Characterisation of Excimer iamp photo-deposited ultrathin oxynitride films

Excimer lamp deposited ultra-thin (< 250A) silicon dioxide and silicon oxynitride films were characterised using spectroscopic ellipsometry (SE) and Fourier transform infrared (FT-IR) spectroscopy. SE analysis of the photo-deposited SiO₂ films revealed no variation in the refractive index (n) of the films for deposition temperatures of 200°C and 300°C. Using a Bruggeman effective medium approximation (EMA), SE analysis was employed to determine both the silicon oxynitride layer thicknesses and compositions as a function of deposition temperatures and gas ratio, defined as (N₂O/(N₂O+NH₃)). From this analysis the optical properties of the silicon oxynitride thin films were extracted. It was observed that the refractive index for the 200°C and 300°C series of samples decreased from n=1.81 to 1.46 and n=1.72 to 1.46 respectively as a function of increasing gas flow ratio. FT-IR analysis revealed spectral features characteristic of Si-O, Si-N, Si-H and N-H bonding. The most significant feature in all recorded spectra was a mixed spectral absorption band ranging from 800cm^-1 to 1300cm^-1. Both the integrated band area and peak wavenumber of this absorption band was found to be sensitive to the degree of nitridation and layer thickness of the thin films. The N-H stretching bond density was calculated from the N-H peak at 3360cm^-1 using appropriate calibration factors. A slight decrease in the N-H bond density with increasing gas flow rate was observed. This variation in bond density was significantly less than that observed for PECVD silicon oxynitride films.