Physically Robust Interconnect Design in CUP Bond Pads

Economic challenges for small size IC design and manufacturing require: reduced die size without adding layers or other costs, bonding with Cu wire instead of Au wire, and maintained or improved reliability. Die size shrinking involves extensive use of circuit under pad (CUP) or bond over active circuitry (BOAC) on IC’s having at least two levels of metal. CUP however, introduces more potential failure modes just as increased probe touchdowns at wafer probe and Cu wire bonding are harsher to the pad and underlying structures. Dielectric cracks under the pad may be more hazardous with circuitry present in the pad sub-layers. In short, bond pad cracks must be prevented while still lowering product cost by routing interconnect circuitry in all pad sub-layer metallization layers. Recommendations for improvement in both CUP and Cu wire bonding in published literature are inadequate for many products. This paper summarizes extensive experimental work and demonstrates practical solutions for improving product yield and reliability issues of bond pads consisting of two or more layers of Al interconnect metallization in SiO2 dielectric. Qualitative CUP layout guidelines are developed for successful crack prevention, using the layout of the CUP circuitry itself to strengthen the pad. Al deformation and SiO2 bending and cracking that would ordinarily be caused by harsh wafer probe or harsh wire bond are prevented, facilitating Cu wire bonding on CUP designs in current and future products. (This work presented previously [1]).