Virtual plasma equipment model: A tool for investigating feedback control in plasma processing equipment

As microelectronics device feature sizes continue to shrink and wafers continue to increase in size, it is necessary to have tighter tolerances during the fabrication process to maintain high yields. Feedback control has, therefore, become an important issue in plasma processing equipment design. Comprehensive plasma equipment models linked to control algorithms would greatly aid in the investigation and optimal selection of control strategies, This paper reports on a numerical plasma simulation tool, the Virtual Plasma Equipment Model (VPEM), which addresses this need to test feedback control strategies and algorithms on plasma processing equipment. The VPEM is an extension of the Hybrid Plasma Equipment Model which has been augmented by sensors and actuators, linked together through a programmable controller. The sensors emulate experimental measurements of species densities, fluxes, and energies, while the actuators change process parameters such as pressure, inductive power, capacitive power, electrode voltages, and mole fraction of gases. Controllers were designed using a response surface based methodology. Results are presented from studies in which these controllers were used to compensate for a leak of N-2 into an Ar discharge, to stably control drifts in process parameters such as pressure and power in Ar and Ar/Cl-2, and to nullify the effects of long term changes in wall conditions in Cl-2 containing plasmas. A new strategy for improving the ion energy flux uniformity in capacitively coupled discharges using feedback control techniques is also explored.