An Examination of Mass Transport Paths in Conventional and Highly Textured AL-CU Interconnect Lines

ABSTRACTThe electromigration lifetime of aluminum interconnect lines shows a strong dependence on the quality of the texture in the metal. This observation has been explained in terms of a decreased diffusivity along the grain boundaries, generally the dominant path for diffusion and void growth. As the texture in the metal is strengthened, atomic order along the grain boundaries is increased, and diffusion is suppressed.In this paper, we examine passivated A1–0.5wt%Cu interconnect lines having either TaAl underlayers or conventional Ti/TiN underlayers. Rocking curve measurements confirm that Al deposited on TaAl has a very strong (111) texture (FWHM ∼ 0.6°) compared to Al on Ti/TiN (FWHM ∼ 6°). Interconnect lines of both types were electrically stressed at 225°C and 250°C for 20 hours at a current density of 3 MA/cm2. The lines were then stripped of passivation, and the void numbers, sizes, and morphologies were determined.Analysis of the void data gave a surprising result: the void volume in the highly-textured Al was greater than in the conventional Al. However, voids in the conventional Al were observed to follow grain boundaries across the width and sever the line, while such destructive void growth was not observed in the highly-textured Al. Instead, voids grew either with a faceted shape or along the line edge, never extending across the line width. Modeling electromigration in the surrounding microstructures shows that void locations in the highly-textured lines are often independent of the surrounding grain boundaries. In this case, the line sidewalls appear to be significant mass transport paths. Since these paths do not cross the line width, voids growth across the line is difficult. This resistance to fatal damage makes highly textured aluminum a promising interconnect material.