'Nanotractor' studies micro-scale friction

Interest in the development of MEMS (microelectromechanical systems) has grown steadily during the past decade. These tiny devices, now used in such applications as auto airbag systems, inkjet printers, and display units, are attractive because they take up little space and require little or no assembly. They also are cheap to produce in batch quantities because they are made with a technology that is already mature -- the microlithography used to make silicon chips.

Arguably, MEMS could prove to be a ubiquitous "disruptive technology" that transforms a variety of engineering disciplines just as the silicon chip transformed electronics engineering. Yet the process of integrating MEMS into new systems has been slow. An overarching reason is that physics -- especially the effects of adhesion and friction -- sometimes works differently at the microscale.

Some assumptions seem intuitively correct because they apply to conventional machines. For example, Amontons' law, first stated 300 years ago, says friction is proportional to force applied normal to (perpendicular to) a surface. Yet relying on this assertion could prove disastrous when applied to devices the size of a pinhead.

At Sandia, where engineers are eager to pack increased capability into very small spaces, the development of tools to manipulate microdevices -- one example is a microtweezers -- has been in high gear for over a decade. More recently, research has turned to the use of nanotechnology as an "enabling technology" to make better MEMS devices.

Stiction

Friction or "stiction" -- the tendency of small parts to adhere to one another -- has proven a significant hindrance to MEMS development. Treating the touching or sliding surfaces of a micromachine with a slippery molecular monolayer can reduce friction. However, there remains the need for a tool to measure the friction between two MEMS surfaces accurately to determine exactly what conditions are most effective in reducing it. With this information in hand, engineers will gain an extremely important tool in modeling MEMS devices prior to their actual manufacture.

But creating a device small enough to measure friction on a MEMS device is no easy task, for the tool must be about the width of a human hair.

In order to study friction at the microscale, Sandia's Maarten de Boer and coworkers set about building a polysilicon actuator that would controllably and accurately generate both very low and very high forces, and apply them both perpendicularly and tangentially.

The resulting "nanotractor" design incorporates an actuation plate in its central section and frictional clamps on its two ends. In the clamps, load is applied electrostatically but borne mechanically to develop friction forces. To obtain motion, the leading clamp is fixed in place with a large voltage. The plate is then actuated by attracting it toward the substrate.

Because the actuation plate is now bending, the trailing clamp, which is not loaded, slides a short distance (about 40 nanometers) toward the leading clamp. The trailing clamp is now held fixed with a large voltage, and the voltages on the leading clamp and plate are turned off. The leading clamp then slips forward. This stepping cycle is applied repeatedly to obtain large-scale motion with very high precision.

De Boer and postdoctoral researcher Alex Corwin determined that this nanotractor operates at up to 80,000 cycles a second, with a velocity of up to 3 millimeters per second. A maximum force of 2.5 millinewtons is achieved when the nanotractor stalls out, about 250 times more force than a comb drive.

Defying the law

Working with Corwin, de Boer found that the coefficient of static friction began to increase at low normal loads (below 50 micronewtons). De Boer attributes this deviation from Amontons' law to adhesive forces. They also observed sliding of up to 200 nanometers before the static friction event. This means Amontons' law is also not valid over short sliding distances.

Besides serving as a test structure for model friction studies, the nanotractor actuator is attractive for other uses. Marc Polosky, a Sandia staff member, has demonstrated its use in a MEMS system that performs mechanical logic functions. It also may prove useful for the precise positioning and control of micro-optical elements.

"Modelers are excited by the nanotractor test results, and now face the challenge of understanding and modeling newly observed phenomena such as gross slip prior to sliding," says Sandia staff member Dave Reedy. "Our goal is to develop a capability to perform simulations of MEMS components that accurately predict response in the presence of adhesion and friction."

Technical Contact: Maarten de Boer mpdebo@sandia.gov, 505-844-9509

Media Contact: Michael Padilla mjpadil@sandia.gov, 505-284-5325