The research team at the University of Arizona is developing new chemistries that advance chip-fabrication technology while minimizing their negative effects on the environment. They are doing so by including environmentally benign processing steps that produce fewer hazardous byproducts as part of the design constraints. By characterizing surfaces on a molecular scale, they are able to manage production of chips so there is little or no cleanup after they are produced.

The team, which includes four undergraduates and six doctoral candidates, is part of the Engineering Research Center for Environmentally Benign Semiconductor Manufacturing. Its participants include MIT, Stanford University, UC Berkeley, Arizona State University, Cornell University, University of Maryland and MIT's Lincoln Lab. The team is administered by the University of Arizona and sponsored by the National Science Foundation and the Semiconductor Research Corp.

Assistant professor of chemical and environmental engineering at the university, Anthony Muscat is leader of the 10-member team. "We hope to educate a new breed of engineers who don't see polluting the environment as a trade-off," he says. All of the team's processes are explained in his group's publication so any chip manufacturer can adopt them without royalties.

For example, the team has developed a gas-phase cleaning technology. It eliminates the need to clean, remove, and reinsert partially fabricated wafers into the gas-phase reactor.

The new process, developed by studying molecular-level interactions, allows the chip to remain in the vacuum chamber throughout gas-phase processing. This eliminates not only the need to remove chips to clean them, but also avoids contamination of the chip and reduces waste.

Gas-phase cleaning uses supercritical carbon dioxide. Because supercritical carbon dioxide lacks any surface tension when interacting with solid surfaces, the material is able to penetrate deep etches on a wafer. Very low viscosity is another characteristic of the material, which enables it to dissolve large quantities of unwanted solids on the surface of a wafer.

Muscat's team produces supercritical carbon dioxide by subjecting it to 1,000 pounds per square inch of pressure at about 100°F. A fluid that is dense enough to dissolve both liquid and solid contaminants is created, yet once dissolved the materials easily diffuse through the carbon dioxide.

For the future, the team is characterizing the molecular interfaces between the layers of different semiconductor materials. A semiconductor works as a barrier layer to separate copper from insulation layers. As chips shrink in size, that barrier will shrink as well. To fix this problem, Muscat's team is developing chemical formulations of nitride film. These formulas will stick to copper and self-assemble to two or three molecules thick. The team will continue to concentrate on optimizing cost, speed and environmental impact.

Source: R. Colin Johnson
EE Times
05/20/02
Silicon Strategies