Auto-Tuning Control of a Switched-Mode Power Converter for Tailored Pulse-Shape Biased Plasma Etching Applications

For reaching a high selectivity in plasma etching, it is required to precisely control the plasma ion energy. This can be realized by applying a tailored pulse-shaped voltage waveform to bias the reactor table. The bias waveform is divided into an etching phase and discharge phase, based on the status of the plasma reactor. During the etching phase, the waveform is a negative voltage slope while during the discharge phase, the waveform is a short positive voltage pulse. Recent research has shown that switched-mode power converters can be used to generate this kind of bias waveform. To obtain a narrow ion energy distribution, the voltage slope rate during the etching phase should be accurately tuned. Traditionally, the voltage slope rate is tuned manually by finding the optimal ion energy distribution from the measurements by a retarding field energy analyzer. However, measurements using a retarding field energy analyzer is interactive with the plasma thus affecting the etching process. On the other hand, the manual-tuning is inefficient since iterative retuning is required if the operating condition is changed. In this paper, an auto-tuning method is proposed, which enables the power converter to generate the optimal waveform automatically. The control method is fully based on the measurements of voltage and current waveforms on the converter side. Therefore, it is nonintrusive and does not interact with the plasma etching process.