Pinned Boundary Piezoelectric Micromachined Ultrasonic Transducers (PMUT)

Ultrasound technologies can be applied to a variety of applications, such as medical imaging/therapeutics, materials assessment, flow rate measurement and others. Piezoelectric micromachined ultrasonic transducers (PMUTs) have advantages over the traditional bulk transducers in wide bandwidth, small size, and low cost. This work proposes to boost PMUT’s acoustic performance further including vibrational amplitude, acoustic pressure and electromechanical coupling by using the pinning boundary structure and to explore various new applications. The equivalent circuit model for pinned boundary PMUTs has been developed and validated with simulation results in terms of mode shape and displacement of the piezoelectric diaphragm under the alternative electrical voltage inputs. Prototype devices with pinned boundary are then fabricated without adding any fabrication complexity as compared to the conventional fabrication process of PMUTs by modifying the mask designs. Fabrication limitation of PMUTs with low resonant frequency is also investigated by fabricating prototype chips that operate at a resonant frequency of 338 kHz in air to experimentally detect objects at 22 cm away. Furthermore, testing results show a measured 2.5 times improvement in center displacement and 3.3 times improvement in the pressure output as compared with those PMUTs based on the traditional design with clamped boundary. The measured mode shape deformation results match well with the analytical and simulation results and a dualelectrode pinned PMUT structure is proposed to enable a single chip for both transmitting and receiving functionality without sacrificing the improved acoustic performance. In the area of practical applications, a tilt angle sensing chip based on the PMUT device has been demonstrated based on the amplitude of signals from the receiver PMUT. It is found that the measured sensing results match well with theoretical predictions with an average error of ±0.7 degree within the tilting range of plate between -8 to 8 degrees. A wireless power transfer system is then proposed based on a pinned ring PMUT transmitting array to provide a maximum efficiency of 57% at a focus point 2.7 mm away at a resonant frequency of 250 kHz.