Physical Supercritical Fluid Deposition—A Scalable Process for Controlling Polymer Self-Assembly on Surfaces

Controlling materials self-assembly over a wide range of length scales is vital to advanced applications in a wide variety of fields. The key challenge in developing a fabrication technique is not only to form nanoscale structures reproducibly and robustly, but to direct their placement to form mesoscale and even macroscale structures. To this end, we are investigating self-assembly processes in supercritical fluids. In a correctly chosen solvent system, a solute can exhibit a peak in its saturation solubility with respect to temperature. By holding the solvent at the temperature of the peak and heating the substrate, the solute can be precipitated from the supercritical solvent onto the substrate. The location of the deposition can be controlled by locally heating the substrate, for example, by passing current through lithographically patterned conductive traces. This coupling of solution phase self-assembly and photolithography gives physical supercritical fluid deposition the potential to control material deposition at all length scales. The unique deposition conditions also provide additional means of controlling the nanoscale structure of materials resulting in uncommon morphologies for polymer semiconductors. The self-assembly process takes place in several steps including a solution phase pre-aggregation step that is sensitive to the Rayleigh number of the solution. Following deposition onto the substrate, the presence of solvent additives can facilitate the formation of larger scale structures by increasing the surface mobility of the deposited material.