High-resolution ¹⁹F MAS NMR spectroscopy of fluorocarbon films from pulsed PECVD of hexafluoropropylene oxide

Magic angle sample spinning with speeds of up to 25 kHz has been applied to obtain high-resolution solid-state ¹⁹F NMR spectra of fluorocarbon films deposited by pulsed plasma-enhanced chemical vapor deposition (PECVD) from hexafluoropropylene oxide. Compared to XPS, the common characterization method for fluorocarbon thin-film analysis, ¹⁹F NMR is demonstrated to provide greater structural information which is important in unraveling the complexity of plasma films. Seven fluorocarbon resonances - three CF₃ sequences, three CF₂ sequences, and a CF resonance - were distinguished, based on differences in next nearest-neighbor bonding environment. Total CF_x (x = 1-3) fractions, as quantified by NMR, agreed with independent XPS results. Line width variation of the assigned resonances resulted from isotropic shift dispersion, arising from connectivity permutation statistics and from the relative mobility of fluorocarbon moieties, which can be hindered by cross-links or branch junctions. With lower pulsed plasma deposition duty cycles, resulting films contained more linear CF₂ chains. Cross-linking, branching, and irregular chain terminations, arising from plasma ion bombardment and excessive gas-phase fragmentation during film growth, are reduced. The thermal decomposition mechanism shifted from a loss of CF₃ end groups to a loss of linear chain fragments as a result of changes in film structure with longer deposition pulse off-time. There is evidence to indicate thermal desorption of oligomers or radical-enhanced depolymerization for films deposited at lower duty cycles.