Sammendrag
Piezoelectric thin film ultrasonic transducer was realised and tested for short range distance measurements in water. The transducer structure (154x154 µm2) utilising electric field excitation in thickness and transversal direction by special electrode layout was designed. Displacement and acoustic response in air and water was modelled by FEMLab and structure was adjusted to obtain resonance frequency in 5 MHz in air and ~3 MHz in water. The transducer was fabricated on SOI wafer by chemical solution deposition (CSD) where 32 sequential layers of PZT were deposited with the total thickness of 2 µm. Subsequently, a cavity underneath the piezoelectric layer was wet etched creating a bending membrane with total thickness of ~12 µm. The ultrasonic transducer was poled with 10 V/µm electric field at 150 oC temperature obtaining an effective transversal piezoelectric coefficient of -15 C/m2. After poling the displacement of the unimorph was measured as a function of frequency by fiber-optic laser vibrometer and admittance of the transducer was measured by network analyser in air and water. Performance improvement for conventional electrode design was estimated and modelling and measurement results were compared.