Sound velocity and Young's modulus in plasma deposited amorphous hydrogenated carbon

The first carbon-film sound velocities obtained with the piezoelectric meth od are given. The a-C:H films were deposited from methane using rf plasma c hemical vapor deposition at different substrate biases and thus contain var ying hydrogen concentrations. Measurements of density allowed the Young's m odulii of the films to be calculated. Both the sound velocity and the Young 's modulus reached a maximum as the substrate bias changed from -47 to - 17 5 V. The film with the maximal properties occurred at a bias of -76 V and h ad a sound velocity of 16.4 km/s, a Young's modulus of 589 GPa, an optical gap of 2.16 eV and a density of 2.19 g/cm(3). Although the modulus is 52% t hat of the directionally averaged value of diamond, it produced a sound vel ocity 91% that of diamond due to the lower film density. The film densities were in the range of 1.81-2.43 g/cm(3) with the densest films occurring at the highest bias magnitudes. Optical gap measurements were taken with a ph oto spectrometer and yielded a gap that decreased from 3.86 to 1.40 eV with increasing bias magnitude and thus decreasing hydrogen content. These valu es indicated sp.; concentrations that increased with the gap from 43% to 94 % and these were consistent with Raman spectroscopy results. (C) 2001 Ameri can Vacuum Society.