Sensitivity Tunable Capacitive Type Micro Accelerometer Chang Han Je*, Sungsik Lee, Myung Lae Lee, Sunghae Jung, Ahra Lee, Gunn Hwang, Byoung Gon Yu, and Chang Auck Choi Convergence Component & Materials Research Laboratory Electronics & Telecommunications Research Institute (ETRI) Daejeon, Republic of Korea Tel: +82-42-860-1179, Fax: +82-42-860-5202, *E-mail: chje@etri.re.kr Abstract— In this paper, we present a novel sensitivity tunable capacitive type micro accelerometer which could adjust its sensitivity. The previous bulk-micromachined accelerometer is hard to make a pattern which is smaller than 1um with high aspect ratio because of ICP etching errors such as loading effect and footing. In this paper, by making movable ground electrodes which have MEMS actuators, we present a high sensitivity micro accelerometer with a narrow sensing gap under 1um using conventional bulk micromachining. Unlike previous capacitive type MEMS accelerometers which have anchored ground electrodes, the proposed micro accelerometer has movable ground electrodes. By simply applying DC bias to MEMS actuators, the ground electrodes are moved to the sensing electrodes. The fabricated sensing gap is 1.9um and it could be reduced to 0.8um according to the actuation voltage, V DD . The working sensitivity of the micro accelerometer is 0.410 pF/g at V DD =1.0 V, which is higher than 0.125 pF/g with no actuation, V DD =0 V. I. INTRODUCTION Recently, the accelerometers with high sensitivity and high resolution are needed in various applications such as microgravity sensing for geophysical survey and building safety management, inertial navigation system for automotive, and spacecraft guidance/stabilization. MEMS accelerometers for these applications have been researched due to low cost, mass production, small size and high performance. There are several types of sensing schemes for MEMS accelerometers such as piezo-resistive, resonant, tunneling-current, and capacitive. Among them, the capacitive sensing type has becoming more attractive and promising for high precision micro accelerometers due to its advantages such as high sensitivity, low noise, stable DC characteristics, low power dissipation, and low temperature sensitivity [1]. The capacitive sensing type is based on the detection of the capacitance change between the reference and sensing electrodes due to the acceleration force. So, in other to achieve high sensitivity, the capacitance change should be large. The large proof- mass, large electrode area and small sensing gap are required to increase the capacitance change. Generally, silicon capacitive type micro accelerometers are fabricated using bulk or surface micromachining. Although surface micromachined micro accelerometers can be integrated with interface electronics, due to their small mass they typically susceptible to cross-axis sensitivity, electrical noise, and small capacitance [2]. Bulk micromachined capacitive type micro accelerometer could be a solution for the large mass and large capacitance. However, bulk micromachining also has a limitation of improving sensitivity because of the difficulty in fabricating high aspect-ratio vertical sense/reference electrodes with small sensing gaps [3]. Several fabrication methods have been developed for masking a small sensing gap with high aspect ratio [3, 4]. However, most of them are complicated and difficult. In this paper, we report sensitivity tunable capacitive type micro accelerometers with a narrow sensing gap using a conventional low-resolution bulk micromachining process with MEMS actuator. II. ACCELEROMETER DESIGN A. Working principle Capacitive type physical sensors have a sensing electrode part and a reference electrode part. Generally, the reference electrode is fixed to the sensor body and the sensing electrode part is suspended by the spring. When the acceleration force is applied, the sensing electrode part is moved. Then, the gap between two parts is changed and the capacitance is also changed. The sensitivity increases as the initial sensing gap decreases. Therefore, a small initial gap is the key issue in high sensitivity capacitive type accelerometers. The novel idea of the present accelerometer is the method to control the sensing gap between the reference electrodes and the sensing electrodes. The working principle of the designed accelerometer is shown in Fig. 1. Unlike the conventional capacitive type micro accelerometer, the present accelerometer has a movable reference electrode with MEMS electrostatic actuator. As shown in Fig. 1(a), the reference electrode is suspended by 1-4244-2581-5/08/$20.00 ©2008 IEEE 1020 IEEE SENSORS 2008 Conference