Excimer Laser-Induced Decomposition of Aluminum Nitride
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EXCIMER LASER-INDUCED DECOMPOSITION OF ALUMINUM NITRIDE 2 1 1 V. Esrom( ), J-Y Zhang( ) and A.J. Pedraza( ) I) Asea Brown Boveri AG, Corporate Research, W-6900 Heidelberg, Germany (2) Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200, USA
ABSTRACT A thin film of aluminum, detected by x-ray photoelectron spectroscopy, is left at the surface of aluminum nitride (AIN) substrates exposed to a high intensity excimer laser beam of UV radiation. Due to the presence of this film, there is a decrease in the surface resistivity of the substrate with increasing number of laser pulses. In addition, line profilometry shows a decrease of the surface roughness with the number of pulses. The thermal decomposition of AIN is assumed to take place in two stages. In the first, liquid aluminum is produced together with the evolution of gaseous nitrogen, and in the second, aluminum evaporates. Using a computer model to simulate the laser heating cycle, it is shown that the thickness of the aluminum film saturates at a given laser energy density. The saturation thickness is a strong function of the substrate absorption and reflectivity and, therefore, of the laser light frequency. The influence of the substrate roughness on the electrical resistivity of the aluminum film is discussed. The application of this process to direct laser writing in high density hybrid circuits is illustrated. During hole drilling by excimer laser, a thin aluminum film is continuously produced at the hole walls. This process can also be employed for substrate planarization. INTRODUCTION A series of techniques for selective metal deposition has been developed during the last years that employ organometallic deposits on different substrates [1-51. These deposits decompose upon laser irradiation to leave a metallic film that serves as a surface activator for subsequent electroless plating. Recently, a new technique for surface structuring was developed whereby direct decomposition of AIN can be produced leaving an aluminum film at the substrate surface. This technique, which eliminates the need of additional precursors to activate the substrate surface, uses UV excimer lamp or laser radiation [6]. The outstanding thermal conductivity of aluminum nitride combined with good electrical and mechanical properties has singled out this material as an excellent candidate for electronic packaging applications [7]. In the present work, the changes that excimer laser irradiation produces in the topography, morphology, and electrical resistivity of AIN substrates are investigated. A computer model for simulating the decomposition and ablation of AIN is presented. The experimental results are analyzed using this model. EXPERIMENTAL PROCEDURE Commercially available AIN substrates were irradiated in air with ArF (wavelength, ). = 193 nm), KrF (1 = 248 nm) and XeF (;. = 351 nm) excimer lasers. Cylindrical quartz lenses (Suprasil) with a focal length of 156 mm were used to focus the laser beam to a narrow strip 10 mm long by 10
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