Electromigration lifetime sudies of submicrometer-linewidth Al-Cu conductors

Electromigration in metal conductors used in VLSI circuits raises important concerns especially at submicrometer dimensions. In this paper, we show that the current-carrying capability required in submicrometer MOS technology can be quite severe. We show experimentally that the mean time to fail for Al-Cu conductors increases as the linewidth decreases below about 2 µm and well into the submicrometer regime. Concomitant with this increase in the mean time to fail, there is an increase in σ, the spread of the failure distribution as well, leading to decreased reliability at early times for very narrow lines. Grain size and geometry are used to explain our results. Our studies also show that the applicability of an unenhanced "lift-off" defined Al-Cu metallurgy for submicrometer NMOS application needs careful examination.