Silicon Microsensors: Construction, Design and Performance

Sensors are devices which convert non-electronic variables to currents or voltages. They become microsensors when they are fabricated by modified integrated circuit techniques. Microsensors fall into two classes: replacements for existing devices with physically smaller, higher performance and lower cost structures; and novel sensors which can only be constructed by using micromechanics. These ideas can be illustrated by the results of a major effort in surface micromachined pressure transducers which started with the goal of producing designable, economicaly viable absolute pressure sensors with a scaling reduction factor of at least 100 over existing devices. Since an absolute pressure transducer consists of a vacuum sealed pill box which deforms with applied pressure and a sensing scheme which converts deformation with applied pressure to electronic signals, reliable pill box construction must be addressed first. Mechanical considerations in conjunction with the desired scaling lead to the conclusion that a pill box wall thickness of roughly 2 micrometers would be required. This implies the use of thin films and because of thermal consideration leads to the selection of polysilicon as the most attractive film for pill box construction. High packing densities, a requirement for novel sensors, leads to a pill box construction technique which uses surface micromachining with silicon dioxide as a sacrificial layer which is etched laterally as the technology of choice. This choice becomes defendable if the mechanical properties of polysilicon can be determined and if process design can be implemented to produce repeatable, predetermined built-in strain, Young's modulus, and tensile strength.