TECHNOLOGY FOR CHANGE

MIT Harnesses 'Void' to Nix Friction in MEMS

R. Colin Johnson | Date: 05-21-10 | Comments

Accelerometers, gyroscopes and other micro-electro-mechanical system (MEMS) chips must overcome forces of friction that are disproportionate to their size. Now, Massachusetts Institute of Technology (MIT) scientists hope to reverse that trend by harnessing a power--called the Casimir force--that only manifests at the nanoscale to sidestep friction by causing parts to naturally repel, rather than attract, each other.

MEMS chips like accelerometers and gyros have tiny moving parts inside that perform similar functions as their full-size counterparts, but in a form factor that can be mass-produced on semiconductor fabrication lines at a fraction of the cost. Unfortunately, as mechanical parts are shrunk to the micro- or even nanometer scale, the effects of friction become disproportionately large. Most difficult is starting a MEMS from a dead stop, since it must overcome "stiction" (a word that is a combination of "stick" and "friction"), which measures the force that must be overcome to get a MEMS part moving.

But what if MEMS parts could be architected so that they repel each other at startup, instead of attract? Then friction and its diminutive sibling, stiction, could be easily overcome, making MEMS parts draw less power and be more reliable and longer lived. Now scientists at MIT claim to have demonstrated how naturally repelling architectures can be crafted, by harnessing a force that only manifests at the nanoscale: the Casimir force.

Dutch scientist Hendrik Casimir hypothesized the existence of a nanoscale force driven by the quantum uncertainty principle applied to empty space. Quantum mechanics teaches that empty space is not really empty�the "void" is only statistical because particles are constantly popping into and out of existence even in empty space. Casimir speculated that if parallel plates were placed so close together that most of these particles could not fit between them, then the particles would pile up outside the plates, thereby exerting a force to push the plates together. Now MIT scientists have found a way to harness this effect, dubbed the Casimir force, as a free source of energy that nixes friction in tiny mechanical devices.

New computational techniques developed at MIT confirm that the complex quantum effects known as Casimir forces would cause tiny objects with the shapes shown here to repel, rather than attract, each other.

The researchers proved their technique works by designing a prototype consisting of an ellipsoid plunger that gets inserted into a complementary hole in a flat plate. The shapes of the hole and the plunger ensure that the Casimir force is balanced until the plunger is moved, at which point the Casimir force causes the parts to repel, thus overcoming the forces of stiction.

Though negligible at larger scales, Casimir forces can cause the moving parts of micromachines, like the one shown here, to stick together.

The MIT researchers also showed how well-known calculations that predict the strength of an electromagnetic field between two objects can be used to calculate the amount and direction of the Casimir force as it applies to nanoscale moving parts. Thus empowered, MEMS designers should be able to employ the Casimir force to counter friction in all sorts of new MEMS chips.

Funding was provided by the Army Research Office, the MIT Ferry Fund and the Department of Energy (DoE).