TABLE OF CONTENTS The roots of silicon micromachining technology date back to Bell Laboratories. The research team developing the basics of semiconductor technology discovered a piezoresistive effect in silicon and germanium. The piezoresistive effect creates a resistance change in the semiconductor material in response to stress. This change was approximately two orders of magnitude larger than the equivalent resistance change of metals (used previously for strain gauge applications), promising an attractive option for sensors. The high sensitivity, or gauge factor, is perhaps 100 times that of wire strain gauges. Piezoresistors are implanted into a homogeneous single crystalline silicon medium. The implanted resistors thus are integrated into a silicon force sensing member. Typically, other types of strain gauges are bonded to force sensing members of dissimilar material, resulting in thermoelastic strain and complex fabrication processes. Most strain gauges are inherently unstable owing to degradation of the bond, as well as temperature sensitivity and hysteresis caused by the thermoelastic strain. Silicon is an ideal material for receiving the applied force because it is a perfect crystal and does not become permanently stretched. After being strained, it returns to the original shape. Silicon wafers are better than metal for pressure sensing diaphragms, as silicon has extremely good elasticity within its operating range. Silicon diaphragms normally fail only by rupturing.
The most popular silicon pressure sensors are piezoresistive bridges that produce a differential output voltage in response to pressure applied to...
