Supercritical CO₂ Uses

At the recent SEMI Microelectronic Materials Strategy Symposium (M²S²), Professor Christopher Ober, chair of the materials science and engineering department at Cornell University (Ithaca, N.Y.), presented an overview of recent uses of supercritical CO₂ applied to semiconductor manufacturing. Topics included cleaning and stripping, resist technologies, metal deposition and novel applications. The scope of research depicts a future cleaning with non-toxic, non-flammable, recyclable solvents that eliminate exposure to harmful chemicals without generating aqueous waste.

Photoresist and residue stripping is the first to be integrated into processing lines. Supercritical cleaning and resist stripping takes place inside cleanrooms, contained in a single tool modular — unlike snow cleaning. Resist stripping systems are generally used as a co-solvent with CO₂ to bathe a wafer in a high-pressure chamber. Another tool for CO₂ cleaning systems is polar-head fluorinated and non-fluorinated surfactant molecules for reverse-microemulsion assisted contaminant removal. These cleaning technologies can also be applied to initial wafer surface cleaning. Other photoresist technologies such as drying, selective developing and spinning have been conducted in CO₂.

Chemical fluid deposition (CFD) of metals is another supercritical application. Metals including copper and nickel have been deposited onto silicon by the reduction of organometallic compounds with hydrogen within a supercritical CO₂ carrier. Silylation of silicon wafers and etching of resist have also been conducted.

Inventive supercritical CO₂ applications suggesting novel modifications to the microelectronics-processing scheme are also being researched. Directly patternable dielectric materials have been demonstrated from CVD deposited F-containing precursors. Other applications include polymer foaming by supercritical fluid to form low-k interconnect materials, and removal of metals and inorganic contamination by reacting the inorganic contamination with a conversion agent before removing with a solvent in supercritical fluid CO₂.

For additional information on clean processing, go to www.semiconductor.net/clean