Plasma Initiated Chemical Vapour Deposition—From the Growth Mechanisms to Ultrathin Low-k Polymer Insulating

Plasma processes are highly versatile methods that enable the formation of a vast variety of materials directly in thin films form. Nevertheless, the many reactive species composing plasmas are responsible for a non-negligible number of side reactions, yielding altered chemistries compared to conventional polymerization processes. In this work, we demonstrate how the combination of ultrashort nanosecond plasma discharge (t_ON ≈ 100 ns) and long plasma off-time (t_OFF = 0.1 – 100 ms) promotes plasma-induced polymerisation over plasma-state polymerisation. During the plasma on-time, a defined number of radical and neutral fragments, which can play both the roles of polymerization initiation or termination groups, are produced. When using vinylic monomers, the conventional polymerization pathway is strongly favoured during the off-time, yielding linear polymer core and unprecedented molecular weights for an atmospheric-pressure plasma processes. In addition to the significance of the monomer structure, the saturation ratio, i.e. the monomer partial pressure over its saturated vapour pressure (P_M/P_sat), is demonstrated as a key parameter of the thin film’s growth. While low P_M/P_sat values result in the prevalence of gas phase reactions, excessively high P_M/P_sat values lead to the formation of poorly cured polymer layers, when operating at low plasma pulse frequency. Taking advantage on the understanding gained on the nanosecond pulsed plasma deposition of polymer layers, ultrathin (<i>ca. </i>10 nm) and conformal polymer layers with low leakage current densities (<i>ca. </i>10^-9A/cm² at 20 V) are prepared. The careful selection of the monomer structure enables to decrease the dielectric constant from 4.2 to 2.8.