Closed-loop control of composition and temperature during the growth of InGaAs lattice matched to InP

Spectroscopic ellipsometry (SE) and diffuse reflection spectroscopy (DRS) a re used to control the Ga mole fraction and substrate temperature, respecti vely, during the growth of InGaAs lattice matched to InP. Ga mole fraction is controlled to within +/-0.002 of its target value and substrate temperat ure is controlled to within +/-2 degrees C of its target value. The same gr owth under constant thermocouple control would result in a 50 degrees C ris e in real substrate temperature and a Ga composition 1% above its target va lue. In both cases, feedback control is achieved using a nested proportiona l-integral-derivative (PID) control loop, where, the inner loop consists of the conventional Eurotherm-thermocouple feedback loop and the outer loop u pdates the thermocouple setpoint using a PID control loop implemented in th e control software. For composition control, the Ga cell thermocouple setpo int is increased (decreased) 0.2 degrees C for every 0.002 that the Ga mole fraction, given by the SE sensor, deviates below (above) the target value. During substrate temperature control, the thermocouple setpoint is updated based on the temperature difference between the DRS sensor and the user se tpoint. Frequency loop shaping, based on a dynamical model of the system ob tained from an identification experiment, is used to tune the outer PID loo p. (C) 1999 American Vacuum Society. [S0734-211X(99)01003-3].