List the types of pressure sensor and show the process steps for the fabricating the piezoresistive pressure sensor.

Based on the type of applications pressure sensors are used in. They can be categorized into many types. Most common types of pressure sensors that have been widely wed are :-

1. Strain Gauge type

2. Capacitive pressure sensor

3. Piezo electric pressure sensor

• 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 piezoresisters.

• A metal layer typically aluminium is then sputter deposited and patterned in the shape of electrical conductors and band pads.

• A Square opening is pattened and etched in the silicon nitride layer on the backside 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 epitoxial layer and the silicon is electro chemically etched from the back side in a solution of potassium hydroxide.

• Naturally the font side must be protected during the etch. The etch stop 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 water on the backside producer an a absolute pressure sensor which measure 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 provides vent parts.