Decentralized control of wafer temperature for multizone rapid thermal processing systems

Decentralized control is shown through analysis and experimentation to be a n appropriate strategy for wafer temperature control in certain multizone r apid thermal processing (RTP) systems. An input-output controllability anal ysis is conducted to illustrate that the direction associated with the refe rence command (set-point) corresponding to a spatially uniform temperature trajectory specification is nearly in alignment with the "most" controllabl e direction associated with the maximum singular value for a multiple conce ntric lamp configuration. Consequently, the control structure need not alte r the directionality of the plant and, thus, can be achieved by a simple de centralized controller where the lamps are paired individually to sensors t o achieve a multiloop structure where all interactions are not taken explic itly into account. This result is shown to produce acceptable performance e ven for an ill-conditioned plant since the directions corresponding to the smaller singular values are irrelevant to the uniform temperature control c riteria. Moreover, straightforward nonmodel-based tuning of the controller is enabled due to the simplicity of the decentralized control structure.