Water distribution within immersed polymer films

The emergence of immersion lithography for the extension of current lithography tools requires a fundamental understanding of the interactions between the photoresist and the immersion liquid such as water. Neutron reflectometry was used to measure the water concentration depth profile within immersed photoresist films. The bulk of the films swelled to the equilibrium water concentration. However a gradient in water concentration was observed near the polymer/substrate interface. Dependent on the relative hydrophilicity of the polymer and the substrate, either a depletion or excess of water was observed at the interface. Using HMDS treated silicon wafers as the substrate results in approximately 17 % water by volume at the interface. The interfacial concentration decreases (or increases) to the bulk water solubility limit approximately 40 Å from the substrate. As the total film thickness approaches this length scale, the substrate induced concentration gradients lead to a film thickness dependent swelling; enhanced or suppressed swelling is witnessed for the excess or depleted interfacial concentrations, respectively. Variation of the substrate surface energy allows for tuning of the interfacial water concentration, ranging from 30% to less than 1% water by volume.