Resist and Residue Removal Using Gas-Expanded Liquids
Spuller, Matthew Thomas
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Each new generation of integrated circuits and other nano-structured devices is produced at ever decreasing length scales. The extension of conventional liquid-phase processes for the manufacturing of these devices is uncertain. This work investigates the ability of liquids to wet nanoscale features. A model for wetting time is derived that may be used to identify those geometries for which wetting is critical. Conditions under which wetting time is significant may result in low yield and poor uniformity, and may require alternate-phase processing. Furthermore, the dependence of wetting time on the properties of the fluid are quantified so that fluids may be designed to have optimal properties and thus optimal performance for wetting. The resulting model can be used as a tool to predict future processing requirements, and when necessary, to design novel processes implementing alternative phase fluids such as gas-expanded liquids (GXLs). This study also quantitatively predicts specific effects associated with modified transport properties for dissolution and transport in nanoscale features. The use of GXLs is a particularly promising alternative to conventional liquid-phase processes. GXLs have superior mass transport properties relative to liquids, but can maintain the solvent strength necessary for IC process steps such as post-etch residue removal and photoresist stripping. In addition, the environmental benefits associated with CO2-based processes can be substantial. Conceptual demonstration of the use of carbon dioxide (CO2)-expanded liquids for photoresist and residue removal has been performed. GXLs containing up to 75% CO2 are equally as effective as the pure solvents for removal of PHOST-based films. These experiments indicate that GXLs have potential applications in photoresist stripping and post-etch residue removal, in which cost savings due to reduced solvent use can be substantial. The removal of films with GXLs has been evaluated primarily with x-ray photoelectron spectroscopy (XPS). Additionally, an in situ optical technique has been developed for film and GXL diagnostics. This technique has been used to evaluate the response of PHOST-based thin films to GXLs and to monitor density changes of liquids upon gas expansion.