Nanosphere Template Lithography for the Fabrication of Aluminum Plasmonic Antenna

The use of aluminum (Al) in plasmonics has garnered significant attention recently due to its UV optical properties and low material costs. Traditional plasmonic metals such as silver and gold have favorable visible and NIR optical properties and have been successful used as surface enhanced Raman (SERS) substrates and as refractive index sensors. Although silver and gold (Au) have excellent plasmonic properties, they have limited use in UV surface enhanced spectroscopies. Al is one metal that supports plasmon resonances at these low wavelengths. One of the challenges of using Al as a plasmonic material is that it rapidly forms a terminal oxide layer that is chemically robust and makes nanostructuring difficult. Consequently much of the work that has been done with Al plasmonics has used electron beam lithography (EBL) to create a pattern in an organic mask layer.[1, 2] While this technique allows for excellent size and shape control, it requires expensive equipment and is limited to the production of templates over a small substrate area. One technique used to fabricate nanostructures over large substrate areas is nano-sphere template lithography (NTL) which uses dispersed polystyrene nanospheres as templates. NTL has been employed in the fabrication of gold and silver nanodisks, nanorings, and nanocrescents.[3, 4] NTL has seen limited success in Al plasmonics, as it has only been used to produce hole arrays. This is because NTL requires a directional etch step to remove the bulk metal film. The robust native oxide of Al it makes it difficult to use reactive ion etching to remove the metal film while leaving the polystyrene templates intact. Here we deposit a copper mask at various incident angles to create a hole-array that can be filled with Al. This method opens up NTL for the production of Al nanoantennas with optical properties tunable throughout the UV and NIR wavelengths and can also be used for other metals such as gold (Au).