Mechanisms of ambient-pressure plasma-enhanced chemical vapor deposition – studies of dielectric barrier discharges with short gasresidence times (“FiloSurf”)

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The current state of knowledge about physico-chemical mechanisms of plasma-enhanced chemical vapor deposition (PECVD) at ambient pressure is comparatively poor, in spite of its considerable potential for a wide range of applications such as solar-cell production, corrosion protection, coating in glass and optics industries, or for powder coating of pharmaceutical products. In general, it is even an open question, whether radicals or charge carriers are the main contributors to film formation. Experimental evidence of an ionic deposition pathway has recently been reported for hexamethyldisiloxane (HMDSO) by one of the proposers. The corresponding studies took advantage of the short residence time of an argon-HMDSO mixture in a single-filament dielectric barrier discharge (SF DBD) through which the gas flowed with high velocity. The finding of a cationic film-formation pathway is strongly supported by recent time- and space-dependent modeling studies by a second proposer obtained for plane-parallel DBDs in argon with tetramethylsilane. The central idea of the present proposal is to make use of special characteristics of DBDs with short gas-residence times, particularly SF DBDs as well as plasma-sheet DBDs, in order to get new insights into mechanisms of ambient-pressure PECVD using DBDs in mixtures of argon with addition of small amounts of different molecular gases. A third proposer will emphasize aspects of discharge physics: regimes, development and key species densities. The admixtures to be studied are hexamethyldisilane (HMDS) as well as hydrocarbons, such as methane, ethane, ethene, and ethyne. The three applicants have long-term expertise in plasma-based surface engineering, experimental plasma physics, and plasma modeling, respectively. They will join their efforts to study the role of excited Ar atoms as energy carriers for ionization, dissociation, and excitation processes experimentally, in particular laser absorption and optical emission spectroscopy, as well as by numerical modeling. The aim is to carry out the role of all four levels of the first excited manifold Ar(1s5) - Ar(1s2). The role of ionic deposition in DBD-based PECVD as well as of the composition, structure and properties of the films grown are in the focus of the film growth experiments and the modeling studies. For these investigations, sinusoidal and nanopulsed voltage operation will be compared. The small precursor turnover in the DBDs will also be used to validate primary reactions in complex plasma-chemical models by means of optical spectroscopy and gas chromatic/mass spectrometry analysis of stable products. Results of the project will substantially promote the state of knowledge regarding DBD-based PECVD. On the one hand, they will help to validate and advance existing plasma-chemical models of DBDs in such mixtures. On the other hand, they will be of considerable practical relevance for future applications.