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. The single crystals obtained by this method have a diameter >=50 mm and no visible growth strips in a fishtail pattern when a 2-mm thick sample is observed between crossed polarizers. These crystals have an index of refraction uniformity Deltan of <1 ppm and a stress birefringence of <1 nm/cm at 193 nm, so that optical elements suitable for DUV lithography can be made from them.