Using the atmospheric pressure plasma chemical vapor deposition (CVD) technique, further investigations were performed to improve the density of silicon nitride (SiN_x) films deposited at extremely high deposition rate. The films were prepared on Si(001) wafers at atmospheric pressure in a very high frequency (150MHz) plasma of gas mixtures containing He, H₂, SiH₄ and NH₃. Si-N, Si-H and N-H bond densities of the SiN_x films were studied as functions of NH₃/SiH₄ ratio and the H₂ concentration by Fourier transformation infrared (FTIR) absorption spectroscopy. Etching rate with buffered HF (BHF) solution, refractive index and relative permittivity of the SiN_x films were also studied as a function of Si-N bond density of those films. It is found that by decreasing NH₃/SiH₄ ratio or increasing H₂ concentration, Si-N and Si-H bond densities increase, while N-H bond density decreases. It is also noticed that N-H bond density is larger than Si-H bond density within the present deposition conditions, indicating that the SiN_x films are N-rich. These facts suggest that elimination of excessive hydrogen atoms bonded with N atoms at the film-growing surface is an important factor to form SiN_x film with a dense Si-N network. The highest Si-N bond density and then the lowest hydrogen content are obtained for the film deposited with the H₂ concentration of 20% and the input power of 1000W.