Highly responsive curved aluminum nitride pMUT

Abstract

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.

Piezoelectric micromachined ultrasonic transducer of flat membrane with boosted transmitting performance

“…Therefore, the zero-bending pMUT is proven to perform much better than the reference pMUT, with enhanced transmitting sensitivity of more than 450% of the reference pMUT. Compared to recently reported AlN pMUT with similar size [5], which even employs 3-D coupling mechanism for highly response purpose, transmitting sensitivity of the zero-bending pMUT is much better to it (45 nm / V).…”

MEMS Ultrasound Transducers for Endoscopic Photoacoustic Imaging Applications

“…AlN and PZT are dominant piezoelectric materials in pMUT fabrication. AlN films for pMUTs are typically deposited by sputtering with a thickness of 0.7–2 μm [ 124 , 125 , 126 ]. High residual stress and low deposition rate (less than 25 nm/min) are the limiting factors of sputtering AlN thin films [ 127 ].…”

“…In 2014, Akhbari et al presented an AlN pMUT based on a curved diaphragm with a diameter of 140 μm and a radius of curvature of 1065 μm, showing a 50 times higher DC transmission sensitivity than that of a planar device with the same dimension [ 126 ] ( Figure 24 a). In 2015, the same group also proposed a bimorph AlN pMUT that had two 0.95 μm thick AlN layers sandwiched by three Mo electrodes, showing a four times higher central displacement sensitivity than that of a unimorph AlN pMUT under similar actuation conditions [ 149 , 150 ] ( Figure 24 b).…”

“… Schematic drawing or optical images of ( a ) curved diaphragm pMUT (Reproduced with permission from IEEE [ 126 ]), ( b ) bimorph pMUT (Reproduced with permission from IEEE [ 149 ]), ( c ) pMUT array with an isolation trench between cells (Reproduced with permission from IEEE [ 151 ]), ( d ) flexurally suspended pMUT (Reproduced with permission from IEEE [ 152 ]), ( e ) ring-shaped pMUT (Reproduced with permission from TRF [ 153 ]), and ( f ) dual-electrode pMUT (Reproduced with permission from Elsevier [ 145 ]). …”

Recent Progress on Photoacoustic Imaging Enhanced with Microelectromechanical Systems (MEMS) Technologies

“…The PMUT provides different benefits compared to CMUT including (i) relatively higher capacitance as compared to CMUT, (ii) does not require high polarization voltage, and (iii) has a compatible matching impedance with sample [ 91 , 92 , 93 ]. Several types of PMUT-based US transducers are widely used for biomedical applications, such as the catheter type, dome-shape array, and concave array type [ 94 , 95 , 96 ]. W. Liao et al first reported the two-dimensional PMUT array with 144 elements for the PAI [ 97 ] system.…”