Fabrication and Characterization of a Microsensor for Detecting Simultaneously Both Earth’s Magnetic Field and Acceleration

Abstract

This paper presents a resonant microsensor for detecting simultaneously both the Earth's magnetic field and its acceleration. The structure design is based on the Lorentz force and lateral driving. The process uses a silicon-on-glass wafer and gold-silicon eutectic bonding for the wafer-level hermetic packaging. A prototype sensor with resonant frequencies in a range of around 2 kHz shows Q factors of about 7. In total, 2 mW is consumed in the current driving element. When 1 g and 0.35 G were applied to the sensor, a sensitivity of 104 mV/g and 5 mV/G was measured.Recently there has been increasing demand for an electronic compass in a variety of mobile electronic systems. 1 Until now, a variety of magnetometers for portable applications have been developed, including Hall-effect devices, anisotropic magnetoresistance devices, giant magnetoresistance devices, magneto-diodes, magnetotransistors, flux-gates, and so on. 2,3 However, semiconductor-based magnetometers exhibit limited sensitivity for small magnetic fields in the range of Earth's magnetic field, exhibit large temperature dependency and offsets, and require complicated fabrication processes. 3-5 Despite high resolution ͑nT Ͻ 1͒, the flux-gate consumes significant power ͑hundreds of mW͒ during operation. In addition, the realization of very small search coil and micromachined flux-gate magnetometers is usually a challenge because of the difficulty in fabrication of miniaturized coils. 6-8 Only a few Lorentzforce-based resonant magnetometers have been reported, 9-13 but because the resonant structure is easily affected by external acceleration, they have difficulty excluding the interference of unintended acceleration signal in the magnetic signal channel when acceleration due to the external shock or tilting is applied to the sensor. 10 In this work, we have conceived a microsensor for detecting the magnetic field using the Lorentz force and differentiating the magnetic field and the acceleration using the modulated frequency difference. This method can integrate a z-axis magnetometer and an x-axis ͑or y-͒ accelerometer in one microstructure and provide advantages in the assembly process and size minimization. Device Design and FabricationA conceptual schematic of the proposed microsensor is shown in Fig. 1. The operational principle is based on the Lorentz force arising from a current-carrying conductor in the magnetic field. 13 To realize this concept, a conducting line is formed on one side of the spring of a silicon microelectromechanical system ͑MEMS͒ structure and stationary sensing electrodes are formed on the upper cap wafer as shown in Fig. 1a. A movable silicon mass is suspended over a glass substrate by means of L-shaped springs, which are designed to be more easily movable in the x direction, whereas the suspensions in the y-and z direction are significantly stiffer and hence suppress corresponding motions. Consequently, the Lorentz force and applied acceleration laterally displaces the suspended structure.If an ac current flows through the co...

A Microelectromechanical Systems-Based Vertical Magnetometer–Accelerometer with the Modulated Frequency Difference in One Microstructure

“…7. 16) First, the ac current frequency (B) is far away from the acceleration frequency (A). From eqs.…”