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Getting the drop on thin films.

Getting the drop on thin films

A thin layer of vapor-deposited tin forming on a hot sapphire substrate starts out as a collection of tiny droplets. As more tin atoms come to rest on the surface, new droplets form and older droplets grow and coalesce until a complete layer eventually forms. Two physicists have now developed a simple model that mimics this process, realistically reproducing the way the distribution of droplet sizes changes over time.

Fereydoon Family of Emory University in Atlanta and Paul Meakin of Dupont's Central Research and Development Department in Wilmington, Del., start with the idea that two distinct mechanisms govern a vapor-deposition process: Droplets grow by absorbing vapor and by coalescing when they touch. In their model, droplets of a fixed size are randomly added to a surface. Whenever two droplets touch or overlap, they fuse into a larger droplet. Computer simulations of this process match experimental observations (see illustrations) that the zones around large droplets tend to have fewer small droplets than found elsewhere on the surface. Such depleted regions are left behind by the coalescence of two large droplets, before new droplets have a chance to accumulate there.

This model seems to apply best to a vapor-deposition process in which the substrate's temperature is near or just above the condensing material's melting point, as in the case of tin deposition on a sapphire surface at 230[deg.]C. "Similar studies could provide valuable insights and introduce new approaches for theoretical and experimental investigations of the kinetics of droplet formation and thin-film growth," the researchers report in the July 25 PHYSICAL REVIEW LETTERS. Thin films play an increasingly important role in a variety of applications, from microelectronics to biochemical sensors.
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Title Annotation:research on thin film formation
Publication:Science News
Date:Aug 20, 1988
Words:285
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