A robust CUP pad structure design with thin pad metal for Cu wire bonding

In order to save the manufacturing cost of ULSI semiconductor device, there are two significant approaches; shrinking chip size by adopting CUP Pad (circuit under pad) and reducing package cost by using Cu wire bonding as a substitute for Au wire. However, these two approaches have a conflict that CUP pad is more sensitive to higher mechanical stress induced by Cu wire bonding than mechanical stress from Au wire. Thick pad metal is a way to enhance the strength of CUP pad but it brings enlargement of top metal design rule, which can increase chip size. Therefore it is highly required to develop a robust CUP pad structure with thin pad metal in order to apply Cu wire bonding without any negative effect on chip size. This study deeply investigated on the crack in CUP pad structure of ULSI with thin top metal caused by high mechanical stress of Cu wire bonding. FIB cross-sectional analysis for the crack revealed how and where the crack is started and propagated over the whole pad. Mechanism of pad crack was discovered by strain and strength analysis on each film layer which forms bonding pad structure. Cu wire bonding experiment was performed with several pad structures at various bonding conditions, the outcome of the experiment delivered major factors to enhance the strength of pad structure against physical damage during Cu wire bonding process. This study concentrates on a robust CUP pad structure of ULSI with thin pad metal for Cu wire designed with considering the major factors.