The electromigration short-length effect in Ti-AlCu-Ti metallization with tungsten studs

Publisher Website
Google Scholar
Abstract
The electromigration short‐length effect has been investigated by testing a two‐level structure with Ti‐AlCu‐Ti stripes and interlevel tungsten (W) stud vias. This investigation represents a complete study of the short‐length effect using a technologically realistic test structure. Lifetime measurements and resistance changes as a function of time were used to describe this phenomenon, where the latter approach provides new insights into the electromigration behavior of multilayered metallizations. A linear increase in resistance was followed by a resistance change with time that approached zero. For the same product of current density and stripe length, longer stripes increased in resistance to higher values than shorter stripes. The sigma of the lognormal distribution increased as the current density decreased and/or as the maximum allowed resistance change increased. The lifetime, or t50, at relatively small current densities did not obey Black’s empirical equation. Rather, the lifetime data obeyed a modified version of this equation that includes a critical current density jc as a new parameter. As an alternate approach to quantifying the short‐length effect, we propose a novel and practical model for determining jc that focuses on the apparent saturation of the resistance increase with time of the W stud chains. Unlike the modified Black’s model, the resistance saturation approach allows one to estimate lower bound limits of jc. The threshold product, (jL)c, is determined from the modified Black’s model and from the resistance saturation model for stripe lengths of 50, 70, and 100 μm. Both models indicate that jc strongly depends on the fail criterion, or the magnitude of the resistance change, but is independent of temperature in the range 175–250 °C. No evidence was found of a (jL)c below which no electromigration‐induced damage occurs.