Si Nx films deposited on silicon in a plasma-enhanced chemical vapor deposition (PECVD) reactor from a mixture gas of Si H4 and N H3 are investigated as an alternative method for low-stress membrane fabrication. We verified that the stress in the silicon nitride films decreased as a function of deposition pressure. A low tensile stress in the film of 170 MPa was obtained at a pressure of 750 mTorr and the refractive index of the film was 1.8. After KOH wet etching from the back side of the silicon substrate, a flat square (5×5 mm) membrane with thickness of 500 nm could be successfully fabricated. The chemical composition of the film was analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, with lower deposition pressures producing less Si-H bonding.