Moisture stability and structure relaxation processes in plasma-deposited SiOF films

Abstract

Fluorinated silicon dioxide (SiOF) films have been prepared by remote plasma enhanced chemical vapor deposition using SiF4, O2, H2, and He gases. Fourier transform infrared spectroscopy has been used to study structural changes in SiOF films caused by interaction with atmospheric moisture. It is shown that the increased incorporation of fluorine into the SiOF network during film deposition gradually increases the value of the average Si–O–Si angle in the SiOF network, 〈θ〉, from 138° to 153°. It is concluded that the value of about 144° corresponds to nearly relaxed SiOF network and therefore can be considered as an equilibrium angle for SiOF films. Subsequent increase in the fluorine concentration in the film results in 〈θ ) larger than the equilibrium 〈θ〉 (“overequilibrium”) and causes structural relaxation of the SiOF network towards the equilibrium 〈θ〉 during the post-deposition period. Both “under-” and “overequilibrium” values of 〈θ〉 seem to be the sources of SiOF film structural instability and increased reactivity with moisture. However, film hydrolysis and structural relaxation processes in the SiOF films characterized by “underequilibrium” values of 〈θ〉 are effectively suppressed by F presence in moderate concentrations. On the contrary, SiOF films characterized by “overequilibrium” 〈θ〉 have low density network and heavily absorb atmospheric water, which strongly promotes both structural relaxation and hydrolysis during the post-deposition period.