Mechanical characterization of micro-bump for aggressive bump scaling

Fine pitch micro-bump (μbump) interconnect has been gaining interest in multi-level die stacking such as DRAM to logic. The bumping process typically includes seed layer deposition, bump plating and seed etch. Seed etch is often a wet etch process, during which etchants will remove the Cu seed layer and the adhesion promoter such as Ti. Seed etch becomes very challenging when the bump size scales below ∼25 μm diameter due to under-cut, i.e. the Cu seed layer under the μbump (or even the bottom part of the μbump) is smaller than the μbump. A small under-cut will reduce the contact area between the bump and the substrate and will influence its mechanical and electrical performance. This paper investigates the impact of under-cut on the mechanical properties of fine pitch micro-bumps. A shear test is used to study the shear strength of Ø25 to Ø7.5 μm bumps, where the dimension of the bumps is ∼one magnitude smaller than the range comprehended in standard JEDEC shear test. It is found that the under-cut not only decreases the shear strength but also changes the failure mode. FEM also shows the change of deformation location when a μbump has an under-cut.