Electron microscopy characterization of a molybdenum diffusion barrier in metallizations for chip carriers

Mo layers have been studied as potential diffusion barriers for Au–Sn solder bonds in micro/optoelectronic device packaging. Solder was electroplated as alternating AuSn and Au5Sn multi-layers on wafers covered with Ti as an adhesion layer, followed by Mo as the diffusion barrier and Au as a capping layer. Samples were annealed at 340–420 °C for as long as 20 min. Scanning and transmission electron microscopy (SEM and TEM) were utilized to characterize interfacial reactions. Mo was found to be metallurgically stable, relative to the Au–Sn solder and the other metallization components, at temperatures up to at least 420 °C. However, the effectiveness of Mo as a barrier can be compromised by two factors. One of these is related to surface roughness associated with AlN or Al2O3 carriers. Non-uniform metallization coverage can lead to breaks in the Mo barrier, resulting in contact between the carrier and molten solder during bonding applications. In addition, thermal stresses generated during heating and cooling can lead to cracking and spalling of the Mo and adhesion layers, exposing the carrier material to molten solder. Pre-annealing can help to relieve the thermal stresses and prevent spalling.