Patent ID: 7868708

Claim:
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, said low-stress crystal having 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 a diameter of the crystal and an index of refraction uniformity Δn of less than 1 ppm; wherein said low-stress crystal is obtainable by a method comprising the steps of: a) dipping a single crystal of said formula (I) that is held at a temperature under a melting point of a crystal raw material of said formula (I) in a melt of the crystal raw material so as to form a solid-liquid phase boundary surface; b) drawing said single crystal dipped into the melt in step a) out from the melt vertically with respect to a melt surface so as to grow the single crystal in a particular orientation; c) conducting heat away during the drawing of the single crystal in step b); d) during the drawing of the single crystal in step b), rotating the single crystal and the melt relative to each other with a controllable rotation speed; e) detecting at least one characteristic surface temperature of the melt surface and/or of a crystal surface of the single crystal close to the melt surface by measuring heat energy radiated therefrom; and f) when temperature fluctuations of the at least one characteristic surface temperature are detected, increasing or decreasing said rotation speed in order to control said at least one characteristic surface temperature and to adjust said phase boundary surface between the single crystal and the melt so that said phase boundary surface is planar.