Microwave plasma assisted chemical vapour deposition was used to produce silicon nitride films on silicon substrates from mixtures of methane and nitrogen. Deposition temperatures varied from 800 to 1000 degrees C and pressure varied from 53.2 to 79.8 x 10(2) Pa. Gas mixtures with low methane content resulted in no reaction. Gas mixtures with high methane content produced an amorphous carbon film on the silicon wafer surface. At intermediate methane contents, the process produces a mixture of alpha and beta silicon nitride. A mechanism is proposed according to which the silicon surface is chemically etched by the activated methyl radicals forming Si(CH3)(4), which then reacts with nitrogen atoms (or ions) to form the silicon nitride. The morphology of the individual crystals evolves from platelets to needle-like depending on the deposition conditions, and on the surface coverage of the silicon surface.