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Applied Optics

Applied Optics


  • Editor: Joseph N. Mait
  • Vol. 49, Iss. 10 — Apr. 1, 2010
  • pp: 1849–1858

Deformation-free form error measurement of thin, plane-parallel optics floated on a heavy liquid

Jiyoung Chu, Ulf Griesmann, Quandou Wang, Johannes A. Soons, and Eric C. Benck  »View Author Affiliations

Applied Optics, Vol. 49, Issue 10, pp. 1849-1858 (2010)

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We describe a novel method for measuring the unconstrained flatness error of thin, plane-parallel precision optics. Test parts are floated 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 nontransparent 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.

OCIS Codes
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(220.3740) Optical design and fabrication : Lithography
(220.4840) Optical design and fabrication : Testing

ToC Category:
Instrumentation, Measurement, and Metrology

Original Manuscript: January 15, 2010
Revised Manuscript: March 1, 2010
Manuscript Accepted: March 4, 2010
Published: March 29, 2010

Jiyoung Chu, Ulf Griesmann, Quandou Wang, Johannes A. Soons, and Eric C. Benck, "Deformation-free form error measurement of thin, plane-parallel optics floated on a heavy liquid," Appl. Opt. 49, 1849-1858 (2010)

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