New 'nano-positioners' may have atomic-scale precision

WEST LAFAYETTE, Ind. - Engineers have created a tiny motorizedpositioning device that has twice the dexterity of similar devices beingdeveloped for applications that include biological sensors and morecompact, powerful computer hard drives.

The device, called a monolithic comb drive, might be used as a"nanoscale manipulator" that precisely moves or senses movement andforces. The devices also can be used in watery environments for probingbiological molecules, said Jason Vaughn Clark, an assistant professor ofelectrical and computer engineering and mechanical engineering, whocreated the design.

The monolithic comb drives could make it possible to improve a class ofprobe-based sensors that detect viruses and biological molecules. Thesensors detect objects using two different components: A probe is movedwhile at the same time the platform holding the specimen is positioned.The new technology would replace both components with a single one - themonolithic comb drive.

The innovation could allow sensors to work faster and at higherresolution and would be small enough to fit on a microchip. The higherresolution might be used to design future computer hard drives capableof high-density data storage and retrieval. Another possible use mightbe to fabricate or assemble miniature micro and nanoscale machines.

Research findings were detailed in a technical paper presented in Julyduring the University Government Industry Micro/Nano Symposium inLouisville. The work is based at the Birck Nanotechnology Center atPurdue's Discovery Park.

Conventional comb drives have a pair of comblike sections with"interdigitated fingers," meaning they mesh together. These meshingfingers are drawn toward each other when a voltage is applied. Theapplied voltage causes the fingers on one comb to become positivelycharged and the fingers on the other comb to become negatively charged,inducing an attraction between the oppositely charged fingers. If thevoltage is removed, the spring-loaded comb sections return to theiroriginal position.

By comparison, the new monolithic device has a single structure with twoperpendicular comb drives.

Clark calls the device monolithic because it contains comb drivecomponents that are not mechanically and electrically separate.Conventional comb drives are structurally "decoupled" to keep oppositecharges separated.

"Comb drives represent an advantage over other technologies," Clarksaid. "In contrast to piezoelectric actuators that typically deflect, ormove, a fraction of a micrometer, comb drives can deflect tens tohundreds of micrometers. And unlike conventional comb drives, which onlymove in one direction, our new device can move in two directions - leftto right, forward and backward - an advance that could really open upthe door for many applications."

Clark also has invented a way to determine the precise deflection andforce of such microdevices while reducing heat-induced vibrations thatcould interfere with measurements.

Current probe-based biological sensors have a resolution of about 20nanometers.

"Twenty nanometers is about the size of 200 atoms, so if you arescanning for a particular molecule, it may be hard to find," Clark said."With our design, the higher atomic-scale resolution should make iteasier to find."

Properly using such devices requires engineers to know precisely howmuch force is being applied to comb drive sensors and how far they aremoving. The new design is based on a technology created by Clark calledelectro micro metrology, which enables engineers to determine theprecise displacement and force that's being applied to, or by, a combdrive. The Purdue researcher is able to measure this force by comparingchanges in electrical properties such as capacitance or voltage.

Clark used computational methods called nodal analysis and finiteelement analysis to design, model and simulate the monolithic comb drives.

The research paper describes how the monolithic comb drive works whenvoltage is applied. The results show independent left-right andforward-backward movement as functions of applied voltage in color-codedgraphics.

The findings are an extension of research to create an ultra-precisemeasuring system for devices having features on the size scale ofnanometers, or billionths of a meter. Clark has led research to createdevices that "self-calibrate," meaning they are able to preciselymeasure themselves. Such measuring methods and standards are needed tobetter understand and exploit nanometer-scale devices.

The size of the entire device is less than one millimeter, or athousandth of a meter. The smallest feature size is about threemicrometers, roughly one-thirtieth as wide as a human hair.

"You can make them smaller, though," Clark said. "This is a proof ofconcept. The technology I'm developing should allow researchers topractically and efficiently extract dozens of geometric and materialproperties of their microdevices just by electronically probing changesin capacitance or voltage."

In addition to finite element analysis, Clark used a simulation toolthat he developed called Sugar.

"Sugar is fast and allows me to easily try out many design ideas," hesaid. "After I narrow down to a particular design, I then use finiteelement analysis for fine-tuning. Finite element analysis is slow, butit is able to model subtle physical phenomena that Sugar doesn't do aswell."

Source: Purdue University

This illustration depicts a tiny device called a monolithic comb drive, which might be used as a high-precision "nanopositioner" for such uses as biological sensors, computer hard drives and other possible applications. The device was created by Jason Vaughn Clark, an assistant professor of electrical and computer engineering and mechanical engineering at Purdue University.

(Photo Credit: Birck Nanotechnology Center, Purdue University)