Paper LB-WeA8
Nanocrystalline Silicon Film Growth Morphology Control through RF Waveform Tailoring

Wednesday, October 20, 2010, 4:20 pm, Room Cimmaron

Deposition of device-quality 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 H₂-SiH₄ mixtures. Higher voltages increase the deposition rate, but also increase the ion bombardment energy which degrades the film, thus limiting the deposition rate of high quality films to »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. Films were deposited in Ar/SiF₄/H₂ plasmas and were characterised using in-situ spectroscopic ellipsometry.

We demonstrate the application of RF waveform tailoring to generate an electrical asymmetry in a capacitively coupled plasma-enhanced chemical vapor deposition system, and its use to control the growth mode of hydrogenated amorphous and nanocrystalline silicon thin films deposited at low temperature (150°C ). A dramatic shift in the DC bias potential at the powered electrode is observed when simply inverting the voltage waveform from “peaks” to “troughs”, indicating an asymmetric distribution of the sheath voltage. By enhancing or suppressing the ion-bombardment energy at the substrate (situated on the grounded electrode), the growth of thin silicon films can be switched between amorphous and nanocrystalline modes, as observed using in-situ spectroscopic ellipsometry. The effect is observed at pressures sufficiently low that the collisional reduction in average ion bombardment energy is not sufficient to allow nanocrystalline growth (<100mTorr).