Decoupling the smoothness, diameter and pitch in large silicon nanowire arrays made by metal-assisted chemical etching (MACE)

J. Yeom, D. Ratchford, C.R. Field, P.E. Pehrsson
Naval Research Laboratory, US

Keywords: metal-assisted chemical etching, nanosphere lithography, nanowire

Summary:

The combination of nanosphere lithography using polystyrene nanosphere nanomasks with metal-assisted chemical etching (MACE) is a fast and inexpensive way to create very large arrays of vertically aligned silicon nanowires (VA-SiNWs) for applications ranging from photonic crystals to chemical sensors. The important array properties like the nanowire density, diameter, and sidewall smoothness, are difficult to independently control, and can profoundly affect the device performance. Fabrication challenges include decoupling the nanowire diameter and pitch, smooth, large-scale reduction of the nanosphere diameter by RIE, and efficient MACE on arrays in which the nanospheres are separated by large stretches of unbroken gold film. Here we present the fabrication of 2D periodic, non-close-packed arrays of nanospheres with precisely controlled diameters, pitch, and roughness, and demonstrate the utility of the sphere arrays as a template to form arrays composed of well-separated, narrow silicon nanowires. Controlled ICP-RIE reduces the self-assembled nanospheres by >75%, while retaining their surface smoothness and hexagonal arrangement. An array of silicon nanowires with a large pitch-to-diameter ratio (e.g., a 50 nm diameter and 500 nm pitch), not commonly found in open literature, is created from such nanosphere arrays. Vivid structure color is demonstrated with different VA-SiNW arrays as potential applications.