SC Fluids Receives Presidential Green Chemistry Challenge Award

The Presidential Green Chemistry Challenge Program promotes pollution prevention and industrial ecology through an Environmental Protection Agency (EPA) Design for the Environment partnership with the chemical industry. The program recognizes outstanding accomplishments in green chemistry to demonstrate the scientific, environmental and economic benefits that green chemistry technologies offer.

Typically, five awards are given annually to industry and government sponsors, an academic investigator, and a small business. SC Fluids Inc. (Nashua, N.H.), this year's recipient in the small-business category, was recognized for further developing SCORR (supercritical CO₂ resist remover) technology (see " Using CO₂ Fluid for Cleaner Chipmaking ," Semiconductor International , June 2001), which dramatically reduces environmental liabilities while still achieving International Technology Roadmap for Semiconductors (ITRS) goals.

SC Fluids, under a Cooperative Research and Development Agreement with Los Alamos National Laboratory (Los Alamos, N.M.), further developed the technology and integrated it into a tool for fully automated, single-wafer supercritical fluid process.

The use of SCORR technology significantly reduced emissions, water consumption and energy use, and decreased the use of chemicals (such as chlorofluorocarbon compounds, sulfuric acid, acetone, methyl ethyl ketone and isopropyl alcohol) by 95-99% by replacing them with supercritical carbon dioxide.

SCORR technology was used for photoresist and residue removal — cleaning and stripping photoresist and residue from wafers and photomasks. Researchers indicated that two factors affect a polymer's ability to solubilize CO₂ — the crystallinity and the amount of cross-linking in the polymer.

The presence of crystallinity in a polymer inhibits the sorption of CO₂. Highly cross-linked polymers solubilize less CO₂ than those with less cross-linking. The sorption of CO₂ then results in swelling of the polymer, and is further enhanced if the glass transition temperature is exceeded. Because the solubility of CO₂ is related to the degree of cross-linking in the polymer, it is possible to swell the photoresist and debond it while not affecting the underlying low-k dielectrics.

The instrument is a combination of the swelling of photoresist caused by the diffusion of CO₂ and the cosolvent into the polymer matrix, a debonding and delamination of the film caused by rapid depressurization and sufficient fluid flow to sweep away the debris.

The zero surface tension, gaslike viscosity and diffusivity of supercritical CO₂ allowed it to penetrate high-aspect-ratio structures to clean and dry them.

Results shown in the Figure indicate that, by using the appropriate temperature, pressure and cosolvent, the photoresist can be completely stripped. It also was found that the debonded photoresist remains on the surface if the fluid flow is insufficient.

	The photoresist is swollen, and attack at the edges of the pattern has begun (top). Upon introducing depressurization and repressurization pulses, the photoresist debonds from the surface and is lifted off in sheets (middle). CO2 and cosolvent at the appropriate temperature and pressure swell the polymer, along with rapid decompression to debond and delaminate it from the surface; fluid flow then sweeps it off the wafer (bottom). (Source: SC Fluids)

"The use of supercritical fluids can also be used for critical point drying of MEMS devices to prevent image collapse post-develop in the photolithography process, and possibly in the future for photoresist development," said Laura Rothman, SC Fluids vice president of technology. "Another new application is for deposition of materials and is called 'chemical fluid deposition.'"

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