Micro pressure sensors are used to monitor and measure minute gas pressure in environments or engineering systems, e.g. automobile intake pressure to the engine.
They are among the first MEMS devices ever developed and produced for "real world" applications
Micro pressure sensors work on the principle of mechanical bending of thin silicon diaphragm by the contact air or gas pressure.
a) Back side pressurized b) Front side pressurized
The strains associated with the deformation of the diaphragm are measured by tiny "piezoresistors" placed in strategic locations on the diaphragm.
These tiny piezoresistors are made from doped silicon
They work on the similar principle as "fail strain gages" with much smaller sizes ( in $\mu$ m), but have much higher sensitivities and resolutions
Wheatstone bridge for signal transduction
$r_1$, $r_3$ = resistance induced by longitudinal and transverse stresses
$r_2$ , $r_4$ = reference resistors
fig shows Top view of silicon die.
The fabrication process of a typical pressure sensor relies mostly on steps standard to the IC industry, with the exception of the precise forming of the thin membrane using electrochemical etching.
An n-type epitaxial layer of silicon is grown on a p-type {100} wafer.
A thin, preferably stress-free insulating layer is deposited or grown on the front side of the wafer, and a protective silicon nitride film is deposited on the back side.
The piezoresistive sense elements are formed by locally doping the silicon p-type using the masked implantation of boron, followed by a high temperature diffusion cycle.
Etching of the insulator on the front side provides contact openings to the underlying piezoresistors.
A metal later, typically aluminium, is then sputter-deposited and patterned in the shape of electrical conductors and bond pads.
A square opening is patterned and etched in the silicon nitride layer on the back side.Double - sided lithography ensures, that the backside square is precisely aligned to the sense elements on the front side.
At this point, electrical contacts are made to the p - type substrate and n- type epitaxial layer, and the silicon is electrochemically etched from the back side in a solution of potassium hydroxide.
Naturally, the front side must be protected during the etch. The etch stops as soon as the p-type silicon is completely removed and the n - type layer is exposed.
The process forms a membrane with precise thickness defined by the epitaxial layer
ANodic bonding in vacuum of a glass wafer on the back side produces on absolute pressure sensor, which measures pressure on the front side in reference to the cavity pressure.
For differential or gauge type pressure sensors, precisely drilled holes in the glass wafer provide vent parts
fig (a) : Fabrication steps for piezoresistive gauge or bulk micromachined pressure sensor.
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