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Abstract:
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Lithography, universally considered to be the backbone of nanotechnology, has been consistently undergoing several developments to achieve a resolution of 10 nm or less. Such resolution can be attained by using traditional electron-beam excitation and chemical protocols. Interestingly, optical lithography, which is a process of transferring features on a photosensitive material using light, has also been evolving to meet the demands of the semiconductor industry. For more than three decades, optical lithography and the semiconductor industry cooperatively work together where the former acts as the engine that powers the so called nanotechnological revolution.
Two-photon photopolymerisation (TPP), a nonlinear optical phenomenon, provides an alternative route for fabricating three dimensional (3D) nanostructures. Such a fabrication technique has outshone any other optical lithography in terms of resolution and flexibility. The resolution of TPP is characterized by the size of voxel. Moreover, the size of the polymerized spot is strongly influenced by several parameters. Carefully combining the optimal parameters (e.g., photon density, wavelength, scanning speed, material and its processing) in the experiments allows one to fully fabricate nanostructures with line widths below the diffraction limit.
In this thesis, a spin-coated negative photoresist (SU-8 5) on glass substrate is used to demonstrate nonlinear lithography. The effects of sample thickness, laser power, scanning speed were studied by using a custom-built TPP experimental setup with a Ti-Sapphire (TiSa) femtosecond (fs) laser as an excitation source. The results showed that optimal fabrication of nanostructures can be achieved using sample thickness of 5 μm, laser power of 43 mW and scanning speed of 15 μm/s. The results obtained were similar to the result mentioned in the reference ‘S. Kawata, H. Sun. Two-photon photopolymerisation as a tool for making micro-devices. Elsevier 208-209 (2003), pp. 153-158.’ The smallest linewidth drawn with these optimized parameters was 358 nm. Additionally, the aspect ratio of the sample and its reproducibility were investigated. The aspect ratio matches with the results revealed in ‘H.J. Kong et al. Ultrafast Laser Induced Two Photon Polymerization of SU-8 High-Aspect-ratio structure and nano wire. Journal of the Korean Physical Society 54 (2009)1, pp 215-219’. Implementation of vectorial scanning was another aspect of this work. Here, the spin coated photoresist on glass substrate is guided along the fixed laser focus to photopolymerize and subsequently fabricate a plurality of nanostructures ranging from simple (e.g. lines) to arbitrary (e.g. like star, TUT emblem and maps). In future, the current workstation will be improved by introducing polarization and tailored light distribution, stimulated emission depletion (STED) concept, and the spectroscopy of photoinitiators to achieve fully controllable photo-induced processes at sub-diffraction resolution. /Kir11 |