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Robust CUP Pad Structure Design with Thin Pad Metal for Cu Wire Bonding
Keywords: Cu wire bonding, CUP pad (circuit under pad) , ULSI semiconductor manufacturing
In order to save the manufacturing cost of ULSI semiconductor device, there are 2 significant approaches; shrinking chip size by adopting CUP Pad (circuit under pad) and reducing package cost by Cu wire bonding as a substitute for Au wire. However, these 2 approaches have the conflict that CUP pad is more sensitive to higher mechanical stress required for Cu wire bonding than Au wire. Thick pad metal is a way to enhance the strength of CUP pad but it brings the enlargement of top metal design rule, which can cause to 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 x-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 on various bonding conditions, which delivered major factors to enhance the strength of pad structure against physical damage during Cu wire bonding process. This study concluded a robust CUP pad structure of ULSI with thin pad metal for Cu wire designed with considering the major factors.
ChangHee Han, Senior Manager
TLI Inc.
Seongnam-si, Gyeonggi-do 463-828,
Republic of Korea

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