Ultrasmooth thin film and nano-scale structure fabrication to extend the bounds of optical microscopy
Plasmonic devices including superlenses, hyperlenses, and far-field superlenses are specially fabricated elements which can improve the resolution of optical microscopy past inherent theoretical limits. However, their fabrication is extremely difficult as they often require ultrasmooth thin films and finely structured silver (Ag). While Ag has an ideal response for these types of lenses, its fabrication in such devices is challenging. Hence, this thesis investigates viable methods of producing ultrasmooth Ag thin films and nano-scale Ag features in order to advance research in plasmonic devices. Optimum plasmon response requires the fabrication of ultrasmooth thin silver films, which presents several challenges, such as high surface roughness and high optical loss. Thin 1 nm seed layers were fabricated in advance of Ag layers in order to improve the surface properties of Ag. We found that a 1 nm germanium (Ge) seed layer results in a 400% reduction in surface roughness down to 0.64 nm(RMS), but offers increased optical loss by about 3% over Ag alone. However, an inert atmosphere high temperature anneal of a Ge/Ag stack results in preferential grain growth, further reducing surface roughness to 0.61 nm(RMS), while also improving transmission by up to 14% over Ag alone. Similar procedures were conducted on copper (Cu) and silver oxide (AgOx) seed layers. While Cu results in very smooth Ag films of 0.61 nm(RMS) for films < 10 nm thick, performance deteriorates at Ag thicknesses above 10 nm, which are preferred for the plasmonic applications identified above. Furthermore, AgOx produces very rough surfaces on substrates which are amorphous–a property which is essential for our use. However, AgOx on crystalline substrates produced smooth surfaces of 0.3 nm(RMS) and may be useful for other plasmonic applications. Interference lithography (IL) was selected as the method to create the periodic nano-scale structures. The IL equipment was modified with the addition of bandpass light filters, 5 μm pinhole Fourier filters, and air vortex shields. Also, elimination of both external vibration and time-dependant vacuum lines are included. With this IL environment, we were able to produce periodic gratings anywhere from 1 μm-300 nm pitch through rigorous optimisation of photoresist, exposure, and development processes. Ultimately, this lead to the fabrication of high contrast, 200 nm period gratings for use in a far-field superlens. The designs and procedures outline within will result in increased performance and production of far-field superlenses with limited equipment, therefore facilitating increased performance of optical microscopes.