Pulsed Laser Deposition Of Silicon Nitride Thin Films By Laser Ablation Of a Si Target In Low Pressure Ammonia

We report the deposition of Si-N films by multipulse excimer laser (lambda = 308 nm, (tau)FWHM = 30 ns) ablation of Si wafers placed in a slow flow of NH3 in the pressure range (1 mu bar-1 mbar). The films are deposited on to a Si collector placed parallel to the Si target. We succeeded in depositing pure amorphous Si3N4 films at a pressure of 1 mbar of NH3. The deposition rate reached a maximum value of 0.2-0.3 nm per pulse. At lower pressures, the deposited films consist of a fine mixture of three amorphous phases (amorphous stoichiometric silicon nitride, amorphous non-stoichiometric silicon nitride and amorphous silicon). The amorphous silicon is prevalent in films deposited at a pressure of several to several tens of mu bars. Droplets of polycrystalline alpha-Si are sometimes visible on the film surface. The experimental evidence, is analysed with a view to elucidating the participation in the chemical synthesis of the three main stages of the process: the substance expulsion from the target by laser ablation, the transition through the gas of the expulsed substance and it's final impact on the collector. We conclude that silicon nitride is mostly synthesized during the impact on the collector of the flow of the ablated substance.