Effect of electromigration on the Cu-Ni cross-interaction in line-type Cu/Sn/Ni interconnect

摘要

The line-type Cu/Sn/Ni interconnects were used to determine the effect of electromigration (EM) on the Cu-Ni cross-interaction under the current density of 1.0×104 A/cm2 at 150 °C for 100 h and 200 h. For the purpose of comparison, the line-type Cu/Sn/Ni interconnects were also aged at 150 °C for 100 h and 200 h. After soldering, Ni3Sn4 and Cu6Sn5 IMCs formed at the Sn/Ni and Sn/Cu interfaces, respectively. No cross-interaction occurred during soldering. The Cu concentration in the IMCs at the Sn/Ni interface increased with the increasing aging time, and the original Ni3Sn4 IMC at the Sn/Ni interface transformed into (Cu0.56Ni0.44)6Sn5 IMC after being aged at 150 °C for 200 h; while the IMC at the Sn/Cu interface remained Cu6Sn5 even after being aged at 150 °C for 200 h. A thermal-electric finite element simulation showed that no thermomigration (TM) effect occurred. During EM, the direction of electric current played an important role. When Cu atoms were under downwind diffusion and Ni atoms were under upwind diffusion (electrons flowed from Cu side to Ni side), more Cu atoms were driven to the opposite side than that of the aging case. The original interfacial Ni3Sn4 IMC at the Sn/Ni interface (anode side) had already transformed into (Cu0.53Ni0.47)6Sn5 after EM at 150 °C for 100 h, which was faster than the aging case. After EM for 200 h, (Cu0.60Ni0.40)6Sn5 IMC layer formed at the Sn/Ni interface. When Cu atoms were under upwind diffusion and Ni atoms were under downwind diffusion (electrons flowed from Ni side to the Cu side), Cu atom diffusion was retarded, while Ni atom diffusion was enhanced. More Ni atoms were dissolved into the Sn. The interface betwe- - en Ni and Sn became uneven after EM for 100 h and became rougher after EM for 200 h. Large Ni3Sn4 particles precipitated in the Sn near the Sn/Ni interface (cathode side). The size of Ni3Sn4 particles that precipitated in the Sn increased with the increasing EM time. Cu atoms were very sensitive to the direction of electron flow. When Cu atoms were under downwind diffusion, more Cu atoms could diffuse across the solder to the Ni side and alter the interfacial IMC type. When Cu atoms were under upwind diffusion, few Cu atoms could diffuse across the solder to the Ni side and no interfacial IMC transformation occurred at the Ni side. Regardless of electron flow direction, few Ni atoms could diffuse across the solder to the Cu side due to its high diffusion activation energy.