Moisture absorption studies of fluorocarbon films deposited from pentafluoroethane and octafluorocyclobutane plasmas

The moisture absorption and diffusion characteristics of fluorocarbon films deposited from pentafluoroethane (PFE) and octafluorocyclobutane C₄F₈ plasmas are presented. The moisture absorption studies were carried out using a quartz crystal microbalance in a controlled environment. X-ray photoelectron spectroscopy and Fourier transform infrared (IR) spectroscopy were used to monitor the changes in bulk and surface chemical structure and composition of the deposited films. The equilibrium moisture uptake at relative humidity >90% was lower than 0.13 wt.% for films deposited from PFE or C₄F₈ monomers. Humidity cycling measurements showed no moisture chemisorption in the deposited films. Attenuated total reflectance infrared spectroscopy (ATR-IR) spectra of the deposited films indicated negligible change in the bulk composition of the deposited films. The estimated diffusivities of water in the deposited fluorocarbon films were of the order of 10⁻¹⁰ cm²/sec, and films deposited from C₄F₈ monomer showed higher diffusivity as compared to films deposited from PFE monomer. The equilibrium moisture uptake is affected by the presence of polar groups, the F/C ratio, and the O/C ratio. The relatively high diffusivity of water in the fluorocarbon films is attributed to the lack of polar groups in the deposited films. Adsorption onto the surface followed by diffusion into the bulk is proposed as the mechanism for moisture absorption in the fluorocarbon films. Finally, the moisture uptake of the fluorocarbon and hydrofluorocarbon films is compared to that of a conventionally used microelectronic polymer, polyimide (PI 2611), in order to evaluate the effect of polar groups and fluorine content on diffusion and equilibrium moisture uptake.