Fundamental aspects of substrate biasing: ion velocity distributions and nonlinear effects

Ion bombardment of the substrate is a significant parameter in plasma processing such as dry etching or thin film deposition. The ion bombardment is described by ion velocity distribution functions (IVDFs), which were here measured quantitatively at a sinusoidally and non-sinusoidally biased electrode. The electrode voltage was monitored and controlled in the frequency domain using fast Fourier transformation. IVDF measurements were performed by a floating retarding field analyzer. A full modulation of the IVDF by arbitrary bias waveforms is only achieved if sufficiently high sheath voltages are used. If the applied sheath voltages become too low, the IVDFs are only partly determined by the RF bias waveforms and the system response becomes nonlinear. An analytical sheath model is derived from the experimental data, which accounts for arbitrary bias waveforms as well as for collisional and nonlinear effects in the sheath. It is shown that a combined DC and RF biasing of the electrode is required to gain full control over the ion bombardment of the substrate.