Paper PS2-TuM2
Argon-Methyldisiloxane-Oxygen Fed Atmospheric Pressure DBDs for SiO₂-like Thin Film Deposition

Tuesday, November 10, 2009, 8:20 am, Room B2

Organosilicon compounds, such as for instance hexamethyldisiloxane (HMDSO), mixed with oxidants (i.e. O₂ or N₂O) and noble gases (i.e. Ar, He), are widely used both in low pressure and atmospheric pressure plasma enhanced chemical vapour deposition (PE-CVD) of SiO₂-like coatings. In particular very recently atmospheric pressure dielectric barrier discharges (DBDs) fed with organosilicon monomers have been addressed as an attractive route towards the deposition of thin films. Since the deposition mechanism is not definitively known intense research efforts should be directed to the identification of the main reaction steps and to the correlation of the plasma chemistry with the coatings properties. For this reason in this work we report our recent results on the deposition of SiO_x thin films with atmospheric pressure DBDs fed by argon (Ar) in mixture with oxygen (O₂) and different methyldisiloxanes, i.e. hexamethyldisiloxane, pentamethyldisiloxane and tetramethyldisiloxane. The characterization of the deposited films was carried out by XPS, FTIR and SEM. The quali-quantitative determination of stable by-products contained in the exhaust gas, and formed by plasma activation, was performed by gas chromatography coupled with mass spectrometry (GC-MS ). The influence of feed composition, in terms of chemical structure of the organosilicon compound and of the oxygen-to-monomer feed ratio, on the properties of the films as well as on monomer depletion and by-products concentration, was investigated.

Results show that in the absence of O₂ polymer-like coatings are deposited. Oxygen addition to the feed leads to a decrease of the carbon content of the film which is more evident when the number of methyl groups in the monomer is lower. GC-MS analyses allowed to appreciate that many linear and cyclic compounds, containing up to five silicon atoms, are formed in the plasma. As an example, in the case of HMDSO, the presence of species containing the dimethylsiloxane (-Me₂SiO-) repeating unit appears to be indicative of oligomerization processes (e.g. chain propagation, ring formation, and expansion reactions) which bring to linear and cyclic compounds with general formulas Me-(Me₂SiO)_n-SiMe₃ (n = 1-4) and (Me₂SiO)_n (n = 3 - 4), respectively. The extent of unreacted monomer does not depend significantly on the feed composition even if the O₂-to-HMDSO feed ratio is varied in a wide range (i.e. 0-25). However, O₂ addition influences the quali-quantitative distribution of by-products.

The results allow to support hypotheses on the nature of films precursors as well as to clarify some aspects of the overall deposition mechanism and of plasma-surface interaction.