The deposition chemistry, optical properties, and structure of plasma-deposited fluorinated silicon nitride films are studied and compared to available results of other researchers. Fluorinated films show an increase in deposition rate and optical band gap, and a decrease in refractive index and film stress compared to unfluorinated silicon nitride films. Fluorine concentration in the films ranges from 5 to 25 atomic percent. The total hydrogen concentration of fluorinated films is lower than that of unfluorinated films. The improved thermal stability of fluorinated films as compared to unfluorinated films is explained by a significant decrease in hydrogen bonded to silicon. All films hydrolyze to some extent and films with large fluorine concentrations hydrolyze rapidly to silicon dioxide. The microstructure of fluorinated films consists of clustered silicon-fluorine and nitrogen-hydrogen regions within an amorphous silicon-nitrogen matrix.