Contract Wafer Bumping of Compound Semiconductors

There are a number of qualified technologies for backend processing of traditional silicon based semiconductors. GaAs wafers have several unique properties that make these established technologies inadequate, including: the presence of air bridges, gold bond pads, and the bulk material properties of the GaAs. In this paper, we describe a process flow and a set of materials which enable the pad resurfacing, UBM deposition, and solder bumping of GaAs wafers. The gold bond pads are resurfaced using a resist liftoff technology and TiCu sputtering. The combination of these allows for pad resurfacing while protecting the air bridges and GaAs. The UBM is deposited on these new TiCu pads by using a thin film layer to again protect the air bridges and GaAs, followed by electrolessly plating nickel and gold. The solder bumping is accomplished using a laser based sphere drop process which is fluxless. To complete the backend processing, the bumped GaAs wafers are then diced and sorted into waffle packs. Details will be discussed relative to the processing conditions, materials used, and yields.