Scanning interferometer for aspheric surfaces and wavefronts

• US 7,218,403 B2
• Filed: 02/24/2005
• Issued: 05/15/2007
• Est. Priority Date: 06/26/2002
• Status: Expired due to Term

• Current Assignee: Zygo Corporation (Ametek Incorporated)
• Original Assignee: Zygo Corporation (Ametek Incorporated)
• Inventors: Kuchel, Michael
• Primary Examiner(s): Toatley, Jr.; Gregory J.
• Assistant Examiner(s): Lyons; Michael A.
• Application Number:
  US11/064,731
• Publication Number:
  US 20050157311A1
• Time in Patent Office: 810 Days
• Field of Search: 356/457, 356/458, 356/489, 356/495, 356/511, 356/512, 356/513, 356/515
• US Class Current: 356/513
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 11/255 for measuring radius of curvature measuring diameter G01B11/08
• CPC Class Codes: G01B 2290/45 Multiple detectors for detecting interferometer signals
• CPC Class Codes: G01B 2290/70 Using polarization in the interferometer
• CPC Class Codes: G01B 9/02019 contacting different points on same face of object
• CPC Class Codes: G01B 9/02039 by matching the wavefront with a particular object surface shape
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02072 by calibration or testing of interferometer
• CPC Class Codes: G01B 9/02081 simultaneous quadrature detection, e.g. by spatial phase shifting
• CPC Class Codes: G01B 9/02087 Combining two or more images of the same region
• CPC Class Codes: G01M 11/025 by determining the shape of the object to be tested measuring contours or curvatures by optical means G01B11/24
• CPC Class Codes: G01M 11/0257 by analyzing the image formed by the object to be tested
• CPC Class Codes: G01M 11/0271 by using interferometric methods

Abstract

Interferometric scanning method(s) and apparatus for measuring optics either having aspherical surfaces or that produce aspherical wavefronts. A test optic is aligned and moved with respect to a scanning axis relative to the origin of a known spherical wavefront that is generated with a reference surface to intersect the test optic at the apex of the aspherical surface and at radial zones where the spherical wavefront and the aspheric surface possess common tangents. The test surface is imaged onto a space resolving detector to form interferograms containing phase information about the differences in optical path length between the reference surface and the test surface while the axial distance which the test optic moves relative to the spherical reference surface is interferometrically measured. The deviation in the shape of the aspheric surface from its design in a direction normal to the aspheric surface is determined and reported.

Claims

1. An interferometric scanning method for measuring rotationally and non-rotationally symmetric test optics having aspherical surfaces, said method comprising the steps of:

generating at least a partial spherical wavefront along a scanning axis through the use of a decollimator carrying a spherical reference surface positioned along said scanning axis at a known location;

aligning a test optic with respect to said scanning axis and selectively moving said test optic along said scanning axis relative to said spherical reference surface so that said spherical wavefront intersects the test optic at the apex of the aspherical surface and at one or more annular zones where the spherical wavefront and the aspheric surface intersect where their tangents are common;

imaging the test surface onto a space resolving detector to form interferograms containing phase information about the differences in optical path length between the spherical reference surface and the test surface;

interferometrically measuring the axial distance, ν

, by which said test optic moves relative to said spherical reference surface; and

analyzing said interferograms to determine the deviation in shape of said aspheric surface compared with its design in a direction normal to the aspheric surface based on the axial distance, ν

, and the phase information contained in said interferograms and providing output data of said deviation.

2. The interferometric scanning method of claim 1 wherein, prior to movement of the test optic with respect to the spherical reference surface, the axial distance, ν

, is set equal to zero and the initial gap, g, the axial distance between the reference surface and the test optic, is measured.

3. The interferometric scanning method of claim 1 where the common tangency where said spherical wavefront intersects the test optic occurs in a circular region near the apex of said aspherical surface and otherwise in said one or more annular zones spaced radially with respect to the axis.

4. The interferometric scanning method of claim 2 wherein said axial circular region and said annular zones are simultaneously imaged onto separate detectors using imaging optics of different magnification with the higher magnification imaging optics being used to image said axial circular region.

5. The interferometric scanning method of claim 4 wherein the average optical path length difference between the reference surface and the test optic around the annular zones is used in subsequent calculations to minimize to first order the effects of tilt and tip.

6. The interferometric scanning method of claim 5 wherein the location of the apex of said aspheric surface within said circular region is determined by interpolation.

7. An interferometric scanning method for measuring rotationally and non-rotationally symmetric test optics having aspherical surfaces, said method comprising the steps of:

generating at least a partial spherical wavefront from a known origin along a scanning axis through the use of a decollimator carrying a spherical reference surface positioned along said scanning axis at a known location with respect to said known origin;

aligning a test optic with respect to said scanning axis and selectively moving said test optic along said scanning axis relative to said spherical reference surface so that said spherical wavefront intersects the test optic at the apex of the aspherical surface and at one or more radial positions where the spherical wavefront and the aspheric surface intersect at points of common tangency;

imaging the test surface onto a space resolving detector to form interferograms containing phase information about the differences in optical path length between the spherical reference surface and the test surface at the one or more positions where points of common tangency occur and generating electrical signals carrying said phase information;

interferometrically measuring the axial distance, ν

, by which said test optic moves with respect to said origin; and

determining the deviation in shape of said aspheric surface compared with its design in a direction normal to the aspheric surface based on the axial distance, ν

, and the phase information contained in said electrical signals and providing output data of said deviation.

8. The interferometric scanning method of claim 7 wherein said step of determining said deviation in the shape of said aspheric surface comprises the steps of:

calculating the optical path length differences, p, between the center of the test optic and the one or more radial positions based on said phase differences contained in said electronic signal; and

calculating the coordinates, z and h, of the aspherical surface wherever said circles of curvature have intersected the aspherical surface at common points of tangency and in correspondence with the interferometrically measured distance, v and calculated optical path lengths, p.

9. The interferometric scanning method of claim 7 wherein, prior to movement of the test optic with respect to the spherical reference surface, the axial distance, ν

, is set equal to zero and the initial gap (g), the axial distance between the reference surface and the test optic, is measured.

10. The interferometric scanning method of claim 9 wherein the coordinates of the test optic are given by abscissa, z, and ordinate, h, and wherein the test optic and said reference surface are moved relative to one another by incremental amounts, Δ

ν

=fct(Δ

h).

11. The interferometric scanning method of claim 10 wherein measurement data is generated at the apex of the test optic to obtain, d₀, the axial separation between the test and reference surfaces, and d, the normal separation of the test and reference surfaces where points of common tangency occur.

12. The interferometric scanning method of claim 11 further including calculating the measured axial distance as, ν

_m =g−

d₀, and the measured optical path length difference at the zone of the test optic as p_m=d−

d₀.

13. The interferometric scanning method of claim 12 wherein the design optical path length difference, p_d is computed independently of h as p_d=p_d(ν

m).

14. The interferometric scanning method of claim 13 wherein the measured ordinate, h_m, is computed as h_m=(R₀+ν

_m−

p_m)·

√

{square root over (p)}_m·

(2−

P_m).

15. The interferometric scanning method of claim 14 wherein the deviation in the normal direction between the design and measured values of the test surface are given as Δ

n=p_m−

p_d where Δ

n=fct(Δ

h_m).

16. The interferometric scanning method of claim 7 wherein said points of common tangency where said spherical wavefront intersects the test optic occur in a circular region near the apex of said aspherical surface and in one or more annular zones spaced radially with respect to the axis.

17. The interferometric scanning method of claim 16 wherein said axial circular region and said annular zones are simultaneously imaged onto separate detectors using imaging optics of different magnification with the higher magnification imaging optics being used to image said axial circular region.

18. The interferometric scanning method of claim 16 wherein the average optical path length difference between the reference surface and the test optic around the annular zones is used in subsequent calculations to minimize to first order the effects of tilt and tip.

19. The interferometric scanning method of claim 7 where, prior to measuring the gap, g, an offset in the abscissa between the design and measured curves for the shape of the aspheric test surface is calculated to provide a correction for the origin of the measurement of the axial distance, ν

_m.

20. The interferometric scanning method of claim 13 wherein the design optical path length difference, p_d, is computed independently of h as p_d=p_d(ν

_m) by eliminating h from the two following equations;

21. A scanning method for measuring rotationally and non-rotationally symmetric test optics having aspherical surfaces, said method comprising the steps of:

generating at least a partial spherical wavefront from a known origin along a scanning axis through the use of a decollimator carrying a spherical reference surface positioned along said scanning axis upstream of said known origin;

aligning a test optic with respect to said scanning axis and selectively moving said test optic along said scanning axis relative to said known origin so that said spherical wavefront intersects;

the test optic at the apex of the aspherical surface and at one or more radial positions where the spherical wavefront and the aspheric surface intersect at points of common tangency;

imaging the test surface onto a space resolving detector to form interferograms containing phase information about the differences in optical path length between the spherical reference, surface and the test surface at the one or more positions where points of common tangency occur and generating electrical signals carrying said phase information;

interferometrically measuring the axial distance, ν

, by which said test optic moves with respect to said origin and calculating the optical path length differences, p, between the center of the test optic and the one or more radial positions based on said phase differences contained in said electronic signal,

calculating the coordinates, z and h, of the aspherical surface wherever said circles of curvature have intersected the aspherical surface at common points of tangency and in correspondence with the interferometrically measured distance, ν and

calculated optical path lengths, p; and

determining the deviation in shape of said aspheric surface compared with its design in a direction normal to the aspheric surface and providing output data of said deviation.

22. Interferometric scanning apparatus for measuring rotationally and non-rotationally symmetric test optics having aspherical surfaces, said apparatus comprising:

a source for generating a collimated beam of radiation along a scanning axis;

a decollimator carrying a spherical reference surface positioned at a known location along said scanning apparatus to receive said collimated beam and generate at least a partial spherical wavefront;

a precision positioning arrangement for holding and aligning a test optic with respect to said scanning axis and selectively moving said test optic along said scanning axis relative to said spherical reference surface so that said spherical wavefront intersects the test optic at the apex of the aspherical surface and at one or more annular zones where the spherical wavefront and the aspheric surface intersect where their tangents are common;

at least one space resolving detector;

an optical arrangement for imaging the test surface onto said space resolving detector to form interferograms containing phase information about the differences in optical path length between the spherical reference surface and the test surface;

an interferometer for measuring the axial distance;

ν

, by which said test optic moves relative to said spherical reference surface; and

a control and analysis arrangement for analyzing said interferograms to determine the deviation in shape of said aspheric surface compared with its design in a direction normal to the aspheric surface based on the axial distance, ν

, and the phase information contained in said interferograms.

23. The interferometric scanning apparatus of claim 22 wherein said precision positioning arrangement and said control and analysis arrangement are configured and arranged so that, prior to movement of the test optic with respect to the spherical reference surface, the axial distance, ν

, is set equal to zero and the initial gap, g, the axial distance between the reference surface and the test optic, is measured.

24. The interferometric scanning apparatus of claim 22 where the common tangency where said spherical wavefront intersects the test optic occurs in a circular region near the apex of said aspherical surface and otherwise in said one or more annular zones spaced radially with respect to the axis.

25. The interferometric scanning apparatus of claim 24 further including another detector and wherein said optical arrangement is further configured to simultaneously image said axial circular region and said annular zones onto said detectors separately using imaging optics of different magnification with the higher magnification imaging optics being used to image said axial circular region.

26. The interferometric scanning apparatus of claim 25 wherein the average optical path length difference between the reference surface and the test optic around the annular zones is used in subsequent calculations to minimize the effects of tilt and tip to first order.

27. The interferometric scanning apparatus of claim 26 wherein the location of the apex of said aspheric surface within said circular region is determined by interpolation.