Basic idea

• Flexible diaphragm serves as one electrode of a capacitor,
• Other electrode is located on a substrate beneath it,
• Average gap between the electrodes, and thus the capacitance, changes as the diaphragm deflects in response to applied pressure.

Capacitance between two parallel plates with surface area A, separated by a distance d by a dielectric with dielectric permittivity e, is given by:

Capacitance changes DC related to the electrode displacement Dd can be can be approximated as:

Relative change of capacitance DC/C is not linear with respect to deformation and thus the pressure

Relationship is reproducible and its full scale value can be 20 - 200 %.

To design capacitive sensors with high sensitivity, it would suffice to make the plate area large and the gap distance narrow.

Technological factors as membrane dimension, fabrication accuracy and reproducibility,
and damping of the membrane movement if the gap is filled with gas or liquid, limit these values.

Concepts of semiconductor pressure sensors

• Movable electrode - thin, square silicon bulk-micromachined membrane that is deflected
  by pressure applied to it