Methacrylate-based polymer films useful in lithographic applications exhibit different glass transition temperature-confinement effects at high and low molecular weight

Abstract

The effects of confinement on polymer films are important in applications related to photoresists. To optimize resolution, methacrylate polymers used in photoresists are often low molecular weight (MW). We use ellipsometry and fluorescence to study how the glass transition temperature (Tg) is affected by confinement in silica-supported films of low and high MW poly(1-ethylcyclopentyl methacrylate) (PECPMA) and poly(methyl methacrylate) (PMMA). With decreasing nanoscale thickness, Tg is nearly invariant for high MW (Mn = 22.5, 188 and 297 kg/mol) PECPMA but decreases for low MW PECPMA, with Tg – Tg,bulk = −7 to 8 °C in a 27-nm-thick film (Mn = 4.1 kg/mol) via ellipsometry and −15 °C in a 17-nm-thick film (Mn = 4.9 kg/mol) via fluorescence. Fluorescence studies using a 20-nm-thick dye-labeled layer in multilayer, bulk PECPMA films reveal a much greater perturbation to Tg in the free-surface layer for low MW PECPMA, which propagates tens of nanometers into the film. The effect of MW in single-layer monodisperse PMMA films is even more striking; Tg increases with confinement for high MW but decreases for low MW, with Tg – Tg,bulk = 9 °C in a 12-nm-thick film (nominal MW = 509 kg/mol) and −16 °C in a 17-nm-thick film (nominal MW = 3.3 kg/mol). The strong influence of MW on confinement effects in PECPMA and PMMA is in contrast to the previously reported invariance of the effect with MW in supported polystyrene films, reconfirmed here.