William H. Lytle

Application of a CFD tool in designing a fountain plating cell for uniform bump plating of semiconductor wafers

Fountain plating is a widely used method for electroplating bumps for flip chip and tape automated bonding (TAB) applications. Ideally, the plating process should produce metal bumps of uniform height (both within the bump and across the wafer) and flat-end surface in order to make good bonding to a device package. However, varying degrees of deposit uniformity have always been an issue in electroplating due to the current density distribution on the wafer, the electric field between the anode plate and the wafer, and the plating solution flow motion. The problem of nonuniform bump plating is especially serious in indium. Plated indium bumps tend to be much thicker on the sides corresponding to the direction of flow, creating nonuniform geometry and possible electric shorts. An improvement on fountain style plating cells has been developed for better control of deposit uniformity during bump plating operations. By applying a computational fluid dynamics (CFD) tool to analyze the flow motion inside the fountain plating cell, a favorable plating solution flow path, is created so that uniform flow will reach the wafer surface. Experiments are performed to verify the CFD model. Indium bumps are plated to a 10-cm diameter wafer in the fountain cell, Photographic results illustrate that when the anode plate is too close to the solution inlet plane, over-plating occurs, resulting in tear drop or comet shaped bumps. However, with a proper distance between the anode and the inlet plane, uniform bumps are created.