Laser-induced chemical vapor deposition of hydrogenated amorphous silicon. I. Gas-phase process model

Abstract

In the laser-induced chemical vapor deposition (LICVD) process, a CO2 laser beam impinges on a gas mixture parallel to the substrate upon which the film is deposited. Since heating of the reactant gases is accomplished only via the absorption of infrared photons, the reaction zone can be controlled precisely. The LICVD technique is a cold-wall thermal process allowing independent control of both the gas and substrate temperatures. In this paper, we propose a model for LICVD of silane (SiH4) and growth of hydrogenated amorphous silicon (a-Si:H) thin films in which the film growth is controlled by gas-phase homogeneous thermal decomposition of the SiH4. The peak gas temperature Tg depends on many process parameters, namely, gas partial pressures, laser power, substrate temperature, and cell geometry. Due to the extreme sensitivity of the growth rate G to the values of the partial pressures and laser power, these parameters must be fixed to within ±1% variation in order to control G to ±50% and prevent powder formation. LICVD gas-phase chemistry involves the production of SiH2 for the thermal decomposition of SiH4 and higher polysilanes (Si2H6, Si3H8, etc.) resulting from reactions between SiH2 and SiH4. SiH2 and possibly higher diradicals produced in the laser beam then diffuse to the substrate and react with the surface layer, thus inducing growth of the a-Si:H film and the concomitant elimination of H2.