Enhancement of adhesion strength of electroless-plated Ni under bump metallurgy by introduction of inductively coupled plasma enhanced bias sputtering Ni seed layer

Abstract

Electroless Ni plating using zincate treatment on Al pad is the most popular under bump metallurgy (UBM) fabrication method because of its low process cost. However, it has a demerit of poor adhesion strength. In this study, instead of using conventional zincate treatment, Ni thin film was used as a seed layer for the fabrication of Ni UBM film using electroless plating. 60-nm-thick Ni seed layer was deposited by inductively coupled plasma enhanced bias sputtering (ICPBS). The bombardment of energetic ions causes the compressive stress in the Ni seed layer and the atomic mixing at the interface. The introduction of Ti buffer layer successfully keeps the Ni seed layer from swelling due to the hydrogen generated by reducing agent in the electroless plating solution. The adoption of Ni seed layer deposited by ICPBS method enables the fabrication of electroless-plated Ni UBM film with maximum of 20 times superb adhesion strength measured by scratch test compared to that fabricated by conventional technique.