GC-MS Investigation of Hexamethyldisiloxane-Oxygen Fed Cold Plasmas: Low Pressure Versus Atmospheric Pressure Operation

This study deals with the investigation by means of gas chromatography-mass spectrometry (GC-MS) of the exhaust gas of an rf low pressure (LP) glow discharge and of an atmospheric pressure (AP) FDBD fed with hexamethyldisiloxane (HMDSO), O-2, and Ar. The influence of feed composition on monomer depletion and on the qualitative-quantitative distribution of stable by-products formed by recombination of plasma intermediates is investigated. Without O-2 addition to the feed almost comparable values of HMDSO depletion are observed both at low and AP. Oxygen addition does not influence the monomer depletion at LP while it induces a slight decrease of the depletion at AP. Whatever the working pressure, O-2 controls the overall chemistry of the plasma, since it influences the concentration of by-products (e.g., silanes, silanols, linear, and cyclic methylsiloxanes). At AP evidences of the importance of methyl abstraction from HMDSO molecule have been obtained, while at LP the prevalence of Si-O bond rupture, of fragmentation, and oligomerization reactions is observed. The comparison of results from the GC-MS investigation of the exhaust gas with FT-IR spectra of the deposited coatings allows to enhance hypotheses on the formation of silanols in the gas phase and in the deposit.