Cu bonding to Cu low K wafers: a systematic study of the mechanical bonding process

Wire bonding is a widely used interconnection technique. Traditional processes use Au bond wire and Al metal layers. Nowadays there is a trend to go over to Cu bond wire to benefit from its better electrical performance characteristics. However during the bonding process, high pressure is put on the bond pad structure. These bond pad structures can contain low K dielectric materials, which have a low dielectric constant, but also a very low stiffness (standard range varies from 1-10 GPa, depending on the porosity in the dielectric). In this study, we investigate the deformations and stresses in wire bond and bond pad structure during the bonding process. The yield stress of the bond determines the pressure on the bond pad structure. In a first part, a comparison between simulations and experiments gives estimates for the yield stress of Cu bond wire and Au bond wire. A second part of the study investigates the stresses and deformations in the bond pad structure. Several parameters such as thickness of the low K dielectric layer and capping are investigated. Mainly a thicker and stiffer capping is showed to lower local stresses under the edge of the bond.