The characterization of PECVD thin-film TEOS glass on gallium arsenide using SAW techniques

The elastic constants of amorphous thin-film tetraethylorthosilicate (TEOS) glass deposited on gallium arsenide (GaAs) were determined using surface-acoustic-wave (SAW) techniques. The TEOS glass was deposited by low pressure plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 350°C on high resistivity (001) oriented GaAs. Film thicknesses of 200, 500 and 1000 nm were selected for the SAW measurements. The density of the as deposited films was approximately 2200 kg/m³. Linear arrays of thin-film aluminum interdigital electrodes with harmonic generation capabilities were patterned on the GaAs wafers for SAW propagation along the [110] axis of the GaAs. SAW resonant-frequency measurements were made in the range from 30 MHz to above 1.0 GHz from which phase-velocity values were calculated corresponding to film-thickness to acoustic-wavelength ratios from 0.002 up to 0.27. The two independent elastic constants were obtained for each film thickness by fitting a theoretical velocity-dispersion curve to the measured velocities. The elastic constants for the amorphous films were in the range c₁₁ =85 to 77 GPa and c₄₄=29.5 to 27.0 GPa decreasing as the film thickness increased. These elastic-constant values are near those reported for bulk fused quartz. The SAW propagation-loss frequency characteristics were measured up to 1.0 GHz and the loss of the composite film and substrate averaged approximately twice that of GaAs alone