National Research Development Corporation

Gordon Charles Joyce

Donald Thomas Hurle

Geoffrey W Green

Kathryn Elizabeth McKell

Donald Thomas James Hurle

A method and apparatus for growing single crystal of GaAs, etc uses the Czochralski techniques. Control of crystal (,) diameter is by a closed loop (,) control of melt (,) temperature in response to crystal weight signals (,) W or dW/dt. The invention injects a test signal St (,) into the control loop and performs a signal processing (,), e.g. cross correlation, on St and crystal weight signal. Peak amplitude of correlation values is related to the growing crystal shape. This is used by comparison with reference values (,) to control the growing-out phase from seed diameter (,) to full diameter of the crystal (,).

Growth of semiconductor single crystals

In crystal growing apparatus in which a crystal is vertically pulled from an electric heated melt the weight of the crystal is measured by a load cell whose output is compared with an expected function to form a control loop for controlling heater power, the improvement comprises a correction circuit for applying to the control loop a signal compensating for a time differential (.lambda.a) of an error signal in the load cell output. The correction circuit may be a feedback or a feed forward loop.

Automatic control of crystal growth

Automatic control of the growth from the melt of a crystal in the Czochralski technique by weighing the crystal, comparing the weight with its expected value in a comparator and using the difference in a servo-loop as a servo-control signal is known. The servo-control signal modulates the melt heater power which adjusts the temperature isotherms of the melt in order to maintain the crystal cross-sectional area constant. It has been found that a thermal delay which occurs when heat is conducted from the melt crucible to the crystal/melt interface can cause the crystal cross-sectional area to depart from its uniform expected value. The effect of the thermal delay is compensated for by a phase advance circuit which is connected to the output of the servo-loop comparator.

Apparatus for automatically controlling crystal growth

The Czochralski growth of a crystal may be automatically controlled to provide uniform crystal cross-sectional area by weighing the crucible containing the melt from which the crystal is grown and deriving a weight signal which is compared with its expected value to produce a feedback loop control signal to control the melt heater power. However a magnetic field will be associated with the melt heater current and the interaction of this field on the crucible can produce a levitation force which is recorded erroneously as part of the weight signal. This unwanted force can make the closed-loop control unstable. The error component in the weight signal is cancelled in the invention by deriving an equal and opposite component by monitoring the current supplied to the melt heater and adding the derived component to the weight signal in the feedback loop.

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Malvern, United Kingdom

Gordon Charles Joyce