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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.

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