A Bulk-Micromachined Three-Axis Capacitive MEMS Accelerometer on a Single Die

This paper presents a high-performance three-axis capacitive microelectromechanical system (MEMS) accelerometer implemented by fabricating individual lateral and vertical differential accelerometers in the same die. The fabrication process is based on the formation of a glass-silicon-glass multi-stack. First, a 35-μm (111) silicon structural layer of an Silicon-On-Insulator (SOI) wafer is patterned with deep reactive ion etching (DRIE) and attached on a base glass substrate with anodic bonding, whose handle layer is later removed. Next, the second glass wafer is placed on the top of the structure not only for allowing to implement a top electrode for the vertical accelerometer, but also for acting as an inherent cap for the entire structure. The fabricated three-axis MEMS capacitive accelerometer die measures 12 × 7 × 1 mm³. The x-axis and y-axis accelerometers demonstrate measured noise floors and bias instabilities equal to or better than 5.5 μg/√Hz and 2.2 μg, respectively, while the z-axis accelerometer demonstrates 12.6 μg/√Hz noise floor and 17.4 μg bias instability values using hybrid-connected fourth-order sigma-delta CMOS application specific integrated circuit (ASIC) chips. These low noise performances are achieved with a measurement range of over ±10 g for the x-axis and y-axis accelerometers and +12/-7.5 g for the z-axis accelerometer, suggesting their potential use in navigation grade applications.