Low-temperature direct copper-to-copper bonding enabled by creep on (111) surfaces of nanotwinned Cu

Abstract

Direct Cu-to-Cu bonding was achieved at temperatures of 150–250 °C using a compressive stress of 100 psi (0.69 MPa) held for 10–60 min at 10−3 torr. The key controlling parameter for direct bonding is rapid surface diffusion on (111) surface of Cu. Instead of using (111) oriented single crystal of Cu, oriented (111) texture of extremely high degree, exceeding 90%, was fabricated using the oriented nano-twin Cu. The bonded interface between two (111) surfaces forms a twist-type grain boundary. If the grain boundary has a low angle, it has a hexagonal network of screw dislocations. Such network image was obtained by plan-view transmission electron microscopy. A simple kinetic model of surface creep is presented; and the calculated and measured time of bonding is in reasonable agreement.

Copper-to-copper direct bonding on highly (111)-oriented nanotwinned copper in no-vacuum ambient

“…There were 5 voids in the nt-Cu joint and 12 in the polycrystalline Cu joint. It was reported that the surface diffusion of Cu on (111) planes is faster by 3–4 orders in magnitude than other major planes 17 , 18 . Therefore, the (111) surface facilitates the diffusion of Cu atoms and then voids in the bonding interface can be filled by the Cu atoms.…”

“…(111) oriented and nanotwinned Cu (nt-Cu) has attracted much attention due to its high surface diffusivity on the (111) plane 17 , 18 . The rapid surface diffusion has enabled low-temperature direct bonding in the range of 150 to 200 °C under an ordinary vacuum of 10 −4 to 10 −3 torr.…”

“…First, surface diffusion by creep to achieve bonding of two surfaces. Second, grain growth to eliminate the bonding interface 16 – 18 . There are several literatures addressing the first step.…”

Low Temperature Cu–Cu Bonding Technology in Three-Dimensional Integration: An Extensive Review

“…Recently, Panigrahi et al demonstrated an optimized Cu rich metal-alloy (Manganin) passivation technique that not only helps in passivate the Cu surface but also inherently enhanced Cu (111) oriented plane at the interface which results in high quality reliable bonding at 140 C [52]. Besides, Liu et al proposed a novel surface modification technique to form nanotwinned Cu surface at the interface in order to enhance the diffusion capability during thermocompression cycle and subsequent bonding was performed at 150˚C with a nominal compressive stress of 0.69 MPa [53]. Also, microstructure behavior of the bonded Cu-Cu interface was examined using FIB imaging for different deposition conditions at different bonding conditions and different deposition conditions as shown in Fig.…”

Stress-enhanced Cu-to-Cu Bonding for MEMS Packaging

“…(4) Recently, modified processes to enhance direct Cu bonding have been suggested. (5)(6)(7)(8)(9)(10)(11) Fujitsu Laboratories proposed a surface grain refining pretreatment, which combines diamond cutting, organic acid vapor, and vacuum ultraviolet treatments. (12,13) Diamond cutting is performed to acquire a fine crystal layer underneath the surface, and formic acid vapor removes Cu oxides on the cut surface.…”