A study on electromigration by driving force approach for submicron copper interconnect

Abstract

Electromigration (EM) is one of the major reliability issues for metal interconnect in integrated circuits (ICs). The ever increasing complexity of interconnects requires more demanding reliability of each component. The prime interest of this work is to investigate the EM failure physics for submicron Cu interconnections based on driving force approach including the effect of surrounding materials. For EM in interconnect, there are various extrinsic weaknesses which are having significant impact on failure characteristics. One good example of extrinsic weakness is stress gradient which exists in the interconnect core material due to thermo-mechanical mismatch at typical EM test condition. From the very beginning of the EM history, the electron wind force (EWF) is believed to be the most responsible driving force for atom or vacancy migration, although it is well known that the surrounding materials and the interconnect processing history highly influence the failure mechanisms. Apart from the EWF, the other driving forces such as forces from stress gradient, temperature gradient, surface tension etc. are believed to be only modify the atomic flux (AF) and corresponding atomic flux divergence (AFD) created by EWF. It is the purpose of this study to verify that the above mentioned forces can act as potential driving force in EM. The obvious sources of AFDs are the non-uniform distributions of current density, temperature, and stress fields in the metallization. These non-uniformities arise due to the presence dissimilar materials surrounding the ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library VII metallization and hence responsible for various extrinsic weakness against EM mass transport. In investigating the EM failure physics, the key reliability process parameters are identified to improve the EM reliability. In order to investigate the various driving forces and their effects on EM reliability for submicron interconnects, AFD based model is developed using finite element method. By introducing the above mentioned combined driving force approach and conducting EM experiments for submicron Cu interconnects, various EM characteristics are described. The model predicts that the stress free temperature (SFT) is one of the important process-related parameter that influences interconnect EM reliability. It is also predicted that by lowering the SFT, EM lifetime can be improved, which is verified by conducting EM experiments. In order to obtain critical void volume, resistance change profile etc., a dynamic EM model is described for submicron Cu dual damascene (DD) line-via test structure. The model predictions are found to agree well with experimental results. One of the EM reliability concern is the current crowding in line-via interconnects. The effect of current crowding is described in the light of width dependence and EM failure characteristics. A modified Black's equation is prescribed for line-via interconnects in the presence of current crow...