The influence of pattern size on the profile and microstructure of electroplated copper pad, redistribution layer and via for advanced packaging

As the packaging technology is developing towards miniaturization, the size reduction of the interconnection structure might affect their properties and reliability of the packaging. Therefore, the influence of pattern size on the profile and microstructure of electroplated copper was explored in this study. Different sizes of pads (40~105μm in diameter), redistributed layers (RDL, 8~20μm in width) and via (outer diameter in the range of 40~105μm, inner diameter in the range of 20~85μm) were electroplated by a commercial electrolyte. Firstly, the pattern size had a important effect on the flatness. As analyzed by confocal laser scanning microscope (CLSM). The flatness of pad was 24 % ~ 34.5 %, and showed a decreasing tendency with the increase of diameter. The flatness of RDL was 16.5 % ~ 34 %, and gradually deteriorated with the line width increases. The via cannot be completely filled when the outer diameter is greater than 60μm. And the flatness of via firstly decreased (from convex to flat), then increased (from flat to concave). Secondly, the microstructure of electroplated copper with different patterns was characterized by focused ion beam (FIB), which showed a little difference when the pattern size changed. Finally, thermal shock reliability test was also conducted on the samples to evaluate their thermal mismatch situation. It seemed that small pads can induce larger thermal mismatch under thermal shock test.