Unsticking MEMS

Exotic materials could combat the Casimir effect, a kind of quantum-mechanical stickiness

Researchers at Los Alamos National Laboratory, in New Mexico, think they may have the answer to a vexing problem called stiction, which causes ultrasmall components of microelectromechanical systems (MEMS) to stick together. This impediment to micromovement is caused by the Casimir effect (after the Dutch theoretical physicist Hendrik Casimir), an odd attractive force that influences only objects that are very close together. As MEMS components are shrunk to a scale of hundreds of nanometers or less, many engineers predict that the Casimir effect will become more of a problem.

”The Casimir force is the ultimate cause of friction in the nanoworld,” says Ulf Leonhardt, a theoretical physicist at the University of St. Andrews, in Scotland. ”Micro- or nanomachines could run smoother and with less or no friction at all if one can manipulate the Casimir force.”

To understand the Casimir effect, recall that a vacuum only seems to be empty space but is actually full of virtual particles and their antiparticle equivalents, which flit into existence and then annihilate one another so fast that they cannot be detected. In 1948, Casimir theorized that these fleeting particles would draw two uncharged metal plates together if the plates were placed very close to each other.

Virtual photons exert pressure as they bounce off the plates. Only photons of a wavelength shorter than the separation between the plates can form there. But in the region surrounding the plates, many more photons with longer wavelengths will be formed as well. The net effect is that there are more photons bouncing off the outside of the plates than between them, and the excess pressure pushes the plates closer together. As the plates grow closer, this effect grows stronger, because even fewer photons are able to form.

Now Felipe da Rosa, Diego Dalvit, and Peter Milonni of the theoretical division of Los Alamos National Laboratory are saying that the Casimir effect, which is normally attractive between two surfaces, could actually be made repulsive—and thus reduce stiction—if those surfaces had a layer of metallic-based metamaterials. Metamaterials are specifically engineered to have properties that do not occur naturally, such as the ability to bend light the wrong way. The method proposed by the Los Alamos team for generating this repulsive effect has yet to be tested experimentally, but experts say it is promising.

The Los Alamos theory is based on an idea of Timothy H. Boyer's, professor of physics at the City University of New York, who theorized in a 1975 paper that magnetic materials with special properties would turn the Casimir force on its head. That's because the virtual photons would induce electromagnetic fields in the plates. With the right material between the plates or perhaps coating them, the induced fields would push the plates apart so strongly that they would overwhelm the pressure of virtual photons squeezing the plates together. Recently, Harvard University professor Federico Capasso and his graduate student Jeremy Munday showed that ethanol between two gold plates would produce some of that effect, reducing the Casimir attraction by 80 percent. But for actual repulsion, simple ethanol won't do.

”You need a strong magnetic material, with unique magnetic effects,” explains Dalvit. ”There's no hope with standard, naturally existing materials. But with the new metamaterials, we've found that you can have the famous Casimir repulsion.”

Dalvit and his colleagues have performed detailed calculations on metamaterials and found that they should fight stiction. He says that experiments to prove it are already under way.