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

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.