Micro-Structuring of Silicon by Pulsed-Laser Ablation Under Reactive Atmospheres
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MICRO-STRUCTURING OF SILICON BY PULSED-LASER ABLATION UNDER REACTIVE ATMOSPHERES. S. JESSE*, A. J. PEDRAZA*, J. D. FOWLKES*, J. D. BUDAI** AND D. H. LOWNDES** *Dept. of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 [email protected]. **Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056. ABSTRACT Micro-holes hundreds of µm-deep below the initial surface surrounded by 40- µm-tall micro-cones protruding over the initial surface form in silicon as a response to multiple UV pulsed-laser irradiation in an SF6 atmosphere. The micro-holes and micro-cones are arranged in a characteristic ensemble displaying a self-organized pattern. Similarly, deep-holes and micro-column ensembles form when the irradiation is performed in an oxygen-rich atmosphere. The formation mechanism of microcolumns and microcones in these reactive atmospheres has been studied using scanning electron microscopy, profilometry and x-ray diffraction. The growth of cones under SF6 atmospheres was studied in-situ using an ICCD gated camera coupled to a long distance microscope. The images obtained with the ICCD camera show that the fluorescent plume is correlated with the amount of ablated material. There is also a strong correlation between the plume and the growth of the cones. The cones grow in the region where the laser-generated plume is intense. When the flux of silicon-rich material ceases because the holes are very deep the balance between redeposition and ablation is tilted toward the latter and the cones begin to recede. Laser ablation is not only a function of the nominal fluence but also of the surface roughness and microstructure. These results strongly support the growth mechanism whereby laser-ablated silicon-rich molecules and clusters are transported to the tip and side of the cones by the laser- generated plasma.
INTRODUCTION Laser-induced ablation can be due to several processes including mechanical spallation, expulsion of small droplets from a transiently laser-melted layer, desorption of atoms and ions, and evaporation [1]. The presence of a reactive atmosphere can augment ablation by producing compounds that are easily removed by laser irradiation. The action of the laser on the target can produce very drastic changes in 2 surface topography. When silicon is irradiated in air with 1000 laser pulses at a power of 130 MW/cm , a dense array of high aspect ratio columns forms on the surface. Theses columns are ~ 20 µm long and 2 – 3 µm in diameter (figure 1) [2 – 7]. In silicon, we have observed that evolution of cone-like structures is extremely dependent on the irradiation atmosphere. An array of very large and complex conical structures around deep microholes is formed if the irradiation is performed in sulfur hexafluoride (SF6) [6,8,9]. Both, the columnar and conical features just described, protrude above the original wafer surface. A possible mechanism that explains these morphological details is redeposition of surrounding ablated silicon on top of the emerging feature
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