Low temperature transport of Al into and through copper starting with Cu/Al/SiO2 bilayers

The transport of Al into Cu is studied by annealing Cu/Al/SiO2 bilayers. In order to minimize the effects of contaminants, samples were prepared and annealed in all-metal ultrahigh vacuum systems. In situ resistivity was used to continuously monitor the transport of Al into the bulk of the copper films. It is observed: that Al begins to move into the copper at low temperature (100–150 °C); that during an isothermal anneal, there is an initial rapid increase in Al concentration in the copper followed by a very slow increase; the equivalent of about 45 A of Al remains bound at the SiO2 interface or at the surface of the copper and is not free to dissolve in the copper. These data are not consistent with simple one-dimensional bulk diffusion but are in good agreement with a model of three-dimensional copper grains in which there is rapid transport to saturate the grain boundaries followed by diffusion of Al from the grain boundaries into the copper grains. One important observation is that because of the role of grain boundaries, it is possible to transport Al through copper films without a significant increase in the resistivity of the copper films. The potential application of this system for microelectronic interconnects in modern microelectronics is discussed.