Two-Photon initiated polymerization (TPIP) has shown great promise for fabrication of complex micro- and nano-structures. The method has been used to fabricate such structures over small areas (< 1 mm2) because of slow fabrication speeds and resulting long fabrication times. In order for TPIP to reach practical application in a commercial setting fabrication times need to be reduced by orders of magnitude. We report results on a highly photosensitive initiation system for photoresists based on free radical and cationic polymerization, where photosensitivity is increased 102- to 103-fold compared to previously reported photoinitiation systems. Threshold writing speeds are determined for critical exposure conditions, including laser power, type and concentration of photoinitiation system, and photoresist type. Surface roughness, a critical parameter in applications such as optics and microfluidics, for example, is also used to determine threshold writing speed. The utility of the approach is demonstrated by making a cell phone keypad light guide from a microreplication tool fabricated using the highly photosensitive photoresist.
ABSTRACT:Two-photon fabrication is a powerful method of fabricating complex microstructures. Superresolution by methods analogous to stimulated emission depletion (STED) has been described previously, enabling sub-100 nm imaging with 800 nm light. STED-related methods of enhancing imaging resolution require photoresists with exposure conditions for which the photoresist exhibits negative contrast, i.e., image density decreases with increasing exposure from the depletion beam. We have observed decreasing voxel size with increasing exposure during two-photon initiated polymerization of acrylate- and methacrylate-based photoresists, that is, negative imaging contrast, γ < 0, independent of the type of photoinitiator. Negative contrast is not observed in epoxy-type photoresists containing photoacid generators. An investigation of the exposure conditions has led us to conclude that radical-radical recombination at high exposure is responsible for negative contrast. Results of the investigation, discussion of the proposed mechanism for negative contrast and implications for two-photon superresolution will be presented.
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