On-line control of process uniformity in single wafer processes

A methodology for on-line process control of uniformity is developed based on the division of process variability into two categories: that which can be effectively controlled on-line and that which must be optimized off-line. This categorization is based on the physics of the process and equipment. In the case of axisymmetric single wafer processing, the radial uniformity can be controlled on-line, as a number of process parameters will have a direct influence on the radial uniformity. However, circumferential uniformity is not directly influenced by any process parameters and must be optimized off-line. The choice of process parameters to effect on-line control is designed to decouple the successive stages of optimization and control and is guided by the formulation of appropriate performance metrics. The methodology presented simplifies on-line control and recommends a narrower goal for its implementation, one that can be achieved with minimum risk of inadvertently degrading the performance of the process. The methodology is applied to single wafer plasma oxide and polysilicon etching processes. It is shown that radial uniformity is improved by applying on-line control while minimizing the impact on circumferential uniformity, with the result that overall uniformity within a wafer is improved. The methodology is also successfully applied to effect a step change in the radial profile of etching rate.