3D photolithography based on image reversal

In this paper we present a novel fabrication method for the realization of complex 3D multi-layer structures with commercially available photoresists. This method is based on the observation that during an image-reversal process, the post exposure bake (PEB) that is used to reverse the contrast of the exposed pattern reduces the sensitivity of the unexposed photoresist at the same time. In multi-layer lithography, this phenomenon, along with non-uniformly distributed dose can be exploited to eliminate the re-patterning effect of the subsequent exposures and thus makes suspended 3D structures possible. In this presentation we demonstrate this observation experimentally, and fabricate "woodpile" structures (greater than or equal to 3 layers) using the proposed method.