Effect of Tafel slope on the difference deposition rate at 2CD (circle diameter) wafer

Single circle diameter bump wafer is generally used. Recently, 2 circle diameter (2CD) wafer is on the rise. 2CD wafer is a kind of wafer which has two different size vias. In the conventional 2CD wafer, solder layer such as SnAg alloy is electroplated on Cu bump. However, it has become difficult to adjust the bump height. Because the size of the via has become smaller and the density has increased. These changes result in the difference in the thickness on the small and large diameters. Thus, SnAg alloy is plated again on the small vias to adjust the bump height in the reflow process. Considering these circumstances, it is desired that the bump thickness of small vias is thicker than large vias. In order to breakthrough this problem, we made height difference by increasing Cu thickness of small vias only with one Cu electroplating step.