Novell embedded microbump approach for die-to-die and wafer-to-wafer interconnects with variable microbump diameters and down to 5 um interconnect pitch scaling

In this paper a novel solder-based die-to-die or wafer-to-wafer interconnect approach is introduced. This technique allows for microbump interconnects with different diameters on a single die and allows for pitch scaling down to 5μm A metal damascene process is used to create the metal pad layers for soldering. Solder μm bumps are created by semiadditive electroplating. After embedding the solder μbumps in a partially cured polymer, the wafer surface is planarized and the solder surface is exposed. This results in flat die surfaces of the die before bonding. Using a thermo-compression bonding process these flat surfaces can be aligned, the solder reacts with the metal pad to form a solder joint and the polymer can bond and cure to ensure a void less underfill layer for mechanical strength and increased reliability. The selection of suitable metals and polymers as well as the different process steps together with reliability data will be discussed in detail. Electrical yield and quality of bonding is demonstrated using imec 5μm pitch PTCU/W test vehicle for die to wafer bonding.