The annealing effects during energy recoil detection (ERD) analysis on the structure of hydrogenated silicon nitride film have been investigated by using Fourier transform infrared (FTIR) spectroscopy. Hydrogenated silicon nitride films were prepared by plasma enhanced chemical vapor deposition with various substrate temperatures. A 2.5 MeV He-4(++) ion beam was irradiated onto the film in a vacuum chamber at room temperature. The ERD signal was measured after various ion doses in order to determine the loss of hydrogen counts induced by the ion beam. The IR absorption spectrum was obtained in order to follow the film structural change which occurred due to the ion beam. The films deposited at 200 and 300 degrees C show a significant decrease in the ERD count with increasing ion beam dose, while the film deposited at 400 degrees C, shows no significant changes. The IR absorption peak position for Si-N stretching (830 cm(-1)) shifted to smaller wave numbers after ion beam irradiation, while the Si-H stretching (2160 cm(-1)) shifted to the opposite direction. The peak position for N-H (3360 cm(-1)) shows no noticeable changes. Normalized peak area for the Si-N (830 cm) stretching increased after ion beam irradiation. The Si-H peak (2160 cm(-1)) area decreased slightly. The N-H peak (3360 cm(-1)) area decreased significantly. A recombination mechanism of the N and H radicals with excess Si radical coming from Si-Si bonds in Si-rich silicon nitride films has been suggested to explain the LR absorption spectral changes which have occurred due to ion beam irradiation.