A silicon MEMS magnetometer has been developed that utilizes a miniature NdFeB rare earth magnet attached to a silicon platform that is suspended by a dual torsional suspension system. An externally applied out-of-plane magnetic field will cause a magnetic torque to be produced between the external field and the NdFeB magnet, causing a deflection of the suspended silicon platform which can be sensed capacitively. The device measures 5.6 mm X 5.6 mm, with the silicon components being manufactured using bulk micromachining processes. The variable capacitive structure is realized by metalizing the bottom side of the suspended silicon platform to allow the silicon platform to serve as the top electrode. The bottom electrode is provided by a bare pad on a printed circuit board (PCB) to which the frame of the silicon device is attached. This results in a variable capacitance with a nominal value of approximately 3–6 pF, depending on the exact width of the gap. The variable capacitance is large enough to be converted into a variable frequency square wave using a CMOS relaxation oscillator circuit.
To realize a practical device, multiple silicon components were manufactured. First, a silicon component had to be manufactured that included the anchor/frame, torsional springs, and suspended platform. To provide protection against destructive over-ranging of the mechanical components during very high accelerations or external magnetic fields, another silicon component was manufactured that provided mechanical stops at the limits of the useful displacement range. Two other components were also manufactured on the same wafer to provide for a cap over the device. Epoxy was used to bond the NdFeB magnet and the various silicon components together. The fabricated devices behaved similarly to their predicted theoretical performance, with a nominal oscillation frequency around 30 kHz, a sensitivity around 100 nT/Hz, and a noise floor around 50 nT.
Several fabrication and assembly issues had to be solved in order to realize the device. The gap width of the capacitive structure is dependent on the thickness of the agent used to electrically connect the silicon anchor to a pad on a PCB. As it is desirable to minimize this gap width, some experimentation was required to find a suitable agent and assembly method. Additionally, the bonding agent used to attach the silicon anchor to the PCB must be applied at a temperature near the expected operating temperature of the device to prevent large stresses from being applied to the silicon frame due to the difference in the coefficients of thermal expansion between silicon and FR4. Also, during fabrication it was found that large flat areas, where a very uniform etch is critical, required wet KOH etching, while deep reactive ion etching could be used for areas where depth and a high aspect ratio were important.
Significance
This MEMS sensor represents a novel configuration for sensing magnetic fields. Without much optimization, the sensor already exceeds the sensitivity of many commercially available Hall-effect based MEMS magnetometers. As MEMS magnetometers are less developed than alternative magnetometer technologies, they may have more opportunities for improvement.
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