Mounting methodologies to measure EUV reticle nonflatness

Battula, Venkata Siva; Zeuske, Jacob R.; Engelstad, Roxann L.; Vukkadala, Pradeep; Mikkelson, Andrew R.; Peski, Chris K. Van

doi:10.1117/12.835202

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“…The calculated deformation can then be subtracted from the measured flatness error to yield the unconstrained flatness error. This approach was taken by Battula et al [5], who have modeled the deformation of EUVL substrates with FEA and then sought to verify the models using different substrate mounts. It appears that the chief problem of the modeling approach lies in the difficulty of verifying the numerical model as long as the unconstrained flatness error of the mask substrate cannot be measured.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 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.

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Section: Introductionmentioning

confidence: 99%