NIST's M³ Takes Surveying to the Realm of the Atom

Researchers at the Commerce Department's National Institute of Standards and Technology have built a novel tool for surveying, subdividing and helping to develop large tracts of the molecular frontier the technologically rich, but as yet, unmapped domain where tomorrow's integrated circuits and ultraminiature machines will be made.

Now in the early stages of performance testing, NIST's Molecular Measuring Machine, or M3, is designed to operate over vast expanses of atomic real estate. After fine tuning, it will be able to precisely locate molecule-sized features and, with unprecedented accuracy, measure vast distances between those features. The new machine's range is 250,000 times greater than that of most scanning tunneling microscopes, whose needle-like probes can glimpse individual atoms.

M3 is expected to measure within 1 nanometer (billionth of a meter), or the equivalent of a string of about four or five silicon atoms the distance between two points over a square 50 millimeters on a side, an area slightly smaller than a dollar bill folded in half. In contrast, the range of a typical STM is about 0.1 millimeter (thousandth of a meter), or one-tenth the diameter of a grain of sand.

For perspective, M3's anticipated capabilities are akin to being able to locate two widely separated grains of sand in a 2,500 square-kilometer (960 square-mile) patch of desert and then measure the distance between them to within 1 millimeter.

While initial testing is likely to continue for about a year, the machine already has been pressed into practical service a comparison of different methods for measuring the width of electrically conducting lines on integrated circuits. Results of the study were presented earlier this year at the International Conference on Microelectronic Test Structures in Japan.

M3's first functional task was devoted to a technical issue in semiconductor manufacturing. When Clayton Teague, a physicist turned measurement scientist, and his NIST colleagues began work on M3 in 1987, they aimed to develop a machine that could meet the U.S. microelectronics industry's most advanced measurement requirements in the late 1990s. By then, dimensions of the smallest features on leading-edge commercial memory chips will shrink to 0.25 micrometer (millionth of a meter), or about one three-hundredth of the width of a human hair. For the semiconductor industry to continue its decades-long trend of crowding ever more and ever smaller devices on slivers of silicon, it will soon require measurement methods that are accurate to within 0.0025 micrometer, or 2.5 nanometers.

In testing done to date, the machine has made measurements on specimens encompassing only a small part of its range. In succeeding tests, it will measure specimens over the full 50-millimeter span. The measurement process is a painstaking one, but one that can be automated fully. In the comparison of methods for measuring linewidth on integrated circuits, for example, M3 operated for 40 hours straight, making about 1 million separate measurements of an area centered on a chrome line designed to be 750 nanometers wide. It made 16 overlapping images, each one 3 micrometers by 5 micrometers.

Compared with measurements made with other microscope-based techniques, M3's linewidth measurements corresponded more closely with those obtained with an electrical method. Although only suggestive because of the small sampling size and other factors, the results are consistent with other comparisons that found electrical and microscope-based methods to yield systematically differing results.

The explanation for these differences could loom large as the semiconductor industry pushes the limits of measurement accuracy and reliability. For miniaturization trends to persist in commercial chip production, the industry must fully understand the capabilities and limitations of various existing and experimental measurement methods now being considered for future generations of integrated circuits.

A host of tasks await M3. High on the agenda, says Teague, is developing measurement aids to help semiconductor manufacturers align masks during the increasingly complex process of printing circuit patterns. M3 will be used to calibrate a variety of measurement references that manufacturers can use to check the accuracy of their own measurement equipment.

M3 introduces the possibility of using nature's own geometry to validate measurements. For example, interatomic spacings in a crystal, with its highly ordered, regularly repeating arrangement of atoms, could serve as the molecular world's version of a ruler. For measuring squareness, says Teague, the right angles of a cubic crystal lattice would be the ultimate reference.

M3 also provides the means to address important scientific questions, the answers to which will influence efforts to manipulate nature's molecular architecture for practical purposes. "Among the things we would like to study is what happens when you remove a single atom or multiple atoms from the surface of a crystal," Teague explains. "What kind of distortion does that cause and how far does the disturbance propagate over the surface? In other words, how does it affect squareness of the crystal lattice? Then we would like to determine the mechanisms by which it propagates."

Over the coming months, Teague and his team will continue to characterize M3's performance and to map irregularities in the motions of moving parts and other sources of measurement uncertainty. They will develop mathematical rules that, incorporated into software, will correct for predictable sources of error and, thereby, further increase the machine's accuracy. Given that M3 is the only machine of its kind in the world, the jobs of refining and enhancing its capabilities may continue for years, even as it performs measurement services that will help U.S. industry stake their claims on the molecular frontier.

A variety of organizations collaborated with NIST in the development of M3. Partners include Hewlett-Packard Corp.; Zygo Corp.; Intelligent Automation, Inc.; and the Los Alamos, Lawrence Livermore and Oak Ridge National Laboratories.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes economic growth by working with industry to develop and apply technology, measurements and standards.