| High-performance microaccelerometers are needed for position sensing, navigation/guidance, microgravity measurement, tilt control, and platform stabilization. For high-precision measurements, the device should have high sensitivity, low temperature sensitivity, and high long-term bias stability. The interface circuit should have high dc response, low noise, low offset, and high gain stability. This dissertation reports a high-performance micromachined all-silicon accelerometer capable of resolving micro-g levels of acceleration.; The sensor structure, combining the advantages of surface and bulk micromachining, is fabricated from a single silicon wafer. It utilizes the full wafer thickness to attain a large proof mass with integrated trench-refilled polysilicon electrodes above and below the mass. The electrodes are stiff in the sense direction and the air-gap, which is defined by sacrificial oxide, is narrow and well defined. An optimized number of holes are formed through the electrodes to maximize the device performance at atmospheric pressures without any need for low-pressure packaging. Symmetric suspension beams are utilized to provide low cross-axis sensitivity. The accelerometer design was thoroughly simulated and verified experimentally.; N. Yazdi developed the first generation accelerometers at the University of Michigan. However, several fabrication issues critical for the reliable operation of high-performance accelerometers were left unanswered. In order to overcome the shortcomings of the previous design, an improved device structure, design, and fabrication process have been developed. The seven mask, batch fabrication process is simple, reliable, reproducible, and provides a yield of >80%. The trench-refilled polysilicon electrodes are an essential component of the single-wafer technology and a new process was developed to obtain stress-free electrodes. The as-deposited polysilicon at 580°C is annealed in situ for 2hrs. at 625°C. The in situ anneal recrystallizes the amorphous or semi-amorphous polysilicon film, allowing the formation of polysilicon with tensile stress ranging from low to high. Subsequent thermal steps help relieve the stress further, resulting in stress-free electrodes. This fabrication technology allows, for the first time, the fabrication of multi axis micro-g accelerometers on a single die. A novel corner-compensation technique was also developed and utilized in the device fabrication. The angle of the suspension beams was designed to optimize shock resistance.; The device, with a 2mm x 1mm x 0.475mm proof-mass, has a measured sensitivity of ∼1pF/g - ∼1.68pF/g. The temperature coefficients of offset and sensitivity for the packaged device were measured to be 3000ppm/°C and −3960ppm/C. These values are much higher than expected (the intrinsic TCO of the device structure is ∼5ppm/°C), and this is attributed to the mismatch of thermal expansion coefficients of the device structure and its packaging. An improved packaging and assembly scheme is expected to significantly improve the overall performance.; The hybrid subsystem, including both the sensor and its interface circuit, is assembled on a PC board and mounted inside a standard 24-pin DIP package. The measured open-loop sensitivity for the hybrid subsystem is 370mV/g, indicating for the first time, that the current system is capable of resolving about 20μg/√Hz. It is worth noting that the calculated thermo-mechanical noise floor of the device structure at atmospheric pressures is 0.8μg/√Hz. Therefore, the overall resolution of the system can be improved by using better low-noise interface circuits. The hybrid subsystem has been shown for the first time to be capable of withstanding at least 1900g. The results obtained are promising for high resolution and high sensitivity accelerometer systems. |
