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.

Self-generation of spatial subharmonics when two coherent, almost collinear, pump beams propagate in a nonlinear photorefractive crystal of Bi12 SiO20, is reported. The intensity of the self-generated light beam at the half-angle between the two pumps is as high as 40% of the pump beam intensity. This phenomenon is probably a spatial analog of the temporal subharmonics obtained in a cavity containing a nonlinear medium. Application of the subharmonic of half-spatial frequency to collinear Bragg diffraction of a near infrared beam is demonstrated.

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