P1I-1 Acoustical Parameters Characterisation of Aluminium Nitride Thin Film BAW Resonators Using Resonant Spectrum Approach

The determination of piezoelectric properties of an aluminium nitride thin film sandwiched between two electrodes, and acoustically coupled to a supporting substrate, is considered. Measuring the properties of such a BAW composite resonator can be used for film characterisation. The resonant spectrum method has been implemented, which consists of analysing the variation in spacing of parallel resonant frequencies for the resonator. The film parameters were derived from experimental measurements of the insertion loss (s₁₁) of aluminium nitride deposited on glass and LiNO₃ respectively, with top and bottom electrodes. A one-dimensional model was used to simulate the functional behaviour of the two examples of four-layer aluminium nitride based resonators. The modelling technique for multilayer ultrasonic transducers was based on the solutions to the 1-D wave equation. It was possible to obtain good agreement between the experimental and modelled results. The values of velocity and density agreed within 0.5%. The spacing of parallel resonant frequencies could also be closely matched between the experiment and the simulation. Under these conditions, the value of k_t ² of the piezoelectric film was inferred from the model.