(Invited) MEMS Accelerometer Fabricated by Gold Multi-Layer Metal Technology

Abstract

MEMS(microelectromechanical systems) accelerometers are key devices for IoT (Internet of Things) technology. Throughout the technical progress, the resolution of detecting acceleration has been required to be the level of micro-G (1 G = 9.8 m/s2) in spite of small sensor size. In order to solve the problem, we proposed capacitive MEMS accelerometers with gold proof mass and have been developing MEMS accelerometers [1-4] using the multi-layer metal technology [5]. Figure 1 shows the relationship between the Brownian noise (BN ) and the proof mass size of MEMS accelerometers. BN is a crucial factor to determine the resolution of MEMS accelerometer [6]. In this Fig. 1, we compare our developed MEMS accelerometers with the other reported ones. These results suggest that we can obtain the low value of 22 nG/(Hz)1/2 by utilizing gold as a high density material of the proof mass, despite the same size of gold proof mass as that of silicon proof mass [7-11]. We have proved the feasibility of the MEMS accelerometer with the gold proof mass. This paper reviews the feature of our proposed MEMS accelerometer and the issue of its reliability. Figure 2 shows a schematic illustration of the proposed MEMS accelerometer. The MEMS accelerometer consists of suspensions, stoppers, a proof mass as upper electrode, and a fixed electrode. When the upper electrode is moving to the vertical direction by the input of acceleration, the capacitance is detected by the fixed electrode. The stoppers are built to prevent mechanical destruction caused by over-swinging of the upper electrode. Figure 3 shows an SEM photograph of the MEMS accelerometer. The result shows that MEMS accelerometer fabrication process is established by the multi-layer metal technology, which consists of the gold electroplating and the photo-sensitive polyimide film [5]. On the basis of the design concept and the fabrication process, we have developed the MEMS accelerometers described above. From the viewpoint of the usage of gold material for MEMS structures, we have investigated the reliability by conducting the heat cycle and the vibration cycle tests [12, 13]. These tests were carried out on MEMS accelerometers and cantilevers for evaluating the material characteristics. Figure 4 shows the results of applying the continuous vibration to the MEMS accelerometer. It is revealed that the MEMS accelerometer is immune to the vibration. In conclusion, we confirm that our proposed MEMS accelerometer can make a contribution to a new technical development. Reference [1] T. Konishi et al., Jpn. J. Appl. Phys., 52, p. 06GL04, (2013). [2] D. Yamane et al., Appl. Phys. Lett., 104, p. 074102, (2014). [3] D. Yamane et al., APCOT 2016, pp299, (2016). [4] D. Yamane et al., Sensors and Materials, (2019). [5] K. Machida et al., IEEE Trans. On ED, 48, 2273, (2001). [6] B. E. Boser and R. T. Howe, IEEE JSC, 31, p. 366, (1996). [7] J. Chae et al., JMEMS, vol.13, pp.628-635, (2004). [8] H. Qu et al., IEEE Sensors Journal, vol.8, pp.1511(2008). [9] R. G. Walmsley et al., in Proc. IEEE SENSORS2009, (2009). [10] R. A. Dias et al., Sensors and Actuators A: pp. 47 (2011). [11] X. Zou et al., in Proc. Transducers 2015, (2015). [12] T. Konishi et al., MNC 2017, (2017). [13] D. Yamane et al., Transducers 2017,2187, (2017). Figure 1