The properties of deposited polymeric films strongly depend on the density and the state of excitation of the species in the vaporous phase which determine the degree of consecutive reaction paths covering the bandwidth between volume polymerization and surface polymerization. According to Yasuda, the former reaction mainly causes roughened fine-grain deposits, whereas smooth and shiny layers can be generated easily by the latter one [1]. For chemical vapor deposition, the main parameters are gas flow, number density (degree of rarefaction) and gaseous temperature which are extended by a row of additional parameters in plasma-activated processes, beginning with the (absorbed) plasma power and terminating with the delicate parameters obtained by diagnostic tools. These changes in the reaction mechanism were extensively studied for the biocompatible molecule parylene (p-xylylene) which is already deposited on stents for tribological purposes and is intended to cover the inner surface of artificial bladders [2]. We investigated the deposition two types of pure parylene (type N: non-substituted, and type C: substituted by one Cl atom), diluted with different amounts of argon, and a reactive alternative by adding oxygen [3]. The in-situ methods are energy-dispersive mass spectrometry and Langmuir probe analysis which have been correlated with ex-post measurements of the film quality: contact angle (surface energy) and the morphology, but most prominent the content of the aromatic species in the volume of the layer which goes down not unexpected with growing plasma power. Adding oxygen opens the window to a hydrophilic response of the surface. With the knowledge of densities of the dimeric precursor, the monomeric species, and the electron density, we modeled the chemical equilibrium of dissociation, and from the density of the aromatic compounds in the layer, we could follow the track of "safe" polymerization and could also describe the ranges for volume polymerization and surface polymerization [4].

[1] H. Yasuda: Plasma polymerization, Academic Press, New York (1985)

[2] G. Franz, F. Rauter, S. Dribinskiy, JVSTA, to be published

[3 K.G. Pruden, K. Sinclair; J. Polymer Sci. A41, 1486 (2003)

[4] G. Kokkoris, V. Constantoudis, P. Angelikopoulos, G. Boulousis, E. Gogolides; Phys. Rev. B76, 193405 (2007)