Polymer dynamics and diffusive properties in ultra-thin photoresist films

A series of experiments are presented to demonstrate thin film confinement effects on the diffusive properties in poly(tert-butoxycarboxystyrene) (PBOCSt). Bilayer diffusion couple measurements reveal that as the thickness of a PBOCSt film is decreased, the kinetics of the deprotection reaction-front propagation (a process involving both the diffusion and reaction of photochemically activated acidic protons) are dramatically hindered. Incoherent neutron scattering measurements suggest that this retardation can be traced to a suppression of local fast relaxations (200 MHz or faster) native to the PBOCSt polymer. The reduced mobility in the thin PBOCSt films is further confirmed with moisture vapor uptake studies performed on a quartz crystal microbalance (QCM). As the film thickness drops below 500 Å there is a strong reduction in the diffusivity of water in the film. In total, these are the first evidences suggesting that the deviations in lithographic performance with decreasing film thickness observed with the bilayer experiments are due to changes in mobility, not reactivity, within a chemically amplified resist.