Ablation, Melting, and Smoothing of Polycrystalline Alumina by Pulsed Excimer Laser Radiation
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ABLATION, MELTING, AND SMOOTHING OF POLYCRYSTALLINE ALUMINA BY PULSED EXCIMER LASER RADIATION Douglas H. Lowndes,* M. DeSilva,** M. J. Godbole,** A. J. Pedraza,** and D. B. Geohegan* * Solid State Division, Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 37831-6056 ** Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 ABSTRACT The effects of pulsed XeCl (308 nm) laser radiation on polycrystalline A120 3 (alumina, 99.6% pure) and single-crystal A1203 (sapphire) are studied as a function of laser fluence. No laser etching of either material is detected below a threshold fluence value, which is much lower for alumina than for sapphire. Above this threshold, laser etching of both materials is observed following a number of incubation (induction) pulses. This number is much larger for sapphire than for alumina but decreases with increasing fluence for both materials. Laser etching rates for the two materials are similar at high fluences and after the incubation period. Scanning electron microscope images show that alumina melts and flows under repeated irradiation at fluences Ž>0.7 J/cm 2 . Atomic force microscopy and surface profilometry reveal significant smoothing of the asreceived polycrystalline alumina surface after repeated irradiations at moderate fluences (- 1-3 J/cm 2 ). Ion probe measurements for alumina in vacuum confirm the incubation behavior, and reveal that at fixed fluence the (positive) charge collected per pulse saturates after a sufficient number of pulses, as does the etch-plume velocity. The results are interpreted in terms of lasergeneration of a sufficient concentration of absorption centers before efficient ablation/etching of these wide bandgap materials can occur. INTRODUCTION Extensive studies have been carried out of the mechanism for laser etching of sapphire (single crystal A1203, c-A1203) [1-6]. These studies were motivated by the fact that although sapphire has a low vapor pressure and wide energy bandgap, Eg -9 eV, it nevertheless is easily etched using relatively low fluences of ultraviolet (UV) photons with energies of 5-6.3 eV, much less than E Early studies showed that the threshold fluence for laser sputtering of sapphire coincide with the onset of intense emission from excited neutral Al atoms in the etch plume [1]. However, in regions of uniform laser irradiation there was no evidence of thermal melting, thermal-stress-induced fracture, or exfoliational processes, which suggested an electronic sputtering mechanism instead. Subsequent studies using a pulsed photothermal deformation (PPTD) technique revealed a large (-3%) absorption of laser energy at the (front and back) sapphire surfaces, and that this absorption is independent of laser fluence, thus ruling out the possibility that a multiphoton absorption process is responsible for laser etching of sapphire [5]. The fraction of energy absorbed at 193 nm, 248 nm, or 351 nm was independent of laser fluence for power densities below the ablation threshold, indicating
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