The properties of silicon nitride film obtained by plasma‐assisted chemical vapor deposition (plasma CVD) are studied along with the effect of NH3plasma treatment on plasma CVD silicon nitride/silicon interface characteristics. With this NH3plasma treatment of the silicon surface before silicon nitride film deposition, the interface state densities of the silicon nitride/silicon interface are reduced to about 1010cm−2 eV−1and the hysteresis voltage of the capacitance–voltage (C–V) curve is reduced to 0.1 V. These improvements are most pronounced when there is a large H concentration in the silicon nitride film. However, no effect is observed when N2or H2plasma was used instead of NH3plasma. Auger electron spectroscopy analysis of the silicon surface after NH3plasma treatment shows that nitrogen is incorporated in the near surface region of the silicon substrate. This implies that nitridation of the silicon surface occurs due to NH3plasma treatment and the interface characteristics are improved because the silicon nitride/silicon interface is made inside the silicon substrate due to this nitridation.