Phase contrast monitoring for extreme ultra-violet (EUV) masks defect inspection

Patent metadata

Field	Value
Publication number	US-10634623-B2
Application number	US-201715593163-A
Country	US
Kind code	B2
Filing date	May 11, 2017
Priority date	Oct 7, 2016
Publication date	Apr 28, 2020
Grant date	Apr 28, 2020

Abstract

Disclosed are methods and apparatus for inspecting an extreme ultraviolet (EUV) reticle using an optical inspection tool. An inspection tool having a pupil filter positioned at an imaging pupil is used to obtain a test image or signal from an output beam that is reflected and scattered from a test portion of an EUV test reticle. The pupil filter is configured to provide phase contrast in the output beam. A reference image or signal is obtained for a reference reticle portion that is designed to be identical to the test reticle portion. The test and reference images or signals are compared and it is determined whether the test reticle portion has any candidate defects based on such comparison. For each of a plurality of test reticle portions of the reticle, the operations for using the inspection tool, obtaining a reference image or signal, comparing, and determining are repeated. A defect report is generated based on any candidate defects that have been determined to be present.

First claim

What is claimed is:

1. A method of inspecting an extreme ultraviolet (EUV) reticle using an optical inspection tool, the method comprising:
using an inspection tool having a pupil filter positioned at an imaging pupil to obtain a test image or signal from an output beam that is reflected and scattered from a test portion of an EUV test reticle, wherein the pupil filter is configured to provide phase contrast in the output beam to enhance a signal from any defects as compared to a signal from noise on the reticle, wherein the pupil filter is designed based on simulated defect intensity signals for a plurality of defect types as a function of a phase change angle produced by a plurality of pupil filter configurations;
obtaining a reference image or signal for a reference reticle portion that is designed to be identical to the test reticle portion;
comparing the test and reference images or signals and determining whether the test reticle portion has any candidate defects based on such comparison;
for each of a plurality of test reticle portions of the reticle, repeating the operations for using the inspection tool, obtaining a reference image or signal, comparing, and determining; and
generating a defect report based on any candidate defects that have been determined to be present.

2. The method of claim 1 , wherein the pupil filter is configured to provide phase contrast in the output beam so as to enhance a signal strength of a phase object, including a defect, as compared to a non-phase or differently phased object in the test reticle portion.

3. The method of claim 2 , wherein the pupil filter is configured to provide phase contrast in the output beam to introduce a phase change so that a phase between light scattered from a phase object more closely matches light reflected from such phase object and such scattered and reflected light can constructively interfere with each other to enhance the phase object's detected image signal.

4. The method of claim 1 , wherein the pupil filter is a slab of glass that has an etched portion having an etched depth corresponding to an amount of phase change that is introduced into a portion of the output beam that is transmitted through the pupil filter.

5. The method of claim 4 , wherein the etched depth is about 85 nm to provide a phase change of 90°.

6. The method of claim 4 , wherein the etched portion of the pupil filter has a width that substantially matches an illumination area, which corresponds to a reflected portion of the output light, so as to provide a phase change for such illumination area and not outside such illumination area, which corresponds to a scattered light portion of the output light.

7. The method of claim 6 , wherein the illumination area corresponds to a sigma 0.5 illumination and the width of the pupil filter equals about half of an aperture diameter of the imaging pupil.

8. The method of claim 4 , wherein the pupil filter comprises a plurality of half-tone patterns on a side of the glass that is opposite a side that includes the etched portion, wherein the half-tone patterns are sized and arranged to control the transmission of the output light.

9. A method of inspecting an extreme ultraviolet (EUV) reticle using an optical inspection tool, the method comprising:
using an inspection tool having a pupil filter positioned at an imaging pupil to obtain a test image or signal from an output beam that is reflected and scattered from a test portion of an EUV test reticle, wherein the pupil filter is configured to provide phase contrast in the output beam;
obtaining a reference image or signal for a reference reticle portion that is designed to be identical to the test reticle portion;
comparing the test and reference images or signals and determining whether the test reticle portion has any candidate defects based on such comparison;
for each of a plurality of test reticle portions of the reticle, repeating the operations for using the inspection tool, obtaining a reference image or signal, comparing, and determining;
generating a defect report based on any candidate defects that have been determined to be present; and
designing the pupil filter by simulating defect intensity signals for a plurality of defect types as a function of a phase change angle produced by a plurality of pupil filter configurations.

10. The method of claim 1 , wherein the pupil filter is configured to provide phase contrast in the output beam so as to cause a correction in a plurality of intensity tones for a plurality of reticle pattern and defect types.

11. The method of claim 1 , wherein the pupil filter is configured to provide phase contrast in the output beam so as to cause a plurality of focus offsets for a plurality of defects types to be substantially equal.

12. The method of claim 1 , further comprising selecting the pupil filter from a plurality of pupil filters having different phase contrast effects, wherein such selecting is based on analyzing results from a previously inspected reticle area.

13. An inspection system for inspecting an EUV reticle, comprising:
a light source for generating an incident beam;
illumination optics for directing the incident beam onto an EUV reticle;
collection optics for directing an output beam that is reflected and scattered from the EUV reticle in response to the incident beam, wherein the output beam is directed through a pupil filter towards a sensor;
the pupil filter positioned in an imaging pupil of the system, wherein the pupil filter is configured to provide phase contrast in the output beam to enhance a signal from any defects as compared to a signal from noise on the reticle, wherein the pupil filter has a design that is based on simulated defect intensity signals for a plurality of defect types as a function of a phase change angle produced by a plurality of pupil filter configurations;
the sensor for detecting the output beam from the pupil filter and generating an image or signal for such output beam; and
a controller that is configured to perform the following operations:
obtaining a reference image or signal for a reference reticle portion that is designed to be identical to the test reticle portion;
comparing the test and reference images or signals and determining whether the test reticle portion has any candidate defects based on such comparison;
for each of a plurality of test reticle portions of the reticle, repeating the operations for obtaining a reference image or signal, comparing, and determining; and
generating a defect report based on any candidate defects that have been determined to be present.

14. The system of claim 13 , wherein the pupil filter is configured to provide phase contrast in the output beam so as to enhance a signal strength of a phase object, including a defect, as compared to a non-phase or differently phased object in the test reticle portion.

15. The system of claim 14 , wherein the pupil filter is configured to provide phase contrast in the output beam to introduce a phase change so that a phase between light scattered from a phase object more closely matches light reflected from such phase object and such scattered and reflected light can constructively interfere with each other to enhance the phase object's detected image signal.

16. The system of claim 13 , wherein the pupil filter is a slab of glass that has an etched portion having a depth corresponding to an amount of phase change that is introduced into a portion of the output beam that is transmitted through the pupil filter.

17. The system of claim 16 , wherein the depth is ab

Assignees

• Kla Tencor Corp

Inventors

• Zhang Qiang
• Shi Rui-Fang

Classifications

• G03F1/84

  Inspecting · CPC title

• G01N21/33

  using ultraviolet light (G01N21/39 takes precedence) · CPC title

• G01N21/8851 Primary

  Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges (G01N21/8806 and G01N21/93 - G01N21/95692 take precedence; optical measurement of dimensions G01B11/00; optical scanning G02B26/10; image transformation G06T3/00; computerised image enhancement G06T5/00; image processing per se for flaw detection G06T7/0002) · CPC title

• G01N21/95607 Primary

  using a comparative method · CPC title

• G01N2021/8877

  Proximity analysis, local statistics · CPC title