Thermal gas evolution studies on a-C-H-Ta films

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.