In this paper, we report a set of methods to reduce the blind zone and improve the axial resolution and accuracy of an object tracking system based on air-coupled Piezoelectric Micromachined Ultrasonic Transducers (PMUTs). These methods require few computational resources and can therefore be deployed in the field to enable a self-calibrating measuring system. The proposed methodology comprises a calibration procedure and custom waveform design. The automatic calibration procedure identifies optimal inverted pulses to suppress the transmitter's free ringing (ringdown). The inverted pulses are found one at a time, by firstly sweeping the pulse timing, then sweeping the pulse duration. The ringdown energy is used as the objective function to be minimized and energy-based checks guarantee convergence. We achieved suppression in the -30 to -36 dB range. We then experimentally demonstrate that the obtained waveform, which includes a wait time between the driving pulses and the inverted pulses, and is designed to produce a beating effect with the receiver, reduces the blind-zone to less than 1 cm and improves the axial resolution and accuracy of an object-tracker with negligible impact on sensitivity.

An innovative method for the ultrasonic measurement of piezoelectric micromachined ultrasonic transducers (pMUTs) using a laser microphone is presented. Full wafer mapping is achieved, allowing extensive statistical analysis of acoustic performance across many samples by automating the measurement process. This approach provides a valuable method for monitoring and improving device performance, as well as optimizing changes after the assembly phase and package mounting. The results demonstrate the effectiveness of our method for detailed acoustic characterization and provide valuable insights for the optimization and reliability assessment of pMUT devices.

High-frequency airborne ultrasound enables high-resolution distance sensing for various applications, such as robotics, industrial automation, gesture control, and more. This paper presents a 1.5MHz, 12-element 2-D Piezoelectric micromachined ultrasonic transducer (PMUT) array for high temporal and spatial resolution distance sensing in air. Electrical impedance analysis and measured electroacoustic frequency response of the air-coupled PMUT array showed a resonant frequency f r of 1.5 MHz, an electromechanical coupling factor k T 2 of 4.5%, and a -3 dB fractional bandwidth of 5% centered at 1.5 MHz, with high transmit and receive sensitivities of 70 Pa/V and 0.16 mV/Pa, respectively. Distance measurement on a curved surface using two matched PMUT arrays in pulse-echo proved the feasibility of sensitive and accurate high-resolution distance sensing in demanding scenarios.

Micromachined Ultrasonic Transducers (MUTs) are being explored as power converters in wirelessly powered biomedical implants. This paper investigates the role of mechanical support properties in piezoelectric MUTs (PMUTs) on their power conversion efficiency. For this purpose, a finite element model (FEM) of a PMUT array was developed and integrated with an equivalent circuit model (ECM). The study considered different mechanical support scenarios, from rigidly clamped to completely free. These were numerically analyzed and validated by impedance measurements and acoustic power transfer experiments on PMUT prototypes. The results show that reducing the mass of the mechanical support increases the Q factor, leading to a significant improvement in power conversion efficiency, with an efficiency increase factor of 5.6x from the clamped to the free case. This approach can potentially enhance overall power conversion efficiency, reduce the need for matching networks, and enable miniaturization in ultrasonically powered implants.

In the design of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) for wideband immersion operation, the choice of piezoelectric material is crucial for determining transducer performance. The sensitivity, bandwidth, and maximum transmit (TX) pressure are significantly impacted by the selected material, thereby determining the suitability of PMUTs for various application scenarios, such as medical imaging, wearables, and implants. This work reports on an experimental comparative analysis of wideband PMUT performance when designed and fabricated using sol-gel (SG) Lead Titanate Zirconate (PZT), Physical Vapor Deposition (PVD) PZT, and 15% Scandium-doped Aluminum Nitride (ScAlN), aiming to provide insights for optimizing PMUT design by selecting the most appropriate material for specific applications.

We analyze and optimize the transmission of ultrasonic waves through the back-etched cavities of broadband PMUT arrays designed for medical imaging and non-destructive testing applications. We use a simple waveguide model to determine the acoustic impedance matching condition and investigate the effectiveness of the matching approach through Finite Element Modeling (FEM). We show that both the characteristic impedance and the finite elasticity of the cavity-filling material may affect the transducer bandwidth. The theoretical results are validated through electro-acoustic measurements carried out using 2MHz, 12-element, 2-D PMUT arrays.

This work presents a promising microfabrication technique employing the silicon-on-nothing (SON) process to form a $2\ \mu\mathrm{m}$ thick continuous monocrystalline silicon membrane over a vacuum cavity of $1\ \mu\mathrm{m}$ in depth. Utilizing the SON process, high fill-factor piezoelectric micromachined ultrasonic transducer (pMUT) arrays on an 8-inch silicon wafer with cavity widths ranging from $170\ \mu\mathrm{m}$ down to $38\ \mu\mathrm{m}$ have been demonstrated. Devices are designed with 15% scandium-doped aluminum nitride as the piezoelectric layer of the pMUT for both air-coupled and water-coupled applications. The air-coupled pMUTs show a peak displacement frequency from 0.8 to 1.6 MHz with a $Q$ -factor between 120 to 194. The water-coupled pMUT arrays show a transmit pressure measured by a needle hydrophone, in DI water at a distance of 20 mm, ranging between 0.4 to 6.9 kPa/V with peak frequency between 5 to 13.4 MHz and fractional bandwidth 56 to 36%, respectively. The piezoelectric-over-SON process proposed here has the potential to gain traction in low-cost and high-yield pMUT manufacturing.

Ultrasonic ranging with piezoelectric micromachined ultrasonic transducers (pMUTs) can be used in applications such as robotic systems and industrial machinery for precise distance measurements. This work presents a technique of operating pMUTs below the resonance frequency to obtain a wide bandwidth for high resolution time-of-flight (ToF) ranging. Digital signal processing was performed to remove the residual ringing at resonance and recover the wideband echo signal by bandpass filtering and cross-correlation in the frequency domain. The ToF was then determined after transforming back to the time domain. An array of pMUTs with resonance frequencies around 1.1 MHz was pulsed at 500 kHz, and a 4.3 µm standard deviation in ranging resolution was achieved with a pulse repetition frequency (PRF) of over 2 kHz. This enables ultra-precise non-contact distance or vibration measurements within a low-profile form factor.

Micromachined Ultrasonic Transducer (CMUT and PMUT) technologies are playing a fundamental role in the development of novel applications such as ultra-portable medical imaging systems. This paper presents the design, fabrication, characterization, and system integration of a 1-D PMUT array for low-frequency diagnostic imaging applications. The PMUT array was fabricated using a sol-gel PZT thin film-based MEMS technology from STMicroelectronics. The 1-D array was integrated into an ultrasound probe and acoustic characterization and imaging tests were carried out using the ULA-OP 256 open scanner. The two-way frequency response of the PMUT had a center frequency of 2.5 MHz and a -6dB fractional bandwidth of 81%, and peak transmit and receive sensitivities assessed at the transducer surface of 31 kPa/V and 3.2 mV/kPa, respectively. In vitro and in vivo scans of a tissue-mimicking phantom and of a carotid artery, respectively, were successfully carried out demonstrating the potentiality of this PMUT technology for medical imaging applications.

In this work, we present the experimental results of testing piezo-electrically tunable micro-lenses (Tlens(R)) using a single-spot optical setup that combines interferometric sensing of low-frequency dynamic deformations with direct detection of dioptric power, as functions of the driving voltage of the thin-film piezo-actuator. By exploiting low-coherence reflectometry, we detect the driving voltage dependence of the structure optical path-length. Voltage tuning capabilities of the focal length are then detected by means of a collimated red laser beam. Measurements can be carried out in a fast sequence without any lens manipulation or relocation, thus allowing a direct comparison among the lens characteristics, before and after electrical stimuli exploited to generate imprint in PZT thin films.

In this work, we present the results of the opto⁻electro⁻mechanical characterization of tunable micro-lenses, Tlens®, performed with a single-spot optical measuring system. Tested devices are composed of a transparent soft polymer layer that is deposited on a supporting glass substrate and is covered by a glass membrane with a thin-film piezoelectric actuator on top. Near-infrared optical low-coherence reflectometry is exploited for both static and low-frequency dynamic analyses in the time domain. Optical thickness of the layers and of the overall structure, actuation efficiency, and hysteretic behavior of the piezo-actuator as a function of driving voltage are obtained by processing the back-reflected signal in different ways. The use of optical sources with relatively short coherence lengths allows performing interferometric measurements without spurious resonance effects due to multiple parallel interfaces, furthermore, selecting the plane/layer to be monitored. We finally report results of direct measurements of Tlens® optical power as a function of driving voltage, performed by redirecting a He-Ne laser beam on the lens and monitoring the focused spot at various distances with a digital camera.

A series of on-chip test structures where designed, modeled and fabricated for on-chip fracture characterization of 0.7 μm thick polysilicon film. The first test structure is rotational and actuated by combfingers capacitors which develop a force sufficient to load the specimens up to rupture; it allow the determination of Young modulus and rupture strength. The second kind of test structure is actuated in the direction orthogonal to the substrate by means of a system of parallel plate actuators, four specimens placed at the four corners of the device are loaded in bending and torsion; they are conceived in order to estimate the value of the shear elastic modulus of the polysilicon film. The third test structure is again actuated by parallel plate capacitors in the direction orthogonal to the substrate and able to load up to rupture a couple of specimens in bending in the plane orthogonal to the substrate; this device has been built in order to measure the resistance of the polysilicon film in these particular loading conditions and to evaluate the possible influence of grain morphology on the mechanical responses. The three different on-chip test structures are briefly described in the paper and the first obtained experimental results are presented and discussed.

Elastic stiffness and strength of thin polysilicon films were obtained through on-chip test structures with in plane and out of plane loading conditions. Comparative studies were performed to investigate the variation of the ultimate strength of the polysilicon film (caused by the cristalline structure of the material) with different loading conditions.