Fabrication, Characterization, and Comparison of Oxygen-Rich Organic Films Deposited by Plasma- and Vacuum-Ultraviolet (VUV) Photo-Polymerization

Abstract

Thin organic films with oxygen‐bearing functional groups (hydroxyl, carboxyl, …) were prepared by “co‐polymerizing” gas mixtures of ethylene (C2H4) and one of several O‐containing oxidizer molecules, O2, CO2, or N2O. This was done either by vacuum‐ultraviolet (VUV) photo‐polymerization or by low‐pressure r.f. plasma‐assisted CVD. The gas mixture ratio, R, permits one to control total oxygen concentration, [O], as well as the relative proportions of the various functionalities. Analyses were performed by XPS (with or without chemical derivatization using TFAA to determine [OH] content), and IRRAS‐FTIR. Temporal and structural stability (mass change) of deposits under exposure to air or water was examined; altogether, data confirmed different reaction pathways for VUV and PECVD.

Sulfur-Rich Organic Films Deposited by Plasma- and Vacuum-Ultraviolet (VUV) Photo-Polymerization

“…In contrast, here, when using the latter hydrocarbon, PP films yield higher [S] values, up to 48 at% (Figure b). Similar results were also reported for O‐containing films, a clear indication that the plasma‐ and VUV‐based chemistries are quite different and complex. Further, surface oxygen concentrations were always below 5 at% (results not shown), which could be an indication that these films are more stable toward aging compared to nitrogen containing films.…”

“…A related finding occurred when acetylene, C 2 H 2 , was used as source gas . In addition, when using C 4 H 6 , no drop in r was observed, even at higher R values; this is somewhat surprising, because r decreased substantially in previous experiments with C 2 H 4 and N‐ or O‐containing gas mixtures . In those previous works it has been shown that NH 3 , for example, acts as an etchant for organic materials, because with increasing R values atomic H is released during fragmentation, leading to a decrease in r .…”

“…Two primary dissociative processes of similar importance have been reported for ethylene, C 2 H 4 , both leading to the formation of acetylene: …”

“…Briefly, it consists of a stainless steel “cross” chamber, evacuated to high vacuum using a turbo‐molecular pump supported by a two‐stage rotary vane pump (base pressure p = 10 −5 Pa). As in previous experiments, the operating pressure during deposition was maintained near p = 13 Pa (100 mTorr). The flow rate of the high‐purity hydrocarbon source gases C 2 H 4 (99.999%, Megs Inc., Montreal, QC, Canada), F C2H4 , or C 4 H 6 (99.8%, Megs Inc., Montreal), F C4H6 , was kept constant at 10 sccm using mass flow controllers (Brooks Instruments, Hatfield, PA), while the flow rate of H 2 S (99.5%, Megs Inc., Montreal), F H2S , was varied between 0 and 15 sccm, also using a mass flow controller; this yielded values of R (e.g., F H2S / F C4H6 ) ranging from 0 to 1.5.…”

“…In essence, VUV‐assisted processing retains only one energy‐source component of the plasma, the VUV photons, to carry out (photo‐) chemistry, thus potentially allowing for better control of the overall process. Indeed, VUV‐based deposits tend to possess higher amine concentrations, [NH 2 ], likely due to more specific and selective reactions occurring via mono‐energetic photons; “hot” electrons, the main originators of chemical reactions in low‐pressure plasmas, necessarily possess a broad, Maxwell‐Boltzmann‐like energy distribution . In addition, VUV radiation can more readily promote intermolecular cross‐linking, leading to more stable films compared with their low‐pressure plasma counterparts…”