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 °C for 90 s or 150 °C for 15 s, while wafer level bonding can be achieved at 100 °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.

In this paper, three iterative methods (Stokes, Newton and Oseen iterative methods) based on finite element discretization for the stationary micropolar fluid equations are proposed, analyzed and compared. The stability and error estimation for the Stokes and Newton iterative methods are obtained under the strong uniqueness conditions. In addition, the stability and error estimation for the Oseen iterative method are derived under the uniqueness condition of the weak solution. Finally, numerical examples test the applicability and the effectiveness of the three iterative methods.

Ultra-low temperature wafer-level Cu-Cu direct bonding with wetting/passivation scheme has been successfully demonstrated at (1) room temperature with post-annealing at 100 ℃, or (2) 40 ℃ bonding without the post-annealing process. In this bonding structure, a wetting layer and a passivation layer were deposited on the Cu surface to improve the surface conditions and enhance the diffusion behavior of Cu atoms. In addition, the wetting layer can prevent formation of AuCu 3 between passivation and Cu, which is beneficial for Cu bonding at a lower temperature. The proposed bonding structure provides the breakthrough to realize wafer-level Cu-Cu direct bonding with an almost thermal stress-free process, which is key to improve reliability and broaden applications of 3D integration and advanced packaging.