Capacitive sensors: When and how to use them

Abstract

Abstract Capacitive sensors for the detection of mechanical quantities all rely on a displacement measurement. The movement of a suspended electrode with respect to a fixed electrode establishes a changing capacitor value between the electrodes. This effect can be measured and if the mechanical quantity controls the movable electrode, a sensor is realized. Since the value of the capacitor is directly related to its size, and a small capacitor means high noise susceptibility, capacitive sensors should be as large as possible. Capacitive pressure sensors have been developed with success for industrial applications, where large membrane sizes are not a critical issue. However, most centres of expertise in silicon sensors show an interest in exploiting silicon technology to produce capacitive pressure sensors as well. From the above, this miniaturization trend appears to be an unsound idea. On the other hand, the principle of capacitive sensors allows the realization of measuring systems with so far unknown performance. Indeed, the capacitive sensor reveals distinct advantages when compared to its piezoresistive counterpart: high sensitivity, low power consumption, better temperature performance, less sensitive to drift, etc. Nevertheless, only a minor fraction of the market for pressure sensors is taken up by capacitive-type sensors. When observing the characteristics of capacitive sensors, it may seem surprising to encounter so few devices in real-world applications. The reasons for the lack in breakthrough can be found in the design complexity and the requirements for a matched sensing circuit. This paper will extensively discuss the justification of the choice for this research effort, and will elaborate on the techniques to fabricate the devices based on electronic manufacturing procedures. Basically, silicon capacitive sensors differ from piezosensors in that they measure the displacement of the membrane, and not its stress! This has important implications on the final assembled device: less package-induced problems can thus be expected. However, a far more important issue is their extremely high sensitivity, together with a low power consumption. These issues make them especially attractive in biomedical implant devices, or in other telemetry applications, where power is not randomly available. So far, this is the only field of success for these sensors. However, due to the interesting detection principle, new fields of application emerge, offering unique and superior performance when compared to available sensors. Uniaxial accelerometers are a good example, where extremely high cross-sensitivity reduction can be obtained.