INTERMETALLIC FORMATION IN COPPER MAGNESIUM THIN-FILMS - KINETICS, NUCLEATION AND GROWTH, AND EFFECT OF INTERFACIAL OXYGEN/

In situ resistance measurements, x-ray diffraction, Rutherford backsca ttering spectrometry, transmission electron microscopy, isothermal and constant heating rate differential scanning calorimetry and Auger ele ctron spectrometry depth profiles have been used to investigate the in teractions in copper and magnesium thin films leading to the growth of Cu2Mg and CuMg2 intermetallics. The effect of exposing the reacting i nterfaces to controlled exposure of oxygen on the nucleation and growt h kinetics of such intermetallics was also investigated. It is found t hat the first phase to form is CuMg2, at about 200-215 degrees C. It i s determined that the formation of CuMg2 occurs by a two step process consisting of nucleation and growth. The nucleation of CuMg2 takes pla ce in a region composed of a Cu/Mg solid solution. The nuclei form at certain preferred sites and grow in directions both parallel and perpe ndicular to the surface, eventually leading to a continuous CuMg2 laye r. The growth of CuMg2 nuclei in the plane of the original interface o ccurs at a constant rate, whereas the growth in a direction perpendicu lar to the original interface is found to be diffusion limited. In the presence of excess copper Cu2Mg forms at higher temperatures, with co mplete conversion to Cu2Mg occurring at about 380 degrees C. When the Cu surface is dosed with oxygen prior to Mg deposition, ramp rate diff erential scanning calorimetry (DSC) shows that the nucleation and grow th of CuMg2 as well as the growth of Cu2Mg are not disturbed. Dosing t he Mg surface with oxygen results in significant changes in the growth of the two phases. In this case a thin MgO layer is formed at the oxy gen dosed surface, lateral growth of CuMg2 is unaffected, but vertical growth of CuMg2 across the oxygen dosed interfaces is delayed by 25-3 0 degrees C. The growth of Cu2Mg is also shown to be delayed, by 22-54 degrees C due to the interfacial oxygen dose.