GC-MS Investigation of Hexamethyldisiloxane-Oxygen Fed Cold Plasmas: Low Pressure vs. 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), O2, 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 O2 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, O2 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 GCMS 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.