Polarization modulation IR spectroscopy has been applied to the in situ observation of the growth process of photo-CVD silicon nitride thin films. Two bands ascribable to vSiN modes were observed. In the initial stage of the deposition, a band appeared at 965 cm-1, whose intensity was nearly unchanged during the deposition. As the film was deposited further, a new band emerged around 1030 cm-1 which grew rapidly, dominating the spectrum. Upon UV irradiation in vacuum, the 965 cm-1 and vSiN band decreased in intensity, while the band at 1030 cm-1 increased. Similar behavior of the above-mentioned bands was observed upon admission of F2 into the cell. These results suggest the presence of a metastable surface layer, which converts into a stable network of silicon nitride.