Alban Sublet
Dielectric Barrier Discharges (DBD) have been used for more than a century, especially for ozone production. Research conducted within the last twenty years has investigated the discharge mechanisms involved and the different discharge regimes observable (filamentary, glow, Townsned and multi-peaks). These studies highlight the fundamental role of metastable species to establish and maintain a homogeneous discharge. These recent improvements in understanding the physics of DBD's open perspectives for new applications and new interests in atmospheric pressure surface treatment. Working at atmospheric pressure for silicon oxide deposition is of great interest : the possibility of continuous process, no vacuum component costs and maintenance, no loading/unloading time. However, in comparison with a classical plasma enhanced chemical vapor deposition (PECVD) process, the high pressure and thus the high gas density may result in a gas phase chemistry and a larger formation of dust particles. Exploring a new pressure range from 10 to 1000 mbar could be an alternative for this process. In the first part of this work the effect of the pressure on a DBD in non-reactive gases (helium, argon and nitrogen), then in neutral gas/oxygen mixture has been investigated with electrical measurements (discharge current and applied voltage), with high-speed imaging and with time-resolved optical emission spectroscopy. The second part of this work is dedicated to SiOx barrier coating characterization (FTIR-ATR, XPS, AFM and SEM) as a function of pressure in oxygen/hexamethyldisiloxane (HMDSO) gas mixture highly diluted in nitrogen. The exploration of discharge regimes as a function of pressure shows, in nitrogen, a progressive transition from Townsend to multi-peaks regime between 320 and 160 mbar. A detailed study of this regime in helium and nitrogen with high-speed imaging shows that each multi-peak corresponds to a new spatially homogeneous discharge. However, the discharge is not completely extinguished between each pulse and the remaining light emission reveals the metastable activity (excitation transfer or Penning effect). Paschen's curves obtained from electrical characterization of the discharge show an inversion (compared to standard cuves) of argon and helium cuves. This inversion shows the importance of metastable energies and capabilities to ionize almost all impurities, in the case of helium. This explains why in helium a breakdown under a lower electric field than in argon is possible. A detailed study of a glow discharge in helium as a function of pressure and impurities with time-resolved spectroscopy showed the metastables evolution within a discharge and the role of impurities in quenching or creation rate of metastables. This study also shows a 4 minutes time for thermal stabilization of the discharge (electrode heating and thermosdesorption). In helium and nitrogen, the very first microseconds of discharge are filamentary and change after 2-3 periods (∼ 200 µs) to the glow or Townsend regime respectively. Adding oxygen, an electronegative and metastable quencher gas, make the discharge change to filamentary when a proportion of more than 1500 ppm is added at atmospheric pressure. This rate is increased until 2 % in nitrogen at 350 mbar. The presence of a polymer substrate reduced this Townsend working domain due to the increase of impurities in the discharge caused by polymer etching. However, this process in pure nitrogen is very efficient for implanting nitrogen functional groups on the surface of polymer films. An incorporation of 23 % of nitrogen onto a PET surface has been reached. Regarding SiOx thin film deposition, adding HMDSO, even for ∼ 100 ppm make the discharge change to filamentary. A pressure below 40 mbar must be reached to obtain a multi-peak regime. This high pressure process is fast and deposition rate of 17 nm/s could be obtain at 500 mbar. FTIR, AFM and SEM characterisation of the coatings showed an inhomogeneous composition and structure of the layer between entrance and exit of the discharge along the gas flow. These conditions could explain that the best oxygen barrier obtained was 40.5 cm3/(m2· atm · day). Depending on the discharge parameters (frequency, residence time, power and pressure) the coatings are more or less organic. A progressive decarbonification of the layer due to progressive monomer depletion explains this behavior. A higher oxygen rate allows a better film composition homogeneity along the discharge, while a longer deposition time results in a rougher coating but has no effect on the layer composition. The particular geometry of the discharge cell (6 cm by 6 cm electrodes and 2 mm gap) with gas injection from one side, leads to a different chemistry along the gas flow. At the entrance, the coating is smooth (Ra ∼ 5 nm) and dense but organic whereas at the exit it is rough (Ra ∼ 15 nm) but has a quasi-stoechiometric composition. These differences are explained by heterogeneous reactions, comparable to PECVD process (surface chemistry) close to the gas input and a progressive transition to homogeneous reactions at the exit of the discharge (volume chemistry) which result in particle formation of nanometric size. Analysis of these particles by laser light scattering (LLS) shows a pressure threshold of 200 mbar with a constant gas mixture and flow within all the pressure range. At this pressure, the first detectable particles appear at the end of the discharge. From 200 to 1000 mbar, this threshold becomes closer and closer to the discharge entrance, but it always corresponds to a residence time of the gas in the discharge of around 30 ms. Thus this time corresponds to the characteristic formation time of detectable particles in the discharge. We also showed that this threshold varies linearly with the power injected in the discharge, the higher the power the faster the particles appear. Spatio-temporal LLS measurements show a cyclic (Τ ∼ 1-2 s) formation of particles. This behavior is linked to a rapid growth and a trapping of the particle in the discharge when they reach a 200 nm size. Then, when their size or density increases (∼ 240 nm) they are collected on the electrodes or expelled by the flow drag force which becomes preponderant in comparison with the electrostatic trapping force. Finally, they agglomerate at the exit of the discharge up to ∼ 300 nm size. Then a new cycle starts. This global approach of SiOx deposition process by DBD opens new perspectives of applications in a new pressure range and show the key parameters to be adjusted for an industrial application.
EPFL2007
