GB1494342A - Automatic control of crystal growth - Google Patents

Abstract

1494342 Automatic control of crystal growth NATIONAL RESEARCH DEVELOPMENT CORP 25 March 1975 [3 April 1974 16 Aug 1974] 14885/74 and 36288/74 Heading G3N A crystal is grown by pulling from an electrically heated melt. The crystal weight is measured by a load cell and compared with an expected value to derive a control signal which is used to adjust the power supplied to the melt heater. The control signal is compensated for known effects dependent on the rate of change of crystal radius. As shown in Fig. 1, a semi-conductor crystal 4 is pulled from a melt 1 in a crucible 2 by a pull rod 5 suspended from a load cell 6. The rod 5 is raised and rotated by motors 7, 8. In use, the load cell output is differentiated 9 and supplied to a comparator 12 which also receives a reference voltage 13 which is constant if a crystal of uniform radius is required. The comparator output signal is fed to a phase advance network 14 which compensates for thermal lag in heating coils 3 and the melt 1, and the resulting signal controls a supply circuit 16 for the coils 3. Owing to effects of change in density and surface tension, instability of the control loop may be caused by a term proportional to the rate of change of the crystal radius. This is corrected by feeding back the signal derived from the network 14 through a phase retarding thermal lag simulating circuit 18 to a differentiator 20, the output of which is combined with the signal from differentiator 9 in an adder circuit 21 to modify the input to the comparator 12. In a modification, the signal from the load cell 6 representing the crystal weight is compared directly with the output of a potentiometer whose slider moves axially with the pull rod 5. If the potentiometer is linear, this produces a crystal of constant cross-section, Fig. 4 (not shown). In other embodiments, the feedback loop 18-21 is omitted and the signal from network 14 is passed through parallel differentiating and proportional circuit branches to an adder circuit which supplies a control signal to circuits (15, 16), Figs. 3, 4 (not shown). A suitable differentiating circuit including operational amplifiers is also described, Fig. 5 (not shown).