Mar. 10, 1998 ITHACA, N.Y. -- Researchers who work with the incredibly small have long used the scanning tunneling microscope (STM) to make pictures of surfaces with such precision that individual atoms appear as bumps. With it, tiny structures can be built by moving one or a few atoms at a time.
But working one atom at a time is a painfully slow process, especially for commercial applications. One way to speed up the work would be to use an array of tiny STMs working together, each one scanning a very small area.
Now, Cornell University researchers have taken a step toward such technology. They have built and tested an array of microscopic STM "nanoprobes" manufactured on the surface of an ordinary silicon chip.
The work is described in the November 1997 Review of Scientific Instruments by Scott A. Miller and Kimberly L. Turner, who worked with electrical engineering Professor Noel C. MacDonald at Cornell's Nanofabrication Facility. At the time the research was done, Miller and Turner were graduate research assistants in applied physics and in theoretical and applied mechanics, respectively.
Commercial applications of the new technique are still quite far away, says Miller, "but pieces are beginning to come together." The most likely application in the reasonably near future, he says, would be a data storage system that could store more information in a square centimeter than modern computers keep on an entire hard disk. Building microcircuits or tiny machines by moving individual atoms is also theoretically possible but still in the realm of science fiction, says Miller, "but it's always good to keep things like that as goals."
A scanning tunneling microscope consists of a tiny needle that is suspended one nanometer (a billionth of a meter, or about the diameter of an atom) above a surface that conducts electricity. If the surface is grounded and a voltage is applied to the needle, a tiny current flows between the needle and the surface. As the needle is moved across the surface, its height is adjusted so that the current remains constant. The variations in the height of the needle are fed into a computer and used to create an image of the surface.
In the Cornell nanoprobe array, the needles and the structures that move them up and down are tiny "nanomachines" manufactured on the surface of a silicon chip. Each needle is mounted on one end of a cantilevered arm about 150 microns long (a micron is a millionth of a meter) -- about two or three times the diameter of a human hair. The arm is mounted at its center so that it can tilt up and down like a teeter-totter to adjust the distance between the needle and the surface to be scanned. The arm also is attached to a comblike series of vertical plates interleaved between stationary plates mounted on the base. The bar and needle can be made to move by applying an electric charge to the plates.
Like other nanomachines, these devices are made by coating a silicon surface with a pattern made up of material that resists etching, then chemically etching away the uncoated areas. This is done several times, with a different pattern each time, to carve out the desired shape in a series of layers, similar to the way an archaeologist exposes a buried structure. By undercutting some layers, it's possible to make parts that are free to move up and down.
Others have built similar arrays of microprobes, the researchers noted in the article, but most have used exotic materials that require special equipment and manufacturing techniques. The Cornell device is made of ordinary silicon. The way in which the probes are controlled is also innovative and can easily be scaled down in size, the researchers said.
The largest prototype array they have built to date consists of 144 probes, arranged in a square consisting of 12 rows of 12 each, with needles about 200 microns apart. This is mounted on another comblike actuator that can move it in the horizontal plane to scan a surface. The array must be moved only enough so that each needle can scan an area 200 microns square. In the prototype, Miller says, the array has been moved up to 20 microns in each direction. Future development focuses both on increasing the range of movement and fitting more and smaller probes into the same space.
Nanoprobe arrays could store data by depositing small bumps made up of a few atoms -- or perhaps even single atoms -- on a surface to represent binary bits. Placing such bumps 100 nanometers apart, Miller says, would allow storing up to 1.2 gigabytes of information (the total storage on a typical computer hard disk) on a square centimeter of surface. By placing the bumps one nanometer apart, up to 12 terabytes of data could be stored in the same space, he says.
The title of the article is "Micromechanical scanning probe instruments for array architectures." The research was funded by the Defense Advanced Research Projects Administration (DARPA) and the National Science Foundation.
Noel MacDonald, until recently the Lester B. Knight director of the Cornell Nanofabrication Facility, currently is on leave to serve a two-year term as director of the Electronic Technology Office of DARPA. Scott Miller has left Cornell to join Kionics, an Ithaca-based nanotechnology company.
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