Interferometric methods and systems using low coherence illumination
First Claim
1. A method of determining a difference in height between first and second locations on a test surface using a single three-dimensional interferogram, the method comprising the steps of:
- (A) producing and acquiring the single three-dimensional interferogram; and
(B) performing both a scanning white light interferometry analysis and a phase shifting interferometry analysis on the single three-dimensional interferogram to determine the difference in height.
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Accused Products
Abstract
Low coherence illumination is used in interferometric methods and systems which exhibit improved precision and flexibility. According to a first aspect of the invention, both a phase shifting interferometry (PSI) analysis and a scanning white light interferometry (SWLI) analysis are applied to a single 3D interferogram. This allows the precision of PSI to be achieved without being limited by the 2π phase ambiguity constraint. According to another aspect of the invention, a position of a contrast peak of a broad-band 3D interferogram is located by calculating a single Fourier transform coefficient, using the Fourier transform coefficient to determine an analytic equation which corresponds to the derivative of the visibility function, and locating the peak contrast position by setting the derivative equal to zero. The use of analytic equation contrast peak detection improves accuracy and computational efficiency. According to another aspect of the invention, a PSI analysis is applied to a broad-band 3D interferogram generated using a low coherence illumination source. The ability to perform a PSI analysis on a broad-band 3D interferogram promotes flexibility because it allows for a device which can perform both PSI analyses and SWLI analyses without physical alterations to the illumination source.
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Citations
27 Claims
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1. A method of determining a difference in height between first and second locations on a test surface using a single three-dimensional interferogram, the method comprising the steps of:
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(A) producing and acquiring the single three-dimensional interferogram; and (B) performing both a scanning white light interferometry analysis and a phase shifting interferometry analysis on the single three-dimensional interferogram to determine the difference in height. - View Dependent Claims (2)
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3. A method of determining a profile of a test surface, the method comprising the steps of:
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(A) producing a three-dimensional interferogram, including the steps of (1) reflecting a reference light beam and a test light beam, the test light beam being reflected from the test surface, (2) recombining the reference light beam and the test light beam, and (3) varying a relative phase between the reference light beam and the test light beam during the performance of steps (A)(1) and (A)(2); (B) receiving interference data at a plurality of pixels of a photosensitive device, the plurality of pixels including a first pixel which receives light reflected from a first location on the test surface and a second pixel which receives light reflected from a second location on the test surface; (C) determining a difference in height between the first and second locations, including the steps of (1) determining a first indication of the difference in height by (a) determining a first temporal position of a first feature of the light received at the first pixel and (b) determining a second temporal position of a second feature of the light received at the second pixel, (2) determining a second indication of the difference in height by (a) determining a first phase of the light received at the first pixel and (b) determining a second phase of the light received at the second pixel, and (3) combining the first and second indications of the difference in height; and (D) determining respective differences in height between a plurality of additional locations on the test surface and the first location by repeating steps (C)(1), (C)(2) and (C)(3) using light received at a remaining one of the plurality of pixels during each repetition of steps (C)(1)(b) and (C)(2)(b). - View Dependent Claims (4, 5, 6, 7, 8, 9, 10, 11)
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12. An optical interferometer comprising:
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an illumination source, the illumination source producing a light beam; a beam splitter, the beam splitter splitting the light beam into a reference light beam and a test light beam, the test light beam being reflected from a test surface; a phase shifter, the phase shifter varying the relative phase between the reference light beam and the test light beam; a photosensitive device, the photosensitive device having an interferogram imaged thereon, the interferogram being a three-dimensional interferogram assembled from a plurality of frames of interferometric data, the interferometric data being produced by a recombination of the reference light beam and the test light beam, the photosensitive device having a plurality of pixels which receive light from a plurality of respective locations on the test surface, including a first pixel which receives light reflected from a first location on the test surface and a second pixel which receives light reflected from a second location on the test surface; and a computer, the computer being coupled to the photosensitive device, the computer determining relative differences in height between the plurality of locations on the test surface, the relative differences in height being determined based on a combination of a scanning white light interferometry analysis and a phase shifting interferometry analysis both of which are performed on the same three-dimensional interferogram by the computer. - View Dependent Claims (13, 14)
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15. A method of determining a difference in height between first and second locations on a test surface, the method comprising the steps of:
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(A) producing an interferogram using a low coherence illumination source, including the steps of (1) reflecting a reference light beam from a reference surface and a test light beam, the test light beam being reflected from the test surface, (2) recombining the reference light beam and the test light beam, and (3) varying a relative phase between the reference light beam and the test light beam during the performance of steps (A)(1) and (A)(2); (B) imaging the interferogram on a photosensitive device, the interferogram being imaged over time so as to be a three-dimensional interferogram comprised of a plurality of frames of interferometric data, the photosensitive device including a first pixel which receives light reflected from the first location and a second pixel which receives light reflected from the second location; (C) performing a scanning white light interferometry analysis on the three-dimensional interferogram to determine a gross indication of the difference in height, including the steps of (1) determining a first position of a first contrast peak for the light received by the first pixel, (2) determining a second position of a second contrast peak for the light received by the second pixel, (3) calculating an elapsed time interval between the occurrence of the first and second contrast peaks, and (4) calculating the gross indication of the difference in height based on the elapsed time interval and a rate at which the relative phase is varied during step (A)(3); (D) performing a phase shifting interferometry analysis on the three-dimensional interferogram to determine a fine indication of the difference in height, including the steps of (1) calculating first and second weighted sums of intensities, the intensities being intensities of the light received at the first pixel measured at a first sequence of scan positions, (2) calculating a first phase for the light received at the first pixel, including the step of calculating an arctangent of the first and second weighted sums of intensities, (3) calculating third and fourth weighted sums of intensities, the intensities being intensities of the light received at the second pixel measured at a second sequence of scan positions, (4) calculating a second phase for the light received at the second pixel, including the step of calculating an arctangent of the third and fourth weighted sums of intensities, and (5) comparing the first phase with the second phase to yield the fine indication of the difference in height; and (E) combining the gross indication of the difference in height with the fine indication of the difference in height to determine the difference in height. - View Dependent Claims (16, 17)
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- 18. A method of determining a difference in height between first and second locations on a test surface, the method comprising the steps of producing a three-dimensional interferogram using a low coherence illumination source and performing a phase shifting interferometry analysis on the three-dimensional interferogram, the phase shifting interferometry analysis being performed using a first sequence of scan positions for the first location on the test surface and using a different second sequence of scan positions for the second location on the test surface.
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26. A method of determining a difference in height between first and second locations on a test surface using an interferometer, the method comprising the steps of:
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(A) producing an interferogram using a low coherence illumination source, including the steps of (1) reflecting a reference light beam from a reference surface and a test light beam, the test light beam being reflected from the test surface, (2) recombining the reference light beam and the test light beam, and (3) varying a relative phase between the reference light beam and the test light beam during the performance of steps (A)(1) and (A)(2) such that the interferogram is a three-dimensional interferogram; (B) imaging the interferogram on a photosensitive device, the interferogram being imaged over time so as to be a three-dimensional interferogram comprised of a plurality of frames of interferometric data, the photosensitive device including a first pixel which receives light reflected from the first location and a second pixel which receives light reflected from the second location; (C) determining information regarding scan variation throughout a range of scan positions, including the steps of (1) determining a first temporal phase history for the light reflected from the first location, including the step of temporally unwrapping the phase of the light reflected from the first location, (2) determining a second temporal phase history for the light reflected from the second location, including the step of temporally unwrapping the phase of the light reflected from the second location, and (3) temporally stitching the first temporal phase history together with the second temporal phase history to form a stitched temporal phase history; (D) determining a first phase of the light received at the first pixel using a first sequence of scan positions, and determining a second phase of the light received at the second pixel using a second sequence of scan positions; and (E) determining the difference in height using the three-dimensional interferogram, including the steps of determining a phase difference between the first phase and the second phase, a first portion of the phase difference being attributable to the difference in height between the first and second locations, and a second portion of the phase difference being attributable to the variation in phase which occurs during step (A)(3), and phase connecting the first sequence of scan positions with the second sequence of scan positions using the scan variation information By determining the second portion of the phase difference using the scan variation information to determine the second portion of the phase difference using the scan variation information. - View Dependent Claims (27)
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Specification