Experiment and modelling of very low frequency oscillations in RF-PECVD: A signature for nanocrystal dynamics

Very low frequency (VLF) oscillations in the DC self-bias voltage and the optical emission spectroscopy signatures of diluted SiH₄, GeH ₄ and SiF₄ plasmas are documented. The oscillations occur under conditions where nanoparticles are generated in the plasma, and close to the transition between amorphous and nanocrystalline material growth. The frequency, intensity and waveforms of the oscillations are shown to have a dependence on gas temperature, RF-power, total pressure and flow rate. The observation of VLF oscillations is a positive indication of the formation of a nanocrystalline film, but the converse inference is not valid. A macroscopic, zero-dimensional model for the plasma dynamics is proposed incorporating a feedback mechanism involving nanoparticles to generate the VLF oscillations. The requirements for the model to reproduce the experimental results are (i) long particle residence times (>10 s), (ii) slow particle growth rates (0.4 nm s^-1) and (iii) the rapid onset of nucleation suppression by large particles. These conditions allow us to reproduce the undamped oscillations and oscillation frequencies observed experimentally. The long particle residence times may explain the complete crystallization of the nanoparticles in the plasma.