Trace organic contaminants adsorbing on silicon surfaces during transportation of wafers to a reaction chamber in an air ambient cause incubation before film growth starts in low-pressure chemical vapor deposition (LPCVD) of silicon nitride film on silicon substrates. The incubation time for wafers either exposed to clean room air for a long period without being stored in a box or stored in an outgassing plastic box prior to LPCVD is longer than that for wafers transported to the CVD reactor immediately after the previous step. It has been found that the longer incubation time is attributed to not only extraneous oxide grown on the silicon surface but also organic contaminants adsorbed on the surface.