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Curved and bimorph piezoelectric micromachined ultrasonic transducers (PMUT)

  • Author(s): Akhbari, Sina
  • Advisor(s): Lin, Liwei
  • et al.

Ultrasonic transducers have been realized as nondestructive tools for a variety of applications, such as medical imaging, diagnosis, therapy, nondestructive testing, range finding, and gesture recognitions. Ultrasonic transducers fabricated by MEMS (Microelectromechanical Systems) technologies are known to have distinct advantages over conventional ultrasound devices in terms of device resolution, bandwidth, power consumption, and cost. This work focuses on innovative architectures of Piezoelectric Micromachined Ultrasonic Transducers (pMUT) for improved electro-mechano-acoustical energy efficiency and increased sensitivity using CMOS-compatible fabrication. Specifically, curved and dual-electrode bimorph (DEB) pMUT structures have been proposed and demonstrated; pMUT systems in the form of large arrays have been analyzed and simulated; as a proof-of-concept demonstration, a DEB pMUT device has been shown for potential muscle spasm diagnosis application in power-efficient hand-held medical systems.

Highly responsive curved pMUT devices have been constructed using a curved AlN layer. Prototype curved pMUTs have resonant frequencies between 1 to 4 MHz and it has been shown that curved pMUTs can in practice have 50X higher low frequency displacement per input voltage and higher electromechanical coupling comparing to flat pMUTs of the same piezoelectric material, lateral size and resonant frequency. The dynamic equations of motion of spherically-curved piezoelectric elastic shells have also been derived followed by analytical equivalent circuit modeling of curved pMUT and the results match well with the FEA simulations and experimental results.

The concept, basic theory, fabrication, and testing results of dual-electrode bimorph (DEB) pMUT for both air- and liquid-coupled applications have also been presented in this thesis. Both 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 4X of the state-of-the-art pMUT with similar geometry, material, and frequency. The prototype transducers are fabricated in a CMOS-compatible process with radii of 100-230 μm using aluminum nitride (AlN) as the piezoelectric layers with thicknesses varying from 715 nm to 950 nm, and molybdenum (Mo) as the electrodes with thicknesses 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 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/(VNd31)2) among all reported pMUT arrays. The generated acoustic intensity is in the range of 30-70 mW/cm2 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.

Equivalent circuit models of large arrays of curved (spherical-shape) and flat pMUTs have been developed for complex pMUT arrays design and analysis. The exact solutions for circuit parameters in the electromechanical domain, such as: mechanical admittance, input electrical impedance, and electromechanical transformer ratio were analytically derived. The array model includes both the self- and mutual-acoustic radiation impedances of individual transducers in the acoustic medium. Volumetric displacement, induced piezoelectric current, and pressure field can be derived with respect to the input voltage matrix, material and geometrical properties of each individual transducer and the array structure. As such, the analytical models presented here can be used as a guideline for analyses and design evaluations of large arrays of curved and flat pMUTs efficiently, and can be further generalized to evaluate other pMUT architectures in form of single-devices or arrays.

Finally, we have successfully demonstrated DEB pMUT arrays capable of detecting slight variations in mechanical properties of samples with similar characteristics to human muscles. It has been experimentally shown that 5.9% speed of sound difference between two PDMS samples can be detected by a 1D DEB pMUT array operating in pulse-echo mode at 300 kHz and driving voltages as low as 2 Vpp. As such, DEB pMUT arrays can be implemented in battery-powered handheld devices like cell-phones or used as dermal patches with external communication devices for real-time or off-line monitoring of muscle spasm due to ergonomics and repetitive strain.

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