Analysis and modeling of hot wire chemical vapor deposition of silicon films

Our goal was to learn how to deposit uniform polycrystalline silicon films over large surface area for application in photovoltaics and flat panel displays. Silicon film depositions were carried out using a commercially available hot wire chemical vapor deposition chamber and a quantitative model was developed to represent the process. The operating conditions ranged between pressures of 25 and 1000 mTorr with silane gas flowrates in the range of 4--60 sccm and filament and substrate temperatures in the range of 1550--1850°C and 400--600°C, respectively. The effects of silane flowrate, filament temperature, total pressure, substrate temperature and reactant concentration on the exit gas phase composition, film growth rate, and film crystalline fraction are determined.; The mathematical model equations describe a vacuum reactor in which silane cracks over a series of heated tantalum wires, reacts further in the gas phase, and deposits in the form of silicon films on glass substrates. The model considers gas motion in the reactor and the reactions of silane leading to Si film deposition. The model incorporates surface pyrolysis reaction on the hot wire, gas phase reactions, and film growth reactions on the substrate. The model predictions of silane conversion and silicon film growth rate are in good agreement with the experimental results over the range of conditions studied.; We have established that optimum pressure and filament temperature exist to deposit poly-crystalline silicon (poly-Si) films of uniform thickness over a large area; since, both the film crystallinity and the axial variation in film thickness increase with total pressure and filament temperature. An important implication of the study is that a critical ratio of atomic hydrogen flux relative to the total flux of growth precursors is required at the film surface for transition from amorphous to poly-Si films. The flux ratio of hydrogen radical to growth precursors is controlled by the pressure, filament temperature and silane flowrate. For the conditions investigated a flux ratio greater than 15 leads to the deposition of poly-Si films.