Mechanism on preferred orientation roof-type Cu6Sn5 grain formation in micro-bump with (111) nanotwinned copper UBM

The (111) nanotwinned copper films used as a novel under bump metallization (UBM) were applied in micro-bump solder technology for 3D packaging. The orientation and morphology of interfacial Cu₆Sn₅ grains on (111) nanotwinned copper UBMs were investigated by soldering at different temperatures (250 °C and 300 °C). The (11−20) preferred orientation roof-type Cu₆Sn₅ intermetallic compound (IMC) formed on Sn/(111) nanotwinned copper interface at a higher soldering temperature of 300 °C; while random orientation scallop-type Cu₆Sn₅ grains formed on Sn/(111) nanotwinned copper interface at lower soldering temperature of 250 °C. The preferred orientation relationships between and interfacial roof-type Cu₆Sn₅ grains and (111) nanotwinned copper UBMs are {11−20}_Cu6Sn5||{111}_copper in 〈0001〉_Cu6Sn5||〈011〉 _copper and {11–20}_Cu6Sn5||{111}_copper in 〈10−10〉 _Cu6Sn5||〈112〉 _copper. Moreover, the average shear strength of the (111) nanotwinned copper/Sn/(111) nanotwinned copper solder interconnects soldered at 300 °C (40.19 MPa) is 30% higher than that soldered at 250 °C (31.09 MPa). This study provides an insight in controlling the morphology and orientation of interfacial Cu₆Sn₅ grains on the (111) nanotwinned copper used as UBMs for the 3D packaging application.