Wave front aberration metrology of optics of EUV mask inspection system

Patent metadata

Field	Value
Publication number	US-9335206-B2
Application number	US-201314010484-A
Country	US
Kind code	B2
Filing date	Aug 26, 2013
Priority date	Aug 30, 2012
Publication date	May 10, 2016
Grant date	May 10, 2016

Abstract

Disclosed is test structure for measuring wave-front aberration of an extreme ultraviolet (EUV) inspection system. The test structure includes a substrate formed from a material having substantially no reflectivity for EUV light and a multilayer (ML) stack portion, such as a pillar, formed on the substrate and comprising a plurality of alternating pairs of layers having different refractive indexes so as to reflect EUV light. The pairs have a count equal to or less than 15.

First claim

What is claimed is:

1. A test structure for measuring wave-front aberration of an extreme ultraviolet (EUV) inspection system, comprising:
a substrate formed from a material having substantially no reflectivity for EUV light; and
a multilayer (ML) stack portion formed over only one or more portions of a top surface of the substrate and comprising a plurality of alternating pairs of layers having different refractive indexes so as to reflect EUV light, wherein the ML stack portion is arranged over only the one or more portions of the top surface of the substrate so that at least a portion of the top surface of the substrate is exposed without being covered by the ML stack portion.

2. The test structure of claim 1 , wherein each pair comprises (i) a molybdenum (Mo) layer and a silicon (Si) layer, (ii) a Ru layer and a Si layer, or (iii) a Mo layer and a Si layer interfaced with a carbon (C) based barrier layer.

3. The test structure of claim 1 , further comprising a capping layer over the ML stack portion formed from a material that prevents oxidation of the ML stack portion and is substantially transparent, wherein the capping layer is comprised of Ru, diamond-like carbon (DLC), Si or platinum (Pt).

4. The test structure of claim 1 , wherein the alternating pairs of layers of the ML stack portion are non-periodic in relative thicknesses and each alternating pair has a thickness that is optimized so that EUV light diffracted from the test structure substantially fills an entrance pupil area of the inspection system and/or has an optimized peak reflectivity.

5. The test structure of claim 1 , wherein a count of the pairs is equal to or less than ten.

6. The test structure of claim 1 , wherein a count of the pairs is equal to or less than five.

7. The test structure of claim 1 , wherein a period of the ML stack portion is between about 7 and 7.5 nm.

8. The test structure of claim 1 , wherein the ML stack portion has a composition that provides a high contrast between the ML stack portion and the substrate when imaged with EUV light.

9. The test structure of claim 1 , wherein the ML stack portion has a thickness equal to or less than 75 nm.

10. The test structure of claim 1 , wherein the substrate has a refractive index that results in reflectivity of EUV light that is less than 0.1%.

11. The test structure of claim 1 , further comprising a conformal layer over the ML stack portion's top and sidewalls, wherein the conformal layer has a low diffusivity for oxygen and is substantially transparent, wherein the conformal layer is comprised of Ru, boron (B), DLC, SiO2 or Si3N4.

12. The test structure of claim 1 , wherein the ML stack portion is a pillar that has a diameter less than 100 nm.

13. A method of forming a test structure for measuring wave-front aberration of an extreme ultraviolet (EUV) inspection system, comprising:
over a top surface of a substrate, depositing a plurality of alternating pairs of a first layer and a second layer that are reflective to EUV light; and
patterning the plurality of alternating pairs of the first layer and the second layer to form a multilayer (ML) stack portion over only one or more portions of the top surface of the substrate so that at least a portion of the top surface of the substrate remains exposed without being covered by the ML stack portion.

14. An inspection system, comprising:
one or more illumination elements for directing an EUV incident beam onto a test structure comprising a substrate formed from a material having substantially no reflectivity for EUV light and a multilayer (ML) stack portion formed on the substrate and comprising a plurality of alternating pairs of layers having different refractive indexes so as to reflect EUV light, wherein the pairs have a count equal to or less than 15;
one or more imaging elements for detecting an output beam from the test structure and generating an image or signal based on the output beam, wherein the output beam emanates from the test structure in response to the incident beam on the test structure; and
a processor configured for analyzing the image or signal to measure wave-front aberration substantially across a pupil of the inspection system.

15. The inspection system of claim 14 having a numerical aperture (NA) greater than 0.1.

16. The inspection system of claim 14 , wherein each pair comprises (i) a molybdenum (Mo) layer and a silicon (Si) layer, (ii) a Ru layer and a Si layer, or (iii) a Mo layer and a Si layer interfaced with a carbon (C) based barrier layer.

17. The inspection system of claim 14 , wherein the alternating pairs of layers of the ML stack portion are non-periodic and each alternating pair has a thickness that is optimized so that EUV light diffracted from the test structure substantially fills an entrance pupil area of the inspection system and/or has an optimized peak reflectivity.

18. The inspection system of claim 14 , wherein the count of the pairs is equal to or less than 10.

19. The inspection system of claim 14 , wherein the count of the pairs is equal to or less than 5.

20. The inspection system claim 14 , wherein a period of the ML stack portion is between about 7 and 7.5 nm.

21. The inspection system of claim 14 , wherein the ML stack portion has a composition that provides a high contrast between the ML stack portion and the substrate when imaged with EUV light.

22. The inspection system of claim 14 , wherein the ML stack portion has a thickness equal to or less than 75 nm.

23. The inspection system of claim 14 , wherein the substrate has a refractive index that results in reflectivity of EUV light that is less than 0.1%.

24. The inspection system of claim 14 , the test structure further comprising a conformal layer over the ML stack portion's top and sidewalls, wherein the conformal layer has a low diffusivity for oxygen and is substantially transparent, wherein the conformal layer is comprised of Ru, boron (B), DLC, SiO2 or Si3N4.

25. The inspection system of claim 14 , wherein the ML stack portion is a pillar that has a diameter less than 100 nm.

Assignees

• Kla Tencor Corp

Inventors

• Zhang Qiang
• Liu Yanwei
• Sezginer Abdurrahman

Classifications

• G03F1/24

  Reflection masks; Preparation thereof · CPC title

• G21K7/00 Primary

  Gamma- or X-ray microscopes · CPC title

• G02B5/0891

  Ultraviolet [UV] mirrors (apparatus for microlithography exposure G03F7/70; X-ray multilayer structures G21K1/06) · CPC title

• G03F1/52

  Reflectors · CPC title

• G03F1/50

  Mask blanks not covered by G03F1/20 - G03F1/34; Preparation thereof · CPC title