Optical profiler using improved phase shifting interferometry

• US 4,639,139 A
• Filed: 09/27/1985
• Issued: 01/27/1987
• Est. Priority Date: 09/27/1985
• Status: Expired due to Fees

• Current Assignee: WYKO, Inc.
• Original Assignee: WYKO, Inc.
• Inventors: Wyant, James C., Prettyjohns, Keith N.
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): Mottola, Steven J.
• Application Number:
  US06/781,261
• Time in Patent Office: 487 Days
• Field of Search: 356/349, 356/351, 356/359, 356/360, 364/525, 364/575
• US Class Current: 356/497
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 9/02002 using two or more frequencies
• CPC Class Codes: G01B 9/02012 using temporal intensity variation
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02069 Synchronization of light source or manipulator and detector

Abstract

An optical profiler includes a two-beam interferometer. An interference pattern produced thereby is focused onto an array of photocells. Phase shift in a reference beam of the interferometer is produced by accelerating a piezoelectric transducer supporting the interferometer mirror to a constant velocity. The velocity is maintained constant for at least 360° of phase shift, during which four integrated buckets are obtained from each photocell. The outputs of the photodetector array are continuously integrated and effectively read out every 90° of phase shift of the reference beam by a computer that computes a first value of phase corresponding to each photocell output from the first, second, and third integrated buckets produced by that photocell and a second phase value from the second, third, and fourth integrated bucket values obtained from that photocell. The two phase values are averaged, eliminating the effects of sinusoidal noise produced by inaccuracies in the reference beam phase at which integrated buckets are initiated and terminated. Data points at which intensity modulation produced by the reference beam phase variation is less than a noise threshold are masked from phase calculations. The intensity of the interferometer light source is automatically controlled by averaging intensity of the intereference pattern at angles that differ by 180° to cancel out the effects of the interference and obtain an average intensity. The lamp voltage is automatically adjusted to maintain the average intensity.

Claims

1. A method of producing information representing the distribution of the phase difference between first and second interfering beams, the first and second interfering beams producing an interference pattern, the method comprising the steps of:

(a) changing the phase difference between the first and second beams at a constant rate;

(b) guiding the interference pattern into an array of photodetectors;

(c) integrating photocurrents produced by the individual photodetectors, respectively, to produce integrated buckets that correspond to intensities of various points across the interference pattern;

(d) measuring first, second, third and fourth integrated buckets produced by each photodetector at times that correspond to first, second, third, and fourth approximately 90 degree changes in the phase difference between the first and second beams while continuously maintaining the rate of change of the phase difference between the first and second beams at a constant value;

(e) computing a first phase value corresponding to each photodetector from the first, second, and third integrated buckets produced by that photodetector;

(f) computing a second phase value, corresponding to each photodetector, from the second, third, and fourth integrated buckets produced by that photodetector; and

(g) adding the first and second phase values corresponding to each of the photodetectors to cancel sinusoidal error produced by small errors in the approximately 90 degree changes in the phase difference between the first and second beams.

2. The method of claim 1 wherein the second beam is a reference beam and step (a) includes changing the phase of the reference beam at the constant rate.

3. The method of claim 2 wherein the reference beam is produced by varying the phase of a first portion of a beam produced by a light source, and the second beam is produced by reflecting a second portion of the light source beam from a test surface to produce a test beam.

4. The method of claim 3 including using a two-beam interferometer to produce the reference beam and the test beam, and varying the phase of the reference beam by moving a reference mirror reflecting a portion of the light source beam at a constant velocity, and moving the reference mirror by applying a constant slope voltage ramp signal to a piezoelectrical transducer supporting the reference mirror, before performing step (d).

5. The method of claim 4 including allowing the piezoelectric transducer and reference mirror to attain a constant velocity before producing any of the integrated buckets to eliminate nonlinearities in the phase shift between the first and second phases.

6. The method of claim 1 wherein step (c) includes conducting photocurrents of the photodetectors into a plurality of integrated bucket capacitors, respectively, to produce the integrated buckets.

7. The method of claim 6 wherein the measuring of step (d) includes strobing various transfer devices coupling the respective integrated bucket capacitors to corresponding cells of a shift register device to discharge the integrated buckets from the integrated bucket capacitors into corresponding cells of the shift register device and to begin producing of subsequent integrated buckets.

8. The method of claim 7 including sequentially shifting the integrated buckets out of the shift register to produce an analog signal having various analog levels that represent the magnitudes of the various integrated buckets, respectively, amplifying the analog levels, and converting them to digital numbers, and reading the digital numbers representing the various integrated bucket magnitudes by means of a computer.

9. The method of claim 8 including performing steps (e), (f), and (g) by means of the computer.

10. The method of claim 1 wherein step (e) includes computing the first phase in accordance with the equation

11. The method of claim 10 wherein the first beam is a test beam reflected from a test surface, the method including computing the height of variations in the test surface in accordance with the equation

12. The method of claim 10 further including determining if modulation of intensity of the reference beam caused in response to variations in the phase difference between the first and second beams exceeds a predetermined noise threshold before computing the first and second phases.

13. The method of claim 12 including assigning a predetermined mask value to an average phase variable for a particular photodetector if the modulation of the intensity corresponding to that photodetector does not exceed the predetermined noise threshold.

14. The method of claim 12 including determining whether the modulation of the intensity exceeds the predetermined noise threshold by performing the steps of comparing the absolute values of C-B, A-B, D-C, and B-C to the predetermined noise threshold and determining that the modulation of intensity is greater than the noise threshold if at least one of those quantities is greater than the predetermined noise threshold.

15. The method of claim 1 including determining a first group of intensities corresponding to integrated buckets produced by a predetermined group of photocells at a first phase difference between the first and second beams, shifting the phase difference between the first and second beams by 180 degrees, determining a second group of intensities detected by the same group of photocells, respectively, from the integrated buckets produced thereby, adding corresponding first and second intensities to cancel variations due to interference between the first and second beams and to obtain an average intensity value, and adjusting the filament voltage of a light source from which the first and second beams are derived so that the average intensity value equals a predetermined intensity level.

16. The method of claim 1 wherein the small errors are in the range of zero degrees to approximately five degrees.

17. A method of providing information representing the distribution of phase difference between first and second interfering beams, the first and second interfering beams producing an interference pattern, the method comprising the steps of:

(a) changing the phase difference between the first and second beams at a constant rate;

(b) guiding the interference pattern into a photodetector;

(c) integrating the interference pattern by of the photodetector to produce integrated buckets that correspond intensities of various points across the interference pattern;

(d) measuring first, second, third, and fourth integrated buckets produced by the photodetector at times that correspond to first, second, third, and fourth approximately 90 degree changes in the phase difference between the first and second beams while continuously maintaining the rate of change of the phase difference between the first and second beams at a constant value;

(e) computing a first phase value from the first, second, and third integrated buckets;

(f) computing a second phase value from the second, third, and fourth integrated buckets; and

(g) adding the first and second phase values to cancel sinusoidal error produced by small errors in the 90 degree changes in the phase difference between the first and second beams.

18. An interferometric apparatus comprising:

(a) means for producing first and second beams and causing the first and second beams to interfere, producing an interference pattern;

(b) means for changing a phase difference between the first and second beams at a constant rate;

(c) means for guiding the interference pattern into an array of photodetectors, the array of photodetectors including means for integrating photocurrents produced by the individual photodetectors, respectively, to produce integrated buckets that correspond to intensities of various points across the interference pattern;

(d) means for measuring first, second, third and fourth integrated buckets produced by each photodetector at times that correspond to first, second, third, and fourth approximately 90 degree changes in the phase difference between the first and second beams while the rate of change in the phase difference between the first and second beams remain constant;

(e) means for computing a first phase value corresponding to each photodetector from the first, second, and third integrated buckets produced by that photodetector;

(f) means for computing a second phase value, corresponding to each photodetector, from the second, third, and fourth integrated buckets produced by that photodetector; and

(g) means for adding the first and second phase values corresponding to each of the photodetectors to cancel sinusoidal error produced by small errors in the approximately 90 degree changes in the phase difference between the first and second beams.

19. The interferometric apparatus of claim 18 wherein the small errors are in the range from zero degrees to approximately five degrees.

20. The interferometric apparatus of claim 18 wherein the array of photodetectors is included in a self-scanning optical sensor array device having a plurality of clock input conductors and an analog output conductor on which voltages representative of integrated buckets scanned in response to the clock inputs are sequentially output, respectively.

21. The interferometric apparatus of claim 20 wherein the measuring means includes clock and data acquisition circuitry for producing a plurality of clock signals applied to the clock input conductors of the self-scanning optical sensor array device, a computer, an interface circuit coupled between the computer and the clock and data acquisition circuitry, a piezoelectric transducer driver circuit, and wherein the phase difference changing means includes a piezoelectric transducer responsive to the piezoelectric transducer driver circuit and a reference mirror supported by the piezoelectric transducer and reflecting one of the first and second beams and movable to vary the phase of that beam.

22. The interferometric apparatus of claim 21 wherein the clock and data acquisition ciruitry includes an amplifier for amplifying the voltages representative of the integrated buckets produced by the self-scanning optical sensor array device and an analog-to-digital converter for converting the amplified voltages into digital representations of the respective integrated buckets, the interface circuit including means for interfacing between the computer and the output of the analog-to-digital converter to enable the computer to read the digital representations of the integrated buckets in synchronization with the scanning of the self-scanning optical sensor array device.

23. The interferometric apparatus of claim 22 wherein the piezoelectric transducer driver circuit includes an address circuit that is incremented by one of the clock signals in synchronization with the clocking of the self-scanning optical sensor array device, a random access memory couple to the computer for storing information representative of a voltage ramp signal, a digital-to-analog converter for converting data output the random access memory in response to the address circuit for producing the voltage ramp signal, and an output amplifier for producing an amplified voltage ramp signal and applying it to the piezoelectric transducer.

24. The interferometric apparatus of claim 21 including means for allowing the piezoelectric transducer and the reference mirror supported thereby to attain a constant velocity before the measuring means begins measuring the first, second, and fourth integrated buckets.