Development of a fully integrated micromachined piezoresistive accelerometer/vibration sensor with integral air damping for condition monitoring.

摘要

This research presents the design and development of a fully-packaged, micromachined, piezoresistive accelerometer. The complex sensor is fabricated from three crystalline (100) silicon wafers, which are processed and then permanently attached to one another using a novel silicon-silicon bonding procedure. The entire sensor design requires a minimum of ten photomasks. By carefully controlling the depth of the accelerometer's top and bottom cavities, near-critical squeeze-film air damping was obtained. The delicate moving parts of the fully-packaged sensor are located in a micromachined interior chamber, which is permanently sealed from the outside environment using an unique thermocompression wafer-level bonding technique. The new silicon-silicon bonding process utilizes a low temperature setting and does not require the assistance of any externally applied voltage potential. The microaccelerometry research culminated with the introduction of a sensor designed to operate in a low to medium G environment. The finished device possessed a typical output voltage sensitivity of.54 mV/G when operated in the constant voltage mode from a 5 volt supply. Its resonant frequency was measured to be 1.9 kHz. The accelerometer exhibited a TCR and TCS of.11%/{dollar}spcirc{dollar}C and {dollar}-{dollar}.20%/{dollar}spcirc{dollar}C, respectively. Its maximum unaltered cross-axis sensitivity was measured to be approximately 12% of its normal principle-axis sensitivity. It was demonstrated, however, that this value could be dramatically reduced to a value less than 3% by mounting the sensor at a 7{dollar}spcirc{dollar} angle to its horizontal. Valuable information was obtained with regard to the relationship between the physical dimensions of the accelerometer structure and such factors as sensitivity, resonant frequency, and damping. Additional experimental results were obtained regarding the piezoresistive coefficient of various doping profiles and its relationship with temperature. A theory was projected concerning ion implanted doping profiles and the effect of the boron segregation coefficient on its effective piezoresistive coefficient. Advanced features of microaccelerometer include: wafer level silicon-silicon bonding and alignment, integral squeeze-film air damping, built-in overrange protection, corner compensation, stress relief, low-G performance, light weight (20 mg), compact size (140 x 140 mils), digital offset nulling, and on-board circuitry for a companion VLSI signal conditioning IC.