3D micromechanical compass

We have designed and fabricated micromechanical magnetometers intended for a 3D electronic compass which could be embedded in portable devices. The sensors are based on the Lorentz force acting on a current-carrying coil, processed on a single crystal silicon resonator. Sensors for all cartesian components of the magnetic field vector can be processed on the same chip. The vibration amplitude is detected capacitively and the resonance is tracked by a phase-locked-loop circuit. The fabrication process is based on aligned direct bonding of a double side polished and a SOI wafer. Magnetometers measuring the field component along the chip surface have a flux density resolution of about 10 nT/root Hz at a coil current of 100 mu A. Magnetometers measuring the field component perpendicular to the chip surface are currently less sensitive with flux density resolution of about 70 nT/root Hz. Resolution is limited by the fundamental thermomechanical noise. The standard deviation of the signal was less than 1% over a period of 5 days.