Cu-Cu hybrid bonding as option for 3D IC stacking

Abstract

Cu-Cu bonding is seen as possible option to enable 3D-IC integration within a reasonable cost. In case of 3D stacking, the TSVs can be revealed (nails) at wafer back side and bonded directly on a Cu landing pad. This can offer small pitch IOs and save the cost of extra processing like RDL (Re-Distribution Layer) and bumping. In this work we present the results achieved by developing the process for TSV nail reveal in order to enable electrical yielding Cu-Cu thermal compression bonding (TCB). Thus, we have investigated the impact on different TSV nail structures under various TCB conditions for a minimum TSV pitch of 10 μm. Three different TSV nail structures have been defined: No Nail exposed, Flat Surface of TSV, and Dome Shape of TSV (refer to fig 1). TSVs with different height (2um and 5um) are also been generated in case of dome shape. We initially report the results achieved on the different structures in case of different bonding conditions. Later we focus on the best performing structures and demonstrate that the use of an underfill (UF) is necessary to ensure good adhesion between dies.