With the approaching limitation of Moore’s law scaling, the semiconductor industry has turned to advanced packaging technologies to meet the high-performance computing needs. 3D integration and formation of electrical interconnects between multiple vertically stacked Si devices have enabled high speed, high bandwidth connections. Microbumps and through silicon vias (TSV) are the key architectural elements for die-to-die and die-to-wafer stacking as they improve performance of the complete system. Typical solder height is 15-30µm in Cu pillars and it is expected to scale down to less than 10µm in 20µm pitch interconnections. Solder based, fine pitch, micro-bump connections are preferred mainly due to its lower bonding temperature compared with Cu-to-Cu thermo-compression bonding. However, with reducing bump dimensions, several critical reliability issues arise such as the small solder volume transformation and conversion to complete intermetallic compound (IMC), during thermo compression bonding. Studies have also shown that for a 20μm microbump the current density can reach values that are significantly higher than the threshold value of Sn electromigration (EM) and the failure mechanism in microbumps are different from traditional flip chip bump. Two main EM failure mechanisms are reported: void propagation along the IMC/solder interface and the dissolution of under-bump-metallization (UBM).
From a bump processing standpoint, it is common to plate multi-metal films on the same equipment to minimize oxidation between metal depositions as well as to increase production flexibility and minimize risks due to equipment down-times. In an earlier paper, we extensively discussed a new in-line acoustic technique for pillar bump metrology for thickness > 30µm to a few hundred microns. Thickness information on microbumps (<30µm) are currently obtained from either step height measurements or cross-section microscopy. Neither one is suitable for in-line measurements, nor can they be used to study the evolution of IMC. Picosecond Ultrasonics (PULSE) technology is well established in wafer fabs for metal film thickness measurement. The capability of the technique to measure UBM and redistribution layer (RDL) has been previously reported. In this paper, we review microbump characterization.