Electromigration Reliability of Cu Pillar on Substrate Interconnects in High Performance Flip Chip Packages

Abstract

Manufacturing high performance devices with shrinking form factors require a novel packaging approach. The Cu pillar-on-die interconnect is a widely accepted solution to package high performance flip chip devices due to its fine pitch adaptability, good electrical and thermal characteristics and elongated electromigration lifetime. However, the thick Cu pillar increases the stress on the die pad creating reliability issues due to fracture or de-lamination of low-k and extreme low-k (ELK) inter-layer dielectric layers. μPILR™ technology follows a Cu pillar-on-substrate approach that enables both the decoupling the Cu pillar from the ELK layers and enhanced electro-migration performance. This cost-effective alternative technology employs a subtractive etch process to form Cu pillars on substrates with exceptional intrinsic co-planarity. The 3D nature of the pillars offers advantages of increased vertical wetting for high yield in fine pitch assembly and reduction of crack propagation for good thermal cycle performance. Our preliminary investigations suggest that the electromigration lifetime of μPILR interconnects exceed the published lifetime data on various types of flip chip interconnects. In this work, the electromigration performance of two different interconnects will be investigated within Pb-free fine pitch flip chip packages. Interconnects include etched Cu pillar-on-substrate and conventional thin Cu UBM with solder-on-substrate-pad. The package level test vehicle has a large 18x20x0.75mm die with 10,121 interconnects with a minimum pitch of 150 μm packaged on a 40x40x1.19mm substrate with 10 metal layers in a 3-4-3 build up on a core stack. A comprehensive study of electromigration performance of these interconnects will be presented with the experimental determination of their activation energy and current exponent values. The Black's equation will be solved using mean time to failure data obtained from the experiments. A detailed description of the physical changes during the electro-migration failure process due to inter-diffusion and inter-metallic compound formation will be discussed.