Method for determining an imaging quality of an optical system when illuminated by illumination light within an entrance pupil to be measured

Patent metadata

Field	Value
Publication number	US-12492963-B2
Application number	US-202217664773-A
Country	US
Kind code	B2
Filing date	May 24, 2022
Priority date	May 31, 2021
Publication date	Dec 9, 2025
Grant date	Dec 9, 2025

Abstract

To determine an imaging quality of an optical system when illuminated by illumination light within an entrance pupil or exit pupil, a test structure is initially arranged in an object plane of the optical system and an illumination angle distribution for illuminating the test structure with the illumination light is specified. The test structure is illuminated at different distance positions relative to the object plane. An intensity of the illumination light is measured in an image plane of the optical system, the illumination light having been guided by the optical system when imaging the test structure at each distance position. An aerial image measured in this way is compared with a simulated aerial image and fit parameters of a function set for describing the simulated aerial image are adapted and a wavefront of the optical system is determined on the basis of the result of a minimized difference.

First claim

What is claimed is:

1 . A method, comprising:
a) for each of a plurality of different distance positions of a test structure relative to an object plane of an optical system, illuminating the test structure with a specified illumination angle distribution corresponding to a subaperture within a pupil of the optical system; b) for each of the different distance positions, measuring a spatially resolved distribution of an intensity of the illumination light in an image plane of the optical system using a spatially resolving detection device, the illumination light having been guided by the optical system; c) determining an aerial image of the test structure using the measured spatially resolved distribution of the intensity of the illumination light in the image plane at each of the different distance positions; d) comparing the determined aerial image with a simulated aerial image and adapting fit parameters of a function set to describe the simulated aerial image until a difference between the determined aerial image and the simulated aerial image has been reduced to a desired value; e) determining a wavefront of the optical system based on the result of the reduced difference between the determined and the simulated aerial image; f) repeating a) through d) using a different subaperture within the pupil; g) determining a combined wavefront of the optical system by combining the determined wavefronts over the entire pupil; and eliminating a test structure contribution to an influence on the wavefront by the test structure to determine a test structure-independent imaging quality of the optical system, wherein:
a linear system of equations is solved for determining the test structure-independent imaging quality of the optical system while eliminating the test structure contribution; and
the linear system of equations comprises data of the determined combined wavefront prior to the elimination of the test structure contribution, contributions of the test structure, and a transformation matrix.

2 . The method of claim 1 , wherein the desired value is a minimum value.

3 . The method of claim 1 , comprising using the subapertures to scan the pupil.

4 . The method of claim 1 , wherein:
e) comprises determining the test structure contribution for exactly one specified subaperture; and f) comprises using the test structure contribution for the exactly one specified subaperture to determine the test structure-independent imaging quality of the optical system for the second subaperture.

5 . The method of claim 1 , wherein:
a dependence of at least one parameter on a respective coordinate in a solution space to be determined is described by a decomposition into basis functions; and the at least one parameter comprises a member selected from the group consisting of the data of the combined determined wavefront prior to the elimination of the test structure contribution, the contributions of the test structure, and the transformation matrix.

6 . The method of claim 1 , wherein the test structure comprises a pinhole.

7 . The method of claim 1 , wherein the test structure comprises a pinhole comprising an elliptical edge.

8 . The method of claim 1 , wherein:
the pupil comprises an elliptical edge; within the determination of the combined wavefront, there is a representation of a pupil function for an at least sectional description of the pupil on a coordinate grid that is equidistant in mutually perpendicular pupil coordinates and parameterized basis functions that are scaled in accordance with a principal axis ratio of an elliptical edge of the pupil.

9 . The method of claim 1 , wherein:
the pupil comprises an elliptical edge; and there is within the determination of the combined wavefront, a representation of a pupil function for the at least sectional description of the pupil on a coordinate grid that is scaled in mutually perpendicular pupil coordinates in accordance with a principal axis ratio of the elliptical edge of the pupil and parameterized basis functions that are scaled uniformly.

10 . The method of claim 1 , further comprising using a metrology system comprising an imaging optical unit to image the test structure toward a spatially resolving detection device.

11 . The method of claim 10 , comprising using the subapertures to scan the pupil.

12 . The method of claim 1 , further comprising using a metrology system which comprises:
an illumination optical unit configured to illuminate a test structure in an object plane in which the test structure is present; a spatially resolving detection device; an imaging optical unit configured to image the test structure toward the spatially resolving device in an image plane; and a stop comprising an aperture having an elliptical edge, wherein the stop is an illumination pupil plane of the imaging optical unit and/or in an entrance pupil of the imaging optical unit.

13 . The method of claim 12 , comprising using the subapertures to scan the pupil.

14 . A metrology system, comprising:
an illumination optical unit configured to illuminate a test structure in an object plane in which the test structure is present; a spatially resolving detection device; an imaging optical unit configured to image the test structure toward the spatially resolving detection device in an image plane; a stop comprising an aperture having an elliptical edge; and a controller configured so that during use of the metrology system:
a) for each of a plurality of different distance positions of a test structure relative to an object plane of an optical system, the illumination optical unit illuminates a test structure with a specified illumination angle distribution corresponding to a subaperture of the optical system;
b) for each of the different distance positions, the spatially resolving detection device measures a spatially resolved distribution of an intensity of the illumination light in an image plane of the optical system using a spatially resolving detection device, the illumination light having been guided by the optical system;
c) an aerial image of the test structure is determined using the measured spatially resolved distribution of the intensity of the illumination light in the image plane at each of the different distance positions;
d) the determined aerial image is compared with a simulated aerial image and fit parameters of a function set to describe the simulated aerial image are adapted until a difference between the determined aerial image and the simulated aerial image has been reduced to a desired value;
e) a wavefront of the optical system is determined based on the result of the reduced difference between the measured and the simulated aerial image;
f) a) through e) are repeated using a different subaperture within the pupil; and
g) a combined wavefront of the optical system is determined by combining the determined wavefronts over the entire pupil,
wherein the stop is an illumination pupil plane of the imaging optical unit and/or in an entrance pupil of the imaging optical unit, and wherein the controller is further configured so that during use of the metrology system:
a test structure contribution to an influence on the wavefront by the test structure is eliminated to determine a test structure-independent imaging quality of the optical system; and
a linear system of equations is solved for determining the imaging quality while eliminating the test structure contribution; and
the linear system of equations comprises data of the determined combined wavefront prior to the elimination of the test structure contribution, co

Assignees

• Zeiss Carl Smt Gmbh

Inventors

• Gwosch Klaus
• Koch Markus
• Stoppe Lars
• Decker Manuel
• Fischer Lukas

Classifications

• G01M11/005

  Testing of reflective surfaces, e.g. mirrors · CPC title

• G03F7/706

  Aberration measurement · CPC title

• G01M11/0257

  by analyzing the image formed by the object to be tested · CPC title

• G02B27/0081

  with means for altering, e.g. enlarging, the entrance or exit pupil · CPC title

• G03F7/70591

  Testing optical components · CPC title