Analysis of structural changes in plasma-deposited fluorinated silicon dioxide films caused by fluorine incorporation using ring-statistics based mechanism

Fluorinated silicon dioxide (SiOF) films were prepared by remote plasma enhanced chemical vapor deposition using SiF4, O-2, H-2, and He reaction gases. Fourier transform infrared spectroscopy studies accompanied by molecular orbital (MO) modeling were used to explain structural changes in SiOF films caused by F incorporation. On the basis of the results of MO modeling, it was shown that F atoms incorporated into the SiOF network only slightly affect the geometry of ring units, the main building blocks of SiOF film network, and cannot cause strong changes in the value of the average Si-O-Si angle, <theta >. Ring-statistics-based mechanism is proposed to explain the increase in <theta > in SiOF films with F content. It is supposed that interaction of highly reactive F species from the incoming flux with the growing SiOF network during deposition process induces the preferential conversion of the most strained small-order ring units into those of higher order characterized by larger <theta >. As a result, the <theta > in SiOF film increases and film structural homogeneity improves with increasing the F content in the incoming flux. It is assumed that structural changes in SiOF films caused by F incorporation are not the abrupt transition from the ring-built SiOF network to the ring-free chain-like network, but a continuous shift of the ring distribution function towards the large-size high-order rings which causes the increase in <theta > within SiOF network, a reduction in the film density, and an enhancement of the film moisture absorptivity. (C) 1999 American Institute of Physics. [S0021-8979(99)04613-7].