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MECH 466Microelectromechanical Systems
University of VictoriaDept. of Mechanical Engineering
Operate on a sensor principle whereby an electrical resistor will change its resistance when it is subjected to a strain (deformation).
Piezoresistive sensors are used as part of many MEMS devices, including:
-Pressure sensors
-Accelerometers
-Tactile sensors
-Flow Sensors
In other words, these “Piezoresistive-based” applications are sensitive to phenomena that cause beams or thin plates to deform, and this deformation can be measured by resistance change.
MEMS pressure sensor consist of a ‘thin plate’ of silicon with a pressure differential across the plate. The resulting deformation causes strain along the edges of the plate.
With careful design, regions along the edge of the plate can be ‘doped’ to create resistors, which will subsquently exhibit resistance change in proportion to the applied strain.
thin diaphragm
thin diaphragm
thindiaphragm
strainsensors
Diagram of Piezoresistive-based Pressure Sensor [Chang Liu]
For a 3-D solid mass, more general relationships exist.
The resistivity ρij can be expressed as:
Note: the reference for this material is available from the book: [M-H Bao, 2000, Micro mechanical transducers Handbook of Sensors and Actuators vol 8, Editor: S Middelhoek (Amsterdam: Elsevier) chapter 5]
For the case of single crystal silicon, where the x, y and z axes line up with the <100> family of directions, we have the relation:
where values for π can be found in Table 6.1 of the textbook.
Note: for other directions, such as <110>, or <111>, transformation matrices are required as described in the work: [M-H Bao, 2000, Micro mechanical transducers Handbook of Sensors and Actuators vol 8, Editor: S Middelhoek (Amsterdam: Elsevier) chapter 5]
A longitudinal resistor has been fabricated in silicon crystal by doping, and lies in the <111> direction. The piezoresistor is n-type, with a doping concentration of 11.7 Ωcm.
Question: Find the longitudinal guage factor of the resistor, using Table 6.1 and Table 6.2 in the course textbook.
The following designs can be considered as Poor Designs:
(C)
- Sensor Design (in this case size) changes the physical properties of the beam
(D)
Very ThickDeposited Material
Very Deep Doping -Sensor Location (in this case sensitive region is located on both sides of the neutral axis) makes for poor sensitivity, since the tensile and compressive strain-resistances cancel each other out.