Feedback control of substrate temperature during the growth of near-lattice-matched InGaAs on InP using diffuse reflection spectroscopy

Diffuse reflection spectroscopy (DRS) is used to control substrate temperat ure to within +/-2 degrees C of user specified setpoint during the growth o f near-lattice-matched InGaAs on InP. The same growth under constant thermo couple control would result in a 50 degrees C rise in real substrate temper ature. Feedback control is achieved using a nested proportional-integral-de rivative (PID) control loop: the inner loop consists of a conventional Euro therm-thermocouple feedback loop that controls the substrate heater power, the outer loop updates the thermocouple setpoint based on the difference be tween the user setpoint and the substrate (DRS) temperature using a PID con trol loop implemented in the control software. Frequency loop shaping, base d on a dynamical model of the system obtained from an identification experi ment, is used to tune the outer PID loop. In addition, the thermal disturba nces that occur during effusion cell shutter operations must be rejected. I n the simplest case, a single correcting step in the Eurotherm (thermocoupl e) setpoint is input when a shutter is toggled. Through disturbance identif ication and model inversion a more sophisticated disturbance rejection acti on from the controller can be obtained. (C) 1999 Elsevier Science B.V. All rights reserved.