Over pad metallization for high temperature interconnections

Electrolytic CuNiAu over pad metallization (OPM) was qualified for high temperature gold wire bonding applications. Stability of the CuNiAu OPM metal stack was tested through extreme conditions, i.e.: ambient temperature 250°C up to 4000 hours, with satisfactory results. Reliability of CuNiAu OPM was then confirmed in series of tests with ambient temperature up to 175°C, such as high temperature storage life (HTSL), highly accelerated stress test (HAST) and temperature cycle (TMCL), along with Al0.5%Cu bond pad alloy as reference. The CuNiAu OPM test groups showed no functional failure and no change of failure mode in wire bond bump shear test - all samples exhibited the favorable bump shear mode. The reference Al0.5%Cu group, on the other hand, showed degrading bump shear failure mode besides a few ATE test failures. Detailed failure analysis confirmed the integrity of OPM-gold wire bond interface, while revealing interfacial voids induced bump lift type of failure mode in the reference group. Good wafer sorting yield on CuNiAu OPM can be achieved with proper selection of probe type and materials. Wire bond optimization was also necessary to accommodate the unique electro-plated OPM topograph. Wafer handling has to be extra careful in order not to scratch the gold topped bumps that stands 5 μm above the wafer passivation surface. Details are to be discussed in the following sections.