Pulsed plasma enhanced and hot filament chemical vapor deposition of fluorocarbon films

Fluorocarbon films from pulsed plasma enhanced chemical vapor deposition (PPECVD) and hot filament chemical vapor deposition (HFCVD) show a greater range in composition and structure compared to films from conventional CVD processes. Films were deposited using hexafluoropropylene oxide (HFPO), 1,1,2,2-tetrafluoroethane (HFC-134) and difluoromethane (HFC-32) as the feed gases. Film characterization was performed through high resolution solid-state 19F and 13C nuclear magnetic resonance (NMR) techniques. Increasing pulse off-time during HFPO PPECVD resulted in films with more linear CF2 character and reduced the amount of cross-linking/branching, attributed to CF2 chain propagation dominating during the off-time. HFC PPECVD films contained significantly less fluorine and more of carbon unsaturation, attributed to plasma hydrogen scavenging of fluorine to form hydrogen fluoride. Switching from PPECVD to HFCVD with HFPO as the feed gas resulted in films resembling bulk poly(tetrafluoroethylene) (PTFE), as a result of clean thermal breakdown of HFPO to form polymerizing CF2 radicals. Isothermal annealing of PPECVD films revealed two different thermal decomposition pathways: one which involved CF3 loss in more cross-linked films, and one which involved oligomer desorption/chain unzipping in films with a substantial linear CF2 chain component.