Highly sensitive wafer-level packaged MEMS magnetic field sensor based on magnetoelectric composites

For high spatial resolution, deep body region measurement of biomagnetic signals in the pico- and femtotesla regime, dense vector-field sensor arrays are required. Current state-of-the-art sensors, like SQUIDs, are too bulky and not applicable. In this paper for the first time MEMS sensors based on magnetoelectric (ME) composites with vacuum encapsulation using wafer-level packaging technology are presented. Silicon device and cap wafers with 150 mm diameter were fabricated using micromachining processes and bonded afterwards for hermetic sealing. The device wafer contains rectangular cantilever beams with a stack composed of SiO2/Pt/AlN/FeCoSiB, Au metal-bond frames and conduction lines. The lateral dimensions of the cantilever were 200 μm × 900 μm with an overall thickness of 7.8 μm. The cap wafer comprised the vacuum cavities, alignment marks and Au/Sn metal-bond frames. For avoiding degradation of the temperature sensitive amorphous magnetic material [(Fe90Co10)78Si12B10] a special developed low-temperature, hermetic sealed transient-liquid phase bonding process was used to package the sensor devices. Characterization of a sensor showed a giant ME coefficient of 2390 (V/m)/(A/m) and a corresponding sensitivity of 3800 V/T in resonance at 7.1 kHz. A minimum resolution of 30 pT and noise levels as low as 27 pT/Hz1/2 have been reached in resonant operation.