A Differential Capacitive Three-Axis SOI Accelerometer using Vertical Comb Electrodes

Abstract

This paper reports a differentially detecting capacitive three-axis SOI accelerometer using novel vertical comb electrodes. The accelerometer structure was fabricated by surface-micromachining technique and consists of only the device layer of a SOI wafer without lower or upper electrodes. The bottom faces of both movable and fixed electrodes are in the same plane at their initial positions but those heights are different. It is also an important feature that the device applies fully differential detections in all three-axis by using only two pairs of four type capacitors. As an initial result, the capacitance changes against three-axis acceleration were observed. The capacitance sensitivities to X and Z-axis acceleration against Y-axis rotation were 1.

A three-axis SOI accelerometer sensing with both in-plane and vertical comb electrodes

“…However, compared with the sensitivity of Z-axis reported in the previously published works, i.e. 1.14 (Tsuchiya and Funabashi 2004) and 1.3 fF/g (Hamaguchi et al 2007), the Z-axis sensitivity of the presented accelerometer has been greatly improved. The nonlinearity, which is defined as the ratio of the maximum deviation from the best fit line to the full scale output, is measured as 0.5% for the inplane accelerometer, and 2.3% for the out-of-plane accelerometer.…”

“…To realize differential capacitive sensing scheme in Z-axis, in C A and C B , the moving electrodes are lower than the stationary electrodes, while in C C and C D , the moving electrodes are higher. Different from the designs by Tsuchiya and Funabashi (2004) and Hamaguchi et al (2007), the vertical comb electrodes of the presented accelerometer have even gaps (d 0 ) between the moving and stationary fingers, which helps minimize zero-g offset when modulation voltage is applied to the capacitors.…”

“…For convenient discussion, we consider a-b coordinate system, which is 45 o apart from XY coordinate system. When the displacement is along positive direction of a-b-Z axes, we have the following equations (Hamaguchi et al 2007):…”

“…Hsu et al (2010) fabricated a silicon-onglass (SOG) three-axis accelerometer, in which the authors integrated in-plane and out-of-plane accelerometers into a single sensing structure to achieve small size, and made the unbalanced proof mass as the outer frame of the in-plane accelerometers to enhance the output of Z-axis sensing. Vertical comb electrodes, which are composed of high and low comb fingers, were utilized to detect Z-axis acceleration on SOI wafer with differential capacitive scheme by Tsuchiya and Funabashi (2004) and Hamaguchi et al (2007). But in their three-axis accelerometers, both the out-of-plane and in-plane motions were detected by the same group of vertical comb electrodes, so the sensitivity must be decoupled for each of the three axes, and consequently the capacitive sensitivities for all the three axes were reduced.…”

Theoretical and experimental study of capacitance considering fabrication process and edge effect for MEMS comb actuator

“…Piyawattanametha et al (2005) qualitatively analyzed the influence of edge effect to angular vertical comb finger using 2-D finite element method, but it did not give an exact calculation formula. Hamaguchi et al (2007) designed a differential comb actuator to eliminate edge effect, but the influence of nonparallel teeth is not considered.…”