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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Micro/nano patterning of silicon and NiP/Al disks by nanosecond and femtosecond laser sources

Pena Alvarez, Ana Azucena January 2012 (has links)
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.
2

Micro- et nano-usinage par laser à impulsions ultracourtes : amélioration de procédés par des méthodes tout optique / Micro- and nano-processing using ultrafast lasers : all-optical enhancing

Landon, Sébastien 21 October 2011 (has links)
La technique d’usinage par impulsions laser femtosecondes possède de nombreux avantages du fait des spécificités physique de l’interaction laser/matière en mode ultra-bref et est donc susceptible d’intéresser le monde industriel. Néanmoins elle présente aussi certaines limitations, principalement en terme de flexibilité et de productivité, limitant l’accès à ce marché. Pour repousser ces limites, nous proposons d’adjoindre des techniques de contrôle du faisceau, à la fois en taille, et plus généralement en forme, exploités par ailleurs dans d’autres domaines scientifiques (pincettes optiques notamment). Ces techniques reposent sur l’utilisation de modulateurs spatiaux de lumière (SLM). Deux solutions sont proposées : la modulation d’amplitude en configuration d’imagerie, et la modulation de phase pure en configuration de Fourier. Le formalisme, les différentes problématiques et la mise en oeuvre de ces deux techniques au sein d’une station de travail prototype que nous avons développée sont présentés. Enfin, nous mettons en évidence le gain apporté par ces techniques sur des problématiques concrètes, tels que l’usinage de réseaux résonant à l’échelle nanométrique, la réduction du temps d’usinage de ces réseaux (ou d’autres motifs), et l’amélioration de la qualité d’usinage de rainures / Femtosecond laser machining processes present many interesting properties owing to the specificities of the light/matter interaction in ultrafast regime. Thus the process may be of prime interest in industrial applications. However some aspects are not compatible with industrialization: namely a lack of flexibility and productivity. To overcome this limitations we propose to add beam shaping techniques in the process that allow control of the beam both in size and shape. These techniques are based on Spatial Light Modulators (SLM). Two different solutions are proposed: amplitude modulation in a geometrical conjugation scheme, and pure phase modulation in a Fourier scheme. Both are integrated in a prototype workstation. We justify the different choices made during the development by the analysis of the formalisms and specific problematics. Finally, enhancements of the femtosecond laser machining process are practically demonstrated in three different fields: reducing the resolution to nanometric scale, reducing the processing time of different texturations and enhancing the quality of simple grooves by modifying only the beam shape

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