High Photosensitivity Two-Photon Photoresists for Large Area Surface Microstructuring
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High Photosensitivity Two-Photon Photoresists for Large Area Surface Microstructuring Robert J. DeVoe1, Tzu-Chen Lee1, Jeremy K. Larsen1, David A. Ender1, Jennifer J. Sahlin1, Craig R. Sykora2, Cheryl A. Patnaude1, Matthew R. Atkinson1, Michael E. Griffin1, Brian J. Gates1, and David H. Redinger1 1
Corporate Research Labs, 3M Co., 201-3N-05 3M Center, St. Paul, MN 55144, U.S.A.
2
Electronic Markets and Materials Lab, 3M Co., St. Paul, MN 55144, U.S.A.
ABSTRACT 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. INTRODUCTION Two-photon initiated polymerization (TPIP) has emerged as one of the most flexible methods of fabricating complex micro- and nano-structures. Many optical, mechanical, biological and medical applications can be addressed uniquely by TPIP.1,2,3 The method has been limited to fabricating arrays of structures over small areas (< 1 mm2) because of fabrication speeds < 103 μm/sec and resulting fabrication times that can reach days or weeks for areas over 1 mm2.4 In order for TPIP to reach practical application in a commercial setting fabrication times need to be reduced by orders of magnitude. Most work using TPIP has used a spot-writing method, a method that enables fabrication of very complex structures, but has limited throughput, i.e., limited area that can be fabricated in a reasonable timeframe. Work using multiple beams5 and dynamic mask exposure6 has shown some promise in improving throughput. All of these writing strategies are limited by the low efficiency for initiation of polymerization that is intrinsic to the two-photon process. The rate of initiation of polymerization (νip in moles/liter/sec) in TPIP is proportional to the square of the light intensity (I2 in watts/cm2), the cross-section for simultaneous absorption of two photons (δ in cm4 sec
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photon-1), the concentration of the light-absorbing molecule ([Dye], moles/liter), and the quantum efficiency of initiation of polymerization (σ, no units):2 νip
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