EP-4742156-A2 - METHOD FOR COMBINING HEIGHT MAPS AND PROFILOMETER FOR THE SAME

EP4742156A2EP 4742156 A2EP4742156 A2EP 4742156A2EP-4742156-A2

Abstract

Inventors

SEPKHANOV, RUSLAN AKHMEDOVICH

Assignees

Mitutoyo Corporation

Dates

Publication Date: 20260513
Application Date: 20231207

Claims (12)

Method for measuring a first height map and a second height map of a sample surface of a sample with a profilometer and combining the first and second height maps to a composite height map, the method comprising: - measuring the first and second height maps with a sensor of the profilometer, wherein the sensor is moved relative to the sample surface so that the first and second height maps are measured in partially overlapping fields of view of the sensor; - combining the first and second height maps to produce the composite height map of the sample surface, characterized in, that the combining of the first and second height maps comprises: - classifying features of the first and second height maps into sharp features and smooth features; - determining, for multiple subregions of the first and second heigh maps, a similarity between the respective subregion of the first and second height maps by using a weighted similarity measure between the first and second height maps in the respective subregion wherein the weights are based on the classification of the features into the sharp features and smooth features; - comparing the similarities between the first and second height map in the multiple subregions; and - combining the first and second height maps by stitching the first and second height maps in one of the subregions based on the comparing of the similarities, e.g. based on the one subregion having a higher similarity, e.g. in the one subregion having the highest similarity.
Method according to claim 1, wherein the method further comprises: - determining a plurality of overlap regions of the first and second height maps by shifting the first and second height maps subsequently relative to each other, and wherein preferably a current overlap region has a different surface area than a preceding overlap region; - determining, after each shift of the first and second height maps, a similarity between the first and second height maps by determining a similarity between the heights of the first and second height map in the respective overlap region by using the similarity measure; - comparing the plurality of similarities by normalizing the similarities based on the respective surface areas of the overlap regions; and - combining the first and second height maps by stitching the first and second height maps in one of the overlap regions based on the comparing of the plurality of similarities, e.g. based on the one overlap region having a higher similarity, e.g. in the one overlap region having the highest similarity.
Method according to one or more of the preceding claims, wherein the sensor comprises multiple pixels, and wherein the first and second height maps are shifted relative to each other by a single pixel of the sensor, and preferably wherein the first overlap region has a surface area corresponding to a single pixel of the sensor.
Method according to one or more of the preceding claims, wherein the similarity measure is based on a correlation for determining a correlation between the heights of the first and second height map in the respective overlap region and wherein the first and second height maps are combined based on the correlations.
Method according to claim 4, wherein the correlations are determined based on a zero-normalized cross-correlation function: C = 1 N 1 sV 1 sW ∑ V i − meanV W i − meanW wherein N is the surface area of the respective overlap region, sV is the standard deviation of heights in the respective overlap region of the first height map, sW is the standard deviation of heights in the respective overlap region of the second height map, V i are the heights in the respective overlap region of the first height map, W i are the heights in the respective overlap region of the second height map, meanV is the mean height in the respective overlap region of the first height map, and meanW is the mean height in the respective overlap region of the second height map.
Method according to one or more of the preceding claims, wherein the sharp features have higher weights than the smooth features.
Method according to one or more of the preceding claims, wherein the features are classified based on a gradient thereof and/or wherein the weights are based on the gradients.
Method according to one or more of the preceding claims, wherein the method further comprises: - determining one or more flat regions in the first and/or second height maps; - determining modified first and second height maps by providing a variation, e.g. a random variation, in the height values of the flat regions; and - determining the similarity, e.g. correlations, between the first and second height maps in the first overlap region using the similarity measure.
Method according to claim 8, wherein the one or more flat regions are determined based on a classification of features in the height maps, e.g. wherein flat regions are regions having substantially flat features, e.g. having a gradient below a predetermined threshold value.
Profilometer, for example an optical profilometer, comprising a sensor having pixels for measuring a first height map and a second height map of a sample surface of a sample, and a processor configured for performing the method according to one or more of the preceding claims, wherein the profilometer preferably comprises a sample holder for holding the sample.
Profilometer according to claim 10, wherein the processor is configured for: - causing the sensor to measure the first and second height maps, wherein the sensor is moved relative to the sample surface so that the first and second height maps are measured in partially overlapping fields of view of the sensor; - combining the first and second height maps to produce the composite height map of the sample surface; and - outputting a composite height map of the sample surface based on the combined first and second height maps, wherein the combining of the first and second height maps comprises: - classifying features of the first and second height maps into sharp features and smooth features; - determining, for multiple subregions of the first and second heigh maps, a similarity between the respective subregion of the first and second height maps by using a weighted similarity measure between the first and second height maps in the respective subregion wherein the weights are based on the classification of the features into the sharp features and smooth features; - comparing the similarities between the first and second height map in the multiple subregions; and - combining the first and second height maps by stitching the first and second height maps in one of the subregions based on the comparing of the similarities, e.g. based on the one subregion having a higher similarity, e.g. in the one subregion having the highest similarity.
Digital data carrier comprising software for, when run on a processor of a profilometer according to one or more of the claims 10 - 11, causes the profilometer to perform the method according to one or more of the claims 1 - 9.

Description

The invention relates to a method for measuring a first height map and a second height map of a sample surface with a profilometer and combining the first and second height maps to a composite height map. The invention further relates to a profilometer configured for measuring a first height map and a second height map and combining the first and second height maps to a composite height map. Known methods of obtaining a composite height map comprise measuring first and second height maps and choosing a subsurface, also known as template, of at least one of the height maps which is then moved over, a subsurface of, the other one of the height maps to determine a similarity between the two subsurfaces. Generally, the subsurfaces may be chosen based on the presence of recognizable features which may be used to correlate the two subsurfaces, e.g. such as in US8447561B2. In other words, known methods rely on the determining of recognizable features of the height maps and matching these features between them. The known methods have a number of problems which the invention aims to alleviate. In particular, the one or more templates have to be chosen such that the height maps overlap in the region of the templates. In order to prevent incorrect combining of the height maps, e.g. due to the presence of similar features on the height maps, multiple templates may have to be tested. The first aspect of the invention aims to overcome the above mentioned problem. The invention also aims to provide an alternative method to determine an overlap region and thus to obtain a composite height map by stitching the height maps. The aim of the first aspect of the invention is achieved by the method according to claim 1. The invention relates to a method for measuring a first height map and a second height map of a sample surface of a sample with a profilometer, for example an optical profilometer, and combining the first and second height maps to a composite height map. The profilometer may be any suitable profilometer which allows to measure multiple fields of view of a sample surface to obtain two or more height maps which may need to be combined to obtain a composite height map of the sample surface. The first and second height map may be two height maps of a plurality of height maps of the sample surface. For example, the obtained composite height map may further be combined with a third height map to obtain a bigger composite height map. The method comprises measuring the first and second height maps with a sensor of the profilometer. The sensor maybe an optical sensor, however any suitable type of sensor may be used. The sensor may comprise multiple pixels for obtaining pixel data related to the height of the sample surface in a field of view of the sensor. The sensor is moved relative to the sample surface, for example by moving a holder for holding the sample surface relative to an objective of the sensor, so that the first and second height maps are measured in partially overlapping fields of view of the sensor. As a result, the first and second height map comprise a region which contains the same height information and along which region the first and second height maps maybe stitched to obtain a composite height map of, part of, the sample surface. The method further comprises combining the first and second height maps to produce the composite height map of the sample surface. The combining of the height maps comprises determining a first overlap region of the first and second height maps by partially overlapping the first and second height maps, in which first overlap region the first and second height maps overlap. In contrast to known methods, wherein a template of the height maps is chosen which template is then moved over a template of the other one of the height maps, the method of the invention comprises directly taking both first and second height map and partially overlapping them to define the first overlap region. The templates in known methods correspond to a portion of a measured height map. A first similarity, for example based on a correlation or a sum of absolute differences, between the first and second height maps in the first overlap region is determined by determining a similarity, for example a correlation between the first and second height maps in the overlap region using a similarity measure. Thus, after overlapping the first and second height map a similarity, based on the similarity measure, is assigned to the first overlap region, which similarity provides a value for the amount of similarity between the first and second height maps in the overlap region. For example, the first similarity value may be based on a sum of squares of differences between height values of the first and second height maps in the first overlap region. After determining the first similarity, a second overlap region of the first and second height maps is determined by shifting the first and second height maps relative