Three-Dimensional Nanofabrication Using Multiphoton Absorption
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One of the major goals of nanotechnology is to be able to fabricate complex, functional nanostructures at will. In recent years, there has been a tremendous effort from many groups to develop microfabrication techniques that can be adapted to the realm of nanostructures. One driving force for such studies is the desire to break free from the constraints of conventional lithography. Although processes such as photolithography and electron beam lithography have been extremely successful in the production of electronic circuits with ever-decreasing feature sizes, these techniques have significant limitations that preclude their use in many other contexts. These techniques are incompatible with many chemical and biological environments, can create sub-100-nm features only with difficulty, are not capable of fabricating structures with complex 3-D shapes, and generally require expensive facilities. Techniques such as nanoimprint lithography, proximity probe lithography, and soft lithography are being developed to overcome these limitations. For instance, replica molding can be used to fabricate and replicate 2-D patterns with a resolution of 30 nm. It has also been shown that intricate 3-D structures can be fabricated by soft lithography, although generally with feature sizes of 25 µm or greater.
Microstereolithography allows for 3-D microfabrication with a feature size down to few micrometers. UV lasers are focused on a thin layer of a liquid photoresist that undergoes solidification upon absorption of light. A second layer of photoresist is then added and another pattern is imposed over the first one. This process is repeated until the desired structure is completed. A developer solution is then used to wash away the unsolidified material, liberating the freestanding 3-D structure. Prototyping under these conditions is slow. For each layer, fresh resin must be spread flat on the polymerized object, which is the rate-determining step for the entire process. The wavelength of light and the film thickness are the major factors in determining the resolution limit of a few micrometers.
Since the first papers about microstereolithography appeared in 1993, many variations have been proposed and demonstrated that improve resolution and production time. Among these, the one that has allowed the fabrication of the smallest features is two-photon absorption polymerization (TPAP), which was introduced by Strickler and Webb in 1991. It was the work of Maruo et al. in 1997 that first demonstrated the promise of this method for the fabrication of nontrivial 3-D structures with submicrometer resolution. In this article, we will describe the basic principles of TPAP, illustrate how it can be applied to the fabrication of small 3-D structures with high accuracy, and discuss new developments that should bring this technique into the realm of nanofabrication.