Cobalt UBM for fine pitch microbump applications in 3DIC

In this paper we report results and challenges of replacing Cu with Co as UBM (under bump metallization) in microbumps for 3D technology applications. Cobalt has softer and single IMC (intermetallic compounds) and according to calculations using Cobalt as UBM can reduce consumption of UBM material by solder which is attractive for sub 10um pitches of microbumps. However, cobalt oxidizes very fast which results in poor wetting by solder as shown in Figure 1. This Figure shows two SEM images of cross section of 20um (left) and 50um (right) pitches microbumps from IMEC test vehicles where poor solder wetting is observed. It can be seen than in both cases Sn is deformed during TCB (thermo-compression bonding) bonding but due to oxide formation on cobalt bumps there is no reaction between Sn and Co. Such a joints may have weak electrical connection however, it is not suitable for a reliable device. Therefore surface treatment/passivation is required for cobalt bumps.