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In this study we discuss superiority of Cobalt for using in 3D interconnection as alternative metal to Cu. Specimens composed of pure Sn and Co or Cu is aged under same aging condition varied time and temperature below melting point of Sn. Thickness of IMC (intermetallic compound) formed at the interface is then calculated for extraction of growth rate and kinetics like activation energy and power factor. Compared with each factors extracted from calculation, IMC formation and growth of Co/Sn has stronger time and temperature dependence than IMC of Cu/Sn. Furthermore, no voids was observed at any interface in Co/Sn bonding.

Thermal compression bonding (TCB) process in combination with a pre-applied underfill material has been developed and investigated for assembling 20 μm pitch Sn-based micro bumps. It is found bonding force has a profound impact on the joint formation behavior. A low bonding force produces bump joints with heavier underfill entrapment and incompletely reacted solder. A higher bonding force leads to more solder squeezing-out, leaving a thin and completely reacted inter-metallic compound (IMC) layer in the joints. Electrical measurement of the daisy chains on the as-bonded chips does not reveal any significant difference between the samples made with different bonding forces. The reliability of the two types of joints were further studied in two post-bonding tests, namely the resistance measurement of daisy chains at an elevated temperature and stack-level thermo-cycling test. Both tests show a better reliability performance from the bump joints with less underfill entrapment and completely reacted IMC layer.

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.

Cu pumping of through silicon vias (TSV) may result in deformations of the Cu/low-k interconnect wiring above the TSVs and affect the back-end-of-line (BEOL) metal and dielectric reliability. We investigate the impact of Cu TSVs on the BEOL reliability, including stress induced voiding (SIV) of Cu vias on top of the TSV and the dielectric reliability of both inter- and intralevel low-k materials in Cu damascene interconnects. Possible solutions to mitigate the reliability risks are also discussed.