PROBLEM TO BE SOLVED: To provide an annealing furnace and an annealing method, which are suitable for obtaining a metal fluoride single crystal having low double refraction and useful as an optical member of a semiconductor production apparatus, especially a large-sized metal fluoride single crystal having a diameter of 200 mm.;SOLUTION: The annealing furnace comprises a heat-insulating vessel (2) for covering the whole side periphery and the upper and lower parts of the fluoride metal single crystals (1) and a heater (4) arranged at the outside of the heat-insulating vessel (2). The heat-insulating vessel (2) is preferably constituted of a heat-insulating material having a coefficient of thermal conductivity in the thickness direction of ≤2 W/m×K-1. In such an annealing furnace, the temperature distribution in the heat-insulating vessel can be made narrow. Thereby, strain can be efficiently released even for a single crystal in which strain is hardly released and a large-sized single crystal having a small double refraction can be obtained by annealing a metal fluoride single crystal (e.g., calcium fluoride) by using the annealing furnace.;COPYRIGHT: (C)2007,JPO&INPIT
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