| 摘要 |
PROBLEM TO BE SOLVED: To provide a fluoride single crystal whose section cut perpendicularly to the direction of the crystal-growing axis has a diameter of≥50 mm and which offers a sufficient resolution performance when used as a stepper lens material, a method for growing the fluoride single crystal, and a lens made of the fluoride single crystal. SOLUTION: In the fluoride single crystal, the section cut perpendicular to the direction of the crystal-growing axis A has a nearly circular shape and a diameter of≥50 mm, and a ratio B2/B1 satisfies the relation: 0.1≤B2/B1≤10, wherein B1 is the refractive index uniformity at the center C and B2 is the refractive index uniformity at the periphery O of the section, respectively. The method for growing the fluoride single crystal comprises a growing step wherein the single crystal is grown in a crystal-growing furnace having a temperature gradient in the direction of crystal growth, a re-heating step wherein the single crystal is uniformly heated to a temperature below its melting point in the crystal-growing furnace and a cooling step wherein the single crystal is cooled. The lens is made of the fluoride single crystal. COPYRIGHT: (C)2007,JPO&INPIT
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