A High Fill-Factor Annular Array of High Frequency Piezoelectric Micromachined Ultrasonic Transducers

Abstract

This paper presents a 1.2-mm diameter high fill-factor array of 1261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz in fluid for intravascular ultrasound imaging. At 1061 transducers/mm 2 , the PMUT array has a 10-20 times higher density than previous PMUT arrays realized to date. Aluminum nitride (AlN)-based PMUTs described in this paper are fabricated using a process compatible with the fabrication of inertial sensors, radio frequency (RF) resonators, and CMOS integrated circuits. The PMUTs are released using a front-side sacrificial etch through etching holes that are subsequently sealed by a thin layer of parylene. Finite element method and analytical results, including resonant frequency, pressure sensitivity, output acoustic pressure, and directivity are given to guide the PMUT design effectively, and are shown to match well with measurement results. Due to the PMUTs thin membrane (750-nm AlN/800-nm SiO 2 ) and small diameter, a single 25-µm PMUT has approximately omnidirectional directivity and no near-field zone with irregular pressure pattern. PMUTs are characterized in both the frequency and time domains. Measurement results show a large displacement response of 2.5 nm/V at resonance and good frequency matching in air, high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz, when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused high-output pressure acoustic beam. [2014-0114] Index Terms-Aluminum nitride (AlN), piezoelectric, ultrasonic, transducer, micromachined ultrasonic transducer (MUT), piezoelectric MUT (PMUT).

Mechanical, Corrosion and Biological Properties of Room-Temperature Sputtered Aluminum Nitride Films with Dissimilar Nanostructure

“…The fabrication of flexible piezoelectric pressure sensors using AlN thin films deposited on polymeric substrates have been reported [ 24 ], together with their use for measuring human muscle movements [ 25 ] and monitoring the respiration and heartbeat during sleep [ 26 ]. AlN thin films have been recently employed to realize piezoelectric micro-machined ultrasonic transducers used for gesture recognition [ 27 ], intravascular ultrasound imaging [ 28 ], or ultrasonic fingerprint sensing [ 29 ]. Also, AlN is seen a very promising material for realization of various microfluidic and biosensing applications, with integrated lab-on-chip diagnostic systems being envisaged [ 30 ].…”

Ultrahigh Frequency Ultrasonic Transducers Design with Low Noise Amplifier Integrated Circuit

“…In addition, the signal amplitude of the ZnO based transducer is rather low for a good performance in acoustic microscopy due to the weak piezoelectric behavior. Another important non-ferroelectric piezoelectric material, Aluminum nitride (AlN), possess better chemical and thermal stabilization, better compatibility with the complementary metal oxide semiconductor (CMOS) technology than ZnO [8,9,10]. Furthermore, the much higher longitudinal wave velocity benefits AlN for ultrahigh frequency application.…”

Broadband piezoelectric micromachined ultrasonic transducers based on dual resonance modes

“…Front-side etching enables thinner PMUT membranes, but makes it more difficult to precisely define the released PMUT's dimensions, a parameter which has a strong effect on the resonant frequency. While the PMUT boundaries can be precisely defined using a buried sacrificial layer [11], this buried layer increases fabrication complexity and cost. Here, we proposed a novel process, where front-side etching is used to release PMUT membrane and patterned thick metal layers are used to define the effective boundary of each PMUT.…”