Background:
MEMS stands for micro electro mechanical systems. However, the term “MEMS” has turned into something of a catch-all, encompassing miniature devices that utilize a variety of operating principles, including electrostatic, thermal, magnetic, piezoelectric, fluidic, and optical. Furthermore, these MEMS devices are used in some very diverse applications. So it’s worthwhile to take a little time to delve into the history of MEMS development.
The origin of the field of MEMS (and nanotechnology in general) is often attributed to Richard Feynman’s 1959 talk, “There’s Plenty of Room at the Bottom”. Here, Feynman proposed, among other things, manipulation at a very small scale. He pointed out that in fact there are certain advantages (such as efficiency and speed) to be obtained by operating at the micro level.
It has been a long road from Feynman’s vision to successful commercial-ization of MEMS technology. Prototype MEMS devices have been demonstrated in laboratories since the 1970s, but it was not until Analog Devices introduced their MEMS accelerometers in the early 90s did MEMS technology become commercially important. Since then, there has been an explosion of both research and commercial interest in MEMS.
Fabrication:
MEMS devices vary widely in terms of operating principles and applications. What they have in common is in the way that they are fabricated; i.e. MEMS devices are built by micromachining.
Just as macro scale parts can be machined into the desired shape, MEMS devices are shaped by micromachining. Micromachining techniques are usually classed as either bulk micromachining or surface micromachining. Bulk micromachining is generally used for simpler structures – basically, starting from a substrate of a given material (e.g. silicon), the unwanted parts are etched away to leave the desired structure. This is done for pressure sensors, for example. More complex structures are also possible with the use of wafer bonding; i.e. stacking the face of a bulk micromachined wafer on top of the face of another wafer.
While bulk micromachining builds MEMS devices out of the substrate, surface micromachining builds devices using thin films on top of the substrate. This has several implications – 1) the substrate could be anything (e.g. glass) since it’s properties won’t matter as much; 2) the MEMS device can be much more complex, involving a different combination of materials, released layers, etc; and 3), importantly, surface micromachining is much more compatible to semiconductor fabrication processes used in integrated circuit (IC) technology. This means that surface micromachined MEMS can be integrated with standard logic ICs, potentially benefiting from Moore’s Law just as computer chips do.
Application areas:
- Sensors – accelerometers, pressure sensors, gyroscopes
- Microfluidics – e.g. inkjets
- RF MEMS
- Optical MEMS, or MOEMS
- Bio-MEMS
- Micropower generation
- Data storage
- Mechanical computing
Sensors are by far the most successful MEMS devices. Almost every modern automobile comes with accelerometers for air bag deployment, pressure sensors for the tires, and possibly a gyroscope for detecting yaw. Accelerometers have also found their way into mobile phones and other consumer devices, in order to automatically rotate the screen when the device is tilted for example.
RF MEMS switches are attractive because of their high linearity, low loss, good isolation, and low power consumption. Similarly, RF MEMS resonators can achieve very high Q (quality factor) at high frequencies. Commercial take up is still low but is expected to increase in the coming years. Already, RF MEMS clock reference oscillators (replacing the venerable quartz oscillator) have achieved some success in the commercial market.
Optical MEMS are generally micro mirrors, used for optical switching and for displays. Texas Instruments (TI) has recently made a big push for their MEMS-based Digital Light Processing (DLP) technology as a replacement for plasma and LCDs in big screen TVs.
Bio-MEMS are basically MEMS for biological applications, and it’s a very hot research area right now. Bio-MEMS primarily use microfluidic principles in order to manipulate cells. The end goal generally is the lab-on-a-chip, a mass-produced consumer level device that has the potential to drastically cut down the costs and response times of medical tests.
The other areas mentioned above are mostly still in development. I will probably blog some more about these emerging technologies and the possible directions that they may go into.
Sources:
- Stephen Senturia. Microsystems Design. Springer 2000.
- Gabriel Rebeiz. RF MEMS Theory, Design, and Technology. Wiley 2003.
- Vijay Varadan, et al. RF MEMS and their Applications. Wiley 2003.
- Wikipedia. http://en.wikipedia.org/wiki/MEMS . Accessed on 9 Dec 2008.
Microenergy MEMS blog by Kerwin Khu is licensed under a Creative Commons Attribution-Share Alike 3.0 License.
