MEMS

MEMS stands for "Micro-Electro-Mechanical-System". "Micro" implies that the scale of MEMS structure is very tiny. They range in size from a few microns to a few millimeters. With semiconductor fabrication technology which is the finest manufacturing process, miniature microscopic components such as sensors, valves, pumps and actuators have been developed. Mostly these devices are fabricated from silicon and other materials, because its technology is based on the semiconductor fabrication process. However, unlike semiconductor chips, MEMS is able to perform mechanical or optical functions in addition to electrical functions. As a result, MEMS devices works as inkjet printer heads, automotive air bag sensors, high resolution digital image projectors, pressure sensors, strain gauges, biosensors, and "lab-on-chip" devices.

Related MEMS sites:

Univerity Research Center

1. Wireless Integrated MEMS System WIMS
2. University of Michigan Solid State Electronics Laboratory
3. Alabama Microelectronics Science and Technology Center AMSTC
4. Berkeley Sensor & Actuator Center BSAC
5. MIT Lincoln Laboratory MIT
6. Stanford Nanofabrication Facility SNF
7. Cornell Nanofabrication Facility CNF
8. ECE Department at the University of California at Santa Barbara
9. Helsinki University of Technology Circuit Theory Laboratory
10. Microelectronics Lab at University of Illinois at Urbana-Champaign
11. The U.C. Berkeley Microfabrication Laboratory (Microlab) Home Page
12. UCSD High Speed Devices Group Home Page
13. University of Louisville MicroTechnology Center
14. Thin and Thick film Sensor Lab, University of Pune

Institues & Companies

1. Sandia National Laboratories SNL
2. Smalltimes - Big News in Small Tech
3. Solid State Technology Group at INESC
4. SPIE - The International Society for Optical Engineering
5. The MEMS Exchange
6. National Institutes of Health (NIH)

    

MEMS Accelerometer

In this project, a capacitive type accelerometer was used in order to detect the movement of human hand on MOUSE system. The selected capacitive accelerometer has several benefits compared with the peizoresistive accelerometer. In general, the capacitive sensing method allows less power consumption than other methods because capacitance changes are used as sensing element instead of current or voltage. Also it is insensitive to temperature change except the space between two capacitive electrodes is subject to the thermal expansion coefficient, however, which is about two orders of magnitude less than piezoresistive material. Additionally capacitive sensing allows for response to DC accelerations as well as dynamic vibration.

Based on the fact above, a capacitive interdigitated silicon proof mass was fabricated in order to increase the shunt capacitance. Upon the silicon substrate wafer, a proof mass which is suspended by silicon bridges and interdigitated electrodes are symmetrically located on each side. The air-gap variable capacitors was created with the average distance between the mass and electodes. As the gap decreases with acceleration, the movement increases the capacitance for that plate linearly within small variation, while the distance to the other side increases, decreasing its capacitance. This basic design leads approximately 2.1 mm long by 2.4 mm wide with wafer thickness 500 um. The sensitivity is determined by the mass of the sense element, the distance from the center of mass to the torsion bar axis, and the bridge stiffness which is expressed by spring contant k. Mechanical stops can be added to prevent from stiction or overmovement.

The interdigitated accelerometer increases the total capacitance and its change per unit accleration. The more the shunt capacitance, the larger capacitance changes and it results in wider dynamic range and higer sensitivity with 2.1mm and 2.4 mm sizes. The minimum detectable acceleration was 5.3mg.

Other references & tutorials 

1. Constantine, Friedman and Goldberg MIT (pdf) -a very good tutorial for acclerometer
2. MEMS Market Expectation from Electronic Business