Low Temperature and Pressureless Microfluidic Electroless Bonding Process for Vertical Interconnections

Thermocompression bonding (TCB) process is now being adopted for high density interconnections but the necessity of applying force and heat causes a lot of problems, such as warpage-induced defects, cracking of delicate chips and thermal drift. To address the above issues, we proposed a novel bonding technique called microfluidic electroless interconnection (MELI) process to directly fabricate interconnection between Cu pillars at the temperature below 80°C and without applying any pressure on the chips. It has been shown in our previous researches that the MELI process using electroless Ni and electroless Au could bond vertical interconnection under controlled flow. In order to extend the application range of the MELI to fine pitch, in this study we analyze the feasibility of selective electroless deposition in microchannel by adding stabilizers into electroless Ni and electroless Au solution. Electroless plating can provide a uniform conformal coating on all parts of the surface that have been catalytically activated. However, the extended coating from the sides of the Cu pillar bump shortens the interconnect pitch, which may cause the risk of bridging. In this paper, we successfully achieve selective electroless Ni plating in microchannel by adding 1.5 ppm of lead acetate into the plating bath. As for electroless Au plating, selective deposition in the microchannel can be accomplished by narrowing the gap. In summary, the innovative MELI process provides a low-temperature and pressureless fine pitch bonding technique.