Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.549102
Title: Micro/nano patterning of silicon and NiP/Al disks by nanosecond and femtosecond laser sources
Author: Pena Alvarez, Ana Azucena
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2012
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Abstract:
This PhD thesis presents the outcome of employing both nanosecond and femtosecond pulsed lasers in order to modify the surface structure of a material at the micro and nano scales. Literature review was carried out on micro/nano fabrication technologies involved in the semiconductor industry, which are the basis of many current micro and nano-manufacturing processes. The first experiments concentrated on direct laser scanning of Si to produce surface microstructures. This type of texturing was very effective at reducing surface reflectivity and can be implemented in photovoltaic devices. It was also found that the ablation efficiency can be improved if laser processing is performed in an argon environment where oxidation can be suppressed. Moreover, a significant relationship between laser-texture characteristics (i.e. topography/morphology and periodicity) and total surface reflectance was demonstrated. Short-circuit modelling of the laser texture showed that electrical performance of the cell can be improved by 41.3% in the 360-1100 spectrum, even in the near-infrared for which Si is a weak absorber. From these experimental results, it was also noticed that the laser-generated micro-structures made the surface significantly wettable; but as the laser fluence was reduced, the contact angle of the surface could be changed. This led to the investigation of the wetting properties of nano-bumps produced on Si at fluences below the ablation threshold. Their wetting behaviour was reported for the first time. An effect named as 'invisible marking' in this thesis was demonstrated: vapour condenses into water drops of different size depending on the lattice arrangement of c-Si or a-Si. Such an interaction at the near-ablation threshold was also explored for another type of material: NiP/Al data storage disks. From this research, elliptical bumps with vertical dimension in the sub-nanometre scale were fabricated with extremely high repeatability (± 0.4 nm). In addition, it was found that elliptical bumps can offer better stiction performance than circular shapes, even at ultra-low flying height. This type of laser texture could be utilised as a means for tribological optimization of surfaces that are in close proximity and relative motion. Following the use of low-fluences by nanosecond pulses, this was also applied to scanning over a microsphere lens array. So far, the research on near-field effects produced at the bottom of transparent particles has focused on how to generate parallel nano-patterns by single pulses. However, the present work has demonstrated that a focused beam with a tight-focus can be used to fabricate single lines or shapes rather than repeated patterns. In this way, a femtosecond laser was introduced to meet such a challenge. Moreover, laser-induced periodic surface structures (LIPSS) by fs pulses were also identified along the near-field generated nano-patterns. The evolution of such a periodic, self-assembly structuring was also investigated, and new optical characteristics of structural colour were found.
Supervisor: Li, Lin ; Wang, Zengbo Sponsor: Mexican National Council of Science and Technology (CONACYT)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.549102  DOI: Not available
Keywords: Laser micro/nano processing ; Surface texturing ; Photovoltaics ; Wetting behaviour ; Tribology ; Photovoltaics ; Structural Colour ; Silicon
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