Method and apparatus for making a highly uniform low-stress single crystal by drawing from a melt and uses of said crystal

Abstract

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-x D x ) 3 Al 5 O 12 wherein 0<x<1, 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.

1 . A low-stress crystal of formula (I):
   (A 1-x D x ) 3 Al 5 O 12   (I),
   wherein 0<x<1, A=Lu and D=Pr and/or Ce; and   wherein said low-stress crystal has a stress birefringence of less than 1 nm/cm at 193 nm as measured in a crystal growth direction in a center region with a diameter equal to 80% of the crystal.   
     
     
         2 . The low-stress crystal as defined in  claim 1 , having an index of refraction uniformity of less than 1 ppm. 
     
     
         3 . A lens, prism, optical window, or a laser rod made from the low-stress crystal as defined in  claim 1 . 
     
     
         4 . An optical component for DUV lithography, a stepper, excimer laser, a wafer for LEDs and for photovoltaic devices, a computer chip, an integrated circuit, or an electronic device containing the computer chip and/or the integrated circuit, wherein said optical component is made from the low-stress crystal as defined in  claim 1 .