Hard graphitelike hydrogenated amorphous carbon grown at high rates by a remote plasma

Hydrogenated amorphous carbon (a-C:H) deposited from an Ar-C2H2 expanding thermal plasma chemical vapor deposition (ETP-CVD) is reported. The downstream plasma region of an ETP is characterized by a low electron temperature (similar to 0.3 eV), which leads to an ion driven chemistry and negligible physical effects, such as ion bombardment (ion energy < 2 eV) on the depositing surface. The material properties in ETP-CVD can be controlled by varying the plasma chemistry. In this article we investigate the change in a-C:H material properties by varying the Ar/C2H2 gas flow ratio over a wide range (1.33-150), with emphasis on low gas flow ratios (1.33-5). By changing the Ar/C2H2 gas flow ratio, the gas residence time in the ETP expansion can be tuned, which in turn defines the chemistry of the ETP-CVD. Soft polymerlike a-C:H to moderately hard a-C:H films have been deposited by lowering the Ar/C2H2 gas flow ratio. Recently, under very low Ar/C2H2 gas flow ratios, a hard graphitelike a-C:H material has been deposited. The striking feature of this material is the infrared absorption spectrum in the C - H-x stretching region (2800-3100 cm(-1)), which is a distinct narrow bimodal spectrum evolving from a broad spectrum for the moderately hard a-C:H. This transition was attributed to the absence of end groups (sp(2) CH2 and sp(3) CH3), which favors an enhanced cross-linking in the film in a similar effect to elevated ion bombardment or annealing. Moreover, the hard graphitelike film has an increased refractive index (n) as high as 2.5 at 633 nm with a corresponding mass density of similar to 2.0 g/cm(3).