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Controlled Crystal Pulling With Accelerated Crucible Rotation

IP.com Disclosure Number: IPCOM000081887D
Original Publication Date: 1974-Aug-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Mueller-Krumbhaar, H: AUTHOR [+2]

Abstract

Crystal growth from the melt following the Czochralski technique is improved by the technique of additional periodic accelerated and decelerated crucible and/or crystal rotation (ACRT). This results in a thorough mixing of the melt, enhancing the material transport and homogenizing the growing crystal. However, due to centrifugal forces, the parabolic curvature of the free surface of the melt thereby periodically changes with the square of the crucible rotation rate. The thus created periodic level changes at the crystal-melt interface are compensated, by an appropriate vertical motion superimposed on the given crystal pulling rate. A suitably controlled mechanism operates on either the crystal or the crucible support, for maintaining a nonvarying contact angle at the crystal-melt interface.

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Controlled Crystal Pulling With Accelerated Crucible Rotation

Crystal growth from the melt following the Czochralski technique is improved by the technique of additional periodic accelerated and decelerated crucible and/or crystal rotation (ACRT). This results in a thorough mixing of the melt, enhancing the material transport and homogenizing the growing crystal. However, due to centrifugal forces, the parabolic curvature of the free surface of the melt thereby periodically changes with the square of the crucible rotation rate. The thus created periodic level changes at the crystal-melt interface are compensated, by an appropriate vertical motion superimposed on the given crystal pulling rate. A suitably controlled mechanism operates on either the crystal or the crucible support, for maintaining a nonvarying contact angle at the crystal-melt interface.

In the interesting limit where the crystal cross section is small compared with the crucible cross section and the changes in angular velocity are slow enough, the above formula (near the figure) can be evaluated for the law of control necessary. For a typical example the values are the following: g = 981 cm sec/-2/, R = 3 cm, r = 1 cm, omega(1)/2pi = 0.67 sec/-1/, omega 2 / 2pi = 1 sec/-1/.

The corresponding level change to be controlled is then about Delta h approx. 0.4 mm. If the angular velocity changes every 30 seconds, this gives a rate of 48 mm per hour, which is of the order of normal growth rates of pulled...