Stitching of near-nulled subaperture measurements

• US 20090251702A1
• Filed: 04/08/2009
• Published: 10/08/2009
• Est. Priority Date: 04/08/2008
• Status: Active Grant

• Current Assignee: QED Technologies International LLC
• Original Assignee: OED Technologies International Incorporated
• Inventors: Murphy, Paul, Devries, Gary, Forbes, Greg, Brophy, Christopher
• Granted Patent: US 8,203,719 B2
• Time in Patent Office: Days
• US Class Current: 356/450
• 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 9/02058 by particular optical compensation or alignment elements, e.g. dispersion compensation
• CPC Class Codes: G01M 11/0207 Details of measuring devices
• CPC Class Codes: G01M 11/0257 by analyzing the image formed by the object to be tested

Abstract

A metrology system for measuring aspheric test objects by subaperture stitching. A wavefront-measuring gauge having a limited capture range of wavefront shapes collects partially overlapping subaperture measurements over the test object. A variable optical aberrator reshapes the measurement wavefront with between a limited number of the measurements to maintain the measurement wavefront within the capture range of the wavefront-measuring gauge. Various error compensators are incorporated into a stitching operation to manage residual errors associated with the use of the variable optical aberrator.

Claims

1. A method of expanding the operating range of a metrology system for measuring aspheric test objects comprising steps of:

associating a wavefront propagator with a physical test object in a plurality of different relationships;

acquiring partially overlapping wavefront measurements of the test object in each of the different relationships by measuring a shape of a measurement wavefront with a wavefront-measuring gauge having a limited capture range of wavefront shapes;

reshaping the measurement wavefront with a variable optical aberrator between a limited number of the measurements in the different relationships to maintain the measurement wavefront within the capture range of the wavefront-measuring gauge; and

incorporating compensators in an operation for assembling the partially overlapping measurements into a composite measurement,

wherein the compensators acquire values in the operation for reducing differences between overlapping portions of the measurements that are at least partially attributable to the reshaping of the measurement wavefront by the variable optical aberrator.

2. The method of claim 1 wherein

(a) the variable optical aberrator is adjustable through a range of settings for reshaping the measurement wavefront,

(b) the compensators have both an amplitude and a functional form that defines a change in the shape of the measurement wavefront that is scalable by the amplitude, and

(c) the compensators include partially interlocked compensators that are constrained to acquire a substantially common amplitude over a set of the measurements taken at the same setting of variable optical aberrator.

3. The method of claim 2 wherein the partially interlocked compensators are free to acquire a different substantially common amplitude over a different set of the measurements taken at a different setting of the variable optical aberrator.

4. The method of claim 3 wherein the partially interlocked compensators account at least in part for differences between an estimated change in the shape of the measurement wavefront produced at a particular setting of the variable optical aberrator and an actual change in the shape of the measurement wavefront produced at the particular setting of the variable optical aberrator.

5. The method of claim 4 wherein the estimated change in the shape of the measurement wavefront produced at each setting of the variable optical aberrator is based on a model of the variable optical aberrator.

6. The method of claim 5 wherein the functional form of at least one of the partially interlocked compensators relates a variable within the model of the variable optical aberrator to a change in the shape of the measurement wavefront that is scalable by the amplitude.

7. The method of claim 6 wherein the variable optical aberrator is reconfigurable between settings and the at least one partially interlocked compensator is associated with the reconfiguration of the variable optical aberrator.

8. The method of claim 1 wherein

the wavefront-measuring gauge includes an interferometer,

the measurement wavefront includes a test wavefront and a reference wavefront,

the step of acquiring partially overlapping wavefront measurements of the test object includes propagating the test wavefront into an encounter with the test object and comparing a change in shape of the test wavefront as effected by the encounter with the test object against the reference wavefront, and

the step of reshaping the measurement wavefront with the variable optical aberrator includes changing the shape of at least one of the test wavefront and the reference wavefront to reduce differences between the test and reference wavefronts.

9. The method of claim 1 wherein the wavefront-measuring gauge includes

a wavefront sensor for measuring a limited range of wavefront shapes within the capture range of the wavefront-measuring gauge, and

a wavefront generator that generates the measurement wavefront in a shape that is within the limited range of wavefront shapes measurable by the wavefront sensor.

10. The method of claim 9 wherein

the step of acquiring partially overlapping wavefront measurements of the test object includes propagating the measurement wavefront into an encounter with the test object for changing the shape of the measurement wavefront in accordance with a physical characteristic of the test object and

the step of reshaping the measurement wavefront with the variable optical aberrator includes changing the shape of the measurement wavefront so that a combined effect of the changes in shape of the measurement wavefront effected by the encounter with the test object and the reshaping of the measurement wavefront maintains the shape of the measurement wavefront within the limited range of wavefront shapes measurable by the wavefront sensor.

11. The method of claim 1 wherein the step of reshaping the measurement wavefront with a variable optical aberrator includes evaluating whether the measurement wavefront is within the capture range of the wavefront-measuring gauge and further reshaping the measurement wavefront to restore the measurement wavefront within the capture range of the wavefront-measuring gauge.

12. The method of claim 1 wherein the step of associating a wavefront propagator with a physical test object in a plurality of different relationships includes relatively moving an axis of the wavefront propagator with respect to an axis of the physical test object through a plurality of off-axis positions.

13. The method of claim 12 wherein the step of reshaping the measurement wavefront with the variable optical aberrator includes changing the measurement wavefront to include at least one of astigmatism, coma, and trefoil.

14. A method of assembling a composite measurement of a physical test object from a plurality of partially overlapping measurements of the test object comprising steps of:

acquiring a plurality of partially overlapping data maps measuring a physical property of the test object from a test wavefront that encounters the test object, each data map being extracted from a shape of the test wavefront referenced against a datum;

relatively changing the shape of the test wavefront with respect to the datum independently of the test object between acquisitions of a limited number of the data maps to reduce differences between the shape of the test wavefront and the datum;

evaluating overlapping regions of the data maps to characterize the relative change in the wavefront shape between acquisitions of the data maps; and

modifying the data maps in accordance with the characterization of the relative change in the wavefront shape between measurements for assembling the partially overlapping data maps into a composite data map.

15. The method of claim 14 including a step of assembling the partially overlapping data maps into the composite data map and displaying the composite data map as an external representation of the test object.

16. The method of claim 14 including a step of estimating changes in the shape of the test wavefront effected by the encounters of the test wavefront with the test object and the step of relatively changing the shape of the test wavefront with respect to the datum includes counteracting at least part of the estimated changes in the shape of the test wavefront to reduce the differences between the shape of the test wavefront and the datum.

17. The method of claim 16 including a step of estimating the change in the relative shape of the test wavefront with respect to the datum effected by the step of relatively changing the shape of the test wavefront with respect to the datum, and the step of evaluating overlapping regions of the data maps includes distinguishing between the estimated change in the relative shape of the test wavefront with respect to the datum and an actual change in the relative shape of the test wavefront with respect to the datum for characterizing a systematic error in the data maps.

18. The method of claim 17 including a step of assembling the partially overlapping data maps into the composite data map while suppressing influence of the systematic error in the data maps associated with the relative change in the shape of the test wavefront with respect to the datum on the composite data map.

19. The method of claim 18 wherein the step of evaluating includes defining compensators to account at least in part for differences between the estimated change in the relative shape of the test wavefront with respect to the datum and the actual change in the relative shape of the test wavefront with respect to the datum.

20. The method of claim 19 including a step of maintaining the shape of the test wavefront with respect to the datum between acquisitions of a limited number of the data maps, and the compensators include partially interlocked compensators that are constrained to acquire a substantially common value over a set of the data maps acquired while maintaining the shape of the test wavefront with respect to the datum.

21. The method of claim 14 wherein the step of evaluating includes defining compensators to account at least in part for the relative change in the shape of the test wavefront with respect to the datum, and the compensators include partially interlocked compensators that are constrained to acquire a substantially common value for a given relative change in the shape of the test wavefront with respect to the datum.

22. The method of claim 21 including steps of

incorporating compensators in an operation for assembling the partially overlapping data maps into a composite data map,

assembling the partially overlapping data maps into the composite data map, and

displaying the composite data map as an external representation of the test object.

23. A method of assembling a composite measurement of a physical test object from a plurality of subaperture measurements of the test object comprising steps of:

generating a measurement wavefront,

propagating the measurement wavefront through encounters with both a variable optical aberrator and the physical test object en route to a wavefront-measuring gauge having a limited capture range of wavefront shapes;

stepping an axis of a wavefront propagator relative to an axis of the physical test object through a succession of off-axis positions between the wavefront propagator and the test object, the wavefront propagator being arranged for conveying the measurement wavefront to the test object;

adjusting the variable optical aberrator between a limited number of the off-axis positions for maintaining the shape of the measurement wavefront within the capture range of the wavefront-measuring gauge;

collecting partially overlapping subaperture measurements of the test object with the wavefront-measuring gauge at the succession of off-axis positions;

evaluating mismatched measurements within overlapping portions of subaperture measurements to characterize effects of the variable optical aberrator on the subaperture measurements; and

stitching the subaperture measurements of the test object together into a composite measurement while discounting the characterized effects of the variable optical aberrator on the subaperture measurements.

24. The method of claim 23 in which effects of the variable optical aberrator on the subaperture measurements are modeled, and the step of evaluating includes changing values of variables within the model to reduce the mismatched measurements within overlapping portions of subaperture measurements.

25. The method of claim 24 in which the step of stepping includes relatively rotating the axis of the wavefront propagator about the axis of the test object for collecting a set of radial subaperture measurements of the test object, and including a step of maintaining the variable optical aberrator at a fixed setting between the common radial subaperture measurements.

26. The method of claim 25 in which the step of evaluating includes constraining at least some of the variables within the model to include substantially common values for each of the common radial subaperture measurements.

27. The method of claim 23 in which the steps of evaluating and stitching include incorporating compensators in an operation for assembling the partially overlapping subaperture measurements into the composite measurement, wherein the compensators acquire values in the operation for reducing differences between the overlapping portions of the subaperture measurements that are at least partially attributable to the variable optical aberrator.

28. The method of claim 27 wherein

(a) the variable optical aberrator is adjustable through a range of settings for reshaping the measurement wavefront,

(b) the compensators have both an amplitude and a functional form that defines a change in the shape of the measurement wavefront that is scalable by the amplitude, and

(c) the compensators include partially interlocked compensators that are constrained to acquire a substantially common amplitude over a set of the measurements taken at the same setting of variable optical aberrator.

29. The method of claim 28 wherein the partially interlocked compensators are free to acquire a different substantially common amplitude over a different set of the measurements taken at a different setting of the variable optical aberrator.

30. The method of claim 23 wherein

the wavefront-measuring gauge includes an interferometer,

the measurement wavefront includes a test wavefront and a reference wavefront,

the step of collecting partially overlapping subaperture measurements of the test object includes propagating the test wavefront into an encounter with the test object and comparing a change in shape of the test wavefront as effected by the encounter with the test object against the reference wavefront, and

the step of adjusting the variable optical aberrator includes changing the shape of at least one of the test wavefront and the reference wavefront to reduce differences between the test and reference wavefronts.

31. The method of claim 23 wherein the wavefront-measuring gauge includes

a wavefront sensor for measuring a limited range of wavefront shapes within the capture range of the wavefront-measuring gauge, and

a wavefront generator that generates the measurement wavefront in a shape that is within the limited range of wavefront shapes measurable by the wavefront sensor.

32. A method of synthesizing a composite data map of a test surface of a physical object from a plurality of overlapping sub-aperture data maps of the test surface, comprising the steps of:

collecting the plurality of subaperture numerical data maps of the test surface from a plurality of regions of the test surface with a metrology system, at least a portion of each such map overlapping a portion of at least one adjacent map to create a region of overlapping data;

associating partially interlocked compensators with systematic errors in the data maps that are expected to be common within groups of the subaperture data maps and to differ between the groups of the subaperture data maps;

each of the partially interlocked compensators having an amplitude and a functional form that defines a change in the subaperture data maps that is scalable by the amplitude;

constraining the partially interlocked compensators to acquire a substantially common amplitude within each group of the subaperture data maps while being free to acquire different amplitudes between the groups of subaperture data maps;

identifying amplitudes of the partially interlocked compensators that minimize mismatch data from each of said data maps in said overlapping regions for isolating the systematic errors in the data maps; and

synthesizing the data maps into a composite data map as a representation of the test surface while suppressing influence of the isolated systematic errors.

33. The method of claim 32 including steps of

associating free compensators with systematic errors in the data maps that are expected to differ among all of the subaperture data maps, the free compensators having a free amplitude range;

associating interlocked compensators with systematic errors in the data maps that are expected to be common among all of the subaperture data maps, the locked compensators being constrained to acquire a substantially common amplitude for all of the subaperture data maps; and

each of the free and interlocked compensators having an amplitude and a functional form that defines a change in the subaperture data maps that is scalable by the amplitude.

34. The method of claim 33 in which the step of identifying amplitudes includes identifying amplitudes of the free and interlocked compensators that minimize mismatch data from each of said data maps in said overlapping regions for isolating additional systematic errors in the data maps;

35. A wavefront-measuring system for measuring test objects comprising:

a support having a mounting axis for mounting a test object;

a wavefront propagator that conveys a measurement wavefront to and from the test object along a measuring axis;

a wavefront-measuring gauge that measures a shape of the measurement wavefront against a datum;

adjustable machine axes that provide off-axis motion between the mounting axis of the support and the measuring axis of the wavefront propagator to capture a plurality of subaperture measurements covering partially overlapping areas of the test object;

a variable optical aberrator that relatively changes a shape of the measurement wavefront with respect to the datum;

data structures embodied in a computer-readable medium including compensators each having a variable amplitude and a functional form that defines a change in the shape of the measurement wavefront that is scalable by the amplitude, and

a processor that calculates the amplitudes of the compensators in an operation for assembling the subaperture measurements into a composite measurement by determining the amplitudes of the compensators that minimize differences between overlapping portions of the subaperture measurements including differences that are attributable to the relative changes in the shape of the measurement wavefront by the variable optical aberrator.

36. The system of claim 35 wherein the data structures include a model of the variable optical aberrator that is accessible to the processor for estimating relative changes in the measurement wavefront shape effected by the variable optical aberrator.

37. The system of claim 36 in which the compensators include compensators that account at least in part for differences between an estimated change in the measurement wavefront shape effected by the variable optical aberrator and an actual change in the measurement wavefront shape effected by the variable optical aberrator.

38. The system of claim 36 wherein the variable optical aberrator includes reconfigurable optics that are reconfigurable for incorporating at least one of astigmatism, coma, and trefoil into the shape of the measurement wavefront.

39. The system of claim 37 wherein the variable optical aberrator includes at least two relatively movable optics that are angularly adjustable with respect to each other.

40. The system of claim 39 in which the variable optical aberrator is an adjustable prismatic device having at least two relatively adjustable prisms.