The acoustic properties of PECVD thin-film silicon carbide

The elastic constants of amorphous thin-film silicon carbide (SiC) deposited on gallium arsenide (GaAs) were determined using surface acoustic wave (SAW) techniques. The SiC was deposited by low-pressure plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 380°C on high resistivity (001) oriented GaAs. Film thicknesses of 197 nm and 422 nm were selected for the SAW measurements. The average measured density of several films deposited under identical conditions was 2140 kg/m³. Linear arrays of thin-film aluminum interdigital electrodes with harmonic generation capabilities were patterned on the wafers for SAW propagation along the [110] axis of the GaAs. SAW frequency measurements were made in the range from 30 MHz to above 1.0 GHz from which phase velocity values were calculated for film-thickness to acoustic-wavelength ratios of 0.002 up to 0.15. The two independent elastic constants were obtained by a least-squares fit of the theoretical velocity dispersion curve to the measured velocities. The average constants for the films were c₁₁=184 GPa and c ₄₄=56 GPa. The SAW propagation-loss frequency characteristic was measured and at 1.0 GHz the loss was near 20 dB/cm, which is approximately 7 dB/cm greater than the measured loss for GaAs