Stress field effects on the microstructure and properties of a-C:H thin films

The physical properties of plasma-deposited hydrogenated amorphous-carbon films (a-C:H) are investigated and correlations to the competing intrinsic and extrinsic stress fields are established. The (a-C:H) films are grown on single-crystal <100> silicon substrates in a plasma reactor using acetylene as a precursor. Although constant C2H2 plasma conditions and constant substrate bias and temperature were used during the growth the films display a multilayer structure. The density and the sp(3)-hybridized carbon fraction are shown to vary during deposition. The microstructure and optoelectronic properties of the (a-C:H) thin film evolve during growth as a result of interaction between the intrinsic tensile stress generated within the growing film and the external stress field existing in the Si substrate. During the initial phase of the growth this external stress field enhances the intrinsic stress. The resulting film is of high density and contains a relatively high fraction of sp(3)-hybridized carbon atoms. Eventually, when the (a-C:H) layer becomes thick enough, the intrinsic stress compensates the external stress field. Layers grown under balanced stress conditions show an unusual alignment of the graphitic planes. Finally, when the intrinsic stress becomes dominant, the density of the film and the corresponding sp(3) fraction decrease, leading to a detectable porosity. The behavior of the optical band gap is shown to reflect the evolution of the (a-C:H) microstructure as it is unambiguously correlated to the evolution of the stress field. (C) 2002 American Institute of Physics.