| 摘要 |
PROBLEM TO BE SOLVED: To efficiently manufacture a metal fluoride single crystal that is useful as an optical material for next-generation optical lithography techniques, has small strain (birefringence) and has little minute voids (observed as a cloud or tarnish) leading to the cause lowering transmission factor and contrast. SOLUTION: When annealing a metal fluoride single crystal having a melting point of X<SB>melt</SB>°C, the maximum temperature X<SB>MAX</SB>°C to be reached by this annealing is made to be at least (X<SB>melt</SB>-250)°C and the temperature rising from (X<SB>MAX</SB>-50)°C to X<SB>MAX</SB>°C, and preferably the temperature rising from (X<SB>MAX</SB>-100)°C to X<SB>MAX</SB>°C are carried out at the warming speed of not faster than 5°C/hr. For example, when annealing a calcium fluoride single crystal having a melting point of 1,420°C, the maximum temperature X<SB>MAX</SB>°C is made to be at least 1,170°C (less than 1,420°C). Furthermore in this case, when X<SB>MAX</SB>°C is made to be 1,300°C for example, the warming speed in the range of from 1,250°C to 1,300°C (preferably 1,200-1,300°C) is made to be not faster than 5°C/hr. COPYRIGHT: (C)2007,JPO&INPIT
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