The spatial distribution of polymer photoresist and deuterium labeled base developer highlights a fraction of the line edge that swells but does not dissolve. This residual swelling fraction remains swollen during both the in situ aqueous hydroxide dissolution (development) and water rinse steps uncovering that the final lithographic feature is resolved by a collapse mechanism during the drying step. These new insights into the mechanism of lithographic feature formation were enabled by contrast variant neutron reflectivity methods with nanometer resolution.