Yihsiang Chiu

This paper presents an AlN-based piezoelectric micromachined ultrasonic transducer (PMUT) with a double-beam suspended structure. In order to obtain greater deflection displacement and output sound pressure compared to standard systems, a suspended structure is proposed, achieved by utilizing double-sided deep reactive ion etching of the silicon substrate. This structure is capable of greatly reducing the tensile stress caused by deflection on the membrane edge, thereby improving the piezoelectric micromachined ultrasound transducer performance. Simulations are performed using the finite element method and COMSOL software, with results demonstrating that the suspension structure exhibits a lower resonant frequency and a larger membrane deflection displacement than other methods, implying higher penetration and acoustic energy levels. Thus, the sound pressure is improved by changing the PMUT structure. Moreover, a PMUT with a double-beam is fabricated, and the impedance diagram is determined with an impedance analyzer. The resonant frequency of the transducer was 31.88 kHz, while the sound wave received by the standard lead zirconate titanate (PZT) ultrasonic transducer was converted to a voltage of 277.5 mV. In comparison, the proposed PMUT demonstrates an improved acoustic emission performance. In addition, the electromechanical coupling coefficient of the suspended PMUT is observed to be approximately 3.08%, which is significantly better than that of a fully clamped PMUT. This demonstrates the strong electromechanical conversion performance of the proposed PMUT. Results from the transmission and reception air experiments prove that the proposed double-beam PMUT can potentially be used for ranging and wireless energy transmission applications.

This paper presents a high-accuracy complementary metal oxide semiconductor (CMOS) driven ultrasonic ranging system based on air coupled aluminum nitride (AlN) based piezoelectric micromachined ultrasonic transducers (PMUTs) using time of flight (TOF). The mode shape and the time-frequency characteristics of PMUTs are simulated and analyzed. Two pieces of PMUTs with a frequency of 97 kHz and 96 kHz are applied. One is used to transmit and the other is used to receive ultrasonic waves. The Time to Digital Converter circuit (TDC), correlating the clock frequency with sound velocity, is utilized for range finding via TOF calculated from the system clock cycle. An application specific integrated circuit (ASIC) chip is designed and fabricated on a 0.18 μm CMOS process to acquire data from the PMUT. Compared to state of the art, the developed ranging system features a wide range and high accuracy, which allows to measure the range of 50 cm with an average error of 0.63 mm. AlN based PMUT is a promising candidate for an integrated portable ranging system.