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Centrifugal Process Improves Thin Films

LOS ALAMOS, N.M.,Aug. 29, 2001 - Researchers at the U.S. Department of Energy’s Los Alamos National Laboratory are taking a new thin-film deposition method for a spin.

Hsing-Lin Wang, a researcher in Los Alamos’ Bioscience Division, today at the 222nd national meeting of the American Chemical Society in Chicago described an innovative method for building multiple thin layers of polymer coatings using centrifugal force. The new method, developed by Wang and Jeanne M. Robinson of Los Alamos’ Chemistry Division, drastically reduces thin-film production time and potentially improves the quality of thin-film devices.

Layers of extremely thin films of polymers are used in optical and electrical devices such as light-emitting diodes. For these applications, one or more layers of polymers, each about a millionth of a millimeter thick, are built up on a substrate such as glass, plastic or metal. The layers have specialized functions: Some conduct electrical impulses; others refract, reflect or produce light.

Traditionally, technicians produce multilayered thin film devices by dipping a substrate material into a polymer solution. Layers can be built on top of one another using this dipping technique, which works because each neighboring polymer layer has an opposite electric charge than its neighbor and the layers stick to one another like a stack of magnets. The dipping process takes several minutes or longer to produce a single layer, depending on the required thickness.

The new thin-film deposition method developed at Los Alamos by Wang and his colleagues can produce a single layer in just tenths of a second. Moreover, the new method gives technicians more control over the quality and thickness of the films.

Wang and his colleagues’ innovative method works by spinning the substrate material on a turntable-type apparatus at up to 7,000 revolutions per minute. Once the substrate is spinning, a technician adds a small portion of a polymer to the center of the substrate. The polymer quickly spreads from the center to the edges of the substrate material, producing a quality coating in only a fraction of a second. The same technique can be used for subsequent layers of polymers. Wang and his colleagues have had good results when creating as many as 50 layers of polymers on top of one another, and they believe the technique will work for even more.

In addition to the improved speed in depositing films, Los Alamos’ spin-assembly method also can produce better quality films for some types of polymers. With traditional dipping, a polymer solution can bleed into the layer it is supposed to coat because both substances are in contact with one another for a relatively long time. This blending of layers, known as interpenetration, can reduce the accuracy or performance of some thin-film devices.

Los Alamos’ spin assembly deposition method reduces contact time between polymers and helps ensure that layers remain separate and distinct.

Wang and colleagues looked at the film surfaces using atomic-force microscopy -- which allows them to view minute features -- and ellipsometry -- which allows them to look at the thickness of individual film layers -- and found that the spin-assembly films had less interpenetration and better surface textures than similar films prepared by dipping.

In addition, Los Alamos researchers used neutrons produced by the linear accelerator the Los Alamos Neutron Science Center to look at polymer horizons in multilayer films produced by spin assembly. The neutrons in essence create a topographic map on a molecular scale that allows researchers to see interpenetration or surface irregularities. The researchers are hoping that these neutron snapshots will provide conclusive evidence of the superior layer structures seen with microscopy and ellipsometry.

"Los Alamos’ spin-assembly method shows promise for industry because it reduces production time and enhances the quality of the product" Wang said. "Moreover, this method is something that could be automated for an industrial setting to provide reliable and fast production."

Spin-assembly is not yet perfect. Wang and his colleagues must correct some film thickness issues peculiar to the method. But the Los Alamos research team is confident that their research, which is barely a year old, will overcome these issues.

The research was made possible by Laboratory-Directed Research and Development -- an internal program in which 6 percent or less of Los Alamos’ operating budget is used to fund cutting-edge research that is determined to be innovative or critical to the Laboratory’s strategic mission.

Wang’s collaborators include: Peter Chiarelli, Pomona College; Malkiat Johal, University of South Florida; and Joanna L. Casson and Robinson of Los Alamos National Laboratory.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.

Contact: James Rickman, elvis@lanl.gov, (505) 665-9203

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