NANOSECOND PULSED EXCIMER-LASER MACHINING OF CHEMICALLY VAPOR-DEPOSITED DIAMOND AND GRAPHITE - PART II - ANALYSIS AND MODELING

Analysis of the experimental data presented in Part I of this paper an d those available in the literature revealed that the mechanism of mat erial removal in laser machining of chemically vapour-deposited diamon d is a two-step process: diamond transforms to graphite, and subsequen tly graphite sublimates. The energy fluence required for the formation of graphite is much lower than its removal by sublimation, and both a re sensitive to the wavelength of the laser beam, the impurities prese nt in the film and the environment during machining. When a 248 nm exc imer laser beam interacts with diamond, there is an energy loss of 20% by reflection and 10% by transmission. The remaining 70% energy is us ed for heating the diamond, converting diamond to graphite, and sublim ating graphite. Graphite is removed mostly by physical ablation and to some extent by chemical oxidation with the ambient. A theoretical cal culation based on bond strength estimates that the threshold energy fl uence for the ablation of diamond is 0.37 Jcm(-2). The experimental en ergy fluence was 0.8 Jcm(-2). Experimental results on the material rem oval rates as a function of energy fluence closely follow the Beer-Lam bert equation, suggesting that physical ablation is the determining me chanism. Temperature calculations showed that both diamond and graphit e tend to oxidize in a single laser pulse that contributes to the mate rial removal.