Example MEMS capacitive transducer

The plates of a capacitor are oppositely charged, so they exert an attractive electrostatic force on each other. The force per unit area of parallel capacitor plates separated by vacuum at bias voltage Vb is 

To provide a measure of residual stress (Tm, which is necessary to estimate pull-in voltage, the fabrication process usually involves deposition of a film of the device plate material, commonly polysilicon, onto a solid Si substrate. This structure can then be observed by means of wafer curvature techniques, for example, to infer a value of residual stress in the film layer. Among the system parameters that influence functional characteristics, the residual stress is the most difiicult to assess properly. On the other hand, control of its value provides an additional means for controlling device characteristics. 

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