Shown on this slide is an exploded view of the MEMS sensor that ST uses in their microphones. There is a silicon base, an isolation layer between the membrane and the base, the flexible membrane that forms one side of the capacitor, and a fixed plate that forms the other side of the capacitor. On top of all this are pads that allow electrical contact to both the fixed plate and to the membrane. The acoustic sensor in ST’s MEMS microphone uses a fixed back plate and a moveable diaphragm to convert sound waves into an electrical signal. The diaphragm and back plate are attached to the silicon base which is etched to create a back chamber. Differences in pressure between the outside of the microphone and the back chamber cause the diaphragm to move, which in turn causes a change in capacitance between the diaphragm and the back plate.