Monolithic CMOS—MEMS Pure Oxide Tri-Axis Accelerometers for Temperature Stabilization and Performance Enhancement

A complementary metal-oxide-semiconductor (CMOS)-microelectromechanical system (MEMS) accelerometer with stacked pure oxide layers as mechanical structures was developed. Metal layers were confined to the sensing electrodes and electrical routings; the metal-oxide composite in the CMOS-MEMS accelerometer was distributed in limited regions. This design has two major advantages: 1) the thermal deformation of suspended MEMS structures resulting from a mismatch in the coefficients of thermal expansion of the metal and the oxide in the metal-oxide films is suppressed; and 2) the parasitic capacitance of the sensing electrode routing underneath the proof mass is reduced. Thus, the accelerometer has higher sensitivity and reduced thermal drift. The curvature of the mechanical structures are improved in the temperature span and the noise floor is lowered. In the full temperature span (30 °C-90 °C), change in the radius of curvature per unit change in the temperature was 0.08%/°C for the in-plane accelerometer and 0.37%/°C for the out-of-plane accelerometer. Compared with the typical metal-oxide design, the proposed pure oxide design yielded a >20-fold improvement in radius of curvature change per unit temperature change for the in-plane accelerometer and a fivefold improvement for the out-of-plane accelerometer. Moreover, the noise floor was reduced to 0.40 (x-axis), 0.21 (y-axis), and 0.94 mG Hz
^-1/2
(z-axis), respectively, a 2.2-7.6-fold improvement compared with the metal-oxide design.