METHOD OF MEASURING A SHAPE OF AN OPTICAL SURFACE AND INTERFEROMETRIC MEASURING DEVICE

• US 20120229814A1
• Filed: 03/12/2012
• Published: 09/13/2012
• Est. Priority Date: 09/18/2009
• Status: Active Grant

• Current Assignee: CARL Zeiss SMT GmbH (Carl-Zeiss-Stiftung)
• Original Assignee: CARL Zeiss SMT GmbH (Carl-Zeiss-Stiftung)
• Inventors: FREIMANN, Rolf, BOERBAND, Bernd, SCHULTE, Stefan, HOF, Albrecht, RIEPENHAUSEN, Frank, MANGER, Matthias, NEUGEBAUER, Dietmar, ISSLER, Helmut, BICH, Armin
• Granted Patent: US 8,593,642 B2
• Time in Patent Office: Days
• US Class Current: 356/512
• CPC Class Codes: B29D 11/0098 Inspecting lenses
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 2290/15 Cat eye, i.e. reflection always parallel to incoming beam
• CPC Class Codes: G01B 9/02027 Two or more interferometric channels or interferometers
• 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: G01M 11/005 Testing of reflective surfaces, e.g. mirrors
• 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 11/0271 by using interferometric methods
• CPC Class Codes: G02B 3/02 with non-spherical faces G02B3/10 takes precedence
• CPC Class Codes: G02B 5/10 with curved faces

Abstract

Measuring a shape of an optical surface (14) of a test object (12) includes: providing an interferometric measuring device (16) generating a measurement wave (18); arranging the measuring device (16) and the test object (12) consecutively at different measurement positions relative to each other, such that different regions (20) of the optical surface (14) are illuminated by the measurement wave (18); measuring positional coordinates of the measuring device (16) at the different measurement positions in relation to the test object (12); obtaining surface region measurements by interferometrically measuring the wavefront of the measurement wave (18) after interaction with the respective region (20) of the optical surface (14) using the measuring device (16) in each of the measurement positions; and determining the actual shape of the optical surface (14) by computationally combining the sub-surface measurements based on the measured positional coordinates of the measuring device (16) at each of the measurement positions.

Claims

1. A method of measuring an actual shape of an optical surface of a test object, comprising:

providing an interferometric measuring device generating a measurement wave,

arranging the interferometric measuring device and the test object consecutively at different measurement positions relative to each other, such that different regions of the optical surface are illuminated by the measurement wave,

measuring the positional coordinates of the measuring device at the different measurement positions in relation to the test object,

obtaining surface region measurements by interferometrically measuring the wavefront of the measurement wave after interaction with the respective region of the optical surface using the measuring device in each of the measurement positions, and

determining the actual shape of the optical surface by computationally combining the sub-surface measurements based on the measured positional coordinates of the interferometric measuring device at each of the measurement positions.

2. The method according to claim 1,

wherein the regions of the optical surface illuminated at the different measurement positions overlap with each other and the sub-surface measurements are computationally combined by fitting the overlapping sections of the sub-surface measurements to each other, and correcting the fitting result based on the measured positional coordinates of the measuring device.

3. The method according to claim 1,

wherein the positional coordinates of the interferometric measuring device in relation to the test object at the measurement positions are measured by directing at least three distance measuring laser interferometers onto at least one retro-reflector attached to the measuring device, wherein the at least three laser interferometers are in a fixed positional relationship to each other during the position measurement.

4. The method according to claim 3,

wherein each of the distance measuring laser interferometers is mounted to tilt with respect to two tilt axes.

5. The method according to claim 3,

wherein at least three retro-reflectors are attached to the interferometric measuring device and each measuring laser interferometer is directed at a different retro-reflector.

6. The method according to claim 2,

is wherein the positional coordinates of the interferometric measuring device are determined from the distances measured by the laser interferometers based on the distances between the laser interferometers, which are measured by a fourth distance measuring laser interferometer that is arranged at a location outside the plane spanned by the three distance laser measuring interferometers, the distances to the at least one retro-reflector is measured by each of the four laser interferometers and the distances between the laser interferometers are determined mathematically therefrom.

7. The method according to claim 1,

wherein the interferometric measuring device is positioned relative to the optical test object using a coordinate measuring machine.

8. The method according to claim 1,

wherein a positioning apparatus positions the interferometric measuring device relative to the test object in order to arrange the measuring device in the different measurement positions, which positioning apparatus is configured to tilt the measuring device with respect to at least one axis.

9. The method according to claim 1,

wherein a positioning apparatus positions the interferometric measuring device relative to the test object, in order to arrange the measuring device in the different measurement positions, and said method further comprises measuring further properties of the optical surface by a further measurement device, which is also positioned relative to the test object by the positioning apparatus.

10. The method according to claim 1,

wherein the interferometric measuring device comprises:

a detection apparatus configured to record an interferogram, and imaging optics comprising at least one curved mirror and configured to image the optical test surface onto the detection apparatus.

11. An interferometric measuring device configured for measuring a shape of an optical surface of a test object, comprising:

a detection apparatus configured to record an interferogram, and

imaging optics comprising at least one curved mirror and configured to image the optical surface onto the detection apparatus.

12. The interferometric measuring device according to claim 11,

wherein the imaging optics comprise two wavefront forming surfaces, which are configured such that the optical path utilized for imaging the optical surface onto the detection apparatus comprises a focus point located between the two wavefront forming surfaces.

13. The interferometric measuring device according to claim 11,

wherein the imaging optics comprise at least one parabolic mirror.

14. The interferometric measuring device according to claim 11,

wherein the imaging optics are configured such that the optical path utilized for imaging the optical surface onto the detection apparatus strikes the curved mirror twice.

15. A method of qualifying a wave shaping element having a wave shaping surface configured for adapting the wave front of a test wave to a target shape of an optical test surface, which qualification method comprises:

consecutively irradiating different regions of the wave shaping surface by the measurement wave,

for each irradiated region measuring the wavefront of the measurement wave after interaction of the measurement wave with the respective region of the wave shaping surface,

computationally combining the measured wavefronts, and

analyzing the combined wavefront and thereby determining deviations of the wave shaping effect of the wave shaping surface from a target wave shaping effect.

16. The method according to claim 15,

wherein the measurement wave passes, after interacting with the respective region of the wave shaping surface, through magnifying imaging optics.

17. The method according to claim 15,

wherein the measurement is conducted by a measurement device comprising illumination optics configured to direct the measurement wave onto the wave shaping element and detection optics configured to measure the wavefront of the measurement wave after interacting with the wave shaping surface, and the different regions of the wave shaping surface are consecutively irradiated by the measurement wave by shifting the measurement apparatus and the wave shaping element relative to each other between irradiations of the different regions.

18. The method according to claim 17,

wherein the measurement apparatus is held and positioned relative to the wave shaping element by a coordinate measuring machine.

19. The method according to claim 15,

wherein the wave shaping surface contains a diffractive structure, and wherein the wavefront of the measurement wave is measured after the wavefront is diffracted at the diffractive structure in the respective wave shaping regions in a positive diffraction order and a negative diffraction order.

20. An optical element having an optical surface, which optical surface is adapted to a non-spherical target shape, such that variations of the actual shape of the optical surface with respect to the target shape having a spatial wavelength between 0.015 mm and 2 mm are limited to at most 100 pm RMS.

21. The optical element according to claim 20,

wherein the optical surface is adapted to the non-spherical target shape, such that variations of the actual shape of the optical surface with respect to the target shape having a spatial wavelength between 0.015 mm and 30 mm are limited to at most 100 pm RMS.

22. The optical element according to claim 20,

wherein the target shape of the optical surface is a free form surface having no rotational symmetry and the target shape has a deviation from a best fitting spherical surface for the free form surface of at least 5 μ

m.

23. The optical element according to claim 20,

wherein the target shape of the optical surface is a rotationally symmetric aspherical surface and the target shape has a deviation from a best fitting spherical surface for the aspherical surface of at least 500 μ

m.