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

Abstract

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 source 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 bonds, forming silicon monofluoride and silicon difluoride sites.