Assessment on palladium-coated copper wire bonding for 28nm Cu/low-k chips: Al bond pad and NiPd bond pad

Abstract

In this study, palladium coated copper core wire is evaluated and a series of bond quality test is carried out on Aluminum (1.5μm Al) bond pad and Nickel Palladium (2.5μm Ni/0.3μmPd) bond pad with O0.6mil palladium-coated copper core wire. From the optimization with mechanical chips, it is found that NiPd has larger bond parameters windows than Al bond pad. Ball shear strength of NiPd samples becomes higher after 175°C high temperature aging test for 1K hours. Al bond pad samples have some pad peel with shear strength >8 gf/mil2 after 1K hours aging at 175°C. Cu/low-k chips with 1.5μm thick Al bond pad bonded with 0.6mil palladium-coated copper wires are ball sheared and wire pulled at time zero and after 96 hours aging test, and it passes ball shear and wire pull test. It is observed that NiPd follows the contour of Al surface during the plating on Cu/low-k chip with Al bond pad, and plating scenario applies for Al bond pad with large probe mark. There is no gap being observed in between the bonded ball and NiPd bond pad even though there is a large probe mark. NiPd pad does not form intermetallic after aging test when compare to Al bond pad sample which has Cu-Al intermetallic being found after aging for long hours. NiPd bond pad provides excellent shielding for Cu/low-k structure where there is no deformation of the bond pad after bonded with palladium-coated copper core wire.