Sina Akhbari

Highly responsive curved aluminum nitride pMUT

We have successfully demonstrated highly responsive, curved piezoelectric micromachined ultrasonic transducers (pMUTs) based on a CMOS-compatible fabrication process using AlN (aluminum nitride) as the transduction material. Micro fabrication techniques have been used to control the radius of curvature of working diaphragms from 400~2000 μm and theoretical analysis have been developed for the optimal dimensions of the transducers to boost the electromechanical coupling and acoustic pressure. A prototype device made of a 2μm-thick AlN on a curved diaphragm with a nominal size of 140μm in diameter and a radius of curvature of 1065μm has been fabricated. The measured resonant frequency is 2.19MHz and DC response is 1.1nm/V, which is 50X higher than that of a planar device with the same nominal diameter. As such, this new class of curved pMUTs could dramatically enhance the responses of the state-of-art, planar pMUTs with high electromechanical coupling for various ultrasonic transduction applications, such as gesture recognition and medical imaging.

Bimorph pMUT with dual electrodes

The concept of “bimorph” piezoelectric micromachined ultrasonic transducers (pMUTs) has been demonstrated by utilizing a two active AlN layers structure constructed in a CMOS-compatible process. The prototype device has two 0.95μm-thick AlN layers sandwiched by three 0.15μm-thick Mo electrodes. In a prototype, both an inner circular and an outer annular electrode are designed on a 230 μm in radius, circular-shape diaphragm. When actuated with the inner electrode of 160μm in radius, the pMUT has a resonant frequency of 198.8 kHz and central displacement of 407.4 nm/V. Under the differential drive scheme using the dual-electrodes for large acoustic outputs at a low frequency, the measured central displacement is 13.0 nm/V, which is about 400% higher than that of a unimorph AlN-pMUT under similar actuation conditions. As such, the dual-electrode bimorph pMUT presents the improved operation as compared with the state-of-the-art flat pMUT design to achieve enhanced acoustic outputs.

An analytical solution for curved piezoelectric micromachined ultrasonic transducers with spherically shaped diaphragms

An analytical solution for piezoelectrically actuated spherically shaped diaphragms has been developed to study their dynamic behavior with targeted applications in piezoelectric micromachined ultrasonic transducers (pMUT). The analytical model starts with a curved pMUT composed of a piezoelectric diaphragm with a nominal radius in size, a radius of curvature in shape, and under both possible actuation sources of radial pressure and electric potential. The diaphragm has the piezoelectric material polarized in the direction perpendicular to its surface and sandwiched between two metal electrodes. When an electric field is applied between the two electrodes, the in-plane piezoelectric strain can cause larger out-of-plane deflections than a flat unimorph piezoelectric diaphragm because of the diaphragms spherical curvature with a clamped periphery for high electromechanical coupling factor. Key performance parameters, including mechanical mode shapes, resonant frequencies, dynamic responses, and displacements, with respect to the curvature and size of the diaphragm have been investigated. Both analytical derivations and numerical simulations using finite element analysis have been performed for the optimal design of the electromechanical coupling factor, with varying factors such as mechanical resonant frequency, radius of curvature, nominal radius, and thickness. As such, this work provides theoretical foundations for the design of curved pMUTs with high electromechanical coupling factor compared with planar-shape pMUTs.

Self-curved diaphragms by stress engineering for highly responsive pMUT

A process to make self-curved diaphragms by engineering residual stress in thin films has been developed to construct highly responsive piezoelectric micromachined ultrasonic transducers (pMUT). This process enables high device fill-factor for better than 95% area utilization with controlled formation of curved membranes. The placement of a 0.65 μm-thick, low stress silicon nitride (SiN) film with 650 MPa of tensile residual stress and a low temperature oxide (LTO) film with 180 MPa of compressive stress sitting on top of a 4 μm-thick silicon film has resulted in the desirable self-curved diaphragms. A curved pMUT with 200 μm in nominal radius, 2 μm-thick aluminum nitride (AlN) piezoelectric layer, and 50% SiN coverage has resulted in a 2.7 μm deflection at the center and resonance at 647 kHz. Low frequency and resonant deformation responses of 0.58 nm/V and 40nm/V at the center of the diaphragm have been measured, respectively. This process enables foundry-compatible CMOS process and potentially large fill-factor for pMUT applications.

Enhanced coupling of piezoelectric micromachined ultrasonic transducers with initial static deflection

The enhancement of the vibration amplitude of a clamped circular piezoelectric micromachined ultrasonic transducer (pMUT) with static deflection has been theoretically and numerically investigated for the first time. Using Fourier series expansion of the classic plate theory with in-plane time-dependent piezoelectric stress, the first harmonic motion equation of a pMUT plate with initial displacement was derived. An analytical solution based on a novel Greens function approach was detailed, where additional plate deflection due to the synergy between the static deflection and the applied AC voltage was theoretically predicted and confirmed using Finite Element Modeling (FEM). This unprecedented demonstration opens up new opportunities for understanding pre-deflected pMUTs and implementing optimized designs.

Equivalent Circuit Models for Large Arrays of Curved and Flat Piezoelectric Micromachined Ultrasonic Transducers

Equivalent circuit models of large arrays of curved (spherical shape) and flat piezoelectric micromachined ultrasonic transducers (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. By utilizing the array solution methods previously established for the thickness-mode piezoelectric devices and capacitive micromachined ultrasonic transducers (cMUTs), the single pMUT circuit model can be extended to models for array structures. 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 the form of single devices or arrays.

An equivalent circuit model for curved piezoelectric micromachined ultrasonic transducers with spherical-shape diaphragms

An equivalent circuit model is analytically derived for a clamped piezoelectric elastic shell with a spherical curved shape. The lumped parameters of the electromechanical elements are extracted from the system equations, and the analytical circuit model is compared with acoustic-piezoelectric FEM simulation results. Very good consistencies are achieved in the frequency responses, electromechanical coupled variations, and the device input impedances.

Dual-electrode bimorph pmut arrays for handheld therapeutic medical devices

A CMOS-compatible, aluminum nitride (AlN) based, dual-electrode bimorph piezoelectric micromachined ultrasonic transducer (pMUT) array has been developed. Under an input 5Vac driving voltage, a high acoustic intensity of 30-70mW/cm2 up to 2.5mm deep into tissues can be generated for therapeutic medical applications. The low driving voltage is readily applicable for battery-powered low intensity pulsed ultrasound (LIPUS) medical devices for fracture and tissue healing applications. The fabrication also introduces a novel method using PECVD SiO2 as the barrier layer between AlN films to increase the breakdown voltage and manufacturing yield. Results on a prototype array have shown the highest intensity per voltage squared, per number of pMUTs squared, per piezoelectric constant squared (i.e. In=I/(VNd31)2) among all reported pMUT arrays.

A 2.34μJ/scan acoustic power scalable charge-redistribution pMUT interface system with on-chip aberration compensation for portable ultrasonic applications

A 1- to 8-ch scalable pMUT interface system with on-chip aberration compensation is presented for portable ultrasonic imaging applications. The system overcomes the necessity of area and power reduction for large ultrasonic arrays and presents the first hardware implementation on-chip calibration to improve the signal quality. The Charge Redistribution Transmitter (CR-TX) saves power by 32.8% while driving pMUT array at a wide range from 100 kHz to 5MHz. The CR-TX drives up to 500pF/channel load, which is 33× of the state-of-the-art ultrasonic driver reported to date. The on-chip adaptive beamformer supports five different media scans, has channel scalability and features the first demonstration of on-chip aberration compensation. The 8-ch system fabricated in 65nm CMOS occupies the core area of 700×1490μm, TX drives up to 6V while consuming 2.34μJ/scan.

Correction “Bimorph Piezoelectric Micromachined Ultrasonic Transducers” [Apr 16 326-336]

This paper points out the small typographical author errors found in the above paper [1] to eliminate any confusion for the readers.