(Invited) A 1-mG MEMS Sensor

MEMS (microelectromechanical systems) technology has contributed substantially to the miniaturization of inertial sensors, such as accelerometers and gyroscopes [1]. Nowadays, MEMS inertial sensors have been widely used in a variety of application in our daily life. In recent years, accurate sensing of low acceleration, specifically below 1 G ( G = 9.8 m/s2, gravitational acceleration), has been one of the major interests for MEMS accelerometer development; those highly-sensitive accelerometers are to be used for possible applications such as healthcare and medical use, navigation systems, and micro-gravity monitoring in space [2]. According to the background, we have proposed a design approach of sub-1G MEMS accelerometer with a gold proof mass [3]. The high-density of gold has enabled us to suppress the thermo-mechanical noise, as it is called Brownian noise (BN ), when compared with typical proof mass material such as silicon; as the magnitude of BN is inversely proportional to the proof mass [4], the increase of proof-mass density would be an effective method to have low BN with a minimum proof-mass footprint. In this paper, we present a 1mG MEMS sensor that was designed to be an extension of our previous work. A proof-of-concept device has been developed with a proof mass made of electroplated gold. The target BN was set to be below 1 μG/Hz1/2, which has potential of acceleration sensing with a resolution of 1 mG or even lower. The proof mass was designed to be 2.4 mm × 2.1 mm in area with the thickness of 12 μm, which was nearly four times larger than that of our previous sensor. To reduce the metal warpage, the proof mass was segmented into a number of sub-blocks that were cross-linked by the additional metal layer. The device was fabricated by the post-CMOS (complementary metal-oxide semiconductor) process [6] utilizing gold electroplating; the sensor can be implemented on to CMOS-LSI (large-scale integration). Thus, MEMS structures and sensing circuitry could be integrated as a single chip with a minimum footprint and parasitic elements. Fig. 1(a) shows a top view of the fabricated MEMS device. The proof mass consists of the array of 18 × 21 sub-blocks. Each proof mass corner is suspended by micromechanical spring. A close-up SEM (scanning electron microscope image) is shown in Fig. 1(b). The stopper is used to limit the displacement of the proof mass, which would prevent the self-destruction of the MEMS structure at the input of excess acceleration. The fundamental characteristics of the MEMS device were experimentally obtained by the frequency response, where we measured capacitance and phase as a function of signal frequency; the actual BN was estimated to be 0.16 μG/Hz1/2 [5] that was sufficiently lower than the target value of 1 μG/Hz1/2. Also, we measured the capacitance change between the proof mass and the fixed counter electrode as a function of input acceleration. The experimental results indicate that the MEMS device was capable of capacitive sensing input acceleration below 1G. The evaluation results show that the proposed sensor design could further improve the sensing resolution of MEMS accelerometers, which would be required for possible future applications. The presentation will cover the design, fabrication process, and measurement results of the proof-of-concept device. [1]  N. Yazdi, F. Ayazi, and K. Najafi, “Micromachined Inertial Sensors”, Proc. IEEE, Vol. 86, pp.1640-1659, 1998. [2] G. Krishnan, C. U. Kshirsagar, G. K. Ananthasuresh, and N. Bhat, “Micromachined High-Resolution Accelerometers,” J. Indian Inst. Sci., vol. 87, pp. 333-361, 2007. [3]  D. Yamane et al., “Design of sub-1g microelectromechanical systems accelerometers” Appl. Phys. Lett., vol. 104, issue 7, pp. 074102, 2014. [4] M. Lemkin, and B. E. Boser, “A Three-Axis Micromachined Accelerometer with a CMOS Position-Sense Interface and Digital Offset-Trim Electronis,” IEEE J. Solid-State Circuits, vol. 34, pp. 456-468, 1999. [5] D. Yamane, T. Konishi, T. Matsushima, H. Toshiyoshi, K. Machida, and K. Masu, "A Sub-1G Capacitive Sensor for Integrated CMOS-MEMS Accelerometers," in Proc. 7th Asia-Pacific Conference on Transducers and Micro/Nano Technologies (APCOT2014), EXCO, Daegu, Korea. June 29 - July 2, 2014, P2-55. [6] K. Machida, T. Konishi, D. Yamane, H. Toshiyoshi, and K. Masu, “Integrated CMOS-MEMS Technology and Its Applications,” ECS Trans., vol. 61, pp. 21-39, 2014. Figure 1