The first practical implementation of liquid immersion lithography will use highly purified water (with a refractive index of 1.44 at 193 nm) as the fluid. Since the photon energy is unchanged, resists designed for conventional dry 193 nm lithography in principle can be applied. However, the addition of a fluid between the lens and resist surface introduces several complications (see Figure 2). The liquid-resist interface can foster materials transport into and out of the resist film, and studies have established that photoresist components can be rapidly leached from 193 nm resist films by water. At the high wafer velocities required for manufacturing, breakup of the water layer into small defect-producing droplets is also a concern. The application of a thin overcoat film can both suppress materials exchange between resist and fluid, and provide a means to avoid droplet formation by controlling the surface energy at the fluid interface. An ultimate goal is to devise resist compositions tailored for immersion application that do not require a protective overcoat. In the future, even greater improvements in resolution can be realized by further increasing the refractive index of the fluids, lens materials and resists.