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

초록

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.

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