A new type of magnetometer-accelerometer is developed with a silicon micromachining. The operation principle of the sensor is based on the well known Lorentz force caused by the interaction of a current and an external magnetic field on a suspended conducting beam. The resonant magnetometers that use the Lorentz force have been reported but they have the critical problem of failing to exclude the effect of acceleration when a magnetic field and acceleration are applied simultaneously. Therefore, we have conceived a new resonant micro sensor detecting both acceleration and the geomagnetic field simultaneously and minimizing the interference with each other. To realize this concept, a conducting line is formed on a spring part of a silicon accelerometer having two mass plates as shown in Fig. 1. The upper sensing electrode detects acceleration in the perpendicular direction and a magnetic field in the parallel direction to the substrate. The vertical displacement of the structure is converted into a capacitance change by the mass and the upper sensing electrode, which form a parallel-plate capacitor. The vibrating mass and springs are fabricated on a silicon-on-glass (SOG) wafer and packaged by a silicon-gold eutectic bonding. Gold is evaporated on the silicon of SOG and acts as a current carrying conductor and a eutectic counterpart. Indeed, the resonant operation in high vacuum condition is successful in increasing the sensitivity. But it causes unwanted problems such as the lack of shock reliability, reproducibility, etc. In order to solve this problem, we have tried it in a very low vacuum (about Q-factors below 10). To operate the sensor, an ac current of its mechanical resonant frequency is driven through the conducting line. The measured current flow and resistance are 10 mA and 10 Ω respectively, so 1 mW is consumed in the current driving element. When 1g (= 9.8 m/s2) of acceleration and 70 mTpp of magnetic field (in South Korea) are applied to the sensor, the measured sensitivity are 300 mV/g for accelerometer and 290,000 V/A_T for magnetometer, respectively. This newly developed sensor can be used in a portable navigator such as SmartPhones and PDAs that need a small, low cost and low power electronic compass.
Journal: TechConnect Briefs
Volume: 1, Technical Proceedings of the 2003 Nanotechnology Conference and Trade Show, Volume 1
Published: February 23, 2003
Pages: 368 - 371
Industry sectors: Advanced Materials & Manufacturing | Sensors, MEMS, Electronics
Topics: MEMS & NEMS Devices, Modeling & Applications