A Study About Bonding Properties with Multilayer Porous Structures for Fine Pitch Interconnection

In this report, we investigated characteristics and bonding properties about some multilayer porous bumps. These porous bumps could be formed by plating and electroless process on nano porous Cu (NP-Cu), which caused a change in its porosity. In addition, NP-Cu/Ni, NP-Cu/Ni/Ag, and NP-Cu/SnAg bumps were not dissolved during the etching process, because Ni and SnAg were formed at a protective layer over the entire NP-Cu bumps. From the compression test, the compressive properties were changed by different multilayer porous bumps. The shear strength of NP-Cu/Ni bonding was lower than that of only NP-Cu bonding, and it was increased significantly by NP-Cu/Ni/Ag bonding compared to NP-Cu/Ni bonding. In NP-Cu/SnAg bonding, it was almost the same as that of only NP-Cu bonding. We confirmed that NP-Cu/Ni/Ag and NP-Cu/SnAg bumps could be connected even for bump diameters below $10 \mu ~\mathrm{m}$. Good electrical resistance was shown in NP-Cu/Ni/Ag and NP-Cu/SnAg bonding as reliability tests with a daisy chain pattern. These results indicated that direct bonding between solid phases could be easily achieved in NP-Cu/Ni/Ag bumps without pre-treatment such as chemical mechanical polishing (CMP) and plasma activation process before bonding. Moreover, it also suggested that there was the possibility of partial melting and formation of intermetallic compounds in NP-Cu/SnAg bumps during the bonding process like transient liquid phase (TLP) bonding. The use of some multilayer porous bumps can provide the possibility of easily achieving fine pitch interconnects in a similar method with the SnAg soldering process.