Electrical and optical characterisation of aluminium nitride piezoelectric films on silicon nitride membranes

Aluminium nitride (AlN) is a thin film piezoelectric material having excellent potential for integration with microelectronic systems. We have investigated flexural modes of Si3N4 membrane structures with and without an AlN active layer. AlN films typically 3 μm thick were deposited by RF sputtering. Mechanical excitation was provided acoustically by sweeping the excitation frequency of a 1 MHz air-coupled ultrasonic transducer. Mode shapes were verified by scanning laser vibrometry up to the [3,3] mode, in the frequency range 100 kHz to 1 MHz. Resonant frequencies were identified at the predicted values provided the tension in the layers could be estimated. For a membrane structure incorporating an AlN layer, acoustic and electrical excitation of flexural modes was confirmed by displacement measurements using laser vibrometry and resonant frequencies were compared with analytical calculations.