# US 9,519,093 B2 · Broadband and wide field angle compensator > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 9,519,093 B2 - Document ID: US9519093B2 - Status: Granted - Kind: B2 - Application number: 14/466,369 - Filed: 2014-08-22 - Published: 2015-02-26 - Granted: 2016-12-13 - Earliest claimed: 2013-08-23 - Assignees: KLA-Tencor Corporation - Applicants: KLA-Tencor Corporation - Inventors: Lawrence Rotter; Klaus Flock; Muzammil Arain; David Y. Wang - Web page: https://patentgrep.com/patent/US9519093B2 ## CPC - G01N 21/23: Bi-refringence - G01N 21/211: {Ellipsometry (optical thickness measurement G01B11/06)} - G01N 21/9501: {Semiconductor wafers (manufacturing processes per se of semiconductor devices implementing a measuring step H10P74/20)} - G02B 5/3083: {Birefringent or phase retarding elements (G02B5/3008, G02B5/3016 take precedence; systems for polarisation control G02B27/286; manufacturing phase modulating patterns by lithographic processes G03F7/001)} - G02B 5/3091: {for use in the UV} - G02F 1/13363: Birefringent elements, e.g. for optical compensation - G01N 2021/213: {Spectrometric ellipsometry} - G01N 2201/068: Optics, miscellaneous - 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) - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS - G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS ## Abstract A rotatable compensator configured to transmit non-collimated light over a broad range of wavelengths, including ultraviolet, with a high degree of retardation uniformity across the aperture is presented. In one embodiment, a rotatable compensator includes a stack of four individual plates in optical contact. The two thin plates in the middle of the stack are made from a birefringent material and are arranged to form a compound, zeroth order bi-plate. The remaining two plates are relatively thick and are made from an optically isotropic material. These plates are disposed on either end of the compound, zeroth order bi-plate. The low order plates minimize the sensitivity of retardation across the aperture to non-collimated light. Materials are selected to ensure transmission of ultraviolet light. The optically isotropic end plates minimize coherence effects induced at the optical interfaces of the thin plates. ## Claims ### Claim 1 A parallel plate compensator, comprising: a first birefringent plate having a front surface, a back surface, an optic axis oriented in a plane parallel to the front and back surfaces within a manufacturing tolerance, and a thickness of less than five hundred micrometers; a second birefringent plate having a front surface, a back surface, an optic axis oriented in a plane parallel to the front and back surfaces within the manufacturing tolerance, and a thickness of less than five hundred micrometers, wherein the first birefringent plate and the second birefringent plate are oriented such that the optic axis of the first birefringent plate is aligned approximately perpendicular to the optic axis of the second birefringent plate; and a first optically isotropic plate in optical contact with the front surface of the first birefringent plate, the optically isotropic plate having a thickness of at least five hundred micrometers, wherein a refractive index of the optically isotropic plate is within 15% of a mean refractive index of the first birefringent plate. ### Claim 2 (depends on 1) The parallel plate compensator of claim 1, further comprising: a second optically isotropic plate in optical contact with the back surface of the second birefringent plate, the second optically isotropic plate having a thickness of at least five hundred micrometers, wherein a refractive index of the optically isotropic plate is within 15% of a mean refractive index of the second birefringent plate. ### Claim 3 (depends on 2) The parallel plate compensator of claim 2, wherein the back surface of the first birefringent plate and the front surface of the second birefringent plate are in optical contact. ### Claim 4 (depends on 2) The parallel plate compensator of claim 2, further comprising: a third optically isotropic plate in optical contact with the back surface of the first birefringent plate, wherein the third optically isotropic plate is disposed between the first birefringent plate and the second birefringent plate. ### Claim 5 (depends on 4) The parallel plate compensator of claim 4, further comprising: a fourth optically isotropic plate in optical contact with the front surface of the second birefringent plate, wherein the fourth optically isotropic plate is disposed between the first birefringent plate and the second birefringent plate. ### Claim 6 (depends on 1) The parallel plate compensator of claim 1, wherein a difference in the thickness of the first birefringent plate and the thickness of the second birefringent plate is less than 100 micrometers. ### Claim 7 (depends on 1) The parallel plate compensator of claim 1, wherein the first and second birefringent plates are made from any of a single crystal quartz material, a magnesium fluoride material, a beta barium borate material, a lithium tetraborate material, a cesium lithium borate material, a potassium dihydrogen phosphate material, and an ammonium dihydrogen phosphate material. ### Claim 8 (depends on 1) The parallel plate compensator of claim 1, wherein the optic axis of the first birefringent plate is aligned at ninety degrees with the optic axis of the second birefringent plate with a tolerance of five degrees. ### Claim 9 (depends on 1) The parallel plate compensator of claim 1, wherein the first optically isotropic plate is made from any of a fused silica material, a fused quartz material, a calcium fluoride material, a lithium fluoride material, a barium fluoride material, and a diamond. ### Claim 10 (depends on 1) The parallel plate compensator of claim 1, wherein the field of view is at least 5 milliradians. ### Claim 11 (depends on 1) The parallel plate compensator of claim 1, wherein the parallel plate compensator is at least 10% transmissive for light having any wavelength within a range between 190 and 880 nanometers. ### Claim 12 A parallel plate compensator, comprising: a birefringent plate having a front surface, a back surface, an optic axis oriented in a plane parallel to the front and back surfaces within the manufacturing tolerance, and a thickness of less than 100 micrometers; and a first optically isotropic plate in optical contact with the front surface of the first birefringent plate, the optically isotropic plate having a thickness of at least five hundred micrometers, wherein a refractive index of the optically isotropic plate is within 15% of a mean refractive index of the first birefringent plate, and wherein the parallel plate compensator is at least 10% transmissive for light having any wavelength within a range between 190 and 880 nanometers. ### Claim 13 (depends on 12) The parallel plate compensator of claim 12, further comprising: a second optically isotropic plate in optical contact with the back surface of the birefringent plate, the second optically isotropic plate having a thickness of at least five hundred micrometers, wherein a refractive index of the optically isotropic plate is within 15% of a mean refractive index of the birefringent plate. ### Claim 14 An ellipsometer, comprising: an illumination source configured to generate non-collimated, polychromatic illumination light directed toward a sample; a parallel plate compensator configured to transmit an amount of the illumination light, wherein the rotatable compensator comprises, a first birefringent plate having a front surface, a back surface, an optic axis oriented in a plane parallel to the front and back surfaces within a manufacturing tolerance, and a thickness of less than five hundred micrometers; a second birefringent plate having a front surface, a back surface, an optic axis oriented in a plane parallel to the front and back surfaces within the manufacturing tolerance, and a thickness of less than five hundred micrometers, wherein the first birefringent plate and the second birefringent plate are oriented such that their respective optic axes are aligned approximately perpendicular to one another; and a first optically isotropic plate in optical contact with the front surface of the first birefringent plate, an analyzer configured to extract a polarization state from the amount of light transmitted through the parallel plate compensator; and a detector configured to measure an intensity of an amount of light transmitted through the analyzer at different wavelengths throughout a wavelength range of interest that includes ultraviolet wavelengths. ### Claim 15 (depends on 14) The ellipsometer of claim 14, wherein the compensator is disposed in an optical path between the specimen and the analyzer. ### Claim 16 (depends on 14) The ellipsometer of claim 14, wherein the compensator is disposed in an optical path between the specimen and the polarizer. ### Claim 17 (depends on 14) The ellipsometer of claim 14, wherein the parallel plate compensator is rotatable about an axis that is perpendicular to a face of the parallel plate compensator. ### Claim 18 (depends on 14) The ellipsometer of claim 14, further comprising: a second optically isotropic plate in optical contact with the back surface of the second birefringent plate, the second optically isotropic plate having a thickness of at least five hundred micrometers, wherein a refractive index of the first and second optically isotropic plates is within 15% of a mean refractive index of the first and second birefringent plates. ### Claim 19 (depends on 14) The ellipsometer of claim 14, wherein the back surface of the first birefringent plate and the front surface of the second birefringent plate are in optical contact. ### Claim 20 (depends on 14) The ellipsometer of claim 14, wherein the parallel plate compensator is at least 10% transmissive for light having any wavelength within a range between 190 and 880 nanometers.