Method for making an all-silicon capacitive pressure sensor

DETAILED DESCRIPTION It is an object of this invention to provide an improved method for fabricating an all-silicon monolithic capacitive pressure sensor.
It is another object of this invention that such a method entails a minimal number of processing steps to form a micromachined capacitive sensing element for the pressure sensor.
It is still another object of this invention that such a method yields a pressure sensor that operates within a range in which the sensing element is sufficiently deflected to contact a thin dielectric situated on a fixed capacitor plate, thereby yielding a sensor characterized by enhanced linearity and overpressure protection.
It is still another object that such a method is conducive to optimizing the size and geometry of the sensing element in order to promote its reliability and performance characteristics.
In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows.
The present invention provides an all-silicon monolithic capacitive pressure sensor that includes a base wafer and a diaphragm bonded to the base wafer.
The diaphragm overlies, but is electrically insulated from, a doped region in a cavity formed in the surface of the base wafer.
Both the diaphragm and the doped region are of the first electrical conductivity type, such that the diaphragm forms a flexible capacitor plate and the doped region forms a fixed capacitor plate of the pressure sensor.
According to the invention, the diaphragm forms a hermetic seal around the fixed capacitor plate, such that a gap between the diaphragm and the doped region is isolated from the ambient atmosphere surrounding the pressure sensor.
The gap has a depth of about two to seven micrometers, such that at least a portion of the operating range of the pressure sensor occurs while the diaphragm is contacting a thin dielectric situated on the doped region.
As a result, the capacitive output signal of the pressure sensor is produced by changes in contact area between the diaphragm and the thin dielectric in response to pressure applied to the diaphragm