Shape Evolution of High Aspect Ratio Copper Bumps used for Wafer Level CSP

Abstract

Electrodeposited via-posts are important elements in wafer level CSP. Cross sections of these via-posts were observed for the via-posts with less than 100 μm cathode length. Heights of via-posts becomenon-uniform with the narrower via-post pitches. Via-posts become the higher for lower portion of electrode and lower for the upper portion of electrode. The uniformity increases for wider via-post pitches. The uniform height via-posts form via-post pitch of for longer than 700 μm at 100 μm cathode length. Uniform height forms for pitch of longer than 400 μm at 75 μm cathode length, longer than 250μm at 50 μm cathode length and longer than 100 μm at 25 μm cathode length. The solution becomes dilute because of consumption of copper ion at the cathode via. This dilute solution emerges from the via and becomes upper stream flow, which travels along the resist surface and flows into the vias atupper portion of the electrode. The upper stream flow mixes with bulk electrolyte along the resist surface and recovers its concentration. With longer resist surface distance, the flow mixes more with bulk electrolyte and increases recovery. This concentration recovery along the resist surface causes the height of via-posts as uniform.