Micromachined inertial measurement unit fabricated by a SOI process with selective roughening under structures

A reduced micromachined inertial measurement unit (MIMU) was fabricated by a proposed two-step release silicon-on-insulator (SOI) process to meet the requirements of land vehicle navigation. The release process consists of two steps, that is, dry release using notching effect and wet release using hydrogen fluoride (HF) solution. By applying the presented layout design rules, most of the area of the device was released in the dry-etching process, but boundaries of the proof mass and the suspension beams were not released until the wet etching. Through such a process configuration, a smooth surface was obtained under some thickness-senstive structures such as suspension beams and a rough surface under some thickness-non-sensitive structures such as proof mass. The smooth surface avoids both the defects and the loss to the thickness brought about by the notching effect that is helpful to improve the reliability of the MEMS devices. The rough surface avoids the stiction under structures with a large area. A reduced MIMU was fabricated by such a process. The test results showed that scale factors of the X/Y-axis accelerometer and Z-axis gyroscope were about 19.11 mV/g and 15.6 mV/°/s at one atmosphere, respectively. The bias stability of the gyroscope was about 129.6°/h with a resolution of 0.0224 /°s/ Hz.