Experimental study of the plasma gas phase in low-pressure radiofrequency discharges of oxygen and hexamethyldisiloxane is presented. The plasma phase has been studied by means of optical emission spectroscopy. Mass spectroscopy of the neutral and of the charged species has been performed too, directly sampling the plasma gas phase, by a dedicated spectrometer. We also measured the ion energy distribution. We have studied the influence of the operating conditions on the plasma gas-phase composition which plays a primary role in the formation process of SiO₂ films, which are known for their important applicative uses.

In this study, we examine the statistical properties of asymmetric surface dielectric barrier discharges (SDBD) produced by applying a periodic high voltage between two conducting displaced electrodes, located at the opposite sides of a flat dielectric panel. Here, the asymmetry refers to the fact that the lower electrode is fully covered with an insulating material, while the upper one, glued onto the dielectric surface, is otherwise left exposed to the air. Such a configuration allows the formation of a thin layer of plasma above the insulating surface. A single cycle signal consists of two well-separated half-cycle patterns, denoted as forward and backward strokes, corresponding to positive and negative voltages, respectively. They display a quite complex discharge pattern constituted by a sequence of individual peaks (bursts) of varying current and time duration. Specifically, we find that backward stroke bursts carry a positive mean charge $⟨Q⟩\simeq 0.3$ nC and mean current $⟨I⟩\simeq 35$ mA, with a mean duration $⟨\tau ⟩\simeq 15$ ns, while forward stroke bursts have a negative mean charge $⟨Q⟩\simeq -0.1$ nC, a mean current $⟨I⟩\simeq -20$ mA, and a mean duration $⟨\tau ⟩\simeq 11$ ns. The statistical analysis suggests that power injection can be tailored to produce the active agents in the plasma needed for a particular application. We also determined discharge spatial correlation patterns from measurements of the associated stimulated optical emission. The optical excitations occur as a result of the ionizing effect of the electromagnetic waves which ignite the discharge, followed by the electric current flow. In particular, we point out that one of the phases of the discharge is compatible with a cathode directed streamer phenomenon (backward stroke), while the mechanism acting for a forward stroke has a different structure.

A characterisation of a low pressure SF6 discharge was undertaken, by means of plasma diagnostics, including radiofrequency electrical probes, Langmuir probes and optical emission spectroscopy. Chemical kinetics modeling of the discharge was performed too. This was aimed to the understanding of a reported transition in the plasma induced surface modification on poly(ethylene terephthalate) (PET) fibres by SF6 plasma treatment which can be characterized as a striking wettability modification reverse displayed by plasma treated fibres as the SF6 pressure in the discharge exceeds a threshold value. Here we have investigated the modification also on different polymeric surface such as that of a PET film and of cellulose (paper). We point out that the observed transition could be understood as a switch between an increased surface hydrophilicity, induced by plasma treatment at low pressure, which produces also surface etching and activation, and an increased surface hydrophobicity, imparted by plasma treatment at higher pressures, due to extended surface fluorination.