# US 10,481,505 B2 · Method for determining an imaging aberration contribution of an imaging optical unit for measuring lithography masks

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- Number: US 10,481,505 B2
- Document ID: US10481505B2
- Status: Granted
- Kind: B2
- Application number: 16/280,494
- Filed: 2019-02-20
- Published: 2019-08-22
- Granted: 2019-11-19
- Earliest claimed: 2018-02-21
- Assignees: Carl Zeiss SMT GmbH
- Applicants: Carl Zeiss SMT GmbH
- Inventors: Markus Koch; Dirk Hellweg; Renzo Capelli; Martin Dietzel
- Web page: https://patentgrep.com/patent/US10481505B2

## CPC

- G03F 7/706: {Aberration measurement}
- G01M 11/0242: {by measuring geometrical properties or aberrations}
- G01M 11/0264: {by using targets or reference patterns}
- G03F 1/84: Inspecting
- G03F 7/70083: {Non-homogeneous intensity distribution in the mask plane}
- G03F 7/70641: {Focus}
- G03F: PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR; (phototypographic composing devices B41B; photosensitive materials or processes for photographic purposes G03C; electrophotography, sensitive layers or processes therefor G03G)
- G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR

## Abstract

Determining an imaging aberration contribution of an imaging optical unit for measuring lithography masks involves firstly focus-dependently measuring a 3D aerial image of the imaging optical unit as a sequence of 2D intensity distributions in different measurement planes in the region of and parallel to an image plane of an imaging of an object by use of the imaging optical unit. A spectrum of a speckle pattern of the 3D aerial image is then determined by Fourier transformation of the measured 2D intensity distributions having speckle patterns. For a plurality of spectral components in the frequency domain, a focus dependence of a real part RS(z) and an imaginary part IS(z) of said spectral component is then determined. From the determined values of the focus dependence of the real part RS(z) and the imaginary part IS(z), a contribution made to the speckle pattern spectrum by a mask structure, which contribution is to be eliminated, is then separated from an imaging aberration contribution made to the speckle pattern spectrum by the imaging optical unit. The imaging aberration contribution is then represented. This results in a method for determining the imaging aberration contribution of the imaging optical unit having little additional time expenditure in comparison with the measurement time on the respective lithography mask.

## Claims

### Claim 1

A method for determining an imaging aberration contribution of an imaging optical unit for measuring lithography masks, the method comprising the following steps:
a) focus-dependently measuring a 3D aerial image of the imaging optical unit as a sequence of 2D intensity distributions in different measurement planes in the region of and parallel to an image plane of an imaging of an object by use of the imaging optical unit;
b) determining a spectrum S({right arrow over (ν)}) of a speckle pattern of the 3D aerial image by Fourier transformation of the measured 2D intensity distributions having speckle patterns;
c) determining, for a plurality of spectral components S(νxi, νyi) in the frequency domain, a focus dependence of a real part RS(z) and an imaginary part IS(z) of said spectral component;
d) separating from the determined values of the focus dependence
aa) a contribution (H) made to the speckle pattern spectrum (S({right arrow over (ν)})) by a mask structure, which contribution is to be eliminated, from
bb) an imaging aberration contribution (Θ) made to the speckle pattern spectrum by the imaging optical unit; and
e) representing the imaging aberration contribution (Θ).

### Claim 2 (depends on 1)

The method according to claim 1, wherein separating involves eliminating a further contribution made by a defocus aberration (Θd) during the measurement of the 2D intensity distributions.

### Claim 3 (depends on 1)

The method according to claim 1, wherein for preparing the representation of the imaging aberration contribution (Θ), the latter is approximated with the aid of a linear combination of a set of orthogonal functions (Zi).

### Claim 4 (depends on 3)

The method according to claim 3, wherein coefficients (zn) of a function expansion that arises in the course of the approximation are represented.

### Claim 5 (depends on 3)

The method according to claim 3, wherein Zernike functions are used as a set of functions during the preparation of the representation.

### Claim 6 (depends on 1)

The method according to claim 1, wherein the method is carried out on an unstructured section of a lithography mask.

### Claim 7 (depends on 1)

A method for correcting an imaging aberration (Θ) of an imaging optical unit of a metrology system for measuring lithography masks, the method comprising the following steps:
a) determining an imaging aberration contribution (Θ) of the imaging optical unit by a method according to claim 1; and
b) correcting the imaging aberration contribution (Θ) by readjusting optical components of the imaging optical unit taking as a basis the imaging aberration contribution (Θ) determined.

### Claim 8 (depends on 7)

The method of claim 7 in which determining the imaging aberration contribution (Θ) of the imaging optical unit comprises eliminating a further contribution made by a defocus aberration (Θd) during the measurement of the 2D intensity distributions.

### Claim 9 (depends on 7)

The method of claim 7 in which for preparing the representation of the imaging aberration contribution (Θ), the latter is approximated with the aid of a linear combination of a set of orthogonal functions (Zi).

### Claim 10 (depends on 7)

The method of claim 7 in which the coefficients (zn) of a function expansion that arises in the course of the approximation are represented.

### Claim 11 (depends on 7)

The method of claim 7 in which the Zernike functions are used as a set of functions during the preparation of the representation.

### Claim 12 (depends on 7)

The method of claim 7 in which the method for determining the imaging aberration contribution of the imaging optical unit is carried out on an unstructured section of a lithography mask.

### Claim 13 (depends on 1)

A metrology system for carrying out a method according to claim 1, comprising an illumination optical unit for illuminating the lithography mask to be examined and comprising an imaging optical unit for imaging the object towards a spatially resolving detection device.

### Claim 14 (depends on 13)

The metrology system according to claim 13, comprising at least one displacement actuator for displacing an imaging component of the imaging optical unit.

### Claim 15 (depends on 14)

The metrology system according to claim 14, wherein the displacement actuator is signal-connected to a central open-loop/closed-loop control device of the metrology system.

### Claim 16 (depends on 13)

The metrology system of claim 13 in which the metrology system is further configured to eliminate a further contribution made by a defocus aberration (Θd) during the measurement of the 2D intensity distributions.

### Claim 17 (depends on 13)

The metrology system of claim 13 in which the metrology system is further configured to approximate the imaging aberration contribution (Θ) with the aid of a linear combination of a set of orthogonal functions (Zi).

### Claim 18 (depends on 17)

The metrology system of claim 17 in which the metrology system is further configured to represent coefficients (zn) of a function expansion that arises in the course of the approximation.

### Claim 19 (depends on 17)

The metrology system of claim 17 in which Zernike functions are used as a set of functions during the preparation of the representation.

### Claim 20 (depends on 13)

The metrology system of claim 13 in which the metrology system is configured to carry out the method for determining the imaging aberration contribution of the imaging optical unit on an unstructured section of a lithography mask.