Abstract
The Czochralski (CZ) crystallization process is used to produce monocrystalline silicon, which is used in solar cells and electronics. The temperature of the crucible containing the molten silicon must be tightly controlled to achieve high silicon quality. SINTEF Materials and Chemistry in Trondheim, Norway, owns and operates a CZ process. At this plant the crucible is heated by a heating element encircling the crucible. The heating element power is manipulated using a triode for alternating current (TRIAC). Presently the crucible temperature is controlled using a single-loop PID controller, which output manipulates the TRIAC. This paper suggests using cascade control for controlling the crucible temperature. The cascade inner loop is a power control loop, and the cascade outer loop is the temperature control loop. Hence, the output of the temperature controller will be the reference (setpoint) to the power controller. The main motivations for using cascade control are (i) fast and effective rejection of power disturbances, and (ii) robustness to parameter variations in the inner loop. This paper also presents model-based PID tuning of the power controller. Different PID tunings are compared by the bandwidth of the control loop and Bode diagrams of the closed loop transfer function and the sensitivity function. An integral controller (I controller) gives best control performance. Robustness to parameter variations in the inner process is simulated using Bode diagrams of the closed inner loop for increased and decreased process gain.
