METHOD AND APPARATUS FOR MAKING A HIGHLY UNIFORM LOW-STRESS SINGLE CRYSTAL BY DRAWING FROM A MELT AND USES OF SAID CRYSTAL

摘要

The method of making uniform low-stress crystals includes immersing a seed crystal held at a temperature under its melting point in a melt in a crucible and drawing it from the melt. The crystal and/or melt are rotated relative to each other and a planar phase boundary surface is maintained between them by detecting a surface temperature of the melt and/or crystal and controlling temperature fluctuations by increasing or decreasing the rotation speed. A low-stress crystal of formula: (A_1-xD_x)₃Al₅O₁₂ wherein 0x1, A=Lu and D=Pr and/or Ce, is preferred. These crystals have an index of refraction uniformity Δn of 1 ppm and a stress birefringence of 1 nm/cm at 193 nm, so that they are suitable for making optical elements for DUV lithography.