Controlling negative ions in plasma using tailored voltage (Vol. 48, No. 5-6)

Calculated profile of negative ions for different voltage waveforms

Plasma processing of materials has wide applications in science and industry. In a capacitively coupled plasma, a feedstock gas, often diluted with hydrogen, is partially ionized with an electric discharge. In the most basic configuration, the key plasma parameters of ion flux and ion energy are strongly linked: increasing the applied sinusoidal voltage amplitude increases both. With a multiple harmonics waveform, it is possible to decouple ion flux and energy, obtaining increased processing speed while maintaining the ideal ion energy for surface reactions. This concept has been formulated and exploited for positive ions. Hydrogen plasmas, however, produce also negative ions which are important in other applications like heating systems in nuclear fusion.

The authors investigated, using a comprehensive numerical model, the effect of tailored voltage waveforms on the location, peak density and dynamics of negative hydrogen ions and the influence of molecular physics on their production rate. They found that under appropriate conditions it is possible to concentrate negative ions in a desired position. This is due to a complex interplay between plasma electrical quantities, particle energy distributions and transport phenomena.

P. Diomede, B. Bruneau, S. Longo, E. Johnson and J.-P. Booth, Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: vibrational kinetics and negative ions control, Plasma Sources Sci. Technol. 26, 075007 (2017)