Development Of A Silicon Based Flexure Accelerometer With High-Precision And Large Dynamic Range

Accelerometer is considered as a key device in inertial surveying system, which has been widely used in systems for geology and resource exploration, gravity aided navigation and earthquake detection. As the variation of gravitational signal is quite weak, the ultra-high resolution accelerometer is strongly required. However, Most reported MEMS accelerometers are used in low precision applications, which focus on form factor and low cost. Flexure accelerometer has been widely used in high-precision measurements. This thesis combines stability of silicon based MEMS with the technology of flexure accelerometers. Aiming at the application in precision gravity detection, the silicon based flexure accelerometer should have range of ± 1g and nano-g resolution.The structure of the accelerometer was designed and optimized by finite element analysis. Deep reactive ion etch with high aspect ratio was applied for the fabrication. A large proof mass with a dimension of 18* 15mm2 was used to decrease the mechanical noise. Design of folded springs in series was introduced for lower resonant frequency. Intermediate frames were introduced into the springs to increase the cross-axis rigidity. Unequally beams spacing were designed to reduce decreasing of operating range caused by mass of springs and frames. Shock stops and dampers were designed to constrain proof-mass in case of overload. The mechanical structure was optimized with a resonant frequency of 15 Hz, a mechanical thermal noise of 1 ng/VHz and an operating range of+ 2.4g. To detect acceleration with large dynamic range, the capacitive detection system which employs the area-changing sensing method combines elementary capacitive pickup electrodes with periodic-sensing-array transducers. Feedback with magnetic force was also designed as well.The design of periodic-sensing-array transducers, elementary capacitive pickup electrodes, feedback coils and sealing ring leaded to a complex structure, which consisted of three metal layers, two insulator layers and silicon substrate. The entire fabrication process was composed of 125 steps. Each step was investigated individually at first. However, when combing together, there were numbers of problem caused by stability and compatibility between different fabrication technologies. For instance, to improve the stability of PSPI in high temperature while encapsulation, a new annealing process with gradient temperature heating method was proposed. To remove deep reactive ion etching residuals, a new process combining plasma ashing with organic rinse was employed. To resolve problem caused by stability of SiO2, Cr layers was used for protection. Finally, the device was successfully fabricated in our lab.In order to characterize the accelerometer, detect circuit, shielding fixture and tilt method system was employed for the calibration. The sensitivity of periodic-sensing-array transducers was about two orders of magnitude smaller than the design value. In addition, the output of elementary capacitive pickup electrodes present a poor linearity. After analysis and experiments, we believed that these drawbacks were caused by distance between electrodes, parasitic capacitance, and capacitance edge effect. In order to eliminate these drawbacks, the accelerometer was redesigned. A thick PSPI layer was used as dielectric to decrease the parasitic capacitance. An inverted device measurement method was used to reduce distance between electrodes and capacitance edge effect. The modified accelerometer achieved sensitivity of 55pF/g by periodic-sensing-array transducers and 0.4pF/g by elementary capacitive pickup electrodes, which presented almost the same value as design. The static noise floor of accelerometer was tested to be 86ng/VHz@0.1Hz and 25ng/VHz@1Hz. The device has endured a shock up to ±2.6g, and the full scale output appears around ±1.4g. A linearity response of elementary capacitive pickup electrodes was also obtained. And the feedback system was functioning as well.This thesis include design, manufactory, test, optimizing of silicon based flexure accelerometer with high precision and large dynamic range.