Thermal annealing of fluorocarbon films grown by hot filament chemical vapor deposition

Annealing alters the stability, structure, and morphology of fluorocarbon films grown from hot filament chemical vapor deposition (HFCVD) using hexafluoropropylene oxide (HFPO) precursor. The composition of these films is greater than 97% CF₂. The as-deposited film in this study contains extraneous hydroxyl (OH) and carbonyl/carboxyl (CO/COO) groups, as revealed by Fourier transform infrared (FTIR) spectroscopy. Such groups are found to be thermally labile and lead to two film decomposition regimes, centered at 150 and 300 °C. This is similar to the two decomposition onsets at 150-250 and 300 C observed in irradiated bulk PTFE, which are attributed to the loss of peroxy radicals and COOH groups, respectively. A postdeposition l h 400 °C in situ vacuum anneal reduced the OH and CO/COO groups in the HFCVD film to below FTIR detection limits. This postanneal film is found to be stable up to 400°C with no evidence of structural changes. The surface morphology of the film quenched from the 1 h. 400 °C in situ vacuum anneal, as imaged by optical microscopy (OM), resembles that of the as-deposited film, showing no discernible surface features. However, for a film quenched from the l h 325 °C in situ vacuum anneal, significant crystalline spherulites of up to ∼1 mm in diameter are observed through OM. This is attributed to favorable crystallization in the proximity of the melting temperature of bulk PTFE. Intensity differences in FTIR absorption peaks in this film, compared to the other films, also support the morphological differences seen.