DEVELOPMENT OF TECHNIQUES FOR REAL-TIME MONITORING AND CONTROL IN PLASMA-ETCHING .2. MULTIVARIABLE CONTROL-SYSTEM ANALYSIS OF MANIPULATED, MEASURED, AND PERFORMANCE VARIABLES

Process control strategies have been developed for reactive ion etching of silicon and silicon dioxide in CF4/O2 and CF4/H2 plasmas. Four measured variables, four manipulated variables, and seven performance variables were considered for both gas mixtures. Relative gain array analysis and singular value decomposition were used to select manipulated/process variable control loop pairings for feedback control and to evaluate potential difficulties in control system performance. Singular value decomposition was also used to determine which process variable (measured real-time) correlated best with a given performance variable. Block relative gain analysis of multivariable interactions in the process indicated that single loop feedback control would be inadequate for control of both CF4/H2 and CF4/O2 etching, which was subsequently verified by simulation. Multivariable control (partial decoupling) was much more effective in reducing dynamic fluctuations in the process variables.