Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

• The fabrication process of a typical pressure sensor is done using electrochemical etching (ECE).
• 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 layer, typically aluminum, 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. One practical protection method includes coating with wax such as paraffin and clamping in a fixture.
• 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 Pyrex glass wafer on the back side produces an absolute pressure sensor that measures the pressure on the front side .