The electromigration threshold in copper interconnects is reported in this study. The length-dependent electromigration degradation rate is observed and quantified in the temperature range of 295–400 °C. Based on the Blech electromigration model [I. A. Blech, J. Appl. Phys. 47, 1203 (1976)], a simplified equation is proposed to analyze the experimental data from various combinations of current density and interconnect length, as well as to estimate the electromigration threshold product of current density and line length, (jL)th, at a certain temperature. The resulting (jL)th value appears to be temperature dependent, decreasing with increasing temperature in the tested range between 295 and 400 °C.
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.
Articles you may be interested inEffect of the Ti/TiN bilayer barrier and its surface treatment on the reliability of a Ti/TiN/AlSiCu/TiN contact metallization J.Resistance saturation as a function of current density and stripe length has been investigated for a two-level structure with Ti/TiN/AlCu/Ti/TiN stripes and interlevel W stud vias. A simple model relating the resistance change at saturation to the current density and stripe length is formulated for structures with short stripe lengths and blocking boundaries at both ends. Experimental results for stripe lengths of 25, 50, or 100 m are in good agreement with the model predictions.
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