In situ Fourier transform infrared (FT-IR) spectroscopy was used to study the atomic layer deposition mechanism of hafnium silicate films with dichlorobis[bis(trimethylsilyl)amido] hafnium (HfCl2[N(SiMe3)2]2) and water. The surface species was monitored during atomic layer deposition using vacuum chambers designed for in situ FT-IR spectroscopy studies. Vibrational spectroscopy reveals the gain and loss of surface species during the two surface half-reactions. The behavior of the functional group (such as O–Η, Si–(CH3)x, and C–H was monitored and from that, the temperature dependence of the growth rate and the film composition could be explained. It was also found that Si–O–Si peaks between 1000 and 1200 cm-1 were formed when water was dosed above 300°C, which could explain the incorporation mechanism of Si into the film.