Deformation-Free Form Error Measurements of Thin, Plane-Parallel Optics Floated on a Heavy Liquid

Published

April 1, 2010

Author(s)

JiYoung Chu, Ulf Griesmann, Quandou (. Wang, Johannes A. Soons, Eric C. Benck

Abstract

We describe a novel method for measuring the unconstrained flatness error of thin, plane parallel precision optics by floating them on high-density aqueous metatungstate solutions while measuring the flatness error with an interferometer. The support of the flat optics by the uniform hydrostatic pressure at the submerged face of the flat optic eliminates flatness errors caused by mounting forces. A small, well characterized flatness error results from the bending of the floating flat by the hydrostatic pressure gradient at the edges. An equation describing the bending of thin, flat plates floating on a liquid is derived, which can be used to correct the flatness measurements of arbitrarily shaped plates. The method can be used to measure flatness errors of both non-transparent and transparent parts, and it is illustrated with flatness measurements of photomask blanks and substrates for extreme ultraviolet lithography. The refractive index of a saturated aqueous lithium metatungstate solution was measured at 632.8 nm and was found to be close to the refractive indices of several low thermal expansion optical materials.

Citation

Applied Optics

Volume

49

Issue

10

Pub Type

Journals

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Keywords

Interferometry, EUV lithography, photomask blank

Citation

Chu, J. , Griesmann, U. , Wang, Q. , Soons, J. and Benck, E. (2010), Deformation-Free Form Error Measurements of Thin, Plane-Parallel Optics Floated on a Heavy Liquid, Applied Optics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=904728 (Accessed October 9, 2025)