UoM administered thesis: Phd

  • Authors:
  • Ana Pena Alvarez


This PhD thesis presents the outcome of employing both nanosecond andfemtosecond pulsed lasers in order to modify the surface structure of a material at themicro and nano scales. Literature review was carried out on micro/nano fabricationtechnologies involved in the semiconductor industry, which are the basis of manycurrent micro and nano-manufacturing processes.The first experiments concentrated on direct laser scanning of Si to producesurface microstructures. This type of texturing was very effective at reducing surfacereflectivity and can be implemented in photovoltaic devices. It was also found thatthe ablation efficiency can be improved if laser processing is performed in an argonenvironment where oxidation can be suppressed. Moreover, a significant relationshipbetween laser-texture characteristics (i.e. topography/morphology and periodicity)and total surface reflectance was demonstrated. Short-circuit modelling of the lasertextureshowed that electrical performance of the cell can be improved by 41.3% inthe 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-generatedmicro-structures made the surface significantly wettable; but as the laser fluence wasreduced, the contact angle of the surface could be changed. This led to theinvestigation of the wetting properties of nano-bumps produced on Si at fluencesbelow the ablation threshold. Their wetting behaviour was reported for the first time.An effect named as 'invisible marking' in this thesis was demonstrated: vapourcondenses into water drops of different size depending on the lattice arrangement ofc-Si or a-Si. Such an interaction at the near-ablation threshold was also explored foranother type of material: NiP/Al data storage disks. From this research, ellipticalbumps with vertical dimension in the sub-nanometre scale were fabricated withextremely high repeatability (± 0.4 nm). In addition, it was found that ellipticalbumps can offer better stiction performance than circular shapes, even at ultra-lowflying height. This type of laser texture could be utilised as a means for tribologicaloptimization of surfaces that are in close proximity and relative motion.Following the use of low-fluences by nanosecond pulses, this was alsoapplied to scanning over a microsphere lens array. So far, the research on near-fieldeffects produced at the bottom of transparent particles has focused on how togenerate parallel nano-patterns by single pulses. However, the present work hasdemonstrated that a focused beam with a tight-focus can be used to fabricate singlelines or shapes rather than repeated patterns. In this way, a femtosecond laser wasintroduced to meet such a challenge. Moreover, laser-induced periodic surfacestructures (LIPSS) by fs pulses were also identified along the near-field generatednano-patterns. The evolution of such a periodic, self-assembly structuring was also investigated, and new optical characteristics of structural colour were found.


Original languageEnglish
Awarding Institution
  • Lin Li (Supervisor)
  • Zengbo Wang (Supervisor)
Award date1 Aug 2012