Controller design issues in the feedback control of radio frequency plasma processing reactors

Feedback control has the potential for improving the reliability and performance of radio frequency (rf) plasma processing reactors for microelectronics etching, deposition, and cleaning applications. Implementation of real-time-control strategies has been slowed by lack of analytic or computational tools to design or optimize systems. To address this need, the virtual plasma equipment model (VPEM) has recently been developed for investigating issues related to feedback control in rf plasma processing equipment. The VPEM has been employed to investigate feedback control of inductively coupled plasma processing reactors for polysilicon etching and, in this article, results from these studies are used in a discussion of controller design, control strategies, and validation of the VPEM. It is demonstrated that response surface based controllers best operate in combination with corrections from an unstructured controller such as a proportional-integral derivative, which relaxes the inherent rigidity of the model-based controller. Since the behavior of plasma processing reactors generally changes over time due to, for example, coatings of the walls, it was found advantageous to make the controllers adaptive. (C) 1999 American Vacuum Society. [S0734-2101(99)05003-4].