| Over the last three decades, a large variety of pressure sensors have been developed using both bulk and surface micromachining techniques. The increasing need for more pressure sensors in a wide variety of applications always challenges the researchers to develop better techniques to improve the sensitivity, reliability, yield and yet reduce the cost.; Today, there are two major types of micromachined pressure sensors: piezoresistive and capacitive. However, surface micromachined differential capacitive pressure sensors have not been successfully commercialized because of a major problem: susceptibility to contamination in the gap between the two sensing plates. During the last decade, several micromachined differential capacitive pressure sensors with an open gap have been presented. The micron-sized gap is small and can be easily obstructed by dust particles. The contamination problem is eliminated if the capacitor gap is sealed. Yet, sealing the gap makes the plate deflection sensitive to changes in common-mode pressure. As a result, only structures that contain an isolated gap, sensing absolute pressure changes have been commercialized successfully. Bay et al. suggested that by rigidly joining the external capacitor plates and the incorporation of a third internal stationary electrode the common-mode pressure sensitivity could be eliminated. In this thesis, we examined two contamination-insensitive differential capacitive pressure sensors with a sealed gap. The first device is made of three polysilicon layers including a stationary middle plate and the top and bottom plates that are rigidly coupled together by a series of posts hence deflecting in tandem by a change in differential pressure. Because of the posts and the sealed gap, however, the linked-plate device is insensitive to common-mode pressure and contamination.; The second device consists of two thinner diaphragms for sensing absolute pressure changes on both front side and backside, and a rigid thicker fixed plate on the front. It is a combination of two absolute pressure sensors: the device on the front is sensing the front side absolute pressure while the other is sensing the backside absolute pressure. Thus, the difference of these two absolute pressures gives the differential pressure.; Two different solutions for eliminating contamination in differential capacitive pressure sensors are presented in this thesis. The basic properties of linked-plate structure devices, characteristics of absolute capacitive pressure sensors, various material, process design and fabrication issues are discussed and the contamination-insensitive devices are fabricated and tested. |
