A MEMS DC Current Sensor Utilizing Neodymium Rare Earth Magnets

Abstract

A silicon MEMS DC current sensor 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, such as that produced by a DC current flowing through a nearby current trace, 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 just a simple 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, torsion 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 die to provide for a cap over the device to seal it from the outside environment. Epoxy was used to bond the NdFeB magnet and the various silicon components together. The devices fabricated proved to behave similarly to their performance as predicted by mathematical modeling, with a test current of +/− 5 A causing a variation in the oscillation frequency of the CMOS oscillator circuit of +/− 8 kHz, from a nominal frequency of 26 kHz. 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 through 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 high aspect ratio were important.