Polymeric substrates can be coated at low temperatures by means of an electric discharge fed by organic gases. Since both the composition and the structure of the deposited material can be varied in a wide range by the choice of starting compound and deposition conditions, the properties of the coatings can be adjusted specifically to fulfill many required demands. After a general introduction of the deposition method, a typical deposition apparatus is described and an optimization procedure of the deposition process is proposed. The intermediate position of plasma polymerized materials between common polymers and amorphous solids is demonstrated for films from hexamethyldisilazane. Based on the assumption of a continuous random network structure the density of covalent bonds and the total volume fraction of micro-voids can be evaluated and compared with application relevant properties such as mechanical stiffness and gas permeability. The model of a continuous random network structure may be valid for plasma polymerized material in general.
Thermal induced gas evolution studies were performed on a-C:H:Ta films deposited by a sputtering process using a tantalum target and an argon-hydrocarbon gas mixture. Decreasing hydrocarbon concentration of the plasma atmosphere during the deposition results in an increasing tantal carbid (TaC) volume fraction, perceptible by x-ray diffraction measurements, while the a-C:H volume fraction decreases. Gas evolution spectra, similar to those of a-C:H and a-Si:C:N:H films, suggesting the presence of an a-C:H matrix with a void network structure. TaC forms precipitations in this matrix. The degree of crosslinking of the carbon atom network of the a-C:H matrix, which increases with an increasing volume fraction of TaC in the films, determines the thermal gas evolution concerning temperature and amount of hydrocarbons and argon. As shown also by gas effusion measurements, a more crosslinked matrix can also be obtained by reduction of the hydrogen content of the hydrocarbon gas, for instance, by substitution of methane by ethylene. Since the argon effusion temperature does not depend on film thickness, the argon effusion temperature may serve as a material property, which renders the comparison of different a-C:H materials possible, also in two phase systems like a-C:H:metal. The released hydrocarbon molecules were formed during the heating by alkyl group cleavage. Hydrocarbon gas release temperature is found to depend on the volume fraction of TaC, since bonding states of carbon atoms are influenced by the presence of tantalum, as indicated by x-ray-photoelectron-spectroscopy.
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