Direct integration (DI) of solid state stress sensors with single crystal micro-electro-mechanical systems for integrated displacement sensing

Abstract

MEMS with planar degree of freedom show grate promise as they allow devices with variety of abilities. Still sensing displacements in the plane of the wafer is a complicated task as the sidewalls of the structures are not visible to the photolithography process. This paper introduces one solution to this disadvantage by Direct Integration (DI) of stress sensors made of pn diodes and MOS transistors with MEMS made of micro-beams with planar degree of freedom.Micro-beams with typical cross section of 2 pm x 20 pm are fabricated from single crystal silicon using SCREAM [l] (Single Crystal Reactive Ion Etching and Metalization), process. Sensors are integrated the support of the beams close to their roots. A finite element analysis shows that deformation of the beams induces stress that extends into the support of the beams. By modulating the band gap energy this stress affects the I-V transfer function of the sensor located at the support. This band gap modulation results in modulating secondary properties of solid state devices such as the intrinsic concentration, the charge carrier concentration, the built-in p n junction potential, the junction width, and the mobility of holes and electrons through the piezoresistance tensor.In this paper the DI concept is demonstrated by implementing planar accelerometers and flow sensors that are based on integration with pn diodes and with nMOS transistors. Accelerometers with sensitivity as high as 326 mv/g and flow sensor with amplitude and frequency sensitivities as high as 58 mv/ml/s and 250 Hdml/s are demonstrate.