Control of moisture at buried polymer/alumina interfaces through substrate surface modification

Moisture absorption in poly(4-tert-butoxycarbonyloxystyrene) (PBOCSt) films supported on Al ₂O ₃ sputter coated silicon wafers is measured using neutron and X-ray reflectivity. Accumulation of water at the interface during moisture exposure results in an apparent film-thickness- dependent swelling for ultrathin PBOCSt films. The swelling of a film on Al ₂O ₃ is less than the swelling of a film of the same thickness on SiO _x for films thinner than 20 nm. This is due to comparatively less moisture accumulation at the Al ₂O ₃/PBOCSt interface. A simple, zero adjustable parameter model consisting of a fixed water-rich layer at the interface and bulk swelling through the remainder of the film describes the thickness-dependent swelling quantitatively. The influence of four different Al ₂O ₃ surface treatments on the moisture distribution within PBOCSt films was examined: bare Al ₂O ₃, tert-butylphosphonic acid, phenylphosphonic acid, and n-octyltrichlorosilane. Both the phenyl and the octyl surface treatments reduce the accumulation of water at the polymer/substrate interface. The tert-butyl treatment does not reduce the interfacial water concentration, presumably due to insufficient surface coverage.