Andre Guedes

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.

Improving Magnetic Field Detection Limits of Spin Valve Sensors Using Magnetic Flux Guide Concentrators

The magnetic field detection limit of spin valve sensors in the thermal noise regime is improved from 1.3 nT/Hz1/2 to 0.064 nT/Hz1/2 by incorporating a Co93Zr3Nb 4 soft magnetic flux guide concentrator structure. Linear spin valve sensors were processed down to 20times2 mum2 dimensions, with and without an additional 3500-Aring-thick flux guide concentrator, with a magnetic flux gain factor ranging from 5 to 20. Spin valves show a starting sensitivity of 0.2 %/Oe, improved to 3.8 %/Oe with the flux concentrator. Noise measurements from dc to 500 kHz were performed, indicating similar noise levels with and without flux concentrators. In terms of magnetic field detection, sensors with flux concentrators show a detection limit of 2 nT/Hz1/2 at 10 Hz, compared to 47 nT/Hz1/2 without flux concentrators. No additional 1/f noise is measured in the spin valve sensor response upon the addition of the soft magnetic flux concentrators in the frequency range studied

Hybrid magnetoresistive∕microelectromechanical devices for static field modulation and sensor 1∕f noise cancellation

Low frequency 1∕f noise in magnetoresistive (MR) spin valve sensors was suppressed by modulating an external dc magnetic field at high frequency microelectromechanical system using a (MEMS) microcantilever structure with an integrated magnetic flux guide. With this hybrid MR∕MEMS device, direct detection of dc magnetic fields in the sensor high frequency thermal noise regime was achieved. The microcantilever was actuated using a gate electrode by applying an ac voltage with frequency f, causing it to oscillate at 2f. Measurements show detection of a dc magnetic field at 2f frequency (400kHz), where sensor 1∕f noise is two orders of magnitude lower than dc.

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.

OPTIMIZATION AND INTEGRATION OF MAGNETORESISTIVE SENSORS

This paper addresses challenging issues related to the integration of magnetoresistive (MR) sensors in applications such as magnetic field mapping, magnetic bead detection in microfluidic channels, or biochips. Although sharing the same technological principle for detection (magnetoresistance effect), each application has unique specifications in terms of noise, sensitivity, spatial resolution, electrical robustness or geometric constraints. These differences are of high impact for manufacturing, because some strategies used for sensor optimization compromise the freedom for device architecture.

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.

Hybrid Magnetic Tunnel Junction-MEMS High Frequency Field Modulator for 1/f Noise Suppression

A dc to ac magnetic field transformer was developed using a magnetic tunnel junction (MTJ)/microelectromechanical system (MEMS) mixed device. A MEMS torsionator was fabricated with an incorporated magnetic flux guide, that when actuated by a gate electrode, modulates an external dc field into the same frequency of the micro-torsionator oscillation. Attached to it a MgO based MTJ was fabricated, performing the detection of the generated ac magnetic field. This dc to ac field transformation enables the detection of dc magnetic fields in the high frequency thermal noise regime, where the 1/f noise is typically two orders of magnitude lower. Measurements show the detection of a modulated field at 460 kHz, coming from the MEMS torsionator designed to exhibit high modulation efficiency. This efficiency was of 11%, i.e., 11% of the external dc field is modulated. As a result, from a MTJ exhibiting a noise of SV MTJ=4.4 nT/Hz1/2 at 460 kHz, this hybrid device enables a dc magnetic field detection of 40 nT/Hz1/2. This work acts as a major improvement to a previous one , mainly through the optimization of the MEMS magnetic field modulation efficiency.

On-chip magnetoresistive detection of resonance in microcantilevers

Magnetoresistive spin-valve sensors were used to provide on-chip detection of the mechanical resonance of a thin silicon microelectromechanical systems cantilever. The spin-valve sensor was placed underneath the free end of the cantilever. A CoCrPt thin-film permanent magnet was placed on top of the amorphous silicon/Al cantilever. The cantilever was electrostatically actuated and its deflection creates a change in the magnetic field that can be sensed by the spin-valve sensor. The resonance frequency of the structure in the megahertz range is detected by the measurement of the spin-valve sensor output. Minimum deflection detection limit is determined to be 0.06 A/Hz1/2.