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How MEMS pressure sensor converts pressure into its equivalent electrical parameter, explain with its schematics representation and fabrication process steps.

Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

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• Principle of operation: A pressure sensor has a semiconductor distortion gauge formed on the surface of the diaphragm and it converts changes in electrical resistance into an electrical signal by means of the piezo resistance effect that occurs when the diaphragm is distorted due to an external force (pressure)

• Schematic Diagram and Explanation:

1) The basic structure consists of four sense elements in a wheatstone bridge configuration that measures stress within a thin crystalline silicon membrane.

2) The stress is a direct consequence of the membrane deflecting in response to an applied pressure differential across the front and back sides of sensor.

3) Stress is linearly proportional to the applied pressure membrane deflection is less than one $\mu m$

4) Output normalized to input applied pressure is known as sensitivity.

- Fabrication process:

1) An n-type epitaxial layer of silicon is grown on p-type {100} wafer. A thin preferably stress free insulating layer is deposited or grown on the front side of the wafer and protective silicon nitrite film is deposited on back side as shown in fig 1.

Fig (1) Deposite Insulator.

Fig (2) diffuse piezoresistors.

Fig (3) Deposite, pattern metal And electro chemical etch Of back side cavity.

Fig (4) Anodic bond of glass.

2) Piezoresistive sensors are formed by locally doping the Si, P-type using the boron implantation followed by high temperature diffusion cycle as shown in fig (2).

3) Etching of the insulator on the front side provides contact openings to underlined piezoresistors. A metal layer (Al) Is sputter deposited and patterned in the shape of electrical conductor and bond pads.