There is an ever-increasing need for miniaturization of devices used for telecommunications, and information storage. Emerging photonic and electronic architectures are incorporating designs that involve creating complex three dimensional structures in order to further decrease the foot print of systems. In order to create such 3D structure a variety of approaches are being explored. Progress has been made in using molecular self-assembly to build up structures with unique properties in a "bottoms-up" approach. However, such self-assembly is limited predominantly to forming periodic structures. Fabrication of three-dimensional objects of arbitrary shapes comprised of polymeric, metallic and nanostructured materials with feature sizes approaching those achievable by either photo- or electron-beam lithography is a difficult task. In some cases 3D structures can be fabricated using sequential layer-by-layer processes requiring many separate processing steps that are cumbersome and costly, but also restrictive in the range of 3D structures that can be formed. Alternatively, one can use the technique of stereolithography, for which commercial instrumentation is available to produce 3D structures made of polymers, but this technology is limited to a minimum feature size of about 50 microns.
Two-photon 3D lithography is a powerful new approach to the fabrication of complex microstructures. This process requires the use of focused laser beams to excite molecules by a two-photon absorption mechanism. Using this method of excitation, absorption occurs only in the focal volume providing a way to activate chemical processes to produce patterns in materials with pinpoint control in three-dimensions. By computer controlled scanning of the laser beam focus within a photochemically active precursor material, virtually any three-dimensional structure can be fabricated with submicron resolution. Recently, we have developed highly efficient new two-photon absorbers, which advance the state-of-the-art one hundred fold, and have used them to demonstrate two-photon 3D lithography of polymers in negative tone resists and more recently in positive tone resist and in metallic systems. This work illustrates the promise for a practical fabrication technology with readily available laser sources.
Two-photon 3D lithography may impact future electronic and photonic technology because of: 1) the limitless possibilities for the types of three dimensional structures that can be fabricated, 2) the ability to directly pattern materials ranging from transparent polymers to highly conducting metals, 3) the possibility to create "micromolds" that can be used for templated growth of a vast range of materials, allowing for the integration of disparate materials into devices and 4) the capability to fabricate structures from the nanoscale (< 200 nm) to the microscale, providing a means of integrating nanostructures with well established microscale technologies.