Focus ring geometry influence on wafer edge voltage distribution for plasma processes

Capacitively coupled wafer-bearing cathodes are widely used in etching and deposition processes. Uniform electric field and plasma density across the wafer surface are necessary for process control all the way to the edge of the wafer. Terminating structures at the wafer edge such as focus rings are used to improve uniformity and minimize costly edge exclusion. The focus ring can be viewed as an arbitrary impedance element at the wafer edge that balances the sheath voltage above it and the region above the wafer, minimizing field variation at the wafer edge. To validate this assumption, a one-dimension circuit model with focus rings was developed. The simulations were compared to experimental results measured using hairpin probe, VI probe, and a retarding field energy analyzer (Impedans RFEA). It was found that the focus ring coupling acts as a voltage divider only in high voltage cases, and the sheath voltage drop over the focus ring will increase in low voltage cases and does not rigorously follow the voltage divider model typically used.