Silicon that moves-towards the smart sensor

The author describes an electronic accelerometer based on capacitive measurement. It is based on the simple parallel-plate capacitor: one plate is fixed, while the other moves relative to it under acceleration, which causes the capacitance to change. One example of this approach is the Analog Devices ADXL-50, manufactured by surface micromachining. The ADXL 50 consists of a variable differential air capacitor whose plates are etched from a single 2 mu m piece of silicon. The fixed plates are simple cantilever beams, supported 1 mu m above the chip in free space by polysilicon anchors that form a molecular bond with the chip. The thicker central mass is free to move in a plane perpendicular to the tethers. A series of regular fine elements project from the central mass, each acting as a parallel-plate capacitor. As capacitance is inversely proportional to the distance between the plates, the device has been designed to have the finest gap possible using photolithographic machines, to give the largest possible signal. The operation of the device is briefly discussed.<>