Design consideration for three-axis MEMS accelerometers using an asymmetric proof mass

Abstract

This paper presents three different designs of three-axis accelerometers using a thick epitaxial surface micromachining process. The designs have a common serpentine spring topology, providing a resonant frequency of around 1.5 kHz, with a spring length of 350 μm and have capacitive sensitivities in the region of 5 to 10 fF/g for the X and Y sensing. The Z-axis sensing is performed by means of torsional springs of 370 μm in length. The sensing techniques include varying gap differential combs for the XY-axes; varying gap differential electrodes and varying overlap area for the Z-axis. The acceleration range for all designs is ±10 g and the overall sensitivity was increased by having a large nominal capacitance.

A single mass two-axis capacitive MEMS accelerometer with force rebalance

“…As expected, two sine waves with almost 90 phase difference are obtained. The output data that belong to corresponding axes can be expressed as follows: O x = SF x * a x +C xy * a y + o 1 (1) O y = SF y * a y +C yx * a x + o 2 (2)…”

“…Many studies utilized the single mass sensor structure in multi-axis MEMS accelerometer design fabricated by using bulk micromachining process [1], or CMOS fabrication techniques [2][3][4][5]. Having a singlemass,multi-axis accelerometer design causes some functionality issues which need to be taken care of.…”

“…In order to address this cross-axis sensitivity issue, [4] offered a solution including a change in the mechanical structure of the sensor leading to a different fabrication process which increases the cost of the sensor. Another study, [1], reports very low cross-axis sensitivities by utilizing individual masses sensitive to each axis which causes the increase in die area.…”