A dual-frequency piezoelectric micromachined ultrasound transducer array with low inter-element coupling effects

Abstract

This paper presents a dual-frequency piezoelectric micromachined ultrasonic transducer (PMUT) line array with low crosstalk level, which was fabricated on silicon-on-insulator (SOI) wafers with sputtered piezoelectric thin film (PZT) and Si diaphragm structure. The obtained array consists of 120 of 0.77 MHz and 192 of 2.30 MHz PMUT units in total with minimum interspace of 50 μm. Due to the high piezoelectric coefficient of PZT, the PMUT shows high transmitting sensitivity in air and good effective electromechanical coupling factor. The displacement sensitivities are assessed to be 595 nm V−1 and 112 nm V−1 at the resonant frequencies of 0.77 MHz and 2.30 MHz respectively in air. To reduce the vibration coupling, rectangular grooves in the bottom silicon are designed between the adjacent line elements, and the PMUT units in the array are arranged in a mis-aligned style. Modal analysis for the 0.77 MHz units indicates neighbouring coupling-vibration decreases greatly from 44.5% to 14.8% of the excited vibration when the excited line is driven at 4Vpp, which proves both the grooves and the mis-aligned ranking are effective for coupling effect reduction. Moreover, results indicate the coupling effect between different frequencies can be ignored due to their inherent resonance characteristics. The sound pressures for a single 0.77 MHz line element and 2.30 MHz line element are evaluated to be 53 kPa and 73 kPa at a distance of 1 cm in water. This high performance dual-frequency PMUT line array makes some high resolution imaging methods possible based on PMUT technology, such as those of dual-frequency, total focus.

Piezoelectric Micromachined Ultrasonic Transducers (PMUTs): Performance Metrics, Advancements, and Applications

“…Multifrequency (MF) arrays have been utilized to further improve the bandwidth of high frequency PMUTs for biomedical imaging applications [ 75 ]. A dual-frequency PMUT line array was proposed by Liu et al [ 76 ]. The PMUT elements are designed with sputtered PZT of thickness 3 µm as the piezoelectric layer.…”

“…Another approach in multifrequency PMUT arrays is to use elements of different sizes in the array. This improves the beam profiles and reduces cross coupling between elements [ 76 ]. However, using different element sizes in a PMUT array requires optimization of interelement spacing and pitch for achieving a high fill factor [ 81 ].…”

Piezoelectric Micromachined Ultrasound Transducer Technology: Recent Advances and Applications

“…The array was bonded to CMOS through wafer-level conductive eutectic bonding, achieving individual pixel readout of ultrasound images with high signal to noise ratio (SNR). More recently, Liu et al [130] developed a dual-frequency PMUT linear array, at which 0.77 MHz and 2.30 MHz line elements were alternately arranged in 2 rows and 12 columns. To reduce the vibration couplings between adjacent elements, rectangular grooves in the silicon substrate were fabricated.…”

Dual-frequency piezoelectric micromachined ultrasound transducer based on polarization switching in ferroelectric thin film

“…The inherent manufacturing challenges with stacked arrays have inspired developers to investigate micro-electromechanical systems (MEMS)-based devices, such as capacitive micromachined ultrasound transducers (CMUTs) and piezoelectric micromachined ultrasound transducers (PMUTs). These microfabrication techniques enable the monolithic integration of each designed sub-array with different frequency bands [13][14][15] and unique exural vibration of the membranes, instigating studies on realizing multi-frequency ultrasound operation based on uniform element transducer arrays [16][17][18] . In MUTs, a wellknown approach for achieving multi-frequency ultrasound is excitation at their fundamental and harmonic modes [19][20][21][22] .…”