Bimorph Piezoelectric Micromachined Ultrasonic Transducers

This paper presents the concept, basic theory, fabrication, and testing results of dual-electrode bimorph piezoelectric micromachined ultrasonic transducers (pMUTs) for both airand liquid-coupled applications. Both the theoretical analyses and experimental verifications under the proposed differential drive scheme display high drive sensitivity and an electromechanical coupling energy efficiency that is as high as 4× of the state-of-the-art pMUT with a similar geometry and frequency. The prototype transducers are fabricated in a CMOS-compatible process with the radii of 100-230 μm using aluminum nitride as the piezoelectric layers with the thicknesses varying from 715 to 950 nm and molybdenum (Mo) as the electrodes with a thickness of 130 nm. The tested operation frequencies of the prototype transducers are 200-970 kHz in air for possible ranging and motion detection applications, and from 250 kHz to 1 MHz in water for medical ultrasound applications such as fracture healing, tumor ablation, and transcranial sonothrombolysis. A 12 × 12 array structure is measured to have the highest intensity per voltage squared, per number of pMUTs squared, and per piezoelectric constant squared (In = I/(V Nd
₃₁
)
²
) among all reported pMUT arrays. The generated acoustic intensity is in the range of 30-70 mW/cm
²
up to 2.5 mm from the transducer surface in mineral oil with a driving voltage of 5 Vac, which is suitable for battery-powered therapeutic ultrasound devices.