Self-Assembly in Supercritical Fluids: Using Photolithography for Additive Manufacturing

Photolithography and solution phase self-assembly are the two primary approaches to creating nanostructures. The former is the powerhouse of computing, the latter is the manufacturing method of living organisms; the two share very little common ground. Photolithography is a subtractive process, employed with staggering precision, but typically used with monolithic materials. Solution phase self-assembly is an additive process that can be employed with systems of arbitrary complexity. However, entropy places limits on the precision of the technique, especially over large length scales. In an effort to bridge these non-intersecting methods of manufacturing, we have developed a materials deposition technique that leverages the unique properties of supercritical fluids. Counter to typical solutions, the saturation solubility of a solid solute first increases, and then decreases with increasing temperature as a solution is heated near its critical point. We have leveraged this non-monotonic solubility behavior to deposit films by holding the solution temperature near the solubility maximum and heating the substrate. Because the saturation solubility decreases with increasing temperature, materials are precipitated from the supercritical solvent, forming a thin film. Because the substrates are heated resistively, we can control the local heating by patterning resistive heating elements on the substrate. We accomplished by performing photolithography on a substrate coated with indium tin oxide. When these lines were resistively heated, material deposited onto them. The technique also works when a thin polymer layer is inserted between the electrode array and the supercritical fluid, allowing the photolithographically patterned electrode array to serve as a patterning master for additive manufacturing onto thin and flexible materials. Moreover, the technique does not rely on line of sight mass transfer or cumbersome printing heads. We leveraged this advantage to deposit materials on to the curved interior of an elastomeric hemisphere approximately 3 mm in diameter.