The U.S. Department of Energy's Los Alamos National Laboratory worked with the Hewlett-Packard Company to develop the use of a supercritical carbon dioxide fluid formulation and treatment system to remove coatings called photoresists from semiconductor wafers without the use of toxic chemicals.
"The semiconductor industry is eager to reduce chemical use, and our new system cuts out the toxic chemicals and millions of gallons of water used in the industry's main production process," said Los Alamos project leader Craig Taylor. "Our pulsed-flow system answers the technical problems the industry has had with alternate cleaning methods."
Integrated circuit manufacturers rely heavily on photolithography to create the desired features in each layer of chip circuitry. The process requires the selective removal of hardened coatings, or resists, from a wafer, leaving the intricate circuitry intact. Current wet-stripping technologies apply either corrosive combinations of sulfuric acid and hydrogen peroxide or organic solvents. Both systems produce large liquid waste streams.
In contrast, the Supercritical Carbon diOxide Resist Remover, or SCORR, system uses carbon dioxide under pressure to efficiently remove positive or negative photoresists. It is the only nonhazardous process fully compatible with existing integrated circuit manufacture.
Under increasing pressure and temperature, carbon dioxide gas first reaches a liquid phase, then enters a region called "supercritical" in which it has useful properties of both gas and liquid. Supercritical carbon dioxide expands to fill its container and diffuses into the tiniest pores like a gas. On the other hand, because supercritical carbon dioxide has a high density like a liquid, it can dissolve substances and carry them. These properties make supercritical carbon dioxide an excellent solvent.
Los Alamos' SCORR system delivers supercritical carbon dioxide mixed with a few percent of the nontoxic, nonhazardous organic co-solvent propylene carbonate onto the wafer surface through a novel, pulsed-flow system. SCORR thoroughly strips photoresist from wafers in less than half the time required for wet-stripping or alternative chilled ozone systems.
The mixture costs a fraction of the solvents used in the traditional systems. And both carbon dioxide and propylene carbonate are readily available, nontoxic and recyclable.
"Supercritical carbon dioxide is cheap, fast, very effective and environmentally benign," said Taylor. "This is an important new application for a safe, familiar technology."
Current wet-stripping methods use acid mixtures that are hazardous to workers and are highly corrosive and flammable. The only hazard of the SCORR system is the use of a pressurized vessel for the stripping treatment; however, large-scale commercial use of supercritical carbon dioxide is well known and has been used safely in the food, pharmaceutical and petrochemical industries for many years.
If adopted by the semiconductor industry, the SCORR system 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 carbon dioxide for its final rinse step, it eliminates the need for high volumes of water pulled from reservoirs where fabrication plants are located, including the arid Southwest.
The SCORR system can be used to clean most organic contaminants from other inorganic substrates without altering the substrate surface, and is therefore ideal for many precision-cleaning applications.
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and the Washington Division of URS for the Department of Energy's National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.