Principal Investigator Jing Kong
Bonding technology plays a significant role in electronic packaging as it provides physical and electrical connections between semiconductor chips. Reliability of bonding joints affects the energy consumption and speed of an electronic system. Hence, it is important to have a reliable bonding technology. Copper (Cu) bonding technology is one of the most frequently-used bonding technologies nowadays. However, two critical issues have been limiting the reliability of lead-free Cu bond- ing technology: high bonding temperature (~260 °C) and aging degradation.
We have devised a graphene-based Cu bonding technology that is of low bonding temperature and high reliability. By integrating nanoscale graphene/Cu composite on the Cu substrate prior to thermocompression bonding, Sn-Cu joints can be fabricated at a bonding temperature as low as 150 °C, which is the lowest reported value to date for Cu bonding technology. Specifically, we electrochemically deposit a layer of Cu nanocone array on the Cu substrate and cover it with a graphene sheet, prior to the bonding process. When subjected to heat, microscale Sn solder deforms and replicates the Cu nanocone array morphology, and hence transforming into nanoscale Sn. Compared to microscale Sn, nanoscale Sn has much lower melting points and facile surface diffusion. This phenomenon effectively contributes to the low bonding temperature observed in our bonding technology. The presence of the graphene layer prevents the formation of Cu-Sn intermetallic compounds thus significantly slows down the aging degradation. With the advancement in graphene synthesis and transfer technology, we anticipate the graphene-based Cu bonding technology presented in this work can be integrated into the existing commercial Cu bonding technology for industrial applications in the near foreseeable future.