Nanofabrication, at lateral resolutions beyond the capability of conventional optical lithography techniques, is demonstrated here. Various nanofeatures (nanogrids, nanocraters, nanocurves) were machined, with high spatial resolution (~10-12nm), on thin metallic and semiconductor films by utilizing the enhanced field that exists underneath a scanning microscope tip irradiated with a laser beam. For the first time in the published literature, recrystallization results of thin a-Si films, at the nano-scale, are being presented to demonstrate the utility of this machining scheme as an effective 'nano'-laser source. Attempting to understand the modification mechanism and the physics involved, numerical simulation studies were performed to evaluate the field enhancement underneath the tip and the 'femtosecond laser-thin film' interaction dynamics in general. For the former study Finite Difference Time Domain simulation was carried out to evaluate the spatial distribution of the field intensity in the near field of the SPM probe tip. The later study employed finite-difference numerical method to solve the hyperbolic two-temperature scheme used to model the interaction. Possible applications of thin film structuring and its use as a mask may be in the areas of high-resolution nanolithography, nanofluidics, controlled nanodeposition, ultra high-density data storage, nanoelectronics, nanophotonics and various nanobiotechnology related applications.
Ultra-short pulsed-laser radiation has been shown to be an effective tool for controlled material processing and surface nano/micro-modification because of minimal thermal and mechanical damage. Nanostructuring of a variety of materials is gaining widespread importance owing to ever-increasing applications of nanostructures in numerous fields. This study demonstrates that controllable surface nanostructuring can be achieved by effectively utilizing the local field enhancement in the near field of a SPM probe tip irradiated with femtosecond laser pulses. Results of nanostructuring of various metallic and semiconductor thin film samples utilizing an 800nm femtosecond laser system in conjunction with a commercial SPM in ambient air are presented. Additionally, results from a companion micro-ablation study on gold thin films and numerical Finite Difference Time Domain (FDTD) simulation results for the spatial distribution of the laser field intensity beneath the tip are presented in an effort to achieve better understanding of the laser-material interaction. Flexibility in the choice of the substrate material and the processing environment, high spatial resolution (~10-12nm) and possibility of high processing rates by massive integration of the tips make this method an effective nanostructuring tool. Potential applications of this method include nanolithography, mask repair, nanodeposition, high-density data storage, as well as various nano-biotechnology related applications.
Fundamental questions arise regarding the possibility and nature of melting and the ensuring mechanism of ablation in femtosecond laser processing of materials. A comprehensive experimental study is presented to address these issues in depth and detail. The mechanisms of ultra-fast laser-induced phase-transformations during the laser interactions with materials have been investigated by time-resolved pump-and- probe imaging in both vacuum and ambient environment. The temporal delay between the pump and probe pulses is set by a precision translation stage up to about 500 ps and then extended to the nanosecond regime by an optical fiber assembly. Ejection of material in the form of nanoparticles is observed at several picoseconds after the main pulse. The ignition of surface-initiated plasma into the ambient air immediately following the pump pulse and the ejection of ablated material in the picosecond and nanosecond time scales have been proven by high-resolution, ultra-fast shadowgraphy. To further dissect the origin and evolution of the ablation process, a double pulse experiment has been implemented, whereby both the pump and probe pulses are split into two components each separated by variable temporal delays. A diffractive optical element is used to fabricate micro-channels in silicon wafers.
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