Supercritical Carbon Dioxide

Carbon dioxide has long been the nemesis of environmentalists because of its role in global warming, but under just the right conditions-namely, high pressure and the right temperature-it’s one of nature’s best and most environmentally benign solvents. Decaf-coffee lovers, for instance, benefit from its ability to remove caffeine from coffee beans. During the last few years, carbon dioxide has also made inroads in the dry-cleaning industry, providing a safe cleaning alternative to the chemical perchloroethylene. But it’s on the high-tech front that carbon dioxide may make its biggest impact. “There are huge opportunities,” says University of North Carolina chemist Joseph DeSimone. “I am confident that carbon dioxide will dominate several of the key steps in microelectronics.”

Carbon dioxide could provide the semiconductor industry with a more environmentally sound way to scrub silicon, but it could also allow the continued miniaturization of integrated circuits. And that means faster and cheaper computers and consumer electronics. “The environmental angle will make [chip makers] look good, but they aren’t going to retool only on the basis of that,” says Craig Taylor, a supercritical-fluids researcher at Los Alamos National Laboratory in New Mexico. “What’s interesting to industry is that supercritical carbon dioxide may be an enabling technology for going to smaller dimensions.”

Scientists have known for more than a century that at 75 times atmospheric pressure and 31 C, carbon dioxide goes into an odd state that chemists call “supercritical.” In this state, the liquid and the gas forms of carbon dioxide become indistinguishable: they merge into one fluid with unusual properties. Among the strangest, the viscosity of the fluid drops to almost nothing and its surface tension goes to zero. The low viscosity means it flows unusually well with low resistance, and the zero surface tension means the fluid’s surface doesn’t curl up at the edges and stick to the sides of its container. The net result: supercritical carbon dioxide can flow into crevices and nooks so tiny that other liquid solvents would gum up.

Researchers at Los Alamos, the University of North Carolina and elsewhere have been exploring the possibility that using supercritical carbon dioxide-or liquid carbon dioxide hovering just below the supercritical state-could let them make features on microchips at an unprecedented level of resolution. In photolithography, the fundamental process used in chip making, a photoresist (a light-sensitive material that covers the silicon chip) is exposed to light shined through a “mask”; the exposed photoresist is then washed off, leaving a pattern on the silicon. Existing technology typically uses a water solution to wash away the photoresist. “But the structures are getting so small that the high surface tension of the water itself can be damaging,” explains DeSimone. Just like honey poured over a house of cards, the water can collapse the delicate silicon features. Supercritical carbon dioxide can wash over the structures without demolishing them.

Carbon dioxide could also provide a way of laying down the ultrathin copper wires used in today’s best microchips. Jim Watkins and colleagues at the University of Massachusetts recently found they could dissolve metallic compounds in carbon dioxide and pour the solution into the tight nooks and crannies of trenches etched into the silicon to form the wires. When the researchers add hydrogen gas, the compounds release their metal loads onto the silicon surfaces to create high-quality interconnects thinner than 100 nanometers.

If carbon dioxide can clean up chip manufacturing, it could provide a classic win-win situation for those balancing environmental impact and manufacturing performance in the high-tech industry. Manufacturers will be able to continue to produce chips with the shrinking features needed for tomorrow’s ever faster computers. And those in Silicon Valley can save their water supplies for making decaf lattes.