This article proposes a novel structured aluminum nitrate (AlN) thin film based piezoelectric micromachined ultrasonic transducer (PMUT) with superior ultrasonic performances that can directly be applied for pulse-echo imaging. The bottom electrode of the PMUT is thickened and serves both as the electrical layer and supporting layer. Large numbers of elements are parallelly connected to form square arrays. This method can not only improve the acoustic output but also reduce the total impedance. The ultrasonic transmitting, receiving behaviors of our developed 6.37 MHz (in air) PMUT array containing vibration elements of horizontal × vertical = 20 × 20 are investigated. It can generate superior acoustic output with a normalized value of 1.71 kPa·V-17mm-2 at 3.6 mm away in deionized water media. Thanks to the lager acoustic output and reduced electrical impedance, ultrasonic pulse-echo imaging with our developed PMUT array is realized only with the assistance of a switch to separate the ultrasound transmission and reception process. We have mechanically scanned a mark in steel plate and generate a B-mode image with great accuracy using the recorded pulse-echo signals. These demonstrate the superior acoustic behavior of our developed PMUT array and open an avenue to the development of portable ultrasonic imaging devices.

As $a$ common and natural activity of human beings, eye movement has been regarded as an indicator of human physiological and psychological health; eye movement tracking has been applied to $a$ wide range of domains, such as disease diagnostics, drug therapy testing and human-computer interaction. Conventionally, eye movement is captured by three approaches, i.e., recording video using $a$ camera, measuring time-varying Electrooculography (EOG) signals using electrodes, and detecting reflected infrared light using infrared sensor. However, all these techniques incur certain limitations that impede their adoption for practical eye tracking, such as privacy violation, high computation resources, discomfort and strong light reliability. This paper presents $a$ portable and robust eye tracking technique based on ultrasound. Specifically, piezoelectric MEMS ultrasonic transducer arrays integrated on glasses are used to transmit and receive bio-safe pulse echo ultrasound individually. With experiment on volunteers, the developed devices can accurately track the rotation and direction of eyeball movement in real-time, based on the time of flight (TOF) principle. The tracking device is lightweight (< 25 mg) and compact (millimeters in size), showing $a$ great potential for wearable eye tracking application like virtual reality. [2021-0193]

Acoustic induced nanoparticle patterning and location migration in inhomogeneous media formed in situ .

This article presents the development of small sized piezoelectric micromachined ultrasound transducer (PMUT) with cost effective fabrication process. Improved ultrasound imaging quality requires small sized, high frequency and high performance PMUT cell and further high density arrays. Fabrication with currently well developed and cost effective sacrificial process benefits their mass production and application. To determine small sized AlN PMUT cell, commonly reported elastic layer materials including Si, Si $_{3}$ N $_{4}$ , SiO $_{2}$ are compared by mathematic derivation under the same stack thickness and device frequency. SiO $_{2}$ has the smallest Young modulus and it serving as elastic layer can yield the expected one. Arranging SiO $_{2}$ layer at the top of the device is compatible with phosphor silicate glass (PSG) sacrificial manufacturing process for cost effective fabrication. This fabrication strategy further downsizes cell size since extra device protection layer is omissible. We make 12MHz PMUT linear arrays with cell diameter, thickness and element pitch to be 40 $\mu$ m, 1.9 $\mu$ m and 120 $\mu$ m. Their electrical and ultrasound results are quite uniform at both device and wafer levels. One PMUT line with 20 parallelly connected units can produce pulse-echo signal with over 25dB SNR at 10mm distance. Lateral and axial ultrasound imaging resolutions of $\sim$ 1 mm and $\sim$ 0.3 mm are obtained in practical verification. These indicate the good ultrasound performance of our developed PMUT array. Plus the advantages of small size and cost effective fabrication, using SiO $_{2}$ elastic layer and arranging it at the top of the device is a competitive strategy to develop phased array PMUTs. 2022-0184