Abstract:
Periodic metal and metal-dielectric composite nanostructures have been of interest from
the field of plasmonics and metamaterial fabrication. In order to exploit the behavior of
these unique materials in the visible region of the optical spectrum, these structures
need to be significantly shorter than the wavelength of response, and hence fabrication
of these have posed unique challenges. One of the key fabrication aspects is the metal
thin film deposition. This study has looked at key parameters in PVD which influence
the grain structure and morphology, in one of the metals of interest, Ag, and further
examined how these factors influence formation of these periodic nanostructures. Our
findings indicate small grain sizes formed with high source-to-substrate spacing are
optimal conditions for forming nanostructures with different geometries with size less
than 100nm. Additionally our studies also indicate these conditions provide films with
least agglomeration and a smooth texture which could have significant impact on their
optical behavior. The study also looked at formation of nanostructures through
different processes – (i) additive process via lift-off and electron beam lithography
(EBL) and (ii) subtractive processes of ion beam milling, and reactive ion etch (RIE).
This included examining three metals of interest in plasmonics- Ag, Au and Al. Our
findings indicate that the optimized process is dependent on the metal systems and liftoff
with EBL remains the most flexible option. RIE may be suitable for Al based
systems where we form a volatile species during etch while it may not be as successful
for Au and Ag. For isolated nanostructures as discussed in this paper, ion beam etch is
highly dependent on grain sizes and may have some fundamental limitations in isolating
structures. The structural and morphological characterization of nanostructures has
also been of importance and has been carried out as part of this work with SEM, AFM,
EDS, and TEM studies. The impact and application of these structures could be greatly
enhanced by their formation in a flexible polymer membrane and this was also
demonstrated as part of this thesis. Finally, we also present some optical data of these
nanostructures where we see a difference in extinction coefficient of these structures
based on both geometry and metal choice.
