Electroless copper films deposited onto laser-activated aluminum nitride and alumina
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D. H. Lowndes Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200 and Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056 (Received 13 September 1993; accepted 20 December 1993)
Metallization of ceramic substrates by laser activation and subsequent electroless deposition has been demonstrated recently in aluminum nitride and alumina. However, the bond strength between the electroless copper and the ceramic substrate is weak (less than 14 MPa). Low temperature annealing of electroless copper films deposited on substrates activated at low laser energies strongly increases the adhesion strength. The effectiveness of the annealing for improving the metal-ceramic bonding is dependent upon the laser treatment performed on the substrate prior to deposition. Faster deposition kinetics are obtained for both substrates by increasing the laser energy density. On the other hand, an increase in the laser energy density leads to poor adhesion strengths. The dislocation microstructure produced during laser irradiation in aluminum nitride is analyzed as a possible cause of laser activation. Free aluminum produced by laser irradiation of aluminum nitride and of alumina is discussed as another factor of laser activation. The chemical and microstructural changes taking place in the near-surface region as a consequence of laser-induced processes are correlated with adhesion enhancement promoted by the annealing treatment.
I. INTRODUCTION There has been an increasing demand to improve the performance of high density hybrid circuits. Advances in integrated circuit technology have been held back due to limitations in packaging design. The large amount of power that is consumed in these devices is dissipated in the form of heat, resulting in severe temperature rises in the integrated circuits. In order to support rapid heat dissipation, high thermal conductivity ceramic substrates have been developed. In addition to having a high thermal conductivity, the substrate material must have a low dielectric constant and a thermal expansion coefficient close to that of conductive materials such as copper or gold. Until the last decade, alumina was the material of choice for ceramic packages because it was easy to process and its thermal conductivity was adequate for the device requirements of the time. Over the past few years the increase in heat dissipation requirements has led to a growing interest in aluminum nitride as a substrate material because it has a much higher thermal conductivity than alumina and its thermal expansion coefficient closely matches that of silicon devices. The development of new substrates has in turn stimulated research concerning metallization techniques for the production of integrated circuits. Thin-film J. Mater. Res., Vol. 9, No. 4, Apr 1994 http://journals.cambridge.org
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metallization uses elevated temperatures to adhere the film to the substrate by solid-state reactive bonding. Thick-film metallizati
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