Supercritical CO2 blasts photoresist
09/01/1998
Supercritical CO2 blasts photoresist
Cooperation between Los Alamos National Laboratory and Hewlett-Packard has produced a system that uses supercritical carbon dioxide (CO2) fluid properties to remove photoresists from wafers. The beauty of this process is its elimination of toxic and flammable chemicals and reduction of water consumption. The system has been dubbed the Supercritical Carbon diOxide Resist Remover (SCORR).
Under increasing pressure and temperature, CO2 gas first reaches a liquid phase, then enters a "supercritical" region with properties of both gas and liquid - supercritical CO2 expands to fill its container and diffuses into the tiniest pores like a gas. In addition, because supercritical CO2 has a high density like a liquid, it can dissolve substances and carry them away; supercritical CO2 is an excellent solvent.
SCORR delivers supercritical CO2, mixed with a few percent of the nontoxic, nonhazardous organic co-solvent propylene carbonate, onto a wafer surface through a novel pulsed-flow system. SCORR thoroughly strips photoresist from wafers, reportedly in less than half the time required for wet-stripping or alternative chilled ozone systems.
The solvent mixture is less expensive than the solvents used in traditional systems. And both carbon dioxide and propylene carbonate are readily available and recyclable. The only potential hazard is the use of a pressurized vessel for the stripping treatment; however, large-scale commercial use of supercritical CO2 is well known and has been used safely in the food, pharmaceutical, and petrochemical industries.
The significance of this work is that if adopted by the semiconductor industry, the technology will reduce solvent disposal and water treatment costs and cut expenses for acquiring water use licenses and discharge permits. It will help the industry comply with federal and state environmental regulations because the process greatly reduces hazardous wastes and emissions. And because the system uses pure CO2 for its final rinse step, it eliminates the need for high volumes of water in wafer fabrication plants. - P.B.