Attenuation and velocity measurement at GHz frequencies in thin film Carbon Doped Silicon Oxide

Acoustical attenuation and velocity parameters for Carbon Doped Oxide (CDO) thin films at GHz frequencies are needed to design Coupled Resonator Filters (CRF). CDO exhibits longitudinal acoustical impedance in the range of 2 to 11 MRayls, velocity of 2400 to 4000 m/s, and acoustical attenuation of 300 to 5000 dB/cm @ 1 GHz. A novel short loop acoustical technique is presented to make these measurements. An FBAR resonator, operating at the quarter wavelength (λ/4) frequency f_CDO, excites a CDO thin film. The CDO is made λ/4 thick in order to de-couple the CDO film from the FBAR. From the measured thickness t_CDO and λ/4 frequency of the CDO, the velocity v may be calculated: v=4t_CDOf_CDO. From the measured quality factor Q and wave number k=ω/v, the attenuation constant α-k/(2Q) is calculated. By building matched λ/4 thick resonator/CDO film stacks at different frequencies, the CDO attenuation versus frequency is obtained. A linear (f¹) attenuation, not the usual quadratic (f²), dependence on frequency is found. The measurement technique will be described in detail, as well as limitations on the range of validity. A series of CDO film compositions on FBARS were made. Synthesis of the CDO films was discussed previously. A typical CDO film exhibited acoustical v=3250 m/s, impedance=4.7 MRayls, and α=2000 dB/cm @ 1 GHz, (f¹) dependence. For well matched λ/4 FBAR resonators and CDO films, the short loop results were quite accurate. When the quarter wavelength condition of either the FBAR or CDO film was violated, there is a slight error in the velocity and attenuation measured. In either case the results are compared to those of values obtained by detailed analysis of the measured scattering parameters in a Mason equivalent circuit.