UV Excimer Laser Micromachining,
Laser Ablation and Surface Micro Texturing / Roughening
Marco Mendes, Ph.D., LIA-BLS CLSO, Laser Applications Engineer at JPSA, USA
Ph.D. thesis in Materials Engineering, "Excimer laser micromachining of Al2O3-TiC ceramics. Study of the process", 244 pages (in english)
Thesis is available on line for free and in pdf file, 244 pages, written in english (see details below)
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Table of Contents and Chapter 1 - Introduction
Chapter 2 - Introductory concepts
General properties of laser radiation and the interaction of laser radiation with matter are introduced. The absorption of laser radiation by metals and insulators is discussed, and the general mechanisms of laser ablation are presented.
Chapter 3 - Experimental methods
An overview of excimer laser technology and the fundamental concepts of micromachining using excimer lasers are presented. The characterisation techniques used throughout this study are briefly described. Finally, the properties of the materials studied (Al2O3-TiC, Al2O3 and TiC) are analysed.
Chapter 4 - KrF Nanosecond excimer laser micromachining of Al2O3-TiC ceramics in air
The micromachining of Al2O3-TiC composite ceramics in air using KrF (l=248 nm) nanosecond laser pulses was studied. The influence of the processing parameters was analysed aiming to optimise the machining process. Surface topography, surface roughness, ablation rate and surface finish machining quality were investigated as a function of the following parameters:
- Laser fluence;- Number of pulses;- Laser beam spot size;- Laser beam angle of incidence;- Machining with static or moving sample.
Chapter 5 - Study of the influence of the working atmosphere on the micromachining of Al2O3-TiC ceramics
The effects of the ambient atmosphere on the ablation of Al2O3-TiC using nanosecond duration KrF (l=248 nm) laser pulses are studied. The influence of the gas type and pressure on the ablation rate and surface topography is investigated, and the observed changes are correlated with the ablation plume dynamics. The mechanisms of formation of particles during ablation are studied.
Chapter 6 - Study of the influence of the excimer laser wavelength on the micromachining of Al2O3-TiC ceramics
The micromachining of Al2O3-TiC ceramics using nanosecond ArF (l=193 nm) and XeCl (l=308 nm) excimer lasers was studied. The effects of the laser fluence and number of pulses on the ablation rate and surface topography were analysed. A comparative study of the ablation of Al2O3-TiC using nanosecond laser pulses at different wavelengths (193, 248 and 308 nm) was performed.
Chapter 7 - Femtosecond excimer laser micromachining of Al2O3-TiC ceramics. Influence of the laser pulse duration
The micromachining of Al2O3-TiC ceramics using femtosecond duration laser pulses (l=248 nm) is studied. The results are compared with those obtained when processing is carried out using nanosecond laser pulses at the same radiation wavelength, in order to investigate the influence of the laser pulse duration on the ablation process.
Chapter 8 - Final conclusions and future work
The final conclusions of this study are presented. The versatility of the excimer laser projection technique in the micromachining of Al2O3-TiC as well as the associated difficulties are summarised. Excimer laser processing and ion beam etching of Al2O3-TiC are compared. Indications for future work are presented.
Appendix A - Thermal model
The finite-difference thermal model developed to investigate heating/ablation during irradiation with nanosecond duration laser pulses is presented. This model was used to investigate the heating behaviour of Al2O3, TiC and Al2O3-TiC by considering the fraction of the individual phases in the composite. It allowed to study the influence on heating of various material properties, namely the optical absorption coefficient and thermal conductivity.
Appendix B - Dynamics of the ablation plume
The ablation plume dynamics was coupled to the finite difference model, in order to estimate the pressure and attenuation of radiation in the plume.
Abstract: Laser micromachining of Al2O3-TiC composite ceramics was studied using nanosecond and femtosecond pulse duration excimer lasers. KrF (l=248 nm) nanosecond pulses were used to investigated the dependence of the ablation rate, surface topography, and machining quality on laser fluence, number of pulses, beam spot size, angle of incidence and machining with static or moving sample. The influence of the working gas (He, Ne, N2, air, Ar and Kr) and pressure (10-4-1 bar) on the machining process, and on the formation and accumulation of particles around the treated area was also studied. Plume confinement due to ambient gas effects and laser light attenuation due to scattering and absorption by particles in the ablation plume are related with the reduction in the ablation rate with increasing pressure and atomic/molecular weight of the atmosphere gas. The influence of the radiation wavelength was studied using ArF (l=193 nm), KrF (l=248 nm) and XeCl (l=308 nm) nanosecond lasers. Laser processing leads to a globular/columnar topography when the fluence is lower than a threshold value that depends on the radiation wavelength and atmosphere. The shape and dimensions of these columnar features may be controlled to considerable extent, suggesting the possibility of using the process for microroughening applications. For higher fluences smooth surfaces are obtained, allowing for the machining of relatively accurate shapes. Femtosecond duration laser pulses (l=248 nm), lead to better surface finish, higher accuracy and ablation rate, as compared to nanosecond pulses at similar fluences. Minimisation of thermal effects allows sub micrometer structures to be machied.
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Last updated: 07/01/2009
