Tiny Machines
In 1959 the physicist Richard Feynman gave a talk called “There’s Plenty of Room at the Bottom,” on the possibility of microminiaturization. To encourage progress he offered a prize of $1,000 to anyone who could build an operating electric motor that fit into a 1/64th inch cube, and within months, someone had done it.
A MEMS crankshaft and gear, with a pollen grain, red blood cells, and a 50 micron line included for scale. Note that this distance is about the diameter of a human hair. (Courtesy Sandia National Laboratories, SUMMiTTM Technologies, www.mems.sandia.gov)
An electrostatic comb drive. The structure in-between the two comb assemblies, in the center of the image, restores the combs to their initial positions when the voltage is turned off. (Courtesy Sandia National Laboratories, SUMMiTTM Technologies, www.mems.sandia.gov)
A MEMS drive gear. The shafts extending up and to the left are driven by vibrating motors. (Courtesy Sandia National Laboratories, SUMMiTTM Technologies, www.mems.sandia.gov)
After that, not much happened until the mid-1980s, when physicists and engineers began to apply integrated circuit construction technology—batch processing, lithography, and etching—to produce both the mechanism and the associated electronics out of the same piece of silicon. Thanks to sacrificial layers that are etched away, the gears and rods are fabricated in place and ready to go, with no assembly required. Such devices are known as Micro-Electrical-Mechanical Systems, or simply MEMS. The image shows a MEMS gear and camshaft, with a pollen grain and red blood cells thrown in for scale. This width of this gear is about 50 microns (50 x 10-6 m), roughly a tenth the size of the motor that met Feynman’s challenge. (See Building at the Nanoscale).
MEMS motors are driven by electrostatic forces, so there is no need for winding tiny coils. Two batch-fabricated silicon “combs” form a capacitor, as shown in the diagram. When a voltage is applied to the combs, they attract, with a force that increases with the number and length of teeth in the comb. Mounting the combs on a restoring spring provides a back-and-forth motion as the voltage is switched on and off, and adding a crankshaft enables two combs, mounted at right angles, to spin a wheel, as shown.
MEMS products have been successfully commercialized since the early 1990s, including:
- small, inexpensive accelerometers for automobile air bags
- ink nozzles and reservoirs for inkjet printers
- heads for read-write drives
- arrays of tiltable mirrors for display
The relative size of MEMS-scale forces is quite different from what we experience in the macro-world. For these tiny machines, friction and surface tension are much more important than inertia or weight, since the ratio of surface area to volume varies inversely as the distance and therefore increases with smaller size. For example, for a cube of side s, the ratio of surface area to volume is 6s2/s3 = 6/s. As s decreases, this ratio goes up, making surface effects like friction more significant than inertia or gravity, which both scale with the volume. For the same reason, wear-and-tear from parts rubbing together is correspondingly more important, especially in devices like the comb motor with its back-and-forth movement.
