Hard graphitelike hydrogenated amorphous carbon grown at high ratesby a remote plasma

Hydrogenated amorphous carbon (a-C:H) deposited from an Ar - C 2 H 2 expanding thermal plasma chemical vapor deposition (ETP-CVD) is reported. The downstream plasma region of an ETP is characterized by a low electron temperature ( ∼ 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 / C 2 H 2 gas flow ratio over a wide range (1.33-150), with emphasis on low gas flow ratios (1.33-5). By changing the Ar / C 2 H 2 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 / C 2 H 2 gas flow ratio. Recently, under very low Ar / C 2 H 2 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 ( s p 2 CH 2 and s p 3 CH 3 ), 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 ∼ 2.0 g / cm 3 .