# US 10,871,369 B2 · Systems for and methods of measuring photomask flatness with reduced gravity-induced error > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 10,871,369 B2 - Document ID: US10871369B2 - Status: Granted - Kind: B2 - Application number: 16/527,917 - Filed: 2019-07-31 - Published: 2020-02-06 - Granted: 2020-12-22 - Earliest claimed: 2018-08-02 - Assignees: Corning Incorporated - Applicants: Corning Incorporated - Inventors: Thomas James Dunn; John Weston Frankovich; Robert Dennis Grejda; Christopher Alan Lee; Matthew Ronald Millecchia; Yoshihiro Nakamura - Web page: https://patentgrep.com/patent/US10871369B2 ## CPC - G01B 11/306: {for measuring evenness} - G01B 9/02075: {of particular errors} - G01B 11/2441: {using interferometry} - G03F 1/22: Masks or mask blanks for imaging by radiation of 100nm or shorter wavelength, e.g. X-ray masks, extreme ultraviolet [EUV] masks; Preparation thereof - G03F 1/84: Inspecting - G03F 7/707: {Chucks, e.g. chucking or un-chucking operations or structural details} - G03F 7/70783: {Handling stress or warp of chucks, masks or workpieces, e.g. to compensate for imaging errors or considerations related to warpage of masks or workpieces due to their own weight} - 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 The methods disclosed herein include recording at near-vertical first and second measurement positions respective first and second interferograms of the photomask surface and defining a difference map as the difference between the first and second interferograms. Respective first and second normal forces on the photomask are also measured at the first and second measurement positions. The change in the normal force is used define a scaling factor, which is applied to the difference map to define a scaled difference map. A compensated flatness measurement with a reduced shape contribution due to gravity is obtained by subtracting the scaled difference map from the first interferogram. An interferometer-based flatness measurement system is also disclosed. ## Claims ### Claim 1 A method of interferometrically measuring flatness of a photomask supported at a support location, comprising: recording at first and second measurement positions respective first and second interferograms of a surface of the photomask, and measuring respective first and second normal forces F1 and F2 at the support location, wherein the first and second measurement positions define a difference ΔF=F2−F1 in the normal force; defining a difference map as a difference between the first and second interferograms; and using the difference in normal force ΔF and the normal force F1 at the first measurement position to define a scaling factor S and applying the scaling factor S to the difference map to define a scaled difference map. ### Claim 2 (depends on 1) The method according to claim 1, further comprising: defining a compensated flatness by subtracting the scaled difference map from the first interferogram. ### Claim 3 (depends on 1) The method according to claim 1, wherein the photomask comprises an extreme ultraviolet (EUV) photomask. ### Claim 4 (depends on 1) The method according to claim 1, wherein the photomask is a patterned photomask. ### Claim 5 (depends on 1) The method according to claim 1, wherein the recording of the first and second interferograms is performed using a Fizeau interferometer. ### Claim 6 (depends on 5) The method according to claim 5, further comprising adjusting the Fizeau interferometer between first and second measurement angles θ1 and θ2 that define the first and second measurement positions. ### Claim 7 (depends on 6) The method according to claim 6, wherein the first and second measurement angles θ1 and θ2 are between 1° and 5°. ### Claim 8 (depends on 1) The method according to claim 1, wherein the photomask is supported by an adjustable support apparatus that comprises three support members configured to provide three support locations for the photomask, the adjustable support apparatus configured to place the photomask in a stand-off configuration, the recording of the first and second interferograms performed in the stand-off configuration. ### Claim 9 (depends on 1) The method according to claim 1, further comprising: recording a first plurality of interferograms at the first measurement position and combining the first plurality of interferograms to form the first interferogram; and recording a second plurality of interferograms at the second measurement position and combining the second plurality of interferograms to form the second interferogram. ### Claim 10 (depends on 1) The method of claim 1, further comprising classifying the photomask based on the scaled difference map. ### Claim 11 A method of interferometrically measuring flatness of a photomask, comprising: recording at first and second measurement angles θ1 and θ2 respective first and second interferograms IF1 and IF2 of a surface of the photomask supported by an adjustable support apparatus, wherein θ2>θ1 and wherein θ1 and θ2 are each within 11.46° of vertical; measuring at the first and second measurement angle θ1 and θ2 respective first and second normal forces F1 and F2 of the photomask on the adjustable support apparatus, wherein a force difference is defined as ΔF=F2−F1; defining a difference map DM=IF2−IF1 as a difference between the second and first interferograms IF2 and IF1; defining either a first scaled difference map DM1=(F1/ΔF)·DM or a second scaled difference map DM2=(F2/ΔF)·DM; and defining a compensated flatness measurement CFM by either: i) subtracting the first difference map from the first interferogram (CFM=IF1−DM1); or ii) subtracting the second difference map from the second interferogram (CFM=IF2−DM2). ### Claim 12 (depends on 11) The method according to claim 11, further comprising supporting the photomask on a support apparatus comprising two base support members and a top support member, wherein the top support member is operably connected to a force sensor, and determining the first and second normal forces F1 and F2 using the force sensor. ### Claim 13 (depends on 11) The method according to claim 11, wherein the measurement angles θ1 and θ2 are between 1° and 5°.