Methods for growing large-volume single crystals from calcium fluoride and their uses

Abstract

The method for making a uniform, large-size single crystal of calcium fluoride includes placing a single precursor crystal of calcium fluoride in a tempering vessel provided with a cover; introducing calcium fluoride powder into the tempering vessel and subsequently heating the single precursor crystal, preferably in intimate contact with the calcium fluoride powder, in the tempering vessel together with the calcium fluoride powder for two or more hours at temperatures above 1150° C. to temper the precursor crystal and thus form the uniform, large-scale single crystal of calcium fluoride. The uniform large-sized single crystals of calcium fluoride can be used to make improved lens, prism, light-conducting rod, optical window or other optical component for DUV photolithography, steppers, excimer lasers, wafers, computer chips and electronic devices containing the wafers and chips.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to methods for growing highly homogeneous large-volume single crystals from calcium fluoride by tempering at elevated temperature, especially in an oven, and their uses.[0003] 2. Prior Art[0004] Single crystals, especially calcium fluoride single crystals, are required as starting materials for optical components in DUV-photolithography, such as steppers or excimer lasers. They are conventionally used as lenses or prisms. They are especially used to optically copy fine structures into integrated circuits, computer chips and / or photo-lacquer-coated wafers. For these purposes it is necessary that these optical components have a high degree of uniformity, i.e. that their index of refraction n must be exactly constant throughout the entire crystal. This means that the index of refraction difference An should vary by at most 5.times.10.sup.-6, preferably not more than 2.times.10.sup.-6. Since, on the one hand, the requi...

AI Technical Summary

Problems solved by technology

Although the CaF.sub.2 single crystals are held for a considerable time at a high temperature after crystallization, the stress birefringence usually still amounts to 5 to 20 nm / cm, which is too large for later use.

With this process however it was not possible to improve the stress birefringence of substantially larger calcium fluoride single crystals, i.e. single crystals of a diameter greater than 200 mm, especially greater than 300 nm, and a thickness of 50 to 400 mm, or also of already polished and worked optical elements during economical processing times. During experiments, which shorten the tempering time and / or improve the tempering, the tempering temperature is still further increased, which leads to a great loss in volume for the single crystals.

It has been shown however that no satisfactory uniformity can be attained with sufficient yield with the known methods.