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Two-photon excitation in laser scanning photolithography allows exposure of patterns not possible with conventional one-photon direct writing. In our experiments two red photons from a highly focused subpicosecond colliding pulse mode-locked dye laser are simultaneously absorbed by initiator molecules to affect a photochemical reaction that is normally driven by single-photon absorption using ultraviolet light. The quadratic dependence of the two-photon absorption rate on the incident intensity confines excitation to a submicron focal volume. By scanning this volume in a 3-d pattern through a thick layer of photoresist it is possible to expose arbitrary three dimensionally defined regions. Preliminary results showing half micron wide trenches of very high aspect ratio, and resist structures with undercutting edges, all produced with only a single development step, demonstrate. the potential utility of two-photon excitation in microfabrication.
Wu et al. (Mon,) studied this question.