INDUSTRY NEWS

Los Alamos, H-P develop nontoxic breakthrough for photoresist removal using carbon dioxide

Anew photoresist stripping method that requires no toxic chemicals offers chipmakers the potential to save millions of gallons of water and attendant costs, claim the method's developers. Researchers at Los Alamos National Laboratory and Hewlett-Packard have developed a supercritical carbon dioxide fluid and treatment system for photoresist removal that does not use strippers such as sulfuric acid and hydrogen peroxide.

"Our pulsed-flow system answers the technical problems the industry has had with alternate cleaning methods," says Craig Taylor, project leader at Los Alamos.

The method is called SCORR, for Supercritical Carbon diOxide Resist Remover. Compatible with current manufacturing processes, SCORR uses pressurized carbon dioxide to remove both positive and negative photoresists. The method works because supercritical CO^₂ has the properties of both gas and liquid, the researchers point out. It expands to fill its container and diffuses like a gas. At the same time, its liquid-like high density enables it to dissolve substances and carry them away, giving the supercritical CO^₂ the properties of an excellent solvent.

The carbon dioxide is mixed with a small percentage of propylene carbonate, a nontoxic organic cosolvent. Both are delivered to the wafer surface via a pulsed-flow system that thoroughly strips photoresist from wafers in less than half the time needed to remove it using wet stripping or chilled ozone systems, according to the developers. They add that both carbon dioxide and propylene carbonate are readily available at a fraction of the cost of traditional solvents.

Unlike the corrosive and flammable nature of acid-based strippers, the only hazard presented by the SCORR system is the use of a pressurized vessel for the stripping treatment. Developers says the system will enable chipmakers to reduce the costs of solvent disposal and water treatment.

The system also eliminates the high-volume use of water, lowering expenses for both water use licenses and discharge permits. These benefits would be of particular interest to fabs in arid regions like the southwestern United States, Los Alamos points out.

Using a sample provided by Hewlett-Packard, researchers at the lab recently conducted "worst-case" experiments "to test the ability of the system," says spokesman Ternel Martinez. Photoresists "were subject to ion implantation, and the supercritical carbon dioxide was able to remove them without any problems." Jim Rubin, chief researcher on the project, "said there is no reason why supercritical carbon dioxide cannot replace distilled DI water in the rinsing process," according to Martinez.

The researchers will make a presentation on the SCORR system at Semicon Southwest in Austin, TX, on October 21. For more information contact Ternel Martinez, 510/665-7778.

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