ENGINEERING: MEMS
Physics from another world
Getting microscopic mechanical devices to work in a world where the laws of physics have completely different effects, and then placing them on a piece of silicon, requires a broad range of software. Paul Schreier puts this emerging market under the microscope
MEMS have gone through several major
stages of adoption. The first was the use of MEMS in automobiles in accelerometers for airbags, then using sensors to measure tyre pressure and driver presence. Next came consumer devices including
MEMS devices such as inkjet printer heads, silicon microphones for cell phones, gyroscopes for game controllers and digital micromirrors for video projection. According to iSuppli, applications in consumer electronic devices and mobile handsets ‘…bulldozed their way through the economic crisis, in the process cementing their status as the new locomotive for MEMS.’ During the next decade, a third wave
will see the adoption of MEMS motion sensors in many medical devices, industrial instrumentation, defence applications, robotics and machine health monitoring. ‘Once you have a new enabling technology that can determine orientation and position, people realise how to use it across a large spectrum of applications,’ elaborates Michael Judy, a fellow at Analog Devices, one of the largest suppliers of MEMS devices.
Because of the multidiscipline nature of MEMS devices, it’s necessary to have close links between device creation/simulation software and tools for system design and close links into IC fabrication programs. (Diagram courtesy of Coventor)
whether in automobiles or, more lately, in consumer devices. How does your iPhone know when to switch the image from portrait to landscape? How does the Wii MotionPlus handheld accessory replicate every twist of the wrist or turn of the body on the TV screen? They rely on extremely tiny versions of familiar electrical and
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ven if you don’t know what a MEMS (micro-electromechanical system) device is, the chances are that you’ve already used one,
mechanical devices such as beams, valves, pressure sensors, hinged mirrors and gears, all of which are implemented on a tiny chip. While the human hair might be 100 microns thick, a MEMS feature, such as a beam, can be just a single micron thick. MEMS devices have penetrated so many applications that the numbers of devices are staggering. According to market-research company iSuppli Corp, 4.14bn units will ship this year, with revenue projected at $US 6.54bn.
SCIENTIFIC COMPUTING WORLD OCTOBER/NOVEMBER 2010
When gravity hardly matters Techniques for designing systems at the micro world are, in many cases, no longer applicable at the micro level. As a simple example consider that gravity, as a volumetric force, scales with the cube of the dimensions whereas surface forces scale with the square. If you reduce a body’s dimensions by 1,000 times, the gravitational force drops by 10-9 electrostatic forces drop by 10-6
, whereas . Thus, in
many MEMS devices, you can essentially neglect the weight of components. Another problem becomes adhesion (or stiction, the combination of sticking and friction), often due to electrostatic attraction or capillary action. For structures with thicknesses of a few tenths to several micrometers and
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