Downstream electromigration improvement in 45nm technology

Abstract

The broad time-to-failure distribution and bimodality of downstream electromigration (EM) in 45nm technology node are investigated. Liner void and end of line void at wafer edge on downstream EM structure after M1 CMP is a clear physical vapor deposition (PVD) shadowing effect signature caused by poor liner gap fill capability. Furthermore, void growth study during early electromigration stage effectively indicates that the void is initiated at the bottom corner of the via interfacing with cap layer for early failures and slit void is consistently observed at same location on unstressed sample. These pre-existed voids create poor contact either on via bottom interface or trench and cap layer interface or via bottom corner which produce a difference in failure time resulting in poor time to failure (TTF) spread and lower t50. New Cu seed deposition technique eliminates end of line void and liner void and it turns out to improve downstream EM performance. Optimized post etch treatment (PET) chemicals help to reduce Cu oxidation to improve via bottom integrity and eliminate slit void. This effective post etch treatment was demonstrated to improve downstream EM bimodality behavior to tight mono-modal distribution. Via above to metal below overlay is also one of key factors for downstream EM improvement. There is a strong correlation between the TTF and the via to metal overlay and stringent overlay control is beneficial to improve downstream EM sigma and t50.