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Silicon nitride thin films were deposited on (100) Si wafers in an ultra-high vacuum system using a Si effusion cell and reactive nitrogen from a radio-frequency plasma source. The films were characterized using infrared transmission spectroscopy, infrared reflectance, Rutherford backscattering spectrometry, spectroscopic ellipsometry, specular x-ray reflectivity, wet etching in a buffered-oxide etch solution, and the electrical characterization of metal-insulator-semiconductor capacitors. High-quality, stoichiometric silicon nitride films with a refractive index of 2.05 at 632.8 nm are produced when the deposition temperature is 750 °C. Lower deposition temperatures produce nitrogen-rich silicon nitride films with lower refractive index, lower density, greater tendency toward oxidation in ambient air, faster etching in a buffered oxide etch solution, and greater electrical leakage. A deposition model involving thermal evolution of weakly-bonded excess N is proposed to explain our observations.