Method for measuring a spherical-astigmatic optical surface

Patent metadata

Field	Value
Publication number	US-2016298951-A1
Application number	US-201615187226-A
Country	US
Kind code	A1
Filing date	Jun 20, 2016
Priority date	Dec 19, 2013
Publication date	Oct 13, 2016
Grant date	—

Abstract

Method for measuring a spherical-astigmatic optical surface ( 40 ), includes: a) generating a spherical-astigmatic wavefront as a test wavefront with a wavefront generating apparatus ( 10 ); b) interferometrically measuring wavefront aberrations between the wavefront generating apparatus and the surface which is adjusted to the wavefront generating apparatus such that the test wavefront impinges each point on the surface substantially perpendicularly, plural measurements being taken in which the surface is measured at a number of positions, spherized about the two centers of the radii of the astigmatism and/or rotated by 180° about a surface normal to the surface, such that corresponding interferogram phases are determined; and c) determining the wavefront of the wavefront generation device and a shape of the surface using a mathematical reconstruction method. The spherical-astigmatic surface is then corrected using a suitable processing method, a) to c) being repeated until the wavefront aberrations are smaller than a given value.

First claim

What is claimed is:

1 . A method for measuring a spherical-astigmatic optical surface, comprising:
a) generating a spherical-astigmatic wavefront as a test wavefront with a wavefront generation device; b) interferometrically measuring wavefront differences between the wavefront generation device and the spherical-astigmatic surface adapted to the wavefront generation device such that the test wavefront is incident substantially perpendicularly at each point of the spherical-astigmatic surface, wherein said measuring comprises carrying out a plurality of measurements, in which the spherical-astigmatic surface is measured at a number of positions, spherized about two centers of the radii of the astigmatism and/or rotated by 180° about a surface normal of the spherical-astigmatic surface, and determining corresponding interferogram phases; c) determining the wavefront of the wavefront generation device and a surface form of the spherical-astigmatic surface through a mathematical reconstruction method, according to which the surface of the spherical-astigmatic surface is corrected via a given processing method, and d) repeating said generating, said measuring and said determining until the wavefront differences are below a defined threshold.

2 . The method as claimed in claim 1 , wherein the wavefront of the wavefront generation device is corrected during said determining, and wherein said generating, said measuring and said determining are repeated until the wavefront differences are below the defined threshold.

3 . The method as claimed in claim 1 , wherein the spherical-astigmatic surface is embodied as a calibration element for the wavefront generation device.

4 . A method for measuring a spherical-astigmatic optical free-form surface, comprising:
a) generating a spherical-astigmatic wavefront as a test wavefront with a wavefront generation device calibrated according to the measuring method as claimed in claim 1 utilizing a calibration element as the spherical-astigmatic surface; b) interferometrically measuring regions of the spherical-astigmatic surface, embodied as an optical free-form surface, with the test wavefront, wherein the test wavefront is incident substantially perpendicularly on the free-form surface at each of the regions, wherein the regions of the free-form surface and the test wavefront are displaced in relation to one another and/or spherized, and determining the corresponding interferogram phases; and c) stitching the free-form surface from the regions, wherein deviations of the test wavefront and the spherical-astigmatic free-from surface differ from respective predetermined values in accordance with a mathematical reconstruction method.

5 . The method as claimed in claim 4 , wherein the regions are embodied as sub-apertures of the free-form surface, wherein scanning of the sub-apertures is carried out using the spherical astigmatic test wavefront.

6 . The method as claimed in claim 5 , wherein a relative movement is carried out between the free-form surface and the wavefront generation device in accordance with a predefined trajectory, so as to perform a substantially comprehensive measurement of the free-form surface.

7 . The method as claimed in claim 5 , wherein partial spherizations are carried out in directions of axes of the astigmatic surface of the sub-apertures, wherein each partial spherization is carried out about a center of a radius valid in the corresponding axis.

8 . The method as claimed in claim 5 , wherein the interferometric measurements are carried out repeatedly, rotated respectively by 180°.

9 . The method as claimed in claim 5 , wherein the test wavefront is incident on the free-form surface with a maximum deviation less than 10% from normal incidence.

10 . The method as claimed in claim 4 , wherein the wavefront generation device and the free-form surface are manufactured in an iterative manufacturing process.

11 . A test apparatus for testing a surface form of an optical free-form surface, comprising a test optical unit, comprising:
a wavefront generation device configured to generate a spherical-astigmatic wavefront, adapted to the free-form surface, as the test wavefront, wherein at least portions of the free-form surface are interferometrically respectively tested with the test wavefront and wherein the deviation of the adapted wavefront from the intended form thereof is determined with the method as claimed in claim 4 .

12 . The test apparatus as claimed in claim 11 , wherein the wavefront generation device comprises an adaptation element for changing a wavefront into the test wavefront.

13 . The test apparatus as claimed in claim 11 , configured to generate a computer-generated hologram for each free-form surface to be tested, said hologram generating a wavefront which is adapted to a curvature and a mean astigmatism of the free-form surface.

14 . The test apparatus as claimed in claim 12 , wherein the wavefront generation device comprises a plane or spherical reference surface with an additional optical unit configured to generate an adapted spherical-astigmatic wavefront.

15 . An optical element comprising a free-form surface, wherein an astigmatic component of a deviation of the free-form surface from a best-adapted sphere is at least 80%.

16 . The optical element as claimed in claim 15 , wherein the deviation represents a root-mean-square (rms) value of the deviation.

17 . The optical element as claimed in claim 16 , wherein the deviation represents a peak-to-valley (PV) value of the deviation.

18 . The optical element as claimed in claim 17 , wherein an astigmatic component of an overall deviation of the free-form surface from a best-adapted sphere is between a PV value of approximately 0.5 mm and approximately 20 mm, wherein a basic radius of the best-adapted sphere is between approximately ≧300 mm and approximately infinity.

19 . An imaging optical system, comprising at least one extreme ultraviolet lithography projection lens comprising a plurality of mirrors, at least one of which comprises at least one optical free-form surface as claimed in claim 15 .

Assignees

• Zeiss Carl Smt Gmbh

Inventors

• Schulte Stefan
• Schillke Frank

Classifications

• G01B9/02085

  Combining two or more images of different regions · CPC title

• G01M11/005

  Testing of reflective surfaces, e.g. mirrors · CPC title

• G01B9/02039 Primary

  by matching the wavefront with a particular object surface shape · CPC title

• G01B11/2441 Primary

  using interferometry · CPC title

• G01B9/021

  using holographic techniques · CPC title