The effects of moisture on strain relief of Si-O bonds in plasma-enhanced chemical vapor deposited silicon dioxide films

Plasma-enhanced chemical vapor deposited (PECVD) silicon dioxide films show considerable improvement in film quality following annealing at temperatures significantly below their densification temperature. This study was performed to investigate this improvement phenomenon. PECVD silicon dioxide films were annealed in different ambients. Some of the films were aged for various lengths of time and then annealed in a nitrogen ambient at atmospheric pressure. The change in bonding nature of these films during annealing and aging was studied by Fourier transform infrared spectroscopy. Sizable shifts in the Si-O characteristic peaks and a decrease in the full width at half maximum of the Si-O stretching peak were observed for films exposed to room ambient before annealing, whereas films which were annealed in the chamber following deposition without breaking vacuum did not exhibit any appreciable change in either. The results of this study suggest that moisture, which diffuses into the film during aging and/or from the annealing ambient, reacts with strained Si-O bonds and forms silanol (Si-OH). During annealing, the silanol reacts to reconstruct Si-O bonds and releases water as a by-product. The reason for the improvement in film quality is that the strain of the newly formed Si-O bonds is less than the strain of the original Si-O bonds. The strain of the Si-O bonds in the as-deposited films appears to be sufficient and responsible for activating the Si-OH formation.