Evaluation of Cu surface diffusion on different metal liner materials for Cu reflow applications

Abstract

Cu reflow behavior was studied on various metal liners, including Co, Ru, Ta, Ti, and W. These investigations provide insight into the differences in surface diffusion characteristics of Cu along these metal liner interfaces, as revealed by structural characterization using various electron microscopy imaging techniques. In addition, the electrical properties of these metal liner/Cu stacks were investigated as a function of annealing time by evaluating changes in sheet resistance, to understand the extent to which Cu films remain continuous. Atomic force microscopy, transmission electron microscopy, and scanning electron microscopy observations provided insight into the dimensions of Cu islands formed during thermal annealing. On Ta and Ru liners, Cu surface diffusion was found to be most prominent. Compared to Ta and Ru, more Cu islands were formed per area for Co liner, exhibiting a reduced average Cu island size. Only minimal Cu island formation was observed for Ti and W liners. Graphical abstract: (Figure presented.)