Optimal waveforms for capacitively coupled ionization in nanosecond plasma discharges

Abstract

Optimal voltage waveforms for capacitively coupled plasma discharges are numerically investigated using a particle-in-cell kinetic model. The focus is on nanosecond time scales where the transient plasma response and charge separation in the plasma can be exploited with precise voltage timing. Square wave and sinusoidal waveforms with a dynamic frequency equal to half of the instantaneous plasma frequency lead to higher plasma density in less time. Using such optimal waveforms, the maximum plasma densities can be increased from 2 to 9 times higher over a standard RF waveform. Conversely, the same density can be achieved in the same duration using half the voltage amplitude as compared to an RF waveform. Plasma density based waveform optimization is applicable in cases where system size is larger than electron mean free path and dominated by electron-neutral collisions.