Fine pitch copper wire bonding on 45nm tech Cu/low-k chip with different bond pad metallurgy

Wire bonding technology has been widely used in the semiconductor industry for interconnection between device and substrate. Gold wire has been used in industry for many years; however with the increase in the price of gold in the past few years, copper wire has become an alternative. Copper wires have better electrical and thermal performance than gold wire. However due to coppers hardness, the bonding of copper wire to the soft Al bond pad becomes a challenge especially for devices with sensitive and fragile structures underneath the Al bond pad. An optimized Al remnant is required for achieving good reliability [1–2]. A robust bond pad structure with different bond pad material could reduce the impact to structures under the bond pad, minimize Al splash and remove the concern associated with Al remnant for reliability. In this study, copper wire bonding on bond pads with different hardness will be evaluated; they are Al pad and NiPd on top of Al pad. An ASM Eagle Xtreme bonder was used for the study. The forming gas (95%N
2
5%H
2
) flow rate of 0.4/0.8ℓ per minutes was controlled by a digital flow meter which provides a consistent supply of forming gas. Forming gas maintain an inert environment during the electrode flame-off for free air ball. Capillary with a hole size of 21.6µm, chamfer diameter of 28µm and face angle of 11° was designed for fine pitch bonding with matte surface to enhance the gripping of ball and wire during bonding. A 4N copper with 0.7mil in diameter was used to evaluate the bonding on a Cu/low-k chip with two different bond pad metals with thicknesses of 2.1µm for the Al bond pad and 1.4µm Al/2.5µmNi/0.3µmPd bond pad finish. The NiPd layer used in this study was actually two separate layers which consist of 2.5µm of Ni and 0.3µmPd. Optimized bonding parameters were used on different surfaces to compare the results of bond quality. Comparisons of the deformation of the Al layer and NiPd layer as a result of the bonding was studied. NiPd is harder and could act as a barrier to prevent the direct interaction of the copper wire on the Al bond pad and thus prevent the problems of Al splash especially in fine pitch wire bonding. In this study, target ball size of 34µm were bonded to 55µm bond pad pitch Cu/low-k chips. The effects of the NiPd layer on various parameters were compared with Al bond pad. Evaluated sample were subjected to thermal aging to further understand the effects of temperature on bond quality.