System and method utilizing multi-point autofocus to align an optical axis of an optical assembly portion to be normal to a workpiece surface

A method for operating a metrology system including an optical assembly portion, wherein the optical assembly portion comprises:

• a light source;
• an objective lens that inputs image light arising from a surface of a workpiece which is illuminated by the light source, and transmits the image light along an imaging optical path, wherein the objective lens defines an optical axis of the optical assembly portion which extends at least between the objective lens and the workpiece surface;
• and
• a camera that receives imaging light transmitted along the imaging optical path and provides images of the workpiece surface;
• the method comprising:
• moving the optical assembly portion to position a first workpiece surface of the workpiece within a focal Z autofocus range of the optical assembly portion;
• capturing a first image stack of the first workpiece surface within the focal Z autofocus range using the optical assembly portion, wherein the first image stack comprises a plurality of images of the first workpiece surface and each image of the first image stack corresponds to a different autofocus height;
• determining an autofocus height for at least three locations of the first workpiece surface based on at least three corresponding regions of interest of the first image stack;
• based at least in part on the autofocus heights at the at least three locations, determining adjustment information for rotating the optical assembly portion relative to the first workpiece surface to nominally align the optical axis of the optical assembly portion with a surface normal of the first workpiece surface and for adjusting a distance between the optical assembly portion and the first workpiece surface;
• utilizing the adjustment information for rotating the optical assembly portion to nominally align the optical axis of the optical assembly portion with a surface normal of the first workpiece surface and for adjusting a distance between the optical assembly portion and the first workpiece surface;
• executing a first operation on the first workpiece surface, wherein the first operation comprises at least one of a measurement operation, a points-from-focus (PFF) operation, a machining operation, or capturing one or more images of the first workpiece surface;
• moving the optical assembly portion to position a second workpiece surface of the workpiece within the focal Z autofocus range of the optical assembly portion;
• capturing a second image stack of the second workpiece surface using the optical assembly portion, wherein the second image stack comprises a plurality of images of the second workpiece surface and each image of the second image stack corresponds to a different autofocus height;
• determining an autofocus height for at least three locations of the second workpiece surface based on at least three corresponding regions of interest of the second image stack;
• based at least in part on the autofocus heights at the at least three locations, determining adjustment information for rotating the optical assembly portion relative to the second workpiece surface to nominally align the optical axis of the optical assembly portion with a surface normal of the second workpiece surface and for adjusting a distance between the optical assembly portion and the second workpiece surface;
• utilizing the adjustment information for rotating the optical assembly portion to nominally align the optical axis of the optical assembly portion with a surface normal of the second workpiece surface and for adjusting a distance between the optical assembly portion and the second workpiece surface;
• and
• executing a second operation on the second workpiece surface, wherein the second operation comprises at least one of a measurement operation, a points-from-focus (PFF) operation, a machining operation, or capturing one or more images of the second workpiece surface.

A metrology system, comprising:

• an optical assembly portion comprising:
• a variable focal length (VFL) lens;
• a light source;
• an objective lens that inputs image light arising from a surface of a workpiece which is illuminated by the light source, and transmits the image light along an imaging optical path that passes through the VFL lens, wherein the objective lens defines an optical axis of the optical assembly portion which extends at least between the objective lens and the workpiece surface;
• and
• a camera that receives imaging light transmitted by the VFL lens along the imaging optical path and provides images of the workpiece surface;
• a Z-axis movement mechanism configured to change a distance between the optical assembly portion and the workpiece surface;
• a rotation mechanism configured to rotate the optical assembly portion relative to the workpiece surface to change an angular orientation of the optical axis of the optical assembly portion relative to the workpiece surface;
• one or more processors;
• and
• a memory coupled to the one or more processors and storing program instructions that when executed by the one or more processors cause the one or more processors to at least:
• control at least one of the Z-axis movement mechanism or the rotation mechanism to move the optical assembly portion to position a first workpiece surface of a workpiece within a focal Z autofocus range of the optical assembly portion;
• capture a first image stack of the first workpiece surface within the focal Z autofocus range using the optical assembly portion, wherein the first image stack comprises a plurality of images of the first workpiece surface and each image of the first image stack corresponds to a different autofocus height;
• determine an autofocus height for at least three locations of the first workpiece surface based on at least three corresponding regions of interest of the first image stack;
• based at least in part on the autofocus heights at the at least three locations, control the rotation mechanism to rotate the optical assembly portion relative to the first workpiece surface to nominally align the optical axis of the optical assembly portion with a surface normal of the first workpiece surface and control the Z-axis movement mechanism to adjust a distance between the optical assembly portion and the first workpiece surface;
• execute a first operation on the first workpiece surface, wherein the first operation comprises at least one of a measurement operation, a points-from-focus (PFF) operation, a machining operation, or capturing one or more images of the first workpiece surface;
• control at least one of the Z-axis movement mechanism or the rotation mechanism to move the optical assembly portion to position a second workpiece surface of the workpiece within the focal Z autofocus range of the optical assembly portion;
• capture a second image stack of the second workpiece surface using the optical assembly portion, wherein the second image stack comprises a plurality of images of the second workpiece surface and each image of the second image stack corresponds to a different autofocus height;
• determine an autofocus height for at least three locations of the second workpiece surface based on at least three corresponding regions of interest of the second image stack;
• based at least in part on the autofocus heights at the at least three locations, control the rotation mechanism to rotate the optical assembly portion relative to the second workpiece surface to nominally align the optical axis of the optical assembly portion with a surface normal of the second workpiece surface and control the Z-axis movement mechanism to adjust a distance between the optical assembly portion and the second workpiece surface;
• and
• execute a second operation on the second workpiece surface, wherein the second operation comprises at least one of a measurement operation, a points-from-focus (PFF) operation, a machining operation, or capturing one or more images of the second workpiece surface.

A system, comprising:

• an optical assembly portion comprising:
• a light source;
• an objective lens that inputs image light arising from a surface of a workpiece which is illuminated by the light source, and transmits the image light along an imaging optical path, wherein the objective lens defines an optical axis of the optical assembly portion which extends at least between the objective lens and the workpiece surface;
• and
• a camera that receives imaging light transmitted along the imaging optical path and provides images of the workpiece surface;
• an adjustment mechanism configured to change a distance between the optical assembly portion and the workpiece surface and to rotate the optical assembly portion relative to the workpiece surface to change an angular orientation of the optical axis of the optical assembly portion relative to the workpiece surface;
• one or more processors;
• and
• a memory coupled to the one or more processors and storing program instructions that when executed by the one or more processors cause the one or more processors to at least:
• control the adjustment mechanism to move the optical assembly portion to position a workpiece surface within a focal Z autofocus range of the optical assembly portion;
• capture a first image stack of the workpiece surface within the focal Z autofocus range using the optical assembly portion, wherein the first image stack comprises a plurality of images of the workpiece surface and each image of the first image stack corresponds to a different autofocus height;
• determine an autofocus height for at least three locations of the workpiece surface based on at least three corresponding regions of interest of the first image stack;
• control the adjustment mechanism based at least in part on the autofocus heights at the at least three locations to:
• rotate the optical assembly portion relative to the workpiece surface to nominally align the optical axis of the optical assembly portion with a surface normal of the workpiece surface;
• and
• adjust a distance between the optical assembly portion and the workpiece surface;
• and
• execute an operation on the workpiece surface, wherein the operation comprises capturing a second image stack while the optical axis of the optical assembly portion is nominally aligned with a surface normal of the workpiece surface, for which the second image stack comprises a plurality of images of the workpiece surface and each image of the second image stack corresponds to a different focus position of the optical assembly portion along the direction of the optical axis.

A method for operating a system including an optical assembly portion, wherein the optical assembly portion comprises:

• a light source;
• an objective lens that inputs image light arising from a surface of a workpiece which is illuminated by the light source, and transmits the image light along an imaging optical path, wherein the objective lens defines an optical axis of the optical assembly portion which extends at least between the objective lens and the workpiece surface;
• and
• a camera that receives imaging light transmitted along the imaging optical path and provides images of the workpiece surface;
• the method comprising:
• moving the optical assembly portion to position a workpiece surface within a focal Z autofocus range of the optical assembly portion;
• capturing a first image stack of the workpiece surface within the focal Z autofocus range using the optical assembly portion, wherein the first image stack comprises a plurality of images of the workpiece surface and each image of the first image stack corresponds to a different autofocus height;
• determining an autofocus height for at least three locations of the workpiece surface based on at least three corresponding regions of interest of the first image stack;
• based at least in part on the autofocus heights at the at least three locations, determining adjustment information for rotating the optical assembly portion relative to the workpiece surface to nominally align the optical axis of the optical assembly portion with a surface normal of the workpiece surface and for adjusting a distance between the optical assembly portion and the workpiece surface;
• utilizing the adjustment information for rotating the optical assembly portion to nominally align the optical axis of the optical assembly portion with a surface normal of the workpiece surface and for adjusting a distance between the optical assembly portion and the workpiece surface;
• and
• executing an operation on the workpiece surface, wherein the operation comprises capturing a second image stack while the optical axis of the optical assembly portion is nominally aligned with a surface normal of the workpiece surface, for which the second image stack comprises a plurality of images of the workpiece surface and each image of the second image stack corresponds to a different focus position of the optical assembly portion along the direction of the optical axis.