Engineers Point Way To Better Use Of Nanotubes As Measuring Tips

Findings will appear in the November issue of the journal Nanotechnology.

Researchersattach the tubes to the ends of imaging instruments called atomic forcemicroscopes. Because the tubes are long and slender, their shape isideal for the emerging field of "nanometrology," which is preciselymeasuring structures on the scale of nanometers, or billionths of ameter.

Conventional silicon tips used on the microscopes areshaped like inverted traffic cones. They are fine for measuringrelatively flat surfaces, but they do not readily penetrate crevicesthat often exist in tiny devices and structures, said Arvind Raman, anassociate professor of mechanical engineering at Purdue. The silicontips also wear out quickly, reducing image resolution, whereas thecarbon nanotubes have been shown to retain their accuracy after manyhours of use, said mechanical engineering doctoral student Mark Strus.

Butwhile nanotubes better penetrate the nooks and crannies ofnano-structures, the flexible tubes often stick to the sides of thesestructures due to attractive forces between individual atoms called vander Waals' forces.

"An example I give students is that operatingin a nanoscale environment is like having flypaper everywhere becauseof the attraction of van der Waals' forces," Raman said. "Theseshort-range, inter-atomic forces are very relevant on this size scalebecause a nanometer is less than 10 atoms wide."

Researchers use nanotubes as probes by inducing a vibration in a portion of the microscope assembly called a microcantilever.

"Themicrocantilever, which does all of the surface sensing, can be thoughtof as a very small oscillating diving board on which the silicon tipand nanotube are mounted to the free end," Strus said.

As themicrocantilever vibrates, the nanotube tip comes close to the surfacebut never actually touches the object being imaged. The closer the tipcomes to the surface, the more powerful the attractive van der Waals'forces become. The increasing attraction causes changes in thevibration pattern of the oscillating microcantilever, and the changingpattern is carefully monitored to reveal precise changes in contours onthe surface of the object, yielding an image.

The same forcesthat enable the technology to work, however, also cause the stickingaction of the probe. The vibrating tip sticks to the sides of theobject being imaged, producing "artifacts," or inaccuracies in themeasurements and images.

Strus has led research aimed atoscillating the probes in a manner that prevents nanotubes fromsticking to structures, and new findings could lead to more accuratemeasurements using the slender probes. The journal paper was written byStrus, Raman, C-S Han, senior research manager from the NanomechanicalSystems Research Center at the Korea Institute of Machinery andMaterials, and C.V. Nguyen, a research scientists from the NASA AmesResearch Center in Moffett Field, Calif.

Methods to preciselymeasure structures on the scale of nanometers will become essential asnanostructures are used more often in applications such as computerchips, advanced sensors, microscopic machines and the creation of newmaterials. Precision measurements will be critical for developing newstandards needed to properly develop, study and manufacture productsbased on nanotechnology.

Although some researchers are usingnanotube tips in place of conventional silicon tips, the technique isstill being perfected and has not yet reached widespread commercial use.

"Oneof our points in this paper is that you can avoid getting theseartifacts if you know how to set the parameters," Strus said. "Forexample, you can change your set point or your amplitude and still geta good image with your nanotube."

The researchers showedprecisely how artifacts are created by the sticking nanotubes, whichare about 25 nanometers thick. The researchers also have shown how toavoid these artifacts by adjusting operating parameters of themicroscope to prevent the tube from sticking.

One way to decreasethe sticking is to increase the amplitude, or how far the probe moveseach time it vibrates across the surface. With each oscillation, thetube sweeps close to the surface of the object and then swings in theopposite direction, constantly repeating the motion. As the vibratingprobe sticks to the sides of a structure, the microscope's computerizedcontroller pulls the tip farther from the surface. Then, after the tipis pulled away, it starts vibrating normally again, and the controllerrepositions it closer to the surface, again resulting in the stickingaction. This cycle repeats, causing the image artifacts.

Theresearchers demonstrated how to prevent several specific types ofartifacts while using nanotubes to take images of tiny tungsten postsabout 100 nanometers in diameter and other nano-structures.

Theresearch was funded by the Centre for Nanomechatronics andManufacturing in South Korea, and the work is associated with Purdue'sBirck Nanotechnology Center, which is part of Discovery Park, theuniversity's hub for interdisciplinary research.