Residual stress relaxation and stiffness in spin-coated polymer films: Characterization by ellipsometry and fluorescence

Ellipsometry and fluorescence are used via measurements of film thickness and pyrenyl dye emission spectral shape, respectively, to characterize residual stress relaxation in polystyrene (PS) films. In particular, fluorescence of pyrene-labeled PS (MPy-PS) films, with ∼1 mol% pyrene label, provides sensitivity to film stress relaxation and stiffness by the dependence of the ratio of the first to third vibronic peak intensities (I₁/I₃) of the pyrenyl dye to nanosecond-time-scale molecular caging. Both techniques show that residual stress relaxation occurs over a period of hours despite the PS films being 15-40°C above the film glass transition temperature (T_g). Both techniques also show that film T_g is unaffected by stress relaxation, even when stress relaxation is accompanied by measurable changes in thickness. Fluorescence shows that stress relaxation time follows an Arrhenius temperature dependence with an activation energy of ∼110 kJ/mol, which is consistent with stress relaxation occurring by β-relaxation. Using a bilayer/fluorescence technique with bulk bilayer films, it was observed by I₁/I₃ measurements that a 30-nm-thick MPy-PS layer located at a glass substrate interface is stiffer than a 30-nm-thick MPy-PS layer located at a free surface. Over a 20-400 nm thickness range, fluorescence of MPy-PS films show a significant effect of substrate on molecular caging and hence stiffness, with stiffness increasing in the following order: free-standing films (no substrate) < films supported on a polydimethylsiloxane (soft) substrate < films supported on a glass (hard) substrate.