Immersion lithography has emerged as an alternative to the 157 nm node and as such understanding of the interactions between the photoresist and immersion fluid (water) has become a pressing issue. The water concentration within the model photoresist films when immersed was determined using neutron reflectivity. The substrate/polymer interface was found to play an important role, both in determining the water concentration near the interface as well as the morphology of the film surface. The immersed films were not stable over the course of hours when supported on a silicon wafer with a native oxide surface, but were stable when the substrate was first treated with hexamethyldisilazane (HMDS). Surprisingly depending upon the hydrophilicity of the photoresist film, the HMDS surface can either induce water excess or depletion near the interface. These surface induced concentration gradients lead to an apparent film thickness dependent swelling in thin polymer films. In addition to the substrate surface controlling the film stability when completely immersed, the substrate also influences the surface morphology when in contact with a water droplet. The influence of photoresist film thickness and molecular mass on the surface morphology was examined using atomic force microscopy (AFM). The substrate / polymer interface was found to have significant influence on the moisture absorption and surface morphology of immersed photoresist films.
Citation: Journal of Microlithography Microfabrication and Microsystems
Pub Type: Journals
interfaces, thin films