focus relations are obtained by positioning, with the sample holder, the test surface at different lateral positions relative to the optical axis; obtaining, by the processor and for each of the calibrated contrast through-focus relation, the associated height correction by determining the height of the flat test surface based on the associated calibrated contrast through-focus relation and calculating the difference of the height of the test surface from the determined height; and storing, by the processor, the calibrated contrast through-focus relations, associated tilt angles and associated height corrections.
11 . The structured light microscope according to claim 8 , wherein the processor is further configured for comparing the obtained contrast through-focus relation to multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the comparison includes determining a similarity between the obtained contrast through-focus relation and each of the multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the determined similarity.
12 . The structured light microscope according to claim 11 , wherein the processor is configured for basing the similarity between the obtained contrast through-focus relation and the multiple calibrated contrast through-focus relations on contrast through-focus curves that are associated with the respective contrast through-focus relations.
13 . The structured light microscope according to claim 11 , wherein the processor is further configured for determining the local tilt angle of the sample surface at the location by calculating a weighted average of a first tilt angle associated with the calibrated contrast through-focus relation having the highest similarity with the obtained contrast through-focus relation and a second tilt angle, wherein the second tilt angle is neighbouring the first tilt angle and is associated with the calibrated contrast through-focus relation having the second highest similarity of calibrated contrast through-focus relations associated with tilt angles neighbouring the first tilt angle, wherein the weights of the weighted average are based on the determined similarities.
14 . The structured light microscope according to claim 11 , wherein the processor is further configured for determining the height correction by calculating a weighted average of the height corrections associated with the first and second tilt angles, wherein the weights of the weighted average are based on the determined similarities.
15 . A non-transitory computer readable medium including a computer program that, when run on a processor of a structured light microscope according to claim 8 , causes the structured light microscope to perform a method comprising: projecting an optical pattern on the sample surface; moving the sample surface relative to a structured light microscope between multiple measurement positions along an optical axis of the structured light microscope; obtaining a contrast through-focus relation associated with the location of the sample surface by measuring a contrast of the optical pattern on the location at each of the multiple measurement positions; determining a local tilt angle of the sample surface at the location based on comparing the obtained contrast through-focus relation to multiple calibrated contrast through-focus relations and selecting one of the calibrated contrast through-focus relations based on the comparison, wherein each of the calibrated contrast through-focus relations is associated with a tilt angle; obtaining a height correction for the location based on the obtained local tilt angle; and obtaining the local height by determining an in-focus position of the optical pattern along the optical axis and correcting with the height correction.

Description

FIELD OF THE INVENTION The current inventions relates to a method for determining a local height of a location on a sample surface. The invention further relates to a structured light microscope for determining a local height of a location on a sample surface. The invention further relates to a computer readable data carrier comprising a computer program that, when run on a processor of an structured light microscope according to the invention, causes the structured light microscope to perform the method according to the invention. BACKGROUND OF THE INVENTION A structured light microscope may project an optical pattern onto a sample surface and measure a contrast thereof at a location of the sample surface when the sample surface is at different measurement positions relative to the microscope along an optical axis thereof to obtain a contrast through-focus relation. A maximum of the contrast through-focus relation may be associated to a local height of the location of the sample surface. A height map of the sample surface may be obtained by performing these measurements for multiple locations of the sample surface. Optical systems are subject to aberrations. Aberrations may be defined as deviations of the wavefront from an ideal shape. In a structured light microscope, aberrations may distort the image in lateral directions, e.g. perpendicular to the optical axis, the aberrations may also distort a shape of the obtained contrast through-focus relation which may displace its maximum. Thus, aberrations lead to measurement errors in the height measurements. Generally, the effect of the aberrations may be minimized by using multiple optical surfaces. In the case of structured light microscope, minimizing the effect of the aberrations is complex because the sample has a three-dimensional shape. The way the sample reflects the optical pattern depends on the shape of the sample and affects the height measurement via the aberrations of the system. For example, this may result in obtaining a non-flat height map for a flat sample surface. OBJECT OF THE INVENTION It is an object of the invention to provide a method for reducing the effect of aberrations on a height map obtained by a structured light microscope. SUMMARY OF THE INVENTION The object of the invention is achieved by the method according to claim 1. The method of the invention allows for determining a local height of a location on a sample surface. The local height may be a relative local height, e.g. with respect to another location on the sample surface, e.g. a baseline point. By collecting multiple local heights of multiple locations on the sample surface a height map of the sample surface may be obtained. For each location on the sample surface, the associated local height may be obtained using the method of the invention. The location of the sample surface may also be called a sample surface location herein. The sample surface location may be associated with a pixel or a group of pixels of the structured light microscope. The sample surface may be a surface of a semiconductor or an optical element. The sample surface may be a product obtained from a production process. The method of the invention may be used in a quality control of the production process. The method comprises projecting an optical pattern on the sample surface and moving the sample surface relative to a structured light microscope between multiple measurement positions along an optical axis of the structured light microscope. The structured light microscope may include a projector for projecting the optical pattern and an optical sensor to measure a light reflected of the sample surface. The sample surface may be held in a sample holder during measurement. The sample surface may be moved between the multiple measurement positions. The structured light microscope, or a portion thereof such as the optical sensor or a lens of the optical sensor, may be moved relative to the sample surface between the different measurement positions. An effective distance between the sample surface and the optical sensor may be different in the different measurement positions. Since the measured contrast or focus of the optical pattern projected on the sample surface depends on the effective distance, the measured contrast may be different between the different measurement positions. A maximum contrast, i.e. when the optical pattern is in focus, of the optical pattern may be related to a height of the sample surface. The method includes obtaining a contrast through-focus relation associated with the sample surface location by measuring the contrast of the optical pattern on the location at each of the measurement positions. The contrast through-focus relation may relate the effective distance to the measured contrast. The contrast through-focus relation may be a list of data points, wherein each data point includes an effective distance and an associated contrast. The contrast through-focus relation may a