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NIST,
Partners Report Fast, Versatile Method In a technical feat akin to teaching an old dog to do new tricks, a research team led by National Institute of Standards and Technology scientists report that a routine, X-ray-based method for studying the structure of materials may be the answer to a looming semiconductor industry need—a rugged, high-throughput technology for measuring dimensions of chip circuitry composed of millions of nanometer-scale devices. Writing in a recent issue of Applied Physics Letters, the collaborators describe their initial success in using small-angle X-ray scattering (SAXS), a decades-old research tool, to characterize the size and shape of grid-like patterns with 180-nanometer (billionth of a meter) linewidths. With better than 1 nanometer precision, the team determined the average size of periodically repeating features arrayed in one- and two-dimensional patterns on three chemically different samples. The industry-supplied samples were representative of the intricately patterned polymer masks used to print integrated-circuit designs. Equally important, the researchers write, the measurement technique is rapid and versatile—desirable for industrial applications. It can be used on a wide range of materials and to evaluate the quality of surface and subsurface patterns consisting of features considerably smaller than 100 nanometers. In proof-of-concept experiments, essential data were gathered within a second over an area about 40 micrometers on a side—a large swath, nanotechnologically speaking. Analyses of images assembled from X-rays captured after passing through the specimens yielded high-precision measurements of linewidths, spaces between line edges, and shapes of features. For each, measurements were averaged over the area under focus. In addition, analyses yielded information on the roughness of walls and the edges of lines making up the lattice-like pattern. These variables also are key to quality control in the chip-making process. The collaboration consisted of a four NIST polymers scientists, led by Ronald Jones, and colleagues from the ExxonMobil Research Co., Argonne National Laboratory’s Advanced Photon Source, and the Shipley Co. The reported work leverages a well-established X-ray diffraction method for determining the arrangement of atoms in materials. While beaming through a sample, X-rays are deflected by electrons surrounding atoms in the specimen material. From the angles of the diffracted X-rays, their intensity, and other data gathered with a detector behind the sample, researchers can determine the shapes and locations of individual atoms in a sample as well as the distance between atoms. As semiconductor processing and emerging “nanomanufacturing” methods approach molecular and even atomic scales, the novel application of the non-destructive SAXS technique bridges a crucial gap in measurement capabilities. Scanning tunneling microscopes and related instruments can image individual molecules and atoms, but they are impractical for sampling more than a few structures over a large area. Light scattering methods and other tools can measure and map general contours on a chip or wafer, but cannot easily discern the dimensions of features that make up these patterns. In contrast, technical implementation of the SAXS method actually becomes easier as feature sizes decrease, NIST’s Jones explains. The method should enable precise measurements even as linewidths shrink to 30 nanometers within the next few generations of chips. “Currently, there are no clear solutions for the need to measure interconnect lines, contacts, and other critical dimensions once they drop to 30 nanometers,” Jones says. “At that point, the level of dimensional control required during processing will be less than a nanometer.” Proof-of-concept measurements were made with X-rays at the Argonne National Laboratory’s Advanced Photon Source, a state-of-the-art synchrotron. The NIST team believes, however, that advances in optics and other technologies could make commercial-scale SAXS equipment an option for industrial laboratories, although measurement scans would take minutes instead of seconds. NIST researchers say the SAXS method might be used to make subnanometer measurements of several dimensional features key to semiconductor manufacturing, including linewidth, sidewall angle and curvature, pitch, and line-edge roughness. But the method’s potential utility doesn’t stop there. For example, Jones says the versatile technique can be used for three-dimensional analyses and for evaluating the subsurface copper interconnects that link chip components. In addition, SAXS can be used to measure the geometry of nanometer-sized devices made with a wide variety of materials—organic or inorganic, conducting, semiconducting, or insulating. This work is supported by the Advanced Lithography Program of the Defense Advanced Research Projects Agency and the NIST Office of Microelectronics Programs. The Advanced Photon Source is supported by the U.S. Department of Energy. Additional information on the NIST project on adapting SAXS for critical dimension measurements can be found at: http://polymers.msel.nist.gov/highlights/Critical-Dimension-Metrology-Nanoscale-Structures-Small-Angle-X-ray-Scattering.html Examples of SAXS detector images are available from Mark Bello, 301-975-3776; or mark.bello@nist.gov. As a non-regulatory
agency of the U.S. Department of Commerce’s Technology Administration,
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information on NIST, visit www.nist.gov. created on 11/20/03 |