Surface planarization of Cu/Sn micro-bump and its application in fine pitch Cu/Sn solid state diffusion bonding

Low temperature stacking of dies for 3D integration has been gaining interest due to the thermal sensitivity of some advanced node devices such as DRAM. Sn-based solder joint is considered as a promising approach for making die to die interconnections due to its low bonding temperature and high yield. Previously the Cu/Sn solid state diffusion bonding at 150 °C was reported, during which the bonding pressure is typically of 50 MPa. The remaining challenge for fine pitch Cu/Sn solid state diffusion bonding is how to enable good bonding for micro-bump with high topography. The effect of improving the contact between bumps was studied by applying a fly-cutting process to trim Cu/Sn bump surface. First, 2 kinds of fly-cutting processes, named fly-cutting first and seed etch first, are studied. Then different combinations such as fly-cutted Cu to non-fly-cutted Sn and fly-cutted Cu to fly-cutted Sn are compared. It is found that Cu and Sn micro-bumps with high topography become quite flat and smooth after fly-cutting, which ensure a smooth bonding interface. Moreover, ~80% yielding devices are achieved on 40/15 μm pitch/spacing peripheral array dies at 150°C with NUF serving as an intermediate cleaning agent as well as permanent underfill. In general, fly-cutting is an attractive option in enabling a good solid state diffusion bonding for micro-bump with high surface non-uniformity.