Low-Temperature (70°C) Cu-to-Cu Direct Bonding by Capping Metal Layers

Cu-Cu direct bonding at low temperature with Cr wetting layer and Au passivation has been developed, and the bonding mechanism has been investigated. A chip-level Cu-to-Cu direct bonding can be achieved under a very low thermal budget condition at 70 degrees C for 90 s or 150 degrees C for 15 s, while wafer level bonding can be achieved at 100 degrees C under a low vacuum environment (10(-2) Torr) by deposition of capping metal layers, which can protect Cu from oxidation, as well as reduce surface roughness and grain size of Cu. During the bonding process, Cu atoms diffuse through Cr/Au layers into the bonding interface, forming a new inter-layer without voids to achieve high quality bonding. The phenomenon of recrystallization and effects of grain size have been validated by TEM analyses. This Cu-Cu bonding with capping metal layers scheme can enable bonding with ultra-low thermal budget, excellent bonding quality, good electrical performance and high reliability, showing the great feasibility for the 3D integration applications.

Automated summary

This study developed Cu-Cu direct bonding at low temperature using Cr wetting layer and Au passivation, achieving high quality bonding with ultra-low thermal budget. The bonding mechanism involves Cu atoms diffusing through metal layers to form a new inter-layer without voids, ensuring good electrical performance and high reliability for 3D integration applications.