Use of mass loading to operate a thin film ultrasonic transducer in the 300 kHz–10 MHz frequency range

Thin film ultrasonic transducers have been designed which operate over an important frequency range, 300 kHz to 10 MHz. The transducers were made using piezoelectric aluminium nitride films a few microns thick. The films would have a fundamental thickness mode resonance at 1–3 GHz if fabricated as an unsupported film, however operation at much lower frequencies has been demonstrated when the transducers are fabricated on bulk substrates. This would enables them to be used in ultrasonic non-destructive testing in circumstances where the film can be deposited directly onto the object under test. We have found that the major factors influencing the below-resonance operation of the thin film transducers are the device impedance, the spectrum of the excitation pulse, and any mechanical (mass) loading applied to the back face of the transducer. Results are presented showing that the evolution of device impedance as a function of device area could be predicted using a PSpice model of the thin film transducer. The ability of the transducer to generate longitudinal mode pulses rather than shear wave pulses was found to depend on increasing the mechanical loading at the back face of the transducer. This mechanism for pulse generation was confirmed by Finite Element Modelling using PZFlex.