Novel electroless bumping technologies for MCM-D : bumping of single chips and straight-wall bumping on Cu metallisations

To achieve a higher I/O count, increased speed along with increased performance, the flip-chip technique is one of the driving forces. Although flip-chip results in very reliable connections with high electrical performance, the high cost level must be reduced. Therefore it is essential to use a low cost bumping technique for both chip and interconnection substitute (Cu metallisation). The first part of this paper deals with a novel bumping technology for single chips, whereas the second part focuses on the development of a straight-wall bumping process on Cu bond pads. As conventional Al bonding pads on IC chips are not solderable, an under-bump-metallisation (UBM) is required for reflow, soldering. Electroless nickel/immersion gold (e-Ni/Au) has been widely accepted as UBM for this purpose. Conventionally, the e-Ni/Au bumping on ICs is carried our on wafer level. We developed a technology for the deposition of e-Ni/Au on single silicon chips or dice. Two challenges have to be dealt with first, the handling of the tiny chips/dice for the wet processing and second, the protection of the backside of the chips/dice in order to avoid unwanted nickel deposition. The navel method solves these two problems at one go and is extensively explained in this paper. Also to apply a solderable finish on Cu bond pads, e.g. chip or interconnection substrate with Cu metallisation, the maskless and reliable e-Ni/Au process can be called upon. Normally, the e-Ni/Au bumping on Cu pads differs from the e-Ni/Au bumping on Al pads only by the activation. We are currently investigating an innovative bumping technology in order to achieve Ni bumps with straight, walls on Cu bond pads. This e-Ni/Au process becomes more complex as we need to apply a removable mask and adjust the wet processing, but it should be possible to reach a very fine pitch. The core fundamentals of this bumping technology, as well as preliminary results, are presented in the second part of this paper.