Surface Modification of Aluminum Nitride and of Aluminum by Excimer Laser
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SURFACE MODIFICATION OF ALUMINUM NITRIDE AND OF ALUMINUM BY EXCIMER LASER
A.J.Pedraza0), J.-Y. Zhang(2 and H.Esrom) O)Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 USA (Asea Brown Boveri AG, Corporate Research, W-6900 Heidelberg, Germany ABSTRACT A new technique for selective metallization of aluminum nitride (AIN) has been previously reported (1). It involves the use of an excimer laser to activate the AIN surface followed by electroless plating (Cu,Ni,Au) of the irradiated areas. The mechanism of decomposition of ALN is accompanied by ablation and the formation of an Al film on the substrate surface. Ablation rates are reported here as a function of fluence and number of pulses for three different wavelengths X= 193 nm (ArF), X= 248 (KrF) and X= 351 nm (XeF). The effect of laser wavelength on the ablation rate is discussed. The ablation rates for Al were zlso measured and are compared with the AIN ablation rates. A numerical thermal model is used to analyze the mechanisms of laser ablation of both materials. The evaporation kinetics are incorporated into the model. The Clausius-Clapeyron approximation is used to make a self- consistent calculation of boiling and decomposition temperatures. INTRODUCTION AIN is decomposed thermally when heated at a temperature of 2573 K and also by laser irradiation (1,2). The decomposition products are liquid Al and gaseous N that evolves toward the atmosphere. If the temperature is further raised to 2767 K, the molten aluminum evaporates (3). Excimer laser irradiation produces melting and thermal decomposition of the AIN substrate (2,3). The surface of the substrate, that has initially the typical appearance of a fired AIN ceramic with faceted and protruding grains, becomes much smoother and undulated after the laser treatment. A thin metallic Al film is left on the surface of the irradiated substrate as a product of the decomposition (1,4). The two processes that contribute to thermal ablation of AIN during laser irradiation are decomposition, which is accompanied by a 20% volume reduction due to N evolution, and evaporation of part of the Al. This last process is cumulative and after only a few shots the evaporation rate becomes almost equal to the ablation rate. When an Al substrate is irradiated with an excimer laser the temperature of the near surface region rises, melting occurs and finally, if the boiling temperature is reached, evaporation takes place. If evaporation occurs while the substrate is being irradiated the Al vapor interacts with the laser beam. The ejected vapor can absorb a significant fraction of the incoming energy if the beam intensity is sufficiently high (5). Sputtering of droplets and small clusters has been proposed as an alternative ablation mechanism for metals (6). The possible operation of this mechanism for Al ablation is discussed below. In the present work, ablation of Al and of AIN is investigated using three different excimer laser wavelengths. The ablation rates for both materials are
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