Electromigration study of copper interconnects with side reservoir design
Date of Issue2016
School of Materials Science and Engineering
Reliability issues in copper interconnect/low-k dielectric system, namely electromigration and TDDB (Time Dependent Dielectric Breakdown), have become more crucial as the dimensions of the copper interconnect structures keep shrinking with the technology node. In this project, a new method to mitigate the effects of electromigration of copper interconnect is being proposed. Many researchers had tried to improve the lifetime of the copper interconnect when it undergoes electromigration test, such as introduction of end of line metal reservoir, or improving the capping layer of the copper interconnects. However, no matter how robust the fabrication process of the copper interconnects is, there is always a possibility of a defect or pre-existing void present in the metal interconnects. Fatal voids that are found near the end of cathode vias are the cause of electromigration failures, and recent in-situ electromigration tests have revealed that the voids can drift towards the end of cathode vias, instead of nucleating there. The motivation of this project is to improve the lifetime of copper interconnects in the knowledge that pre-existing voids exist, and when subjected to electrical stressing, could lead to earlier failures due to electromigration. An unique design of a side metal reservoir is being introduced in the project, which is hypothesized to be capable of enhancing the lifetime of the copper interconnects under the proposed electromigration mechanism. The modeling and simulation done in this study suggest that the distance from the cathode end of the interconnect, Lcrit, is important when it comes to the location of where the pre-existing void was nucleated or pinned. When the pre-existing void is nucleated or pinned at the distance smaller than Lcrit, there would not be any new void nucleation at the end of cathode via due to the change in the stress evolution along the metal interconnect. The newly designed side metal reservoir was studied to check its impact towards the lifetime of the metal interconnect under electromigration test. The end of line metal reservoir was used as the reference in this study. It is observed that the side reservoir design is able to enhance the lifetime of the copper interconnect as hypothesized. The effectiveness of various length of the side metal reservoir was investigated as well. From the experiments, it was found that there is a limit on how far the side metal reservoir can extend before losing its effectiveness of trapping the pre-existing voids. Finally, attempts on in-situ electromigration experiment on the side metal reservoir were done. However, it proved to be a challenge due to the lack of a high temperature heater to accelerate the stressing during the in-situ electromigration testing.