Supercritical Fluids as Processing Media for Nanostructured Devices

Supercritical fluids including carbon dioxide offer a combination of properties that are uniquely suited for device fabrication at the nanoscale. Liquid-like densities, favorable transport properties, and the absence of surface tension enable solution-based processing in an environment that behaves much like a gas.These characteristics provide a means for extending “top-down” processing methods including metal deposition, cleaning, etching, and surface modification chemistries to the smallest device features. The interaction of carbon dioxide with polymeric materials also enables complete structural specification of nanostructured metal oxide films using a “bottom-up” approach in which deposition reactions are conducted within sacrificial, pre-organized templates dilated by the fluid.The result is high-fidelity replication of the template structure in a new material. In particular, block copolymer templates yield well-ordered porous silica and titania films containing spherical or vertically aligned pores that can serve as device substrates for applications in microelectronics, detection arrays, and energy conversion. Finally, the synthesis of nanoparticles and nanowires in supercritical fluids is developing rapidly and offers promise for the efficient production of well-defined materials. In this review, we summarize these developments and discuss their potential for nextgeneration device fabrication.