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Cleaner chipmaking method uses carbon dioxide fluid

DOE/Los Alamos National Laboratory : 18 May, 2007  (Technical Article)
Scientists at the Department of Energy's Los Alamos National Laboratory have developed a new technology application that could all but eliminate the use of hazardous corrosives and the production of wastewater in the fabrication of integrated circuits, or chips, for computers.
Chip making is sometimes called a 'clean industry' because of the images of technicians in white lab suits working in ultra-clean rooms with shiny pristine silicon wafers. But it is estimated that on the average day of operations at a chip-making plant four million gallons of wastewater are produced, and thousands of gallons of corrosive hazardous materials, like hydrochloric and sulfuric acid, are used.

The new technology, called SCORR, focuses on photoresist removal, one of the steps in a process called photolithography, where high intensity light along with aggressive acids and corrosives are used to create a chip's tiny integrated circuits by altering the topography of a silicon wafer. Using carbon dioxide at high temperature and pressure, known as supercritical carbon dioxide, in place of the hazardous materials, Laboratory researchers have demonstrated a technology that inexpensively replaces the solvents as well as the tremendous quantities of ultra-pure water that are used to wash those solvents away.

'Carbon dioxide, at pressures above 1,050 pounds per square inch and temperatures above 31 degrees centigrade, becomes supercritical,' said Craig Taylor, who leads the SCORR team in the Laboratory's Applied Chemistry Technologies group. 'In its supercritical phase the gas becomes liquid, but behaves a little like both-giving it the ability to act as a solvent. But SCCO2 alone is somewhat ineffective, so it is combined with minor amounts of a more effective cosolvent, and we've seen that this mixture is quite effective at photoresist removal.

'On top of that, when the pressure and temperature are lowered the SCCO2 returns to its gas phase, leaving the silicon wafer bone-dry and virtually free of any dirt, eliminating the need to rinse with ultra-pure water and dry with isopropyl alcohol. And the best news, carbon dioxide is cheap, nonflammable, nontoxic, biodegradable, recyclable, and plentiful.'

The Los Alamos photoresist removal technology produces virtually zero hazardous waste. It is designed as a closed-loop system that reuses the carbon dioxide in the process, adding no greenhouse gas to the atmosphere. Because of their low vapor pressure, the additive cosolvents are easy to separate from the mixture, and so they, too, are collected and reused.

A key element in the process is a tiny high-pressure sprayer that pulses the SCCO2/cosolvent onto the silicon wafer to assist in dislodging the photoresist. Developed by technician Jerry Barton, the sprayer creates enough surface drag to dislodge the microscopic bits of photoresist already softened up by a minutes-long soaking in the SCCO2/cosolvent mixture. This combined process of soaking and spraying, along with an SCCO2-only wash, has produced results that equal the chip fabrication standards currently accepted in industry.

The early work on this technology was accomplished through a Cooperative Research and Development Agreement with computer manufacturer Hewlett-Packard. Research and development continues with IBM and GT Equipment Technologies, Inc.

In addition to Taylor and Barton, the SCORR team includes Leisa Davenhall, Kirk Hollis, Gunilla Jacobson, Jim Rubin, and Laurie Williams.

'We want to bring this technology to the attention of the computer industry, as well as the public in general, just to emphasize the environmental advantages of SCCO2,' said Taylor. 'Even if you were to set aside the hazards and pollution associated with the corrosive materials used in chip making, you still have the issue of water use-and that's especially critical in the southwest where several large chip fabrication facilities are located. We believe that the SCCO2 process has the potential to save hundreds of millions of gallons of water every year even if it were installed in just one factory, making it not only a very important technological advance, but an environmental advance, as well.'
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