Influence of substrate chemistry on the properties of ultrathin polymer films

Abstract

Controlling defect density in ultrathin polymer films is of great importance to the microelectronics industry. Using polystyrene (PS) as a model system, we demonstrated the influence of the underlying surface chemistry on polymer mobility as manifested through changes in glass transition temperature (Tg) and dewetting behavior. All high molecular weight (Mw=90k) polystyrene (PSH) films exhibited Tg depressions (1–12 °C) with decreasing film thickness (90–18 nm). Self-assembled monolayer (hexadecanoic acid) surfaces show the greatest influence on Tg depression, followed by gold and silicon surfaces. Gold capping of PSH films on silicon had no distinct effect compared to their uncapped counterparts. Low molecular weight (Mw=4k) polystyrene (PSL) films dewet on glass and self-assembled monolayer (SAM) surfaces. PSL films on SAM surfaces dewet thicker and to a greater extent than those on glass. Gold, both as a substrate and as a capping layer, suppressed dewetting for all film thicknesses measured. We curve-fit Tg behavior using a three-layer model with adjustable layer densities directly translating into changes in Tg. We calculated the long-range forces between the polymer film and the substrate. The resulting pressure increases could not directly account for our changes in Tg but coincided with our experimental observations for non-metallic interfaces.