Gigahertz scanning acoustic microscopy analysis of voids in Cu-Sn micro-connects

Gigahertz scanning acoustic microscopy (GHz-SAM) is applied to the characterization of bulk voids in the Cu-Sn material system, often used in micro-connects. An increased demand for the development of miniaturized interconnect technologies, such as micro-connects, means that fast characterization methods are required for the assessment and detection of reliability impacting defects. This study attempts to formulate an analytical technique aimed at detecting micro-structural defects in Cu-Sn micro-connects, such as micro-bumps for 1st level interconnects and solid-liquid interdiffusion bonds for nano- and microelectromechanical systems. To study the potential of the analytical method, a specific electroplating chemistry was used that increases the probability of defect formation in the electroplated Cu film. The chemistry is known under certain electroplating overpotentials to promote hydrogen bubble induced voids within the Cu. The samples containing voids were inspected by GHz-SAM with a highly focused acoustic lens operating at 1.12 GHz. To validate the results, GHz-SAM micrographs were compared with focused ion beam prepared cross-sections of the selected samples. Advances in acoustic transducer technology operating in the GHz frequency band allow for micron sized defect examination of materials with enhanced lateral resolution and sub-surface sensitivity.