This paper presents a fully integrated ultrasound system based on a single piezoelectric micromachined ultrasonic transducer (PMUT) monolithically fabricated with a 0.13 µm complementary metal oxide semiconductor (CMOS) process analog front-end circuitry. The PMUT consists of an aluminum nitride, AlN, squared device with 80 µm side that resonates at 2.4 MHz in liquid environment. The monolithic integration of the PMUT with the CMOS circuitry allows a reduction of the parasitic capacitance, a reduction of the electronic noise contribution and a clear improvement in the Signal-to Noise ratio (SNR ~ 27 dB better) compared to a non-integrated equivalent system. A pulse-echo experiment with the single PMUT-on-CMOS for transmitting and sensing simultaneously is demonstrated, ensuring a 17.3 dB SNR, higher than the minimal necessary for accurate fingerprint images, paving the way towards a pixel sized imaging system with no need of multiple simultaneous PMUTs transmitters. Consuming only 0.3 mW and getting an input-referred noise of 3.26 mPa/√Hz at 2.4 MHz, the proposed pulse-echo system achieves a competitive noise efficient factor in comparison with the state-of-the-art.

This paper presents an analog front-end transceiver for an ultrasound imaging system based on a high-voltage (HV) transmitter, a low-noise front-end amplifier (RX), and a complementary-metal-oxide-semiconductor, aluminum nitride, piezoelectric micromachined ultrasonic transducer (CMOS-AlN-PMUT). The system was designed using the 0.13-μm Silterra CMOS process and the MEMS-on-CMOS platform, which allowed for the implementation of an AlN PMUT on top of the CMOS-integrated circuit. The HV transmitter drives a column of six 80-μm-square PMUTs excited with 32 V in order to generate enough acoustic pressure at a 2.1-mm axial distance. On the reception side, another six 80-μm-square PMUT columns convert the received echo into an electric charge that is amplified by the receiver front-end amplifier. A comparative analysis between a voltage front-end amplifier (VA) based on capacitive integration and a charge-sensitive front-end amplifier (CSA) is presented. Electrical and acoustic experiments successfully demonstrated the functionality of the designed low-power analog front-end circuitry, which outperformed a state-of-the art front-end application-specific integrated circuit (ASIC) in terms of power consumption, noise performance, and area.

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This study presents a squared AlN piezoelectric micromachined ultrasonic transducer (PMUT). Using this PMUT greater level of output pressure and higher reception sensitivity has been achieved, compared with the state-of-the-art. Another outstanding characteristic for this PMUT is that it can be monolithically integrated on CMOS substrate, being remarkably advantageous in relation to the bonding method implemented until now.

Ultrasonic systems driven by multi-frequency continuous waves (MFCW) have been used for range distance measurement, offering high accuracy in long and medium range distance estimation. However, the use of continuous waves in very short-distance measurements causes large errors due to multipath reflections. This paper presents a new strategy to estimate very short relative distances with high accuracy based on the use of multi-frequency pulsed waves (MFPW). The proposed strategy allows to avoid the multipath reflections that appear when continuous waves are used, and it improves the achieved accuracy compared to the original MFCW method. To validate it, an 80 µm square AlScN piezoelectric micromachined ultrasonic transducer (PMUT) was chosen as a transmitter while a hydrophone was utilized as a target and receiver, immersed in fluorinert (FC-70) as a propagation medium. Three independent and consecutive tone-burst signals were transmitted successively. The selected frequencies are f1 = 2.3962 MHz, f2 = 2.327 MHz and f3 = 2.1195 MHz, giving first and second-order resolutions of 6.88 and 0.79 µm/°, respectively. Experimental results show a ±6.2 μm measured range error in a range of 3.5 mm, and therefore it represents a good candidate for ultrasound micro-profilometer applications under liquid operation.

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In this paper, guidelines for the optimization of piezoelectrical micromachined ultrasound transducers (PMUTs) monolithically integrated over a CMOS technology are developed. Higher acoustic pressure is produced by PMUTs with a thin layer of AlN piezoelectrical material and Si3N4 as a passive layer, as is studied here with finite element modeling (FEM) simulations and experimental characterization. Due to the thin layers used, parameters such as residual stress become relevant as they produce a buckled structure. It has been reported that the buckling of the membrane due to residual stress, in general, reduces the coupling factor and consequently degrades the efficiency of the acoustic pressure production. In this paper, we show that this buckling can be beneficial and that the fabricated PMUTs exhibit enhanced performance depending on the placement of the electrodes. This behavior was demonstrated experimentally and through FEM. The acoustic characterization of the fabricated PMUTs shows the enhancement of the PMUT performance as a transmitter (with 5 kPa V−1 surface pressure for a single PMUT) and as a receiver (12.5 V MPa−1) in comparison with previously reported devices using the same MEMS-on-CMOS technology as well as state-of-the-art devices.