Study on ruthenium passivation for reliable cu-cu bonding with a tolerance to surface morphology

Abstract

As the demand of emerging industry for mass data processing such as high-performance computing (HPC) and generative artificial intelligence (AI) rises, high-density interconnect technology becomes even more crucial. However, conventional solder bonding technology can’t be applied due to interfacial reliability issues. Through solderless Cu-Cu bonding, stacking highly integrated devices with a fine pitch of several um would be possible. For the low temperature Cu-Cu bonding, the passivation layer is required to prevent surface oxidation during the bonding process. Here, the Ru passivation layer is proposed for the achievement of metal capping passivation technologies. The feasibility of the Ru passivation layer is proved by verifying the passivation effect of Ru for Cu surfaces and the diffusion of Cu through the Ru layer in thermal conditions. The bonding mechanism of the Ru-passivated Cu after thermal compression bonding (TCB) process is discussed through analysis of the bonding interface and fracture surface. Furthermore, we optimized the proper thickness of the Ru layer for highly reliable bonding and evaluated the bonding properties at various temperatures. Conclusively excellent bonding properties could be obtained even at low bonding temperature of 250 ℃, and the electrical reliability under the thermal cycling test (TCT) is measured. These results can confirm that the Ru passivation layer is a promising candidate for low-temperature Cu-Cu bonding technology.