Scientists at the US Department of Energy’s Los Alamos National Laboratory have developed a new technology application that may eliminate the use of hazardous corrosives and the production of wastewater in the fabrication of integrated circuits, or chips, for computers.
It is estimated that on the average day of operations at a chip-making plant four million gallons of wastewater are produced, along with thousands of gallons of corrosive hazardous materials.
The new technology, called SCORR, focuses on photoresist removal – one of the steps in photolithograpic processing – 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 super critical carbon dioxide (SCOO2) in place of the hazardous materials, Los Alamos researchers are said to have demonstrated a technology that inexpensively replaces the solvents as well as the large 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, non-flammable, non-toxic, biodegradable, recyclable, and plentiful.’
The Los Alamos photoresist removal technology is said to produce virtually no hazardous waste as 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 vapour 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, produced results that equal the chip fabrication standards currently accepted in industry.