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Contrast Enhancement based on Acid Equilibrium for Chemically Amplified Resists

We have developed a method of enhancing the resist contrast governed by the hydrogen-ion concentration in chemically amplified resist films. We consider the distribution of permittivity and the equilibrium constants of acids generated by photoirradiation. The hydrogen-ion concentration in a resist film containing a weak acid increases with increasing permittivity. If the permittivity of exposed areas is higher than that of unexposed areas, the resist contrast is greater than that of resist films whose permittivity is the same in both exposed and unexposed areas. Accordingly, the resolution capability is enhanced and the depth of focus is increased. Calculations based on a model in which an additive whose polarizability is increased by photoirradiation is mixed within the resist, indicated that (1) the resist contrast depends on the amount of polarizability change and the concentration of the additive, (2) the resist contrast takes a maximum value at an optimum exposure dose, and (3) an appropriate combination of available additives and available exposure doses can result in a resist contrast higher than that obtained when strong acids are generated and also higher than the optical contrast.