Casimir-force research holds promise for reducing stiction in MEMS devices


Experimental results match theoretical prediction

Engineers are hoping to exploit the Casimir force, which, theory holds, should be repulsive between objects of certain shapes, a phenomenon that could prevent stiction in moveable microstructures such as in MEMS devices. Researchers are still trying to cancel the unwanted effects of the force that that draws two parallel conducting plates together when a few dozen nm apart. Casimir-force experiments are hard to do, however. Nobody has perfected the technology to position different objects accurately with a nm-scale gap. Microscopic objects tend to warp and bend; corrugations on a flat surface can change the Casimir force between them and even its direction, making experimental results hard to interpret. Now University of Florida researchers Jie Zou and colleagues working with Professor Ho-bun Chan have carved a single device out of silicon capable of measuring the Casimir force between a pair of parallel silicon beams—the first on-chip device capable of doing this. Consisting of one fixed beam the device has another moveable beam attached to an electromechanical actuator. The team starts by measuring the separation between them using a SEM (scanning electron microscope). They then apply a voltage to the actuator, which pushes the movable beam towards the fixed beam. The beams oscillate at a natural frequency, which is easy to measure. The natural frequency depends on the forces on the beams, so as the beams move closer together, the Casimir force changes as does the oscillation frequency, allowing the force to be measured. Other forces are also involved, as residual electrostatic forces. When these are taken into account, results match the theoretical predictions for the Casimir force that beams of this shape should generate. The device solves several problems. Both silicon beams are made in the same lithographic step, so unwanted distortions are not significant. The positioning is easier to control since beams and actuator, all part of the same device, need less calibrating and alignment. Measurements are more straightforward on a single chip. "This scheme opens the possibility of tailoring the Casimir force using lithographically de?ned components of non-conventional shapes," say Zou. Casimir forces accordingly should be exploitable by the next generation of microelectromechanical devices for items such as stictionless bearings, springs, and actuators. University of Florida Department of Physics