STM tip builds golden mounds.

STM tip builds golden mounds

Physicists have devised a simple and speedy method for creating microscopic images and characters. Their new technique -- using a scanning tunneling microscope (STM) to deposit tiny gold mounds onto gold or platinum surfaces -- brings scientists one step closer to making near-molecular-scale electronic devices.

In recent years, many groups have sought to manipulate increasingly small clusters of atoms, with the ultimate goal of making tiny circuits and data storage devices. The STM has proved a helpful tool. Invented in 1981, the instrument works by positioning a tiny metal tip within a few atomic diameters of a surface, close enough to allow electrons to leap, or "tunnel," across the air gap to the surface. The resultant current varies with the width of the gap, allowing scientists to chart surface bumps and grooves as small as individual atoms (SN: 4/1/89, p.200).

Dan Rugar, H. Jonathon Mamin and Peter H. Guethner at the IBM Almaden Research Center in San Jose, Calif., wondered whether they could somehow release clusters of atoms from an STM tip and deposit them onto a surface. "I had the idea the if we could apply [an external] voltage to the tip, we could get some atoms to be emitted," says Rugar.

Scientists have long known that a strong electric field can ionize and eject atoms from a surface, but no one had tried this approach with a solid STM tip. The IBM team chose gold as the tip material because it holds its atoms loosely and does not react in air, disrupting the STM.

Rugar's idea worked. In the Nov. 5 PHYSICAL REVIEW LETTERS, he and his colleagues report that short bursts of voltage applied to the gold tip produced surface mounds typically measuring 100 to 200 angstroms in diameter and 20 to 30 angstroms high, each containing several thousand atoms. The linear dimensions of today's computer-chip features are about 100 times larger.

Other researchers have "written" characters of the same size or smaller using different methods and more complicated setups, Mamin notes. Last April, for instance, another IBM team reported it had written "IBM" by pushing individual xenon atoms into place with an STM, but this required supercold temperatures and a vacuum. "We can operate in room temperature and in air," Mamin says.

The new method works fast, depositing a single mound in a few hundred nanoseconds or less and creating a grid of more than 100 mounds in just a few minutes. The speed is limited only by the time it takes to move the tip between locations, Mamin says.

"We were also surprised how reliably it worked," Rugar adds. "We though that after a few tries the tip would be blunted so that it woundn't be able to emit well." Instead, they found that many of the tips could make thousands of dots.

The researchers now plan to try different materials, including a silicon surface, and will attempt to increase the technique's speed.