Electron beam lithography and induced deposition for nanoplasmonic applications

Abstract

Nanoplasmonics concerns the study of light-metal interactions on a subwavelength scale, exhibiting behaviour able to achieve arbitrary control and manipulation of light at the nanoscale, an important goal for the further development of nanophotonic devices. As the field of plasmonics advances, research is looking beyond the materials and fabrication techniques traditionally employed. This thesis discusses the design, fabrication methods, and characterisation of nanoplasmonic structures. A comparison between two electron beam based fabrication techniques, electron beam lithography (EBL) and electron beam induced deposition (EBID), is presented. The discussion covers both the fabrication technology and the properties of the resulting nanoscale structures: gold nanoscale features obtained via standard EBL, and tungsten structures achieved through EBID. As a mature technology, the well-understood characteristics and reliable fabrication procedures of electron beam lithography are weighed up against the limitations of top-down planar fabrication. Electron beam induced deposition is presented as an alternative technology, able to achieve nanoscale fabrication resolution with a point-and-shoot bottom-up deposition technique, but constrained by the lack of optimised fabrication settings as a result of incomplete understanding of a complex set of patterning parameters. Direct-write EBID technology offers to overcome several of the limitations and challenges of electron beam technology, including three-dimensional and greyscale patterning, and precise alignment and orientation of nanoscale features on arbitrary substrate patterns. This thesis also presents the discussion of chirped plasmonic diffraction gratings as a specific application in nanoplasmonics. Their theoretical design based on Fourier analysis and simulation-based design, their fabrication using both electron beam lithography and induced deposition, as well as the characterisation of their far-field diffraction pattern are discussed in detail. As part of the characterisation step, a Fourier microscopy setup for the measurement of the far-field diffraction patterns of nanophotonic structures in both reflection and transmission was constructed.