Active control of instabilities for plasma processing with electronegative gases

Abstract

Feedback stabilization of instabilities was demonstrated in inductively coupled electronegative plasmas by modulating the radio frequency power in response to a feedback control signal. Two modes were observed and stabilized: a fast bursting mode (designated ‘B’ mode) and a slower oscillatory mode (designated ‘O’ mode). ‘B’ mode is a low duty-cycle non-linear phenomenon characterized by short (<1 µs) spikes on optical signals, at a pulse repetition rate of 10–40 kHz. ‘O’ mode is characterized by continuous oscillations in ion saturation current at lower frequency (200 Hz–1 kHz). Some parameter ranges exhibit both modes. Fluctuations in similar processing plasmas and having similar characteristics have previously been observed and identified with instability due to electron attachment to the electronegative neutral gas species.Because ‘O’ mode exhibits fluctuations in ion density, it may be of more concern than ‘B’ mode for repeatable plasma processing. Stabilization of ‘O’ mode may be accomplished using the ion saturation current signal collected by a Langmuir probe as a measure of the plasma density. The floating potential obtained from the capacitively coupled plasma bias electrode is similar in shape to the ion saturation current, and can also be used as the feedback control signal.The bandwidth required for stabilizing the ‘O’ mode is in the kHz range, whereas the bandwidth required to stabilize ‘B’ mode may be closer to a MHz. In practice, it is far easier to stabilize the ‘O’ mode than the ‘B’ mode, but both may be effectively suppressed. However, since a purely ‘B’ mode does not seem to measurably modulate the ion density, and unless it dramatically changes the average electron temperature it may not be necessary to stabilize it in order to achieve repeatable plasma processing.