Richard Przybyla

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1Vpp input voltage.

In-Air Rangefinding With an AlN Piezoelectric Micromachined Ultrasound Transducer

An ultrasonic rangefinder has a working range of 30 to 450 mm and operates at a 375-Hz maximum sampling rate. The random noise increases with distance and equals 1.3 mm at the maximum range. The range measurement principle is based on pulse-echo time-of-flight measurement using a single transducer for transmit and receive. The transducer consists of a piezoelectric AlN membrane with 400-μm diameter, which was fabricated using a low-temperature process compatible with processed CMOS wafers. The performance of the system exceeds the performance of other micromechanical rangefinders.

An ultrasonic rangefinder based on an AlN piezoelectric micromachined ultrasound transducer

An ultrasonic rangefinder has a working range of 30mm to 450mm and operates at a 375 Hz maximum sampling rate. The worst-case systematic error less than 1.1 mm. The rms noise is proportional to the square of the distance and equals 1.3mm at the maximum range. The range measurement principle is based on pulse-echo time of flight measurement using a single transducer for transmit and receive consisting of a piezoelectric AlN membrane with 400 µm diameter which was fabricated using a low-temperature process compatible with processed CMOS wafers. All circuits are low voltage, enabling integration in standard low voltage circuit technology.

Aluminum nitride pMUT based on a flexurally-suspended membrane

Piezoelectric micro-machined ultrasonic transducers (pMUTs) for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric material. Earlier pMUTs based on a fully clamped membrane design suffer from high sensitivity to residual stress, causing large variations in the operating frequency, and have a reduced dynamic range due to nonlinearity at large drive voltages. Here we evaluate a new design based on a membrane that is supported by three flexures and a thin oxide layer, aimed to release residual stress, extend the mechanical dynamic range and improve the acoustic coupling. The acoustic performance of this flexurally suspended design is compared with a fully clamped one, showing a piston-like mode shape, which translates to improved output sound pressure.

3D Ultrasonic Rangefinder on a Chip

An ultrasonic 3D rangefinder uses an array of AlN MEMS transducers and custom readout electronics to localize targets over a ±45° field of view up to 1 m away. The rms position error at 0.5 m range is 0.4 mm, 0.2 °, and 0.8 ° for the range, x-angle, and y-angle axes, respectively. The 0.18 μm CMOS ASIC comprises 10 independent channels with separate high voltage transmitters, readout amplifiers, and switched-capacitor bandpass ΣΔ ADCs with built-in continuous time anti-alias filtering. For a 1 m maximum range, power dissipation is 400 μW at 30 fps. For a 0.3 m maximum range, the power dissipation scales to 5 μW/ch at 10 fps.

A micromechanical ultrasonic distance sensor with >1 meter range

Ultrasonic distance sensors based on piezoceramic transducers have >1m range and millimeter accuracy but require the use of bulky transducers. Existing micromachined sensors deliver inferior performance, with maximum range in the tens of centimeters. We present theory, design equations, and measured results for a micromechanical ultrasonic distance sensor which approaches the performance of piezoceramic-based solutions. The sensor has a maximum range >1300mm and random errors (3σ) of <1.7mm at 1.3m.

Improved acoustic coupling of air-coupled micromachined ultrasonic transducers

Phased array imaging with micromachined ultrasound transducer (MUT) arrays is widely used in applications such as ranging, medical imaging, and gesture recognition. In a phased array, the maximum spacing between elements must be less than half of the wavelength to avoid large sidelobes. This places a limit on the maximum transducer size which is not attractive since the acoustic coupling drops rapidly for MUT diameters less than a wavelength. Here, we present a new approach to increase the acoustic coupling of small radius MUTs using an impedance matching resonant tube etched beneath the MUT. Impedance, laser Doppler vibrometer (LDV), and acoustic burst measurements confirm a 350% increase in SPL and 8x higher bandwidth compared to transducers without the impedance matching tube, enabling compact arrays with high fill-factor and efficiency.

Piezoelectric micromachined ultrasonic transducers in consumer electronics: The next little thing?

This paper describes air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) for consumer electronics applications including time-of-flight range-finding, proximity and presence sensing, and gesture recognition. These applications require sensors that are small size, low-cost, and ultra-low-power, all of which are characteristics of PMUTs.

Air-coupled aluminum nitride piezoelectric micromachined ultrasonic transducers at 0.3 MHz TO 0.9 MHz

Air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) operating at frequencies ranging from 0.3 MHz to 0.9 MHz were designed, fabricated and characterized. We increased the fractional bandwidth by 51% and improved the piezoelectric coupling over 80% by patterning the diaphragm center into a ring or structural ribs, resulting in a reduction of the PMUTs mass. Pulse-echo testing was conducted in air using PMUTs at frequencies up to 0.9 MHz and the measured acoustic loss versus pathlength was compared to theoretical models. Devices were fabricated in an industrial foundry process using wafer-level bonding of a MEMS PMUT wafer to a CMOS wafer using a conductive metal eutectic bond. This process allows for close integration of PMUT arrays and signal processing circuitry and is used here to study the effects of wafer-level packaging on acoustic performance.

12.1 3D ultrasonic gesture recognition

Optical 3D imagers for gesture recognition suffer from large size and high power consumption. Their performance depends on ambient illumination and they generally cannot operate in sunlight. These factors have prevented widespread adoption of gesture interfaces in energy- and volume-limited environments such as tablets and smartphones. Wearable mobile devices, too small to incorporate a touchscreen more than a few fingers wide, would benefit from a small, low-power gestural interface. Gesture recognition using sound is an attractive alternative to overcome these difficulties due to the potential for chip-scale size, low power consumption, and ambient light insensitivity. Using pulse-echo time-of-flight, MEMS ultrasonic rangers work over distances of up to a meter and achieve sub-mm ranging accuracy [1,2]. Using a 2-dimensional array of transducers, objects can be localized in 3 dimensions. This paper presents an ultrasonic 3D gesture-recognition system that uses a custom transducer chip and an ASIC to sense the location of targets such as hands. The system block diagram is shown in Fig. 12.1.1. Targets are localized using pulse-echo time-of-flight methods. Each of the 10 transceiver channels interfaces with a MEMS transducer, and each includes a transmitter and a readout circuit. Echoes from off-axis targets arrive with different phase shifts for each element in the array. The off-chip digital beamformer realigns the signal phase to maximize the SNR and determine target location.