| MEMS technology focuses on producing small-scale devices in a low-cost batch fabrication of silicon micromachined structures. As the devices shrink in size, the sensor's sensitivity decreases. Making highly-sensitive small-scale sensors, and in particular, accelerometers, poses a great technology challenge. As the accelerometer is scaled down, its fundamental thermal noise increases, thus degrading its sensitivity. In addition, the area of the sensing capacitors decreases, thereby reducing the sensor's nominal capacitance and the device's responsivity and sensitivity. Input parasitic capacitance and interface capacitance limit sensor resolution. A highly-sensitive accelerometer is designed and implemented on an SOI wafer and integrated with a high-resolution capacitive measurement system. The accelerometer's measured sensitivity exceeds the sensitivity of currently available commercial devices. The major contribution of this work is a method developed to get a higher sensitivity from the device by controlling the squeeze film damping in high aspect ratio devices. A unique comb-drive design is implemented which introduces the ability to control of squeeze film damping. Squeeze film damping control allows for the design for a desired quality factor and, therefore, improves sensitivity or flattens the device's response for open-loop operation. Deep-etch RIE (DRIE) technology is used to fabricate the transducer. The fabrication process developed provides large-device scalability and an improved dynamic range. A dual-axis accelerometer is designed with a single proof-mass which improves mode matching and saves silicon area. A novel high-resolution capacitive measurement circuit is implemented in CMOS and integrated with the transducer. The circuit employs a feedback bridge to cancel supply drift and provide high stability and a wide temperature range. An active charge control feedback loop drastically reduces charge injection at the capacitive sensor's common electrode. The charge control feedback reduces the voltage error by more than two orders of magnitude and, therefore, drastically improves the sensor's resolution. |
