Mechanical behavior of BEOL structures containing lowK dielectrics during bonding process

New challenges present themselves for the back end of line of IC processing. Copper lines replace aluminum lines to take advantage of the higher electrical conductance of copper. At the same time, lowK polymers are introduced in the structure in order to reduce the electrical losses. On the other hand, these lowK materials are mechanically weak. From the packaging side, copper wire bonding replaces the up to now commonly used gold wire bonding. Since copper has a higher yield stress than gold, higher forces are required to form a satisfying wire bond. Therefore higher forces act on a mechanical weak structure. In this study, the modeling of the bond deformation and the modeling of the stresses in the bond pad structure are decoupled. In a first phase, the bond deformation is modeled. The pressure in the contact zone with the bond pad is extracted. Due to a high hydrostatic component in the stresses, this pressure can be higher than the yield stress of the bond. The highest pressure is found at the beginning of the bond deformation, where the contact area is still small. Normalized pressure curves are set up where the yield stress of the bond is considered as a parameter. In a second phase, two bond pad structures are compared: copper pad direct on oxide and copper pad on a blanket lowK dielectric layer. The lowK layer provides a deficient mechanical support, resulting in high stresses in the copper pad. A nickel capping, used to prevent the copper pad from oxidizing, redistributes the pressure evenly over the copper pad. When no capping is used, a strong local deformation is found at the edge of the bond.