Thin-Film-Based SAW Magnetic Field Sensors

Abstract

In this work, the first surface acoustic-wave-based magnetic field sensor using thin-film AlScN as piezoelectric material deposited on a silicon substrate is presented. The fabrication is based on standard semiconductor technology. The acoustically active area consists of an AlScN layer that can be excited with interdigital transducers, a smoothing SiO2 layer, and a magnetostrictive FeCoSiB film. The detection limit of this sensor is 2.4 nT/Hz at 10 Hz and 72 pT/Hz at 10 kHz at an input power of 20 dBm. The dynamic range was found to span from about ±1.7 mT to the corresponding limit of detection, leading to an interval of about 8 orders of magnitude. Fabrication, achieved sensitivity, and noise floor of the sensors are presented.

Growth of Highly c-Axis Oriented AlScN Films on Commercial Substrates

“…Since then, AlScN has attracted great attention and has become a promising piezoelectric material for MEMS applications [ 7 , 8 , 9 , 10 , 11 ]. Multiple studies on AlScN-based MEMS magnetroelectric sensors [ 12 ], MEMS energy harvesters [ 13 ], MEMS quasistatic mirrors [ 14 ], and acoustic wave resonators [ 15 , 16 , 17 , 18 , 19 , 20 , 21 ] have been reported.…”

“…In most piezoelectric MEMS devices, AlN or AlScN thin films are grown on metallic seed layers such as molybdenum (Mo) or platinum (Pt) to ensure good c -axis orientation, where full width at half maximum (FWHM) values of the rocking curve measurements are typically larger than for AlN and larger than for Al 0.73 Sc 0.27 N [ 22 , 23 , 24 , 25 , 26 ]. However, metallic seed layers cannot be used for, e.g., piezoelectric thin-film-based surface acoustic wave (SAW) devices [ 21 ], optical waveguides [ 27 ], or MEMS actuators with doped silicon used as bottom electrode [ 28 ]. For these applications, the piezoelectric layer (AlN or AlScN) has to be grown directly on substrates such as (100) silicon (Si), silicon dioxide (SiO 2 ) or epitaxial polysilicon (poly-Si), but still a high degree of c -axis orientation of AlN or AlScN is required.…”

“…Due to its higher piezoelectric response compared to AlN, AlScN films have attracted more interest, thus becoming a focus in piezoelectric MEMS research. Although there is success in growing AlScN films with low Sc concentrations on different nonmetallic substrates (on high-resistivity (100) Si [ 21 ], low-resistivity boron-doped (001) Si [ 17 , 30 ], SiC [ 31 ]), higher Sc concentrations present an increasingly difficult challenge for the growth of AlScN films with exclusive c -axis orientation [ 10 ].…”

AlScN-Based SAW Magnetic Field Sensor for Isolated Closed-Loop Hysteretic Current Control of Switched-Mode Power Converters

“…Furthermore, same as AlN, AlScN can be sputter-deposited on large diameter silicon substrates, enabling low sensor fabrication cost, compared to other single crystal materials used for SAW like quartz or LiNbO3. Previously, an AlScN thin-filmbased SAW magnetic field sensor with a magnetostrictive FeCoSiB layer between IDTs has been presented in [9] by the authors. Similar sensors based on other material systems and substrates have also been demonstrated [10][11][12][13].…”

“…To fill this gap, this work uses an AlScN thin film based SAW magnetic field sensor similar to [9] for closed-loop hysteretic current control of a switched-mode dc-dc power converter.…”