Schott Ag

Schott Glas

Klaus Dupre

Daniel Rytz

Lothar Ackermann

Lutz Parthier

Gunther Wehrhan

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 &#x2267;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 &#x394;n of &#x3c;1 ppm and a stress birefringence of &#x3c;1 nm/cm at 193 nm, so that optical elements suitable for DUV lithography can be made from them.

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

The method provides a uniform low-stress single crystal in a predetermined crystal orientation. The method of making it includes immersing a single crystal held at a temperature under its melting point in a melt of crystal raw material and drawing it from the melt to grow the crystal. The crystal and/or melt are rotated relative to each other during the crystal growth. A planar phase boundary surface is maintained by detecting at least one characteristic surface temperature in an interior of a crucible containing the melt and controlling temperature fluctuations by increasing or decreasing the rotation speed when they occur. The single crystals obtained by this method have a diameter of at least 50 mm and no visible growth strips in a fishtail pattern when a 2-mm thick sample is observed between crossed polarizers. Optical elements suitable for DUV lithography can be made from these crystals.

The process for growing single crystals, wherein crystal material is melted in a crucible and a crystal nucleus is immersed in the molten crystal material and slowly pulled out, wherein the crystal formed during the pulling is kept at a temperature close to melting temperature of the output material. The invention also includes a device for practicing the above process.

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Idar-Oberstein, Germany

Klaus Dupre