Resonant magnetic field sensor with frequency output

This paper presents a new type of micromachined resonant magnetic field sensor, delivering a frequency change as output signal. The cantilever-based microsensor is fabricated using an industrial CMOS process in combination with postprocessing micromachining. The cantilever beam with integrated electrothermal excitation and piezoresistive detection elements is embedded in an amplifying feedback loop, resulting in a stable oscillation at its resonance frequency. By generating a Lorentz force (with the magnetic field to be measured) proportional to the resonator position, the spring constant, and therefore the resonance frequency of the resonant cantilever beam is modified. The measured devices have a fundamental resonance frequency of about 200 kHz, and show a sensitivity of 40 kHz/Tesla at atmospheric pressure. The short-term frequency stability of the resonators in closed-loop operation is below 0.1 Hz, which corresponds to a sensor resolution in the 10 μTesla range.