Capillary-force-directed self-assembly of suspended nanomechanical structures above the electronic circuit layer

This report reviews progress in capillary-force-directed self-assembly fabrication methods together with applications of the suspended structures as fiber-based optical components, cells scaffolds for tissue regeneration, and as templates for suspended microfluidic networks and nanomaterials. Capillary forces can direct polymeric solutions, melts and nanocomposites to form near constant diameter fibers, not just over long distances, as in electrospinning, but over micron to centimeter distances typical of integrated circuits and MEMS. Crude hand-brushing of polymeric solutions over micropillar arrays has produced, in a matter of seconds, uniform arrays of near identical nanofibers and trampolinelike membranes suspended between the micropillars. Direct point-to-point writing has also been accomplished with AFM tips, capillary syringes and electrospinning jets. The brush-on method, while fast, does not produce arbitrary patterns. The direct-write method, while producing arbitrary patterns, is slow. Recently, fast arbitrary patterning has been demonstrated by photopatterning holes through thin suspended polymer films, followed by thermal annealing which causes holes, driven by capillary force, to expand, form threads and thin into fibers. For an ablation threshold of polystyrene of 10 mJ/cm2 at 193 nm there are many adequately intense excimer lasers that could be incorporated into roll-to-roll systems. At a hole exposure threshold of ~1 mJ/cm2 , even current 193 nm wafer stepping projection printers could pattern wafers with arbitrary suspended structures at economically sustainable production rates of greater than 50 wafers per hour. A new method of extending this fabrication method to three-dimensions is also described, that additionally overcomes the need for prefabricated micropillar arrays.