Sep. 14, 2001 AMHERST, Mass. – A research team led by University of Massachusetts chemical engineer James Watkins has developed a new method of depositing copper films within tiny channels etched in silicon wafers. The technique, reported in the Sept. 14 issue of the journal Science, is significant because it offers an efficient way to create the ever-smaller circuitry demanded by the microelectronics industry. “This process really makes possible the fabrication of extremely small features that are necessary for future generations of integrated circuits,” Watkins said.
The team included graduate student Jason Blackburn and postdoctoral research fellows Albertina Cabanas and David Long. The work was funded by the National Science Foundation, the David and Lucile Packard Foundation, and Novellus Systems.
"The microelectronic devices that our society relies on are becoming increasingly complex, and at the same time, are decreasing in size," said Watkins. Historically the processor speed of integrated circuits has doubled every 18 months and this requires making individual components smaller and smaller. Current fabrication techniques are projected to reach their limit within the next few years and the current technology roadmap used by the industry offers no solution to the problem after 2005. "When we build devices for microelectronics there are problems in placing the metal exactly where it is needed. We just can’t do that in the very small features that will soon be required using current techniques in a way that would be practical for industry."
The conventional fabrication methods deposit metals and other materials onto silicon from either a gas or from liquid solution. Both approaches have their own advantages and disadvantages. Watkins and his group took a different view. "We reasoned that individually each of the methods is probably limited in a fundamental way, but if you could combine the most desirable attributes of the methods into a single process, then you could solve the problem. This is possible by depositing the materials from a supercritical fluid." A supercritical fluid is a substance that has some of the properties of a liquid and some of the properties of a gas. Watkins explains, "If you heat and compress a gas like carbon dioxide, it can be used to dissolve a wide range of compounds. The solution, however, does not behave like a liquid but rather like a gas and therefore flows easily over complex surfaces and into narrow gaps." This combination of properties makes Watkins’s process, called "chemical fluid deposition," ideal for the fabrication of tiny devices with complicated features.
The use of carbon dioxide has other benefits. It is non-flammable, non-toxic, and renewable and thus offers environmental advantages to current metal deposition techniques. The generation of contaminated waste water is a major concern for the metal plating industry. Using the new process, it is eliminated entirely.
The work described in the current issue of Science is directed toward the deposition of copper for interconnect structures in integrated circuits. However, Watkins see broader implications. "We are currently developing the technique for other applications including photonic materials and gas separation devices." Photonic materials play a key role in optical communications and data transfer.
Chemical fluid deposition is covered by U.S. patent 5,789,027 authored by Watkins and polymer science and engineering Professor Thomas McCarthy and assigned to the University and numerous other patent applications in process.
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