Structure of fluorine-doped silicon oxide films deposited by plasma-enhanced chemical vapor deposition

The Fourier transform infrared (FTIR) spectra and Raman spectra of fluorine-doped silicon oxide films capped by silicon nitride thin film to prevent water absorption from the air were measured as a function of the fluorine concentration in the films. It was found that fluorine doping diminishes the Si-OH bonds in films. Films without fluorine show a clear OH band attributable to Si-OH bonds, but films with 7.6 at % (F) or more exhibit no OH band in FTIR spectra. The FTIR spectra also show that fluorine in films forms new absorption bands at frequencies ranging from 990 to 920 cm(-1). These absorption bands are assigned as silicon monofluoride sites (940 cm(-1)) and as silicon difluoride sites (925 and 985 cm(-1)). While the silicon difluoride sites increase linearly with increasing fluorine source gas flow, the increase in silicon monofluoride sites saturate at a certain fluorine sourer gas flow [i.e., a fluorine concentration in film of about 7.6 at % (F)]. The Raman spectra show that fluorine doping reduces the planar threefold ring structures in the films. It is surmised that fluorine interchanges preferentially with oxygen in strained Si-O-Si bends, forming silicon monofluoride and silicon difluoride sites. (C) 1997 American Vacuum Society. [S0734-2101(97)02306-3].