Silver Surfaces and Nanostructures for Imaging and Sensing Applications.
Authors:   , Colenso Hamish 
Published: January 2024

<p><strong>Silver is an exciting material in nanofabrication. It exhibits the capability to couple surface plasmons and enhance evanescent modes used in imaging and sensing. However, it is also extremely difficult to fabricate into the desired device structures. Silver initially deposits not as a uniform thin film but as individual islands that alter its plasmonic performance. Even when uniform silver thin films are deposited, they corrode readily in air and dewet from the substrates they are deposited on. Techniques typically used to create silver nanostructures also use halogenated plasmas. These halogenated plasmas generate halide ions, which are undesirable for people or the environment. In this thesis, the performance of thin silver films protected by inert plasmonic capping layers was modelled. The models indicated that 40 nm silver films capped by 8 nm gold films and 20 nm silver films capped by 10 nm platinum films were appealing for use as sensors. The models indicated that a 10 nm silver film capped by an 11 nm gold film and a 10 nm silver film capped by a 15 nm platinum film was appealing for use as lenses. The models indicated that silver layers approaching 10 nm thickness were desirable for these plasmonic bilayer devices and informed the fabrication work that followed.</strong></p><p>Silver thin films, of approximately 10 nm thickness, were deposited on amorphous borofloat substrates. The as-deposited films had an RMS roughness of 2.0 nm and a peak roughness of 26.1 nm. However, the silver was deposited in Volmer-Weber islands. To form a continuous film, the as-deposited films were subjected to thermal anneals between 200 °C and 350 °C. The anneal increased both the surface roughness and grain size of the thin films. The anneal also changed the optical transmission spectrum of the thin films. It was found the use of a seed layer is preferred for silver thin films of approximately 10 nm thickness. The risk of contaminating the silver during transfer between process steps outweighs any benefit obtained from an anneal.</p><p>Silver nanostructures were fabricated using a combination of laser interference lithography and reactive ion etching. A dual exposure interference lithography process was used to create a periodic hole array in a photoresist film. A dual etch reactive ion etch process was developed using helium, argon, and oxygen gases for etching back anti-reflective coating. A plasma containing a halogenated gas mixture typically removes the anti-reflective coating. Removing the halogenated plasma from the effluent gas stream demonstrates an exciting proof-of-concept of an environmentally friendly anti-reflective coating etch.</p><p>It was found for 1000 nm period nanostructures that the duty cycle ratio of the pattern changed by 0.1 as a result of the etch. A 0.1 duty cycle ratio change indicates approximately 50 nm of material was removed in all horizontal directions from the photoresist pattern, enlarging the photoresist hole. At the same time, the etch removes 110 nm of the back anti-reflective coating, revealing the substrate. Arrays of 700 nm period, 0.50 duty cycle ratio silver nanostructures were reliably created using this process. These analytical and experimental results contribute to our understanding of silver nanostructures and their behaviour in optical devices.</p>