Control of Microcrystalline Silicon Deposition by RF Waveform Tailoring

Deposition of microcrystalline silicon thin films at an adequate rate is a key challenge in the fabrication of thin film silicon tandem photovoltaic modules for viable large-scale power generation. Conventionally films are deposited using sinusoidal RF excitation of parallel plate reactors containing lean H$_{2}$-SiH$_{4}$ mixtures. Higher voltage increases deposition rates, but also increases the ion bombardment energy which degrades the film, thus limiting the deposition rate of high quality films to $\approx $1 nm/s for RF excitation at 13.56 MHz. We have investigated plasma excitation using non-sinusoidal waveforms to decouple the injected RF power from the ion bombardment. Various waveforms were generated using a function generator and wide-band power amplifier. Film deposition in Ar/SiF$_{4}$/H$_{2}$ plasmas was characterised using in-situ spectroscopic ellipsometry. Certain waveforms allow the deposited film structure to be switched from amorphous to microcrystalline, simply by changing from the original waveform to its complement.