Single crystal calcium fluoride for photolithography

Abstract

A calcium fluoride single crystal for photolithography, wherein a critical resolved shear stress (tauc) in a <1 1 0> direction on a {0 0 1 } plane of the calcium fluoride single crystal is approximately equal to or larger than a shear stress (tau) expressed by tau=1.5E-2.exp(3E3.T-1), where r is shear stress (MPa) and T is average temperature (K) of the calcium fluoride.

Description

[0001] This application is a divisional application of copending U.S. patent application Ser. No. 10 / 657,166, filed Sep. 9, 2003. [0002] This application claims benefit of foreign priority based on Japanese Patent Application No. 2002-266905, filed on Sep. 12, 2002, which is hereby incorporated by reference herein in it entirety as if fully set forth herein.BACKGROUND OF THE INVENTION [0003] The present invention relates to a method of manufacturing a calcium fluorite (“CaF2”) single crystal and a CaF2 single crystal. The present invention also relates to an optical system that uses, as a light source, ultraviolet light from an excimer laser, such as KrF, ArF, F2 and Ar2, and an exposure apparatus for manufacturing semiconductors, and a device fabrication method. [0004] No end to a demand for higher integration of semiconductor integrated circuits has required a higher level performed for an exposure apparatus, particularly, that of a projection optical system. While resolution in t...

AI Technical Summary

Benefits of technology

This approach stabilizes the birefringence and dislocation density, allowing for the production of high-quality CaF2 crystals with improved productivity, even for larger sizes, and enables the creation of high-performance optical systems for exposure apparatuses, particularly for short-wavelength light sources.

Problems solved by technology

While the excimer laser, such as a KrF (having a wavelenght of 248 nm), an ArF (having a wavelength of 193 nm) and an F2 (having a wavelength of 157 nm), has been regarded as a prospective light source for future exposure apparatuses for this reason, most conventionally used glass materials are not compatible with a shortened wavelength of the light source.

This process may reduce the birefringent index by heating and maintaining CaF2 at a high temperature, above 1000° C. However, when a cooling process after annealing provides rapid cooling, the birefringent index disadvantageously increases again and, thus, the cooling rate should be made low.

On the other hand, the low cooling rate itself would lead to a long processing time and remarkably deteriorate productivity.

However, an approach that uses an empirical rule to obtain cooling rates that do not cause birefringence for variously differently sized CaF2 is disadvantageous in that it is difficult to stably provide manufactured CaF2 with good quality.

In addition, it is necessary to arduously determine proper cooling rates for each size of CaF2 single crystal to be processed.

Problematically, it is difficult to confirm that no birefringence occurs after processing, the quality becomes unstable, and an excessively low cooling rate would lower the productivity.

In addition, the birefringence would occur and the productivity would lower without proper cooling even in a process that associates with heating other than annealing after CaF2 single crystal growth.

The conventional empirical approach has been hard to design a process that serves to stably provide low birefringence and high productivity.

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