VHF-PECVD and analysis of thin nano-crystalline silicon films with a multi-tile plasma source for solar energy applications
Monaghan, Eamonn
(2014)
VHF-PECVD and analysis of thin nano-crystalline silicon films with a multi-tile plasma source for solar energy applications.
PhD thesis, Dublin City University.
This work presents a characterisation of a differentially powered, multi-tile VHF capacitively coupled plasma source, MAMELUKE. The use of a differential multi-tile source imposes a spatial structure on the plasma due to the combination of capacitive and inductive power coupling. Specifically, this thesis concentrates on the PECVD of nano-crystalline silicon for applications in thin film solar manufacturing. We examine films produced by the MAMELUKE source focusing on the capability to produce uniform, high quality films over large areas with high deposition rates. The measurements of film properties are then used in combination with knowledge gained from existing works to infer information about the gas phase chemistry produced by MAMELUKE.
The deposited films are characterised for silicon deposition rate; as measured by profilometry, and lm crystalline fraction; as measured by raman spectroscopy. These measurements are made as function of process parameters and, also substrate position.
Films deposited in MAMELUKE are found to be crystalline in nature at deposition rates far exceeding conventional industrial processes and for areas larger than yet achieved by frequencies exceeding 100 MHz. The high deposition rates achieved are attributed to the unique gas phase chemistry achieved using VHF excitation.
The spatial structure imposed by the multi-tile source is found to have an effect on both the film crystalline fraction and deposition rate, depending on the power and flow settings during deposition. When non-uniformities in the film crystalline fraction are present it is observed that the crystalline fraction at tile centres is consistently lower than the tile edges. This is attributed to a non uniformity in the gas phase chemistry, specifically the atomic hydrogen density, which is due to an un-even delivery of process gas across MAMELUKE.
Capacitive coupling is seen to be dominant at tile centres and inductive coupling dominant at tile boundaries; the effect of this on plasma behaviour and thus film properties is examined. Power coupling is classified as capacitive when tile centre locations display the highest silane activation and thus deposition rate, conversely power coupling is classified as inductive when tile edges display the highest silane activation and thus deposition rate.