Reliability of Silver Direct Bonding in Thermal Cycling Tests

Silver-Silver direct bonding has been developed as an alternative interconnection technique for 3D power integration which can form high-strength and low-resistance Ag joints under solid-state conditions. However, the reliability and failure mechanisms of these direct bonded joints have not been characterized yet. In this study, four direct bonded structures were accelerated aged with two different temperature cycling tests (TCT) in the temperature range between -55 °C and 220 °C. The thermally induced stresses in the metallization during TCT can be relieved by elastic and plastic deformation of the Ag films and the debonding at the interface. At higher temperatures above 150 °C, plastic deformation mechanisms dominate the stress relaxation. Stress-driven void migration along grain boundaries may lead to cracking and interfacial failure. Metallization with a diffusion barrier and large grain size can restrict the void formation and diffusion, therefore, provide a higher temperature capability. The results indicate that both the adhesion of the Ag metallization and the Ag-Ag direct bonding remain stable after 1000 thermal cycles.