An Embedded Trace Redistribution Layer with Rounded-Bottom Cu Geometry and Ti Capping for Enhanced Electromigration Reliability

This paper presents the electromigration (EM) performance of an embedded trace redistribution layer (ETR) in which the Cu trace features a rounded-bottom cross-sectional geometry and is encapsulated by a Ti barrier layer except for the top surface, with an optional top-side Ti cap. The ETR (with and without top-side Ti capping) and the conventional semi-additive-process (SAP) redistribution layer (RDL) are comparatively evaluated in terms of EM reliability. The ETR demonstrates a marked lifetime improvement compared with the SAP RDL. Notably, the Ti-capped ETR exhibits a minimal resistance increase in less than 10% even after a test duration of 4000 h. We discuss the key contributing factors and underlying mechanisms that support these improvements. Transmission electron microscopy (TEM) combined with atomic-percentage mapping confirms the effectiveness of Ti capping as a Cu diffusion barrier, showing continuous Ti coverage and no observable Cu diffusion. Electro-thermal simulations co-locate predicted thermal hot spots with experimentally observed open-failure sites, highlighting temperature-driven EM acceleration and the necessity of a barrier to suppress Cu–polymer interfacial oxidation. Stress simulations, together with EM failure analysis, indicate that the rounded-bottom Cu geometry alleviates local stress concentration and stress gradients, thereby creating conditions favorable for enhanced EM resistance.