Tailored ion energy distributions at an rf-biased plasma electrode

In materials processing applications using low-pressure plasmas, positive ions are typically accelerated by a sheath electric field directed towards the substrate, where they enhance surface reactions. The amplitude of a sinusoidal bias voltage waveform applied to the substrate electrode is used to coarsely control the average energy of bombarding ions through its effect on the dc component of the sheath voltage, generally producing a broad spectrum of ion energies. Manipulation of the bias voltage waveform shape to produce ion energy distributions (IEDs) with one or two narrow peaks at selected energies has been previously utilized to highlight the significant role of the IED in plasma etching. Presented here are direct IED measurements made with a retarding field energy analyzer situated on a biased electrode. Measurements in a 10mTorr helicon argon plasma, in which ion flux and ion energy at the substrate are independently controlled, clearly demonstrate the ability to predictably produce arbitrary IEDs at the substrate by tailoring the shape of the bias voltage waveform. Results for sinusoidal (500 kHz-10 MHz) as well as tailored (500 kHz) waveforms producing one and two peaks with selected flux and energy are presented.