Investigating thermal effects on void formation in electromigration using thermoreflectance imaging

This study visualizes the time-variation of the generation, growth, and pattern of voids induced by the electromigration in Al micro-line type interconnections. The two-dimensional temperature distribution near the voids was measured simultaneously using thermoreflectance imaging (TRI) technique. The effects of temperature distribution caused by Joule heating and heat conduction on the void spatial and temporal characteristics are discussed. Measurements were conducted with the base temperature of the line set to room temperature and high temperature conditions. The measurement results and discussion give new insights into the understanding of the thermal characteristics of the voids and heat transfer properties of interconnections, contributing to the advancement of application design and reliability physics. The thermoreflectance coefficients for Au, Si and Al materials were calibrated, and the temperature of the Au line subjected to Joule heating was measured and compared with numerical computation to validate the present measurement method. The pattern of the void generation at the cathode side of the Al line differed between the room- and high-base-temperature cases. The time-variation of the void area followed the power law, and generated three different slopes over time in the room-base-temperature case. The local temperature and its gradient significantly affected the void growth. Furthermore, the void growth rate and the line temperature near the voids followed an exponential relation in the void growth period. It was also found that the temperature gradient of the line can reduce the size reduction of voids after they were generated. These results highlight that the temperature plays an important role in local void growth characteristics in multiple ways.