Haoran Wang

Microelectromechanical systems (MEMS) refer to miniaturized mechanical and electro-mechanical elements that are fabricated through microelectronic processes

Micromachined thin-film lead zirconate titanate (PZT) retains exceptional piezoelectric and energy-harvesting capabilities, thus offering an excellent platform for enabling efficient piezoelectric microsystems on chip. Here, we design and fabricate thin-film ceramic PZT piezoelectric microelectromechanical systems (MEMS) operating on flexural-mode resonances, by using wafer bonding and chemical mechanical polishing (CMP) techniques. We describe the experimental demonstration of an integrated multimode resonant PZT MEMS temperature sensor. The three resonance modes examined (at 1.1, 3.6, 6.8MHz) all exhibit excellent linearity and responsivity to temperature variations in the range of 25°C to 211°C, with temperature coefficients of resonance frequency (TC f ) at -200ppm/°C to -270ppm/°C for open-loop measurement. We have demonstrated dual-mode temperature sensing with real-time tracking of the resonance frequency using a phase-locked loop (PLL). We have also built a self-sustaining MEMS oscillator and studied the closed-loop TC f . We compare the open-loop and closed-loop TC f s in both air and vacuum and find that the TC f can be affected by heating method, pressure, and measurement scheme. The results extend the understanding of temperature effects on the resonance frequency of PZT MEMS, demonstrate real-time temperature sensing, and pave the way for enabling multifunctional PZT microsystems on chip.

Miniaturized ultrasonic transducer arrays with multiple frequencies are key components in endoscopic photoacoustic imaging (PAI) systems to achieve high spatial resolution and large imaging depth for biomedical applications. In this article, we report on the development of ceramic thin-film PZT-based dual- and multi-frequency piezoelectric micromachined ultrasonic transducer (pMUT) arrays and the demonstration of their PAI applications. With chips sized 3.5 mm in length or 10 mm in diameter, square and ring-shaped pMUT arrays incorporating as many as 2520 pMUT elements and multiple frequencies ranging from 1 MHz to 8 MHz were developed for endoscopic PAI applications. Thin ceramic PZT with a thickness of 9 μm was obtained by wafer bonding and chemical mechanical polishing (CMP) techniques and employed as the piezoelectric layer of the pMUT arrays, whose piezoelectric constant d₃₁ was measured to be as high as 140 pm/V. Benefiting from this high piezoelectric constant, the fabricated pMUT arrays exhibited high electromechanical coupling coefficients and large vibration displacements. In addition to electrical, mechanical, and acoustic characterization, PAI experiments with pencil leads embedded into an agar phantom were conducted with the fabricated dual- and multi-frequency pMUT arrays. Photoacoustic signals were successfully detected by pMUT elements with different frequencies and used to reconstruct single and fused photoacoustic images, which clearly demonstrated the advantages of using dual- and multi-frequency pMUT arrays to provide comprehensive photoacoustic images with high spatial resolution and large signal-to-noise ratio simultaneously.

Facilitated by microelectromechanical systems (MEMS) technology, MEMS speakers or microspeakers have been rapidly developed during the past decade to meet the requirements of the flourishing audio market. With advantages of a small footprint, low cost, and easy assembly, MEMS speakers are drawing extensive attention for potential applications in hearing instruments, portable electronics, and the Internet of Things (IoT). MEMS speakers based on different transduction mechanisms, including piezoelectric, electrodynamic, electrostatic, and thermoacoustic actuation, have been developed and significant progresses have been made in commercialization in the last few years. In this article, the principle and modeling of each MEMS speaker type is briefly introduced first. Then, the development of MEMS speakers is reviewed with key specifications of state-of-the-art MEMS speakers summarized. The advantages and challenges of all four types of MEMS speakers are compared and discussed. New approaches to improve sound pressure levels (SPLs) of MEMS speakers are also proposed. Finally, the remaining challenges and outlook of MEMS speakers are given.

This paper presents a dual-frequency piezoelectric micromachined ultrasonic transducer (pMUT) array based on thin ceramic PZT for endoscopic photoacoustic imaging (PAI) applications. With a chip size of $7\times {7}$ mm ² , the pMUT array consists of 256 elements, half of which have a lower resonant frequency of 1.2 MHz and the other half have a higher resonant frequency of 3.4 MHz. Ceramic PZT, with outstanding piezoelectric coefficients, has been successfully thinned down to a thickness of only $4~\mu \text{m}$ by using wafer bonding and chemical mechanical polishing (CMP) techniques and employed as the piezoelectric layer of the pMUT elements. The diaphragm diameters of the lower-frequency and higher-frequency elements are $220~\mu \text{m}$ and $120~\mu \text{m}$ , respectively. The design methodology, multiphysics modeling, fabrication process, and characterization of the pMUTs are presented in detail. The fabricated pMUT array has been fully characterized via electrical, mechanical, and acoustic measurements. The measured maximum responsivities of the lower- and higher- frequency elements reach 110 nm/V and 30 nm/V at their respective resonances. The measured cross-couplings of the lower-frequency elements and higher-frequency elements are about 9% and 5%, respectively. Furthermore, PAI experiments with pencil leads embedded into an agar phantom have been conducted, which clearly shows the advantages of using dual-frequency pMUT arrays to provide comprehensive photoacoustic images with high spatial resolution and large signal-to-noise ratio simultaneously. [2021-0114]

This paper presents, for the first time, a multifrequency piezoelectric micromachined ultrasonic transducer (pMUT) array for endoscopic photoacoustic imaging applications. With a chip size of 3.5 × 3.5 mm ² , the pMUT array consists of 285 pMUT elements with seven different resonant frequencies ranging from 1 MHz to 8 MHz. The pMUT array has been fabricated on a 9 µm-thick layer of ceramic PZT with high piezoelectric constants. Mechanical vibration tests have shown high voltage-to-displacement gains, e.g., 130 nm/V, 35 nm/V, and 8 nm/V for the pMUT elements at their resonances of 1.2 MHz, 3.7 MHz, and 7.8 MHz, respectively. Ultrasound signals with frequencies from 300 kHz to 8 MHz have been successfully generated and detected by a pair of fabricated pMUT arrays. Furthermore, photoacoustic imaging experiments have been conducted, demonstrating the advantages of using multi-frequency pMUT arrays to provide comprehensive photoacoustic images with high signal-to-noise ratio and high spatial resolution.

Photoacoustic imaging can be used for in vivo early cancer diagnosis due to its cross-sectional imaging capability and high contrast, but there always exists a tradeoff between resolution and imaging depth. For that piezoelectric micromachined ultrasonic transducer (pMUT) arrays with both higher-frequency and lower-frequency pMUT elements are needed for achieving high resolution and large imaging depth simultaneously. In this paper, we present the design, fabrication, and characterization of a high-density and dual-frequency pMUT array based on thin ceramic PZT, which consists of 128 pMUTs operating at 1.2 MHz and 128 pMUTs operating at 3.4 MHz. By using wafer bonding and chemical mechanical polishing techniques, ceramic PZT with high piezoelectric coefficients was thinned down to only 4 μm and employed as the piezoelectric layer of the designed pMUT array. The dual-frequency performance of the pMUT array was characterized with electrical impedance, vibration modes, cross-coupling, and acoustic sensing measurements. With a chip size of 7 ×7 mm ² and integration of 256 pMUTs with two operating frequencies into one array, the developed pMUT array shows a great potential for endoscopic photoacoustic imaging to achieve high-resolution and large imaging depth simultaneously.

One of the challenges for piezoelectric MEMS speakers is their relatively low sound pressure level (SPL) due to the relatively small piezoelectric constants of sol-gel or sputtered lead zirconate titanate (PZT). In this paper, we present a piezoelectric MEMS speaker with improved SPL that is enabled by high-piezoelectric-coefficient ceramic PZT as well as a dual-electrode configuration with phase tuning. A dual-electrode piezoelectric MEMS speaker based on 4 μm thin ceramic PZT has been designed and fabricated with top electrodes consisting of an inner circular-shaped electrode and an outer ring-shaped electrode. The maximum SPL at 9.7 kHz reaches 121 dB with both electrodes activated simultaneously. Measurements have also revealed that 180° is not always the optimal phase delay between the two driving signals to yield the highest SPL. Instead, the output SPL of the MEMS speaker changes significantly with the phase delay and is frequency dependent, peaking at different phase delays for different frequencies. The optimal phase delays in the frequency band from 600 Hz to 10 kHz have been obtained by phase tuning experiments, which shows 2 to 10 dB SPL improvement compared with the single-electrode driving method.

Photoacoustic imaging (PAI) is drawing extensive attention and gaining rapid development as an emerging biomedical imaging technology because of its high spatial resolution, large imaging depth, and rich optical contrast. PAI has great potential applications in endoscopy, but the progress of endoscopic PAI was hindered by the challenges of manufacturing and assembling miniature imaging components. Over the last decade, microelectromechanical systems (MEMS) technology has greatly facilitated the development of photoacoustic endoscopes and extended the realm of applicability of the PAI. As the key component of photoacoustic endoscopes, micromachined ultrasound transducers (MUTs), including piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs), have been developed and explored for endoscopic PAI applications. In this article, the recent progress of pMUTs (thickness extension mode and flexural vibration mode) and cMUTs are reviewed and discussed with their applications in endoscopic PAI. Current PAI endoscopes based on pMUTs and cMUTs are also introduced and compared. Finally, the remaining challenges and future directions of MEMS ultrasound transducers for endoscopic PAI applications are given.

In this paper, we present the design, fabrication, and characterization of a compact 4 × 4 piezoelectric micromachined ultrasonic transducer (pMUT) array and its application to photoacoustic imaging. The uniqueness of this pMUT array is the integration of a 4 μm-thick ceramic PZT, having significantly higher piezoelectric coefficient and lower stress than sol-gel or sputtered PZT. The fabricated pMUT array has a small chip size of only 1.8 × 1.6 mm ² with each pMUT element having a diameter of 210 μm. The fabricated device was characterized with electrical impedance measurement and acoustic sensing test. Photoacoustic imaging has also been successfully demonstrated on an agar phantom with a pencil lead embedded using the fabricated pMUT array.