Laser-Induced Surface Activation of Aluminum Oxide for Electroless Deposition
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LASER-INDUCED SURFACE ACTIVATION OF ALUMINUM OXIDE FOR ELECTROLESS DEPOSITION
A.J. Pedraza('), M.I.Godbole"), M.J. DeSilvaý') and D.H. Lowndes0l • Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 (2) Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056 ABSTRACT A new process for writing a copper pattern into alumina or sapphire substrates is presented. This process employs in sequence ultraviolet laser irradiation and electroless deposition of copper. The laser irradiation activates the substrate and copper is deposited in the activated region when the substrate is immersed in the electroless solution. The laser-activation can be performed months in advance to the deposition. A small amount of metallic aluminum is left on the surface of laser irradiated alumina substrates. The presence of this metallic aluminum appears to play a very important role in the laser surface activation process. In addition to aluminum cluster formation there must be other activation centers that promote copper deposition in laser treated alumina substrates. These catalytic centers are much weaker that the aluminum clusters and have been tentatively identified as surface defects. INTRODUCTION The purpose of this paper is to describe a new process for writing a copper pattern into alumina or sapphire. This process employs in sequence excimer laser irradiation and electroless deposition of copper. Following the presentation of results, the mechanisms by which the electroless deposition of copper in laser irradiated alumina can take place are analyzed. A thin near-surface layer of alumina (- 0.17 um-thick) is melted during irradiation with a XeCI excimer laser (308 nm) (I). Almost no change in molten depth is obtained for laser energy densities in the range of I to 6 J/cm2 (1) and for 10 successive pulses. Alumina and sapphire are significantly ablated only after multiple pulses are applied using the XeCI laser. As the laser energy density decreases, the number of pulses required for initiating the ablation process increases (1). Thermal evaporation appears to be a very important part of the ablation process in alumina irradiated with a XeCI excimer laser (I). Picosecond laser irradiation at 266 nm causes ablation of sapphire at a laser energy density of 10 J/cm2 (2). The ablation process takes place in two stages as a function of the number of pulses, respectively characterized by a low and a very fast ablation rate. It has been concluded that the ablation of sapphire, using an ArF excimer laser at an energy density of 1.5 J/cm2 and very low ablation rates (0.4 nm per pulse), is a photochemical process (3). Alumina and sapphire are almost transparent to the electromagnetic radiation in the ultraviolet range of excimer lasers. For these materials, the strong coupling with the incoming radiation can take place by electron-photon interactions only if there are impurity or defect electronic levels. Rutherford Back Scattering-Channeling data of sapphire show that the
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