Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

• 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.