Effect of bias voltage waveform on ion energy distribution

Ion energy distribution (IED) is one of the primary factors governing the etching or deposition characteristics in plasma-aided microelectronics manufacturing processes. This article explores the influence of rf bias voltage waveform and frequency on the IED. It is demonstrated that the sheath voltage above the wafer is reasonably similar to the rf voltage on the biased substrate. Since the IED correlates well with the sheath voltage if the ion transit time through the sheath is smaller than the rf time period, the IED can be controlled by means of the rf bias voltage. The voltage waveform controls the shape of the distribution while the frequency determines its width. The sinusoidal waveform leads to a distribution that peaks at high energies and gradually decreases with decreasing energy. Square wave results in a sharp step in the IED at high energies, the width of which can be controlled by means of the blocking capacitance. The triangular waveform generates a constant IED over a considerable range of energy. It is also demonstrated that, by utilizing the correlation between the IED and applied voltage waveform, one can design voltage waveforms that produce distributions with specific features.