An amorphous silicon nitride thin film has been fabricated by electron cyclotron resonance plasma irradiation. The growth of the film is carried out by Ar/N-2 mixed plasma irradiation at a low temperature of 400degreesC. It is found that nitrogen partial pressure decisively affects film quality. A SiN film having a structure nearest to stoichiometric construction is obtained by precisely controlling the N-2 mixing ratio at 60%. This implies that the existence of Ar with a suitable partial pressure increases the nitrogen radical concentration in the Ar/N-2 mixed plasma. Under optimum conditions, the as-grown SiN film shows a leakage current more than two orders of magnitude lower than that of thermally grown SiO2 having the same equivalent oxide thickness. A high-resolution transmission electron micrograph shows an atomically flat interface of the Si substrate and SiN film.