TSV residual Cu step height analysis by white light interferometry for 3D integration

Cu pumping, or the extrusion of Cu out of a TSV after being subjected to high temperature conditions, is one of the highest risk failure modes to be overcome in the development of TSV-middle integration for 3D packaging technologies. Typical Cu pumping analyses focus on a low number of data points through brute force measurement using cross sectional analysis. The low number of data points gathered for each condition does not provide results with a high statistical confidence level. In addition, it is most likely that the cross section does not provide measurement along the plane that contains the highest amount of Cu pumping, resulting in inaccurate maximum pumping height measurements. In this study, a Cu pumping measurement technique was developed using a production capable scanning white light interferometry (SWLI) system, which enables the collection of hundreds or thousands of individual TSV Cu pumping data points. This enables an accurate statistical comparison between Cu pumping mitigation schemes. Using this technique, multiple TSV plating process and thermal annealing conditions were compared by varying temperature and time to determine the conditions that resulted in the lowest amount of Cu pumping. In addition, an alternate integration scheme was investigated that includes multiple anneal and CMP planarization steps. The experimental results demonstrate that Cu pumping can be kept to a manageable level for high reliability performance.