# US 11,442,021 B2 · Broadband light interferometry for focal-map generation in photomask inspection > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 11,442,021 B2 - Document ID: US11442021B2 - Status: Granted - Kind: B2 - Application number: 17/063,622 - Filed: 2020-10-05 - Published: 2021-04-15 - Granted: 2022-09-13 - Earliest claimed: 2019-10-11 - Assignees: KLA Corporation - Applicants: KLA Corporation - Inventors: Rui-Fang Shi; Dmitry Skvortsov - Web page: https://patentgrep.com/patent/US11442021B2 ## CPC - G01N 21/8806: {Specially adapted optical and illumination features} - G01B 9/0209: Low-coherence interferometers - G01B 11/0608: {Height gauges} - G01N 21/956: Inspecting patterns on the surface of objects {(contactless testing of electronic circuits G01R31/308; testing currency G07D; manufacturing processes per se of semiconductor devices implementing a measuring step H10P74/20)} - G03F 1/84: Inspecting - G01N 21/45: using interferometric methods; using Schlieren methods - G01N 2021/95676: {Masks, reticles, shadow masks} - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) - G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS - 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) ## Abstract Heights on a surface of a photomask are measured using broadband light interferometry. The heights include heights of patterned areas of the photomask. A focal map is produced from the measured heights on the surface of the photomask. To produce the focal map, the measured heights of the patterned areas are adjusted based on fill factors for the patterned areas. The photomask is inspected for defects, using the focal map. ## Claims ### Claim 1 A method, comprising: measuring heights on a surface of a photomask using broadband light interferometry, the heights comprising heights of patterned areas of the photomask; producing a focal map from the measured heights on the surface of the photomask, comprising adjusting the measured heights of the patterned areas based on fill factors for the patterned areas; and inspecting the photomask for defects, using the focal map. ### Claim 2 (depends on 1) The method of claim 1, wherein: the measuring comprises generating a height image for the photomask, using the broadband light interferometry; and the adjusting comprises adjusting the height image to offset the measured heights of the patterned areas, based on the fill factors. ### Claim 3 (depends on 1) The method of claim 1, wherein: the broadband light interferometry uses visible light; and inspecting the photomask for defects is performed using ultraviolet light. ### Claim 4 (depends on 3) The method of claim 3, wherein the ultraviolet light is 13.5 nm light. ### Claim 5 (depends on 3) The method of claim 3, wherein the ultraviolet light is 193 nm light. ### Claim 6 (depends on 1) The method of claim 1, wherein the broadband light interferometry is Mirau interferometry. ### Claim 7 (depends on 1) The method of claim 1, wherein the broadband light interferometry is Michelson interferometry. ### Claim 8 (depends on 1) The method of claim 1, further comprising calculating the fill factors based on a database of the design for the photomask. ### Claim 9 (depends on 8) The method of claim 8, wherein adjusting the measured heights comprises: determining height corrections for respective measured heights of the patterned areas based on the fill factors, using a predetermined correspondence between measured heights and fill factors; and applying the height corrections to the respective measured heights. ### Claim 10 (depends on 1) The method of claim 1, further comprising: determining reflectivities of the patterned areas based on the broadband light interferometry, wherein the broadband light interferometry comprises respective instances of broadband light interferometry performed using respective wavelength bands of a plurality of wavelength bands; and determining the fill factors based on the reflectivities, using predetermined correspondences between reflectivities and fill factors for the plurality of wavelength bands; wherein adjusting the measured heights comprises: determining height corrections for respective measured heights of the patterned areas based on the fill factors, using a predetermined correspondence between measured heights and fill factors; and applying the height corrections to the respective measured heights. ### Claim 11 (depends on 10) The method of claim 10, wherein: the plurality of wavelength bands comprises a first color and a second color; the predetermined correspondences comprise a first correspondence between reflectivities and fill factors for the first color and a second correspondence between reflectivities and fill factors for the second color; and neither the first correspondence nor the second correspondence is one-to-one. ### Claim 12 A photomask-inspection system, comprising: a broadband light interferometer; photomask-inspection optics; one or more processors; and memory storing one or more programs for execution by the one or more processors, the one or more programs comprising instructions for: measuring heights on a surface of a photomask using the broadband light interferometer, the heights comprising heights of patterned areas of the photomask; producing a focal map from the measured heights on the surface of the photomask, comprising adjusting the measured heights of the patterned areas based on fill factors for the patterned areas; and inspecting the photomask for defects with the photomask-inspection optics, using the focal map. ### Claim 13 (depends on 12) The photomask-inspection system of claim 12, wherein: the instructions for measuring the heights comprise instructions for generating a height image for the photomask, using the broadband light interferometer; and the instructions for adjusting the measured heights comprise instructions for adjusting the height image to offset the measured heights of the patterned areas, based on the fill factors. ### Claim 14 (depends on 12) The photomask-inspection system of claim 12, wherein: the broadband light interferometer is a visible-light interferometer; and the photomask-inspection optics comprise 13.5 nm optics. ### Claim 15 (depends on 12) The photomask-inspection system of claim 12, wherein: the broadband light interferometer is a visible-light interferometer; and the photomask-inspection optics comprise 193 nm optics. ### Claim 16 (depends on 12) The photomask-inspection system of claim 12, wherein the broadband light interferometer is a Mirau interferometer. ### Claim 17 (depends on 12) The photomask-inspection system of claim 12, wherein the broadband light interferometer is a Michelson interferometer. ### Claim 18 (depends on 12) The photomask-inspection system of claim 12, wherein the one or more programs further comprise instructions for calculating the fill factors based on a database of the design for the photomask. ### Claim 19 (depends on 18) The photomask-inspection system of claim 18, wherein the instructions for adjusting the measured heights comprise: instructions for determining height corrections for respective measured heights of the patterned areas based on the fill factors, using a predetermined correspondence between measured heights and fill factors; and instructions for applying the height corrections to the respective measured heights. ### Claim 20 (depends on 12) The photomask-inspection system of claim 12, wherein: the one or more programs further comprise instructions for: determining reflectivities of the patterned areas based on the broadband light interferometry, wherein the broadband light interferometry comprises respective instances of broadband light interferometry performed using respective wavelength bands of a plurality of wavelength bands; and determining the fill factors based on the reflectivities, using predetermined correspondences between reflectivities and fill factors for the plurality of wavelength bands; and the instructions for adjusting the measured heights comprise instructions for: determining height corrections for respective measured heights of the patterned areas based on the fill factors, using a predetermined correspondence between measured heights and fill factors; and applying the height corrections to the respective measured heights. ### Claim 21 (depends on 20) The photomask-inspection system of claim 20, wherein: the plurality of wavelength bands comprises a first color and a second color; the predetermined correspondences comprise a first correspondence between reflectivities and fill factors for the first color and a second correspondence between reflectivities and fill factors for the second color; and neither the first correspondence nor the second correspondence is one-to-one. ### Claim 22 A non-transitory computer-readable storage medium storing one or more programs for execution by one or more processors of a photomask-inspection system that further comprises a broadband light interferometer and photomask-inspection optics, the one or more programs including instructions for: measuring heights on a surface of a photomask using broadband light interferometry, the heights comprising heights of patterned areas of the photomask; producing a focal map from the measured heights on the surface of the photomask, comprising adjusting the measured heights of the patterned areas based on fill factors for the patterned areas; and inspecting the photomask for defects with the photomask-inspection optics, using the focal map.