Interferometric methods and systems using low coherence illumination

• US 6,028,670 A
• Filed: 01/19/1998
• Issued: 02/22/2000
• Est. Priority Date: 01/19/1998
• Status: Expired due to Term

• Current Assignee: Zygo Corporation (Ametek Incorporated)
• Original Assignee: Zygo Corporation (Ametek Incorporated)
• Inventors: Deck, Leslie L.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US09/008,721
• Time in Patent Office: 764 Days
• Field of Search: 356/357, 356/359, 356/360
• US Class Current: 356/497
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 2290/35 Mechanical variable delay line
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02078 Caused by ambiguity
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation
• CPC Class Codes: G01B 9/0209 Low-coherence interferometers

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.

Claims

1. A method of determining a position of a peak of a contrast function, the method comprising the steps of:

(A) receiving light at a pixel of an imaging device, the light being received in the form of a three-dimensional interferogram comprised of a carrier modulated by the contrast function;

(B) digitally sampling an intensity variation of the three-dimensional interferogram to acquire N samples;

(C) determining M frequency domain transform coefficients based on the intensity variation;

(D) using L frequency domain transform coefficients in an equation which determines the position of the peak of the contrast function, the L frequency domain transform coefficients being among the M frequency domain transform coefficients determined in step (C), the L frequency domain transform coefficients corresponding to a low frequency component of the intensity variation.

2. A method according to claim 1, wherein L is equal to one, and wherein the equation has the form z=A tan^-1 (B), where z is the position of the contrast peak, A is a constant, and B is a value which is determined based on the frequency domain transform coefficient.

3. A method according to claim 2, wherein B=Im(c)/Re(c), where c is the frequency domain transform coefficient, Im(c) is an imaginary portion of the frequency domain transform coefficient and Re(c) is a real portion of the frequency domain transform coefficient.

4. A method according to claim 1, wherein L is equal to one, and wherein the frequency domain transform coefficient which is used in the equation is the lowest non-zero frequency domain coefficient transform which can be determined given the number N of samples acquired.

5. A method according to claim 1, wherein the pixel is a first pixel, wherein the contrast peak is a first contrast peak, and wherein the method further comprises the steps of

repeating steps (A) and (B) for light received at a second pixel to determine a position of a second contrast peak;

and

determining a difference in height between first and second locations on a test surface based on a difference in position of the first and second contrast peaks, the first and second locations reflecting the light received at the first and second pixels, respectively.

6. A method according to claim 1, wherein the L frequency domain transfer coefficients are coefficients of a rectified intensity signal, and wherein the separating step further comprises:

subtracting a DC component from the intensity variation to produce a DC-subtracted intensity signal, and then

rectifying the DC-subtracted intensity signal to produce the rectified intensity signal.

7. A method according to claim 6, wherein the rectifying step is performed by squaring the DC-subtracted intensity signal.

8. A method according to claim 1, wherein L is equal to M and M is equal to one.

9. A method of determining a difference in height between first and second locations on a test surface, the method comprising the steps of:

(A) producing an 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, and

(2) recombining the reference light beam and the test light beam;

(B) imaging the interferogram on a photosensitive device, 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, the light received at the first pixel and the light received at the second pixel each having an intensity variation which varies as a function of scan position and which is comprised of a carrier modulated by a contrast function;

(C) determining a first position of a first contrast peak of the light received at the first pixel, step (C) consisting essentially of the following steps

(1) determining a frequency domain transform coefficient based on the intensity variation, and

(2) using the frequency domain transform coefficient in an equation which determines a position of a peak of the contrast function, the equation having the form z=A tan^-1 (B), where z is the position of the contrast peak, A is a constant, and B is a value which is determined based on the frequency domain transform coefficient;

(D) determining a second position of a second contrast peak of the light received at the second pixel, the step (D) consisting essentially of repeating steps (C)(1) and (C)(2) for the light received at the second pixel; and

(E) determining the difference in height between the first and second locations based on a difference in position between the first contrast peak and the second contrast peak.

10. A method according to claim 9, wherein B=Im(c)/Re(c), where c is the frequency domain transform coefficient, Im(c) is an imaginary portion of the frequency domain transform coefficient and Re(c) is a real portion of the frequency domain transform coefficient.

11. A method according to claim 9, wherein the step of imaging the interferogram includes the step of digitally sampling the intensity variation of the light received by the first and second pixels to acquire a number of samples, and wherein the frequency domain transform coefficient is the lowest non-zero frequency coefficient which can be determined given the number N of samples acquired.

12. A method according to claim 9, wherein the step of determining the frequency domain transfer coefficient includes the steps of:

subtracting a DC component from the intensity variation to produce a DC-subtracted intensity signal, and then

squaring the DC-subtracted intensity signal to produce a rectified intensity signal.

13. An optical interferometer comprising:

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 photosensitive device, the photosensitive device having an interferogram imaged thereon, the interferogram being produced by a recombination of the reference light beam and the test light beam, the photosensitive device 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 a difference in height between the first and second locations based on a difference in position between a first contrast peak for light received by the first pixel and a second contrast peak for light received at the second pixel wherein, to determine the position of each of the first and second contrast peaks, the computer determines a frequency domain transform coefficient based on the intensity variation and uses the frequency domain transform coefficient in an equation having the form z=A tan^-1 (B), where z is the position of a respective one of the first and second contrast peaks, A is a constant, and B is a value which is determined based on the frequency domain transform coefficient.

14. An optical interferometer according to claim 13, wherein B=Im(c)/Re(c), where c is the frequency domain transform coefficient, Im(c) is an imaginary portion of the frequency domain transform coefficient and Re(c) is a real portion of the frequency domain transform coefficient.

15. An optical interferometer according to claim 13,

wherein only one frequency domain transform coefficient is determined to determine the first contrast peak, and

wherein only one frequency domain transform coefficient is determined to determine the second contrast peak.

16. A method of determining a difference in height between first and second locations on a test surface, the method comprising the steps of:

(A) producing a three-dimensional interferogram using a low coherence illumination source, 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, and

(2) recombining the reference light beam and the test light beam;

(B) imaging the three-dimensional interferogram on the photosensitive device, 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 a first position of a first contrast peak for the light received by the first pixel, including the steps of

(1) digitally sampling the intensity variation at a predefined rate to acquire a number N of samples,

(2) subtracting a DC component from the sampled intensity variation to produce a DC-subtracted intensity signal,

(3) rectifying the DC-subtracted intensity signal to produce a rectified intensity signal,

(4) determining a frequency domain transform coefficient for the rectified intensity signal, and

(5) determining the location of the contrast peak by using the frequency domain transform coefficient in an equation, the equation having the form z=A tan^-1 (Im(c)/Re(c)), where z is the position of the contrast peak, A is a constant, c is the frequency domain transform coefficient, Im(c) is an imaginary portion of the frequency domain transform coefficient, and Re(c) is a real portion of the frequency domain transform coefficient;

(D) determining a second position of a second contrast peak by repeating steps (C)(1) to (C)(5) for the light received by the second pixel; and

(E) determining the difference in height by comparing the first position of the first contrast peak and the second position of the second contrast peak.

17. A method according to claim 16, wherein for each of the first and second pixels, a total number M of frequency domain transform coefficients are determined based on the intensity variation, M being substantially less than N/2.

18. A method according to claim 17, wherein M is equal to one such that the frequency domain transform coefficient C is the only frequency transform coefficient which is determined.

19. A method according to claim 18, wherein the frequency domain transform coefficient C is the lowest non-zero frequency domain transform coefficient which can be determined given the number N of samples acquired.