# US 2004/0184038 A1 · Method and apparatus for measuring the shape and thickness variation of polished opaque plates > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2004/0184038 A1 - Document ID: US20040184038A1 - Status: Published application - Kind: A1 - Application number: 10/393,883 - Filed: 2003-03-20 - Published: 2004-09-23 - Earliest claimed: 2003-03-20 - Assignees: Phase Shift Technology, Inc. - Inventors: Klaus Freischlad; Shouhong Tang - Web page: https://patentgrep.com/patent/US20040184038A1 ## CPC - G01B 11/30: for measuring roughness or irregularity of surfaces - G01B 9/02021: {contacting different faces of object, e.g. opposite faces} - G01B 9/02027: {Two or more interferometric channels or interferometers} - G01B 9/02028: {Two or more reference or object arms in one interferometer} - G01B 11/06: for measuring thickness {; e.g. of sheet material (thickness measurement by thermal means G01B21/085)} - G01B 2290/45: Multiple detectors for detecting interferometer signals - G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS ## Abstract The present invention consists of a technique and device for measuring the thickness variation and shape of wafers or other polished opaque plates. A combination of two improved phase-shifting Fizeau interferometers is used to simultaneously measure the single-sided distance maps between each side of the wafer and the corresponding reference flat, with the thickness variation and shape being calculated from these data. Provisions are made to determine and eliminate the shape and tilt of the reference surfaces, and also to facilitate the correct overlay of the two single-sided measurements for the calculation of thickness variation and shape. ## Claims ### Claim 1 A method for measuring the thickness variation and shape of polished opaque plates including the steps of: placing a polished opaque plate in a cavity formed between reference flats in two opposing interferometer channels to map the opposite surfaces of the plate; synchronizing the phase shifting of the interferograms in the two interferometer channels; measuring the cavity characteristics of the reference flats forming the cavity; and determining the thickness variations of the plate from the surface mapping of the opposite surfaces of the plate and the cavity characteristic measurements. ### Claim 2 (depends on 1) The method according to claim 1 wherein the step of synchronizing the phase shifting of the interferograms in the two interferometer channels comprises supplying the light from a single tunable light source, the output beam of which is split by a beam splitter to propagate to both interferometer channels. ### Claim 3 (depends on 2) The method according to claim 2 wherein the light source is a wavelength tunable laser. ### Claim 4 (depends on 3) The method according to claim 3 wherein the step of measuring the cavity characteristics of the reference flats forming the cavity includes providing an oversized field of view of the opaque plate in the cavity for measuring the cavity tilt of the reference flats. ### Claim 5 (depends on 4) The method according to claim 4 further including the step of blocking the recording of spurious light from one interferometer channel in the opposing interferometer channel. ### Claim 6 (depends on 5) The method according to claim 5 wherein the step of measuring the cavity tilt is a step of measuring the cavity tilt substantially simultaneously with mapping of opposite surfaces of the plate. ### Claim 7 (depends on 6) The method according to claim 6 further including the step of providing a first frequency phase shift in the interferometer channels to map the surfaces of the plate and providing a second frequency phase shift for measuring the cavity tilt of the reference flats. ### Claim 8 (depends on 1) The method according to claim 1 wherein the step of measuring the cavity characteristics is a step of measuring the cavity tilt of the reference flats substantially simultaneously with mapping of opposite surfaces of the plate. ### Claim 9 (depends on 8) The method according to claim 8 wherein the step of measuring the cavity tilt of the reference flats forming the cavity includes providing an oversized field of view of the opaque plate in the cavity. ### Claim 10 (depends on 9) The method according to claim 9 further including the step of blocking the recording of spurious light from one interferometer channel in the opposing interferometer channel. ### Claim 11 (depends on 10) The method according to claim 10 further including the step of providing a first frequency phase shift in the interferometer channels to map the surfaces of the plate and providing a second frequency phase shift for measuring the cavity tilt of the reference flats. ### Claim 12 (depends on 1) The method according to claim 1 further including the step of blocking the recording of spurious light from one interferometer channel in the opposing interferometer channel. ### Claim 13 (depends on 1) The method according to claim 1 further including the step of providing a first frequency phase shift in the interferometer channels to map the surfaces of the plate and providing a second frequency phase shift for measuring the cavity characteristics of the reference flats. ### Claim 14 An apparatus for measuring the thickness variation and shape of polished opaque plates including in combination: first and second spaced apart reference flats forming a cavity therebetween for placement of a polished opaque plate wherein the first and second reference flats have dimensions such that an oversized field of view is provided for the opaque plate; first and second interferometer devices located on diametrically opposite sides of the cavity to map the opposite first and second surfaces of the plate; a light source; means for synchronizing the phase shifting of interferograms in the two interferometer devices; first and second interferogram detectors; and at least one computer coupled to receive the outputs of the first and second interferogram detectors for determining thickness variations of the plate. ### Claim 15 (depends on 14) The device according to claim 14 further coupling the at least one computer with the means for synchronizing the phase shifting of the interferograms in the two interferometer devices. ### Claim 16 (depends on 14) A device according to claim 14 wherein the light source is a tunable laser light source coupled with the computer to control the phase shifting of the interferograms by wavelength shifting of light. ### Claim 22 (depends on 14) The device according to claim 14 further including means for circularly polarizing the outgoing beam of light directed toward the cavity in each of the interferometer devices with a polarization of the same handedness in both devices. ### Claim 23 (depends on 22) The device according to claim 22 wherein the means for circularly polarizing the outgoing beam of light in the first and second interferometer devices is a quarter-wave plate. ### Claim 24 (depends on 14) The device according to claim 14 wherein the first and second reference flats are coated with a material producing different reflectance for light impinging on each of the first and second reference flats from opposing directions.