US-12618664-B2 - Measuring device for interferometrically measuring a surface form

Abstract

An apparatus ( 10 ) for interferometrically measuring a surface shape ( 12 ) of a test object ( 14 ) in relation to a reference shape ( 41 ) includes (a) a diffractive optical element ( 30 ) generating a test wave ( 32 ) from measurement radiation ( 22 ), whereas a wavefront ( 42 ) of the test wave is adapted to a target shape ( 43 ) of the surface ( 12 ) of the test object ( 14 ) and the target shape is configured as a first non-spherical surface, (b) a reference element ( 38 ) with a reference surface ( 40 ) having the reference shape ( 41 ), the reference shape being configured as a further non-spherical surface, (c) a first holder ( 60 ) configured to arrange the test object ( 14 ) in the beam path of the test wave ( 32 ) in a measurement configuration, and (d) a further holder ( 62 ) configured to arrange the reference element ( 38 ) in the beam path of a reference wave ( 34 ) in the measurement configuration.

Inventors

• Martin Endres

Assignees

• CARL ZEISS SMT GMBH

Dates

Publication Date

20260505

Application Date

20230925

Priority Date

20210325

Claims (19)

1. 1 . A measurement apparatus for interferometrically measuring a shape of a surface of a test object in relation to a reference shape, comprising: a diffractive optical element configured to generate a test wave from measurement radiation, a wavefront of the test wave being adapted to a target shape of the surface of the test object and the target shape being configured as a first non-spherical surface, a reference element with a reference surface which has the reference shape, the reference shape being configured as a further non-spherical surface, a first holder configured to arrange the test object in a beam path of the test wave in a measurement configuration, and a further holder configured to arrange the reference element in a beam path of a reference wave in the measurement configuration, wherein the reference shape deviates from the target shape of the surface of the test object by no more than 500 μm.
2. 2 . The measurement apparatus as claimed in claim 1 , wherein both the first non-spherical surface and the further non-spherical surface are configured as a respective free-form surface.
3. 3 . The measurement apparatus as claimed in claim 1 , wherein the diffractive optical element is further configured to generate the reference wave with a wavefront adapted to the reference shape, such that the reference wave is incident substantially normally to the reference surface at every location of the reference surface.
4. 4 . The measurement apparatus as claimed in claim 3 , wherein the diffractive optical element is encoded at least twice, a first encoding being configured to generate the test wave and a second encoding being configured to generate the reference wave.
5. 5 . The measurement apparatus as claimed in claim 3 , wherein a first region of the diffractive optical element, where the test wave is generated, and a further region of the diffractive optical element, where the reference wave is generated, have an overlap in which at least 20% of an area of a larger of the first and the further regions is arranged.
6. 6 . The measurement apparatus as claimed in claim 1 , wherein the wavefront of the test wave located at the surface of the test object arranged in the measurement configuration deviates by no more than 500 μm from the wavefront of the reference wave located at the reference surface.
7. 7 . The measurement apparatus as claimed in claim 1 , wherein the surface of the test object has a first measurement region which is irradiated by the test wave in the measurement configuration and the reference element comprises a further measurement region which is irradiated by the reference wave in the measurement configuration, wherein respective areas of the first and the further measurement regions deviate from one another by at least 1%.
8. 8 . The measurement apparatus as claimed in claim 1 , wherein the first and the further holders are mounted on an actuation module configured to move both the first and the further holders, whereby the test object and the reference element are arranged in a further measurement configuration, in which respective positions of the test object and of the reference element are interchanged.
9. 9 . The measurement apparatus as claimed in claim 8 , wherein the actuation module is configured to move the first and the further holders, whereby the test object and the reference element are arranged in the further measurement configuration, in which, in addition to the respective positions, the respective orientations and the respective tilt positions of the test object and of the reference element are also interchanged.
10. 10 . The measurement apparatus as claimed in claim 8 , wherein the actuation module is configured to rotate the first and the further holders about a common axis of rotation.
11. 11 . The measurement apparatus as claimed in claim 8 , wherein the actuation module is configured to displace at least one of the first and the further holders in a translation direction and/or to tilt at least one of the first and the further holders.
12. 12 . The measurement apparatus as claimed in claim 1 , wherein the diffractive optical element is configured to generate the test wave and the reference wave with propagation directions which each have deviations more than 5° vis-à-vis a symmetric arrangement of the propagation directions, wherein the propagation directions in the symmetric arrangement are arranged symmetrically in relation to an axis perpendicular to a diffraction pattern of the diffractive optical element.
13. 13 . The measurement apparatus as claimed in claim 1 , wherein the reference shape is adapted to the target shape of the test object surface, such that the reference shape deviates from the target shape of the test object surface by no more than 500 μm, and the first and the further holders are arranged such that a tilt position, vis-à-vis the direction of gravity, of the test object held by the first holder corresponds to a further tilt position, vis-à-vis the direction of gravity, of the reference element held by the second holder, such that the surface of the object and the reference surface are each tilted by the same angle relative to gravity in a central region.
14. 14 . The measurement apparatus as claimed in claim 1 , wherein, in the measurement configuration, the test object and the reference element are arranged in succession with a partially overlapping position in the respective beam paths of the test wave and the reference wave.
15. 15 . The measurement apparatus as claimed in claim 1 , wherein the diffractive optical element is configured to radiate the test wave onto a measurement region of the surface which is extended vis-à-vis a used region of the surface, wherein the used region is a region which is radiated by exposure radiation in a state where the test object is installed in a projection apparatus.
16. 16 . The measurement apparatus as claimed in claim 1 , wherein the target shape of the surface of the test object differs by at least 1 mm from any sphere.
17. 17 . A measurement apparatus for interferometrically measuring a shape of a surface of a test object in relation to a reference shape, comprising: a diffractive optical element configured to generate a test wave from measurement radiation, a wavefront of the test wave being adapted to a target shape of the surface of the test object and the target shape being configured as a first non-spherical surface, a reference element with a reference surface which has the reference shape, the reference shape being configured as a further non-spherical surface, a first holder configured to arrange the test object in a beam path of the test wave in a measurement configuration, and a further holder configured to arrange the reference element in a beam path of a reference wave in the measurement configuration, wherein the reference element has a hole and the diffractive optical element is configured to generate the test wave with a convergent beam path such that a caustic of the test wave is generated in the hole of the reference element arranged in the beam path of the reference wave.
18. 18 . A method for interferometrically measuring a shape of a surface of a test object in relation to a reference shape, comprising the steps of: radiating at least a portion of a test wave, generated by a diffractive optical element, onto the surface of the test object which is arranged in a beam path of the test wave with a first holder, the wavefront of the test wave being adapted to a target shape of the surface of the test object and the target shape being configured as a first non-spherical surface, arranging a reference element in a beam path of a reference wave with a further holder, the reference element comprising the reference surface having the reference shape and the reference shape being configured as a further non-spherical surface, and superimposing the test wave, following an interaction of the test wave with the surface of the test object, with the reference wave, whose radiation was exposed to an interaction with the reference surface, wherein the reference shape deviates from the target shape of the surface of the test object by no more than 500 μm.
19. 19 . The method as claimed in claim 18 , wherein the test wave and the reference wave are generated by radiating measurement radiation onto the diffractive optical element, the test wave being superimposed with the reference wave, following the interaction with the reference surface.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This is a Continuation of International Application PCT/EP2022/057158 which has an international filing date of Mar. 18, 2022, and the disclosure of which is incorporated in its entirety into the present Continuation by reference. This Continuation also claims foreign priority under 35 U.S.C. § 119(a)-(d) to and also incorporates by reference, in its entirety, German Patent Application DE 10 2021 202 911.6 filed on Mar. 25, 2021. FIELD OF THE INVENTION The invention relates to a measurement apparatus and a method for measuring a shape of a surface of a test object in relation to a reference shape by interferometry. BACKGROUND For the highly accurate determination of a surface shape of a test object, such as a microlithographic optical element, formed as a non-spherical surface, for instance as a free-form surface, interferometric measurement arrangements comprising a diffractive optical element are known. The diffractive optical element is, for example, in the form of a computer-generated hologram (CGH) and configured so as to generate a test wave with a wavefront adapted to the target shape of the surface. Diffractive structures required to this end can be determined by a computer-aided simulation of the measurement arrangement together with the target surface and then be prepared on a substrate as CGH. By superimposing the test wave reflected by the surface with a reference wave, it is possible to determine deviations from the target shape very precisely. DE 10 2015 209 490 A1 describes such a measurement arrangement, which is in the form of what is known as a reference mirror interferometer. In this case, a complex coded CGH generates both a test wave with a wavefront adapted to the target shape and a reference wave. Whereas the test wave is reflected back to the CGH from the surface to be measured, the reference wave has a different propagation direction and strikes a flat or spherical reference mirror. From the latter, the reference wave is also reflected back to the CGH. After passing through the CGH again, the reflected test wave and the reference wave come into superposition and thus create an interference pattern on the detector. To obtain great measurement accuracy when measuring the surface of the test object, known flaws of the CGH, for example disturbances in the CGH surface or a CGH alignment state, are frequently taken into account and removed by calculation when the surface shape is determined in conventional fashion. Thus, a calibration of the measurement arrangement that is as accurate as possible is decisive for the accuracy of the surface measurement. To this end, interferograms generated by one or more plane or spherical calibration mirrors are evaluated in known measurement arrangements in order to separate disturbances due to alignment or figure errors in the measurement arrangement from the actual measurement signal. However, as a result of the necessary great complexity of the interferometric measurement arrangement and the long time required for calibration, the use of calibration mirrors for removing, by calculation, errors caused by the CGH causes great outlay. SUMMARY It is an object of the invention to provide a measurement apparatus and a method whereby the aforementioned problems are solved, and in particular a surface measurement of the test object with great accuracy and comparatively little outlay is made possible. According to the invention, the aforementioned object can for example be achieved by a measurement apparatus for measuring a shape of a surface of a test object in relation to a reference shape by interferometry. The measurement apparatus comprises a diffractive optical element for generating a test wave from measurement radiation, a wavefront of the test wave being adapted to a target shape of the surface of the test object and the target shape being configured as a first non-spherical surface, a reference element with a reference surface which has the reference shape, the reference shape being configured as a further non-spherical surface, a first holder configured to arrange the test object in the beam path of the test wave in a measurement configuration, and a further holder configured to arrange the reference element in the beam path of a reference wave in the measurement configuration. In particular, the reference wave has a wavefront adapted to the reference shape. The measurement apparatus is configured in particular to measure a deviation of a shape of the optical surface of the test object from the reference shape by interferometry. According to an embodiment, the measurement apparatus is configured to measure a shape of an optical surface of an optical element of a microlithographic projection exposure apparatus, for instance a projection lens of such a projection exposure apparatus, by interferometry. A non-spherical surface is understood to be an aspherical surface or a free-form surface. An aspherical surface