Ekta Misra

Novel design and integration enhancements in the final polymeric passivation for improved mechanical performance and C4 electromigration in lead-free C4 products

Two key C4 reliability concerns for the current and next generation integrated circuits are electromigration (EM) and “white C4” bumps caused by the stresses induced by die-package interactions. This paper discusses novel design and integration changes in the final polymeric passivation via (FV) in order to mitigate white bump and chip-package interaction (CPI) stresses in the ultra-low k (ULK) BEOL levels and also meet lead-free C4 EM requirements. FV design changes such as strategically offsetting a single or multiple FV vias towards the center of the chip and thus to the compressive side of the C4 bump has been shown to reduce the stresses in the ULK levels due to chip package interactions and hence significantly reduce the number of white bump fails. Changing the shape of the FV via to strategically distribute current more uniformly through the C4 bumps has also been shown to improve the C4 EM performance significantly, while lowering the overall stresses in the chip. Effects of final passivation thickness and via diameter on the white bump stresses will also be discussed. Supporting white-bump, C4 EM and electrical/mechanical modeling data showing the benefits of the design and integration changes will also be discussed in detail in the paper.

Role of FBEOL Al pads and hard dielectric for improved mechanical performance in lead-free C4 products

One of the major reliability concerns of current and next generation integrated circuits is mechanical failure due to stresses induced by the chip-package interactions (CPI). The packaged parts are subjected to thermal-mechanical stresses due to a mismatch of the coefficient of thermal expansion of the Si, lead-free C4 bumps, and the organic flip-chip substrate leading to mechanical delamination or cracking in the weaker low-k/ultra-low K films within the chip. This work discusses the role of Aluminum (Al) pads in the far-back-end-of-line (FBEOL) levels of the chip in CPI stress mitigation of the weak low-k and ultra-low k (ULK) BEOL levels. The affect of the Al pad thickness, size and shape on the CPI stresses have been studied by means of 3D mechanical finite element analysis. “White C4” bump data showing the benefits of increasing the thickness of the Al pads and growing the Al pad size to be larger than the under bump metallurgy (UBM) diameter in alleviating detrimental stresses from the weak BEOL levels is also been discussed in the paper. This paper also outlines through mechanical modeling and “white C4” bump data the reduction in CPI stresses in the weaker BEOL levels with increasing thickness of the FBEOL hard dielectric.

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

Integrated circuits with Pb-free C4s 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.