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Towards Damage-Free Micro-Fabrication of Silicon Substrates Using a Hybrid Laser-Waterjet Technology

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Towards Damage-Free Micro-Fabrication of Silicon Substrates Using a Hybrid Laser-Waterjet Technology TOWARDS DAMAGE-FREE MICRO-FABRICATION OF SILICON SUBSTRATES USING A HYBRID LASER-WATERJET TECHNOLOGY by VIBOON TANGWARODOMNUKUN B.Eng.(1st Hons), M.Eng. A thesis submitted to the University of New South Wales in fulfillment of the requirements for the degree of Doctor of Philosophy School of Mechanical and Manufacturing Engineering The University of New South Wales February 2012 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Tangwarodomnukun First name: Viboon Other name/s: Abbreviation for degree as given in the University calendar: PhD School: Mechanical and Manufacturing Engineering Faculty: Engineering Title: Towards Damage-Free Micro-Fabrication of Silicon Substrates Using a Hybrid Laser-Waterjet Technology Abstract 350 words maximum: (PLEASE TYPE) A novel hybrid laser-waterjet machining technology is developed in this thesis using a new material removal concept to achieve near damage-free micromachining. Using this concept, a laser is used to heat and soften the material while a waterjet is used to expel and remove the laser-softened material in a layer by layer manner, so that material is removed in its solid-state below its melting temperature. Water also takes a cooling action. An experimental rig has been built to realize this novel concept and an extensive experimental investigation has been carried out to understand the process and the effect of various parameters on the process using a single-crystalline silicon as the specimen material. It has been found that near free of heat-affected zone and high material removal rate can be achieved when using this hybrid laser-waterjet technology, as compared to the dry laser micromachining process. Specifically, a laser Raman spectroscopy study has found that a much thinner amorphous layer within 40 nm was formed than that found in the dry laser machining process. In order to understand the coupled effect of laser and waterjet on the material removal process and to predict and control the process on a mathematical and quantitative basis, a temperature-field model has been developed whereby a model for the dry laser machining process is developed first before it is extended to the hybrid laser- waterjet process incorporating the waterjet cooling and expelling effects. The parabolic heat conduction associated with enthalpy method is numerically solved by using an explicit finite difference scheme for predicting the two- dimensional temperature field. The thermal model has been verified by comparing the predicted temperatures with the temperatures measured by an infrared camera. The simulated groove depths are also compared with the experimental data under the corresponding conditions and it is found that they are in good agreement. A simulation study of the hybrid laser-waterjet process is finally reported which provides an in-depth understanding of the material removal process and mechanisms and the interaction between laser, waterjet and material under the coupled effect of laser heating and waterjet cooling and expelling. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). 27 February 2012 …………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS Originality Statement I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged. Signed .................................................................. 27 February 2012 Date .................................................................. ii Copyright Statement I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation. Signed .................................................................. 27 February 2012 Date .................................................................. iii Authenticity Statement I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format. Signed .................................................................. 27 February 2012 Date .................................................................. iv Abstract A comprehensive literature review on laser physics, the underlying science and models in laser machining as well as other novel machining technologies using lasers has been carried out. It is shown that while laser micomachining is the preferred technology in industry, damage-free micromachining of difficult-to-cut materials is still one of the most challenging technologies for processing micro-structures accurately with little or no damage induced by the process. A novel hybrid laser-waterjet micromachining technology is developed in this thesis using a new material removal concept to minimize the thermal damage that normally occurs in traditional laser machining processes. Using this concept, a laser is used to heat and soften the material while a waterjet is used to expel and remove the laser- softened material in an element to element (or layer by layer) manner. Water also takes a cooling action. In this way, the material is removed in its solid-state below its melting temperature. As a result, less laser-work interaction time may be required which gives the potential to increase the material removal rate. An experimental rig has been built to realize this novel concept and an extensive experimental investigation has been carried out to understand the process and the effect of various parameters on the process using a single-crystalline silicon as the specimen material. It has been found that near free of heat-affected zone (HAZ) and high material removal rate can be achieved when using this hybrid laser-waterjet technology, as compared to the dry laser micromachining process. However, the overlap of laser beam with the waterjet has been found to markedly decrease the laser fluence and the cut quality, so that it is recommended to place the laser beam just outside the intersection of the waterjet with the work surface. Plausible trends of the machining performance with respect to the process parameters have been found. It is shown that groove width, depth and HAZ size increase with an increase in laser pulse energy and pulse overlap, while v ABSTRACT focal plane position and waterjet offset distance have to be properly selected for a high ablation performance. Moreover, the groove width and depth increase with an increase in water pressure and waterjet impact angle, while the effect of these parameters on HAZ is not significant. Surface and subsurface damage of silicon in terms of material micro-structural changes has been investigated by using a laser Raman spectroscopy analysis to determine the crystallinity of silicon and the amorphous layer thickness formed after the ablation. When silicon was ablated by the new process, a much thinner amorphous layer within 40 nm was formed than that
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