Plasma Initiated Chemical Vapour Deposition - from the Growth Mechanisms to Ultrathin Low-k Polymer Insulating Layers

Doctoral thesis (2021)

Plasma-assisted approaches are broadly used in thin-film deposition, surface preparation and top-down fabrication. Particularly, plasma processes, which can operate at atmospheric pressure, have ensured ... [more ▼]

Plasma-assisted approaches are broadly used in thin-film deposition, surface preparation and top-down fabrication. Particularly, plasma processes, which can operate at atmospheric pressure, have ensured the simultaneous synthesis and deposition of numerous thin film compositions that have met multiple applications. Yet, the wide variety of reactive species composing plasmas induces a non-negligible amount of side reactions that result in a lack of regularity in polymeric materials compared to the ones formed by wet chemical polymerisation processes. The combination of ultrashort nanosecond plasma discharge (t_ON ≈ 100 ns) and long plasma OFF-time (t_OFF = 0.1 – 100 ms), for the initiation and propagation of the free-radical polymerisation reaction, was recently demonstrated to yield the formation of conventional polymer layers. Based on the current understanding of the process, i.e. significance of the plasma pulse frequency, this thesis aims at gaining a deeper insight in the influence of other important parameters. The nanosecond pulsed plasma deposition of low dielectric constant insulating thin films is studied. Providing additional dimensions to the characterisation, the dielectric layer’s properties such as the leakage current and the dielectric constant, allow to discriminate mechanisms that would not have been identified from other techniques. Hence, ensuring the further development of the fundamental understanding of the nanosecond pulsed plasma approach. From the nanosecond pulsed plasma deposition reaction of different tetra-organosiloxane compounds, the growth mechanisms driving the formation of insulating polymer layers are elucidated. For vinylic monomers, the plasma-induced polymerisation is demonstrated to be highly favour over plasma-state polymerisation at low plasma pulse frequency. This yields the excellent retention of the monomer structure and the prevalence of surface reactions, which are essential to obtain remarkable insulating properties. In addition to the significance of the monomer structure, the saturation ratio, i.e. the monomer partial pressure over its saturated vapour pressure (PM/Psat), 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 reticulated and leaky polymer layers, when operating at low plasma pulse frequency. As such, three different regimes of growth are identified: the monomer deficient regime, the competition regime, and the energy deficient regime. Optimisation of saturation ratio ensures the formation of smooth and conformal low dielectric constant insulating thin films. Taking advantage on the understanding gained on the nanosecond pulsed plasma deposition of insulating polymer layers, the dielectric constant is tuned from the careful selection of the starting monomer compound. Several vinylic cyclo-siloxane and -silazane compounds are notably studied. Dielectric constant values as low as 2.8 are obtained from the monomer possessing the lowest polarisable bonds and the larger ring size, i.e. the 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotrisiloxane, while retaining a low leakage current density in the range of 10^-9 A/cm^2 at 20 V. [less ▲]