Real-time control of substrate temperature using band edge thermometry

The nested loop approach is a convenient way to add an accurate external temperature sensor to the conventional Eurotherm-thermocouple control loop found in molecular beam epitaxy systems. In this case, the outer control loop updates the Eurotherm (thermocouple) setpoint based on the difference between the user setpoint and the value measured by the external sensor, using a proportional-integral-derivative algorithm implemented in control software. This method has the advantage that the Eurotherm-thermocouple control system is left intact, so that, in the event the external sensor or control computer fails, the heater power will continue to be controlled.