APPLICATION OF A CFD TOOL IN DESIGNING A FOUNTAIN PLATING CELL FOR UNIFORM BUMP PLATING OF SEMICONDUCTOR WAFERS

This paper describes a unique and efficient method to design the fount ain plating cell in order to control uniform bump plating of semicondu ctor wafers. Fountain plating is a widely used method for electroplati ng bumps for flip chip and tape automated bonding (TAB) applications. Ideally, the plating process to form the metal bumps should produce me tal 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 packa ge. However, varying degrees of deposit uniformity have always been an issue in electroplating due to the current density distribution on th e wafer, the electric field between the anode plate and the wafer, and the plating solution pow motion. The problem of nonuniform bump plati ng is especially serious in indium. Plated indium bumps tend to be muc h thicker on the sides corresponding to the direction of flow, creatin g 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 compu tational fluid dynamics (CFD) tool to analyze the flow motion inside t he fountain plating cell, a favorable plating solution flow path is cr eated so that uniform flow will reach the wafer surface. Experiments a re performed to verify the CFD model. Indium bumps are plated to a 10- cm diameter wafer in the fountain cell. Photographic results illustrat e that when the anode plate is too close to the solution inlet plane, over-plating occurs, resulting in tear drop or comet shaped bumps. How ever, with a proper distance between the anode and the inlet plane, un iform bumps are created. Detailed descriptions of the CFD modeling and experiment are presented in the paper.