Zone melting large dia. silicon bar - using periodic rotation of bar to prevent short circuits between bar and induction melting coil

摘要

A vertical semiconductor bar, esp. monocrystalline silicon, held at both ends, is zone melted without using a crucible, but using a ring-shaped inductive heating coil. The top holder for the bar is not rotated continuously, but is rotated round its axis during specific periods of time. The discontinuous rotation is pref. achieved by feeding pulses to the motor rotating the holder, esp. using phase control for acceleration from zero speed to max. speed, followed by redn. to zero speed; after a brief pause the cycle is repeated. The max. speed of the holder is pref. 20 R.p.m; the pulse duration is pref. 1 second; and pref. 20 periods or rotation are used per minute. The pulses duration and height may be varied during the process. Method prevents hairs of Si forming at the periphery of the molten zone; such hairs can cause short circuits between the bar and the induction coil, thus stopping the process.