Mitigation of Cu Nodule Formation in High Open Area Products for Electroplated Cu RDL Applications

In the customer's 200 mm RDL applications, Cu nodules were observed by 2D wafer inspection after photoresist (PR) strip and led to an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$&gt;80 \%$</tex> yield loss in wafer. Our investigation revealed that the affected device has a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{1. 5 X}$</tex> larger open area (OA) and higher current setting than the generic devices. A large OA implies that a sufficient supply of Cu ions must be provided within a unit time to facilitate the reduction reaction. However, we found that one cell's plating gap (PG), the distance between wafer and anode, was wider than another and without the compression made by a smaller plating gap, the flow within the gap was decreased. This reduction in ion exchange rate leads to the partial reduction of hydrogen ions into hydrogen gas, thereby affecting the electroplating process. In this study, by adjusting the plating cup' concentricity and conducting various splits with different ASD and PG values, it was confirmed that Cu nodule formation is more likely to happen under conditions of high OA/current output with insufficient mass transfer.