Atomic diffusion bonding of wafers with thin nanocrystalline metal films

Bonding between two flat wafers with thin metal films was studied for 14 thin metal films with various crystal structures and self-diffusion coefficients. Thin nanocrystalline metal films were fabricated on two flat wafers' surfaces using sputter deposition. Bonding of the two metal films on the wafers was accomplished immediately after film deposition in vacuum. For the films, Al, Au, Ag, Cu, Si, Co, Ni, Pt, Ti, Ru, Fe, Cr, Mo, and Ta were used. The wafers were bonded at room temperature over the entire bonded area (1 or 2 in. wafers of either Si or SiO₂) using these metal films. Transmission electron microscopic cross-section images revealed that complete crystalline grains were formed across the original surfaces of the films, probably because of recrystallization occurring at the bonded interface when Al-Al, Ag-Ag, Au-Au, Cu-Cu, and Ti-Ti nanocrystalline films were bonded. A clear interface corresponding closely to the original film surface was visible in the bonded Pt-Pt films. However, fcc-(111) lattices were formed continuously across the films' original surfaces. A thin amorphous layer was formed at the interfaces of Cr-Cr, Fe-Fe, and Ta-Ta bonded films. Experimental results revealed that the two films' bonded structure was related closely to the self-diffusion coefficients of the metals used for bonding. A high atomic diffusion coefficient at the grain boundaries and film surfaces is likely to have enabled bonding at room temperature. Moreover, results obtained using Cu-Cu, Al-Al, Ti-Ti, Cr-Cr, and Ta-Ta films demonstrated that the wafers were bonded even with only 0.2-nm-thick films on both sides. Bonding films of different materials was also achieved, e.g., Ta-Cu films. The bonding technique described herein is promising for use with bonding wafers to fabricate new thin film devices and microelectromechanical systems.