Multiphoton Absorption Polymerization: Issues and Solutions

Abstract

Multiphoton absorption polymerization (MAP) is gaining importance as a means for fabricating 3-D micro-devices. By focusing an ultrafast laser inside a prepolymer resin, radical polymerization can be initiated by two-photon absorption of a photoinitiator. The result is a highly cross-linked solid polymer point, or voxel, which is extended to create complex shapes by scanning the laser beam in a 3-D pattern. The geometric freedom combined with sub-micron resolution provided by MAP is unparalleled by any other microfabrication techniques. However, MAP suffers from three issues; the novelty of the technique itself, the fact that it is inherently a serial process, and the restriction of device materials to cross-linking polymers. To better understand the technique, the MAP fabrication setup is described in detail. Specific techniques of fabrication, such as how to design and wash microstructures, are also described. To address the second issue, micro-transfer molding (TM) has been applied to make high fidelity molds of complex master microstructures, followed by a fast and easy replication step to make duplicate structures. This technique has even been extended to replicate structures with closed-loops, such as arches or coils, which should be topologically impossible to mold and replicate. The third issue has been addressed in two ways, by laser-direct-writing of metal patterns on 3-D substrates and by changing the surface chemistry of the polymer to contain primary amines. Laser-deposited metal can be made conductive by further electroless growth yielding 3-D conducting patterns. The amine surface modification can be used for any number of chemistries, including catalytic metal seeding, which could then be grown into a metal coating. This new flexibility in surface chemistry, along with the enhanced speed of TM, ensures that MAP will be a practical technology to create micro-devices. Numerous electrical, mechanical, optical, and biological applications of MAP are described as well as potential future applications. To date this work has resulted in 9 peer reviewed publications, and 2 more which have recently been submitted.