Aqueous-based photoresist drying using supercritical carbon dioxide to prevent pattern collapse

Abstract

A supercritical drying process was developed to eliminate the capillary forces naturally present during normal drying of photoresist materials. Supercritical carbon dioxide (scCO2), organic solvents and surfactants were used to prevent the collapse of high-aspect-ratio structures fabricated from aqueous-based photoresist. Nondistorted resist lines were patterned with this process with aspect ratios of at least 6.8. Water rinsed resist structures cannot be dried directly with scCO2 due to the low solubility of water in the supercritical phase. In our process we introduced the replacement of the aqueous rinse by n-hexane mediated by a compatible surfactant. The surfactant allowed to incorporate the aqueous phase into micellar microdomains in the organic phase while keeping the interfacial tension at values close to zero. Noncollapsed supercritically dried structures were rewet in n-hexane or water and dried using nitrogen at atmospheric pressure. Under these conditions, the patterns were collapsed as a result of capillary forces acting on the resist walls. The effect of capillary forces on pattern stability were qualitatively compared to the relative surface tension values between n-hexane and water. The advantage of using supercritical CO2 in the drying stage of resists compared to normal drying highlights the importance of eliminating the surface (or interfacial) tension forces created by the rinse fluid, if a reduction of linewidth and spacing between resist lines is pursued, while keeping the aspect ratio constant.