Effect of Bonding Strength on Electromigration Failure in Cu–Cu Bumps

Abstract

In microelectronic packaging technology for three-dimensional integrated circuits (3D ICs), Cu-to-Cu direct bonding appears to be the solution to solve the problems of Joule heating and electromigration (EM) in solder microbumps under 10 μm in diameter. However, EM will occur in Cu–Cu bumps when the current density is over 106 A/cm2. The surface, grain boundary, and the interface between the Cu and TiW adhesion layer are the three major diffusion paths in EM tests, and which one may lead to early failure is of interest. This study showed that bonding strength affects the outcome. First, if the bonding strength is not strong enough to sustain the thermal mismatch of materials during EM tests, the bonding interface will fracture and lead to an open circuit of early failure. Second, if the bonding strength can sustain the bonding structure, voids will form at the passivation contact area between the Cu–Cu bump and redistribution layer (RDL) due to current crowding. When the void grows along the passivation interface and separates the Cu–Cu bump and RDL, an open circuit can occur, especially when the current density and temperature are severe. Third, under excellent bonding, when the voids at the contact area between the Cu–Cu bump and RDL do not merge together, the EM lifetime can be more than 5000 h.

Interfacial Characterization of Low-Temperature Cu-to-Cu Direct Bonding with Chemical Mechanical Planarized Nanotwinned Cu Films

“…The contact regions were under severe compressive stress while the non-contact regions (voids) were subjected to less severe or no stress. This leads to a stress gradient where Cu atoms diffuse from the high localized to lower stress regions [ 41 , 42 , 43 ]. These Cu atoms slowly fill up voids and eventually form a bonding.…”

Recrystallization and Grain Growth in Cu-Cu Joints under Electromigration at Low Temperatures

“…Furthermore, pancake-type void formation was observed at the bonding interface, as shown in Figure 7c. Electron wind force and thermal-induced tensile stress may be the main reasons for the void formation at the bonding interface [36]. Figure 7b is the corresponding EBSD image, which indicates that recrystallization and grain growth took place in the Cu RDL on the top die.…”

“…In this research, NT-Cu joints were successfully bonded at 200 • C. Other than accelerated EM conditions at high temperatures [35,36], EM tests were carried out at 150 • C which is closer to the user condition. Recrystallization was observed after current stressing at 150 • C for 7000 h. Interestingly, a drop in resistance was observed after prolonged current stressing.…”

Solid-phase transient soldering method based on Au/Ni–W multilayer thin-film-modified copper-based structures

“…Solder junctions will get smaller and smaller in size. Soldering technology is the key to realize vertical interconnection [ [6] , [7] , [8] , [9] , [10] ]. Among the various soldering techniques, metal-to-metal soldering provides both good mechanical support and electrical interconnections.…”