In 3D integration industrial context, copper is widely favored over others metals as a bonding material for its exceptional electrical and mechanical properties. It has been already reported that directly bonded structures involving copper layers exhibit typical voids that drastically abound beyond 300 • C. In order to have a better understanding of the voiding process, we specifically designed structures involving materials and surfaces exhibiting different properties. These stacks underwent different bonding processes which mainly differ in the mechanical applied load. For each variation in this study the total volume of voids was estimated throughout a strict protocol. Thus, we demonstrate that voiding phenomena is related to a stress driven vacancy diffusion very comparable to standard metallurgical creep mechanisms. Regarding the origin of the vacancies, among all the possible options, two predominant sources have been identified. Better understanding of these physical phenomena should enable the achievement of advanced wafer assemblies exhibiting much higher reliability and quality.

International audienceWafer level metal bonding involving copper layers is a key technology for three-dimensional integration. An appropriate surface activation can be performed in order to obtain room temperature direct bonding with bonding strength to be compatible with microelectronic device integration. In this paper, we focus on bonding strengthening mechanisms in the low temperature range from 20 to 100 A degrees C. In order to improve the bonding interface closure, several process parameters are studied: water amount available when surfaces are brought into contact and copper layers deposition technique have been identified to be predominant in this low temperature range. Based on metal oxidation theory, several mechanisms are proposed and some calculations on this bonding strengthening are performed. For the first time, activation energies of this phenomenon are obtained for copper layers deposited by different method in the specific environment of the bonding interface. This study gives guidelines and recommendations for the integration of this metal bonding technique at low temperature

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