FBEOL no-aluminum pad integration in Pb-free C4 products for environmental, cost and reliability benefits

Integrated circuits with Pb-free C4's have two major reliability concerns: thermal-mechanical stress induced mechanical fails within the Si chip and C4 electromigration (EM). Decreasing feature sizes, and increasing power and performance requirements have exacerbated these concerns and necessitated the development of innovative solutions to address the reliability issues and support the building of more robust and reliable packaged parts. Aluminum (Al) pads have been historically used in the Far Back End of Line (FBEOL) levels of the Si chip primarily for mechanical and Chip Package Interactions (CPI) benefits. Previous studies have shown that the Al pads used in legacy FBEOL process integration can serve as a stress redistribution layer to dissipate the detrimental thermal-mechanical stresses from reaching the underlying weaker BEOL ULK/Low-K levels. Aluminum processing in the wafer fab however typically uses carcinogenic chemicals such as hexavalent chromium for passivation and for corrosion inhibition. The current work evaluates non-Al FBEOL structures for the obvious environmental reasons but also as means for reducing processing costs and fab cycle times. Mechanical finite element analysis have been performed to determine the effect of the FBEOL structural changes on the stresses in the ULK/Low-k BEOL levels. White “C4” bump and C4 EM data comparing the Al pad structure to the non-Al pad structures will also be reviewed and some of the key process integration challenges with the non-Al structures will be discussed.