CHARACTERIZATION OF EXCESS SI IN NONSTOICHIOMETRIC SIO2-FILMS BY OPTICAL AND SURFACE-ANALYSIS TECHNIQUES

Optical and surface analysis techniques such as infrared spectroscopy, ellipsometry, and Auger electron spectroscopy (AES) and x-ray photoel ectron spectroscopy (XPS) have been used to study the way in which exc ess Si is incorporated in nonstoichiometric SiO2 films deposited by a low pressure chemical vapor deposition technique. It has been found th at for this range of silicon content, the Si atoms in excess either cl uster into a well-defined second phase embedded in a SiO2 matrix, or f orm a SiO2 type of material depending on the postdeposition thermal tr eatments given to the film. The oxides are deposited at a substrate te mperature in the range of 650 to 750 degrees C using a mixture of N2O and SiH4 gases. The excess Si is introduced by reducing the nitrous ox ide to silane gas ratio during the deposition (this ratio was in the r ange of 4 to 200). After deposition, a densification process consistin g of a 1000 degrees C anneal in a nitrogen ambient for 30 min was give n to some of the samples. The refractive index, as determined by ellip sometry, is sensitive to the excess of silicon in the films, although its behavior was similar for both annealed and as-deposited samples. T he volume percent of excess silicon was estimated using spectroscopic ellipsometry for some samples. Infrared spectroscopy, on the other han d, shows a clear shift of the stretching vibration peak of Si-O-Si bon ds toward lower wave numbers as the Si excess in the film is increased in the case of as-deposited films, while the annealed samples do not present this effect. Also, as-deposited samples show an absorption pea k, at 890 cm(-1), that could be associated with partially oxidized sil icon. AES and XPS spectra show a distinct difference in the peaks asso ciated to Si bonded to either oxygen or another silicon atom for both types of samples indicating that at least a considerable part of the e xcess Si in the as-deposited samples is in the form of SiOx, while in the annealed samples it is mostly forming a second phase within a sili con dioxide matrix.