Micro-bump structure

A bump structure, applicable for package structures, comprising:

• at least a bonding pad, disposed on a semiconductor substrate, wherein the bonding pad is ring-shaped and has a central hollow;
• a protection layer, disposed on the bonding pad and on the semiconductor substrate and partially covering the bonding pad;
• a ring-shaped metal layer, disposed within an opening of the protection layer, wherein the ring-shaped metal layer at least covers sidewalls of the opening and a portion of the protection layer surrounding the opening, the ring-shaped metal layer has a hollow and a portion of the semiconductor substrate is exposed through the hollow of the ring-shaped metal layer and the central hollow of the bonding pad;
• and
• a centrally-concave metal structure, disposed on the ring-shaped metal layer, disposed in the hollow of the ring-shaped metal layer and the central hollow of the bonding pad and covering the exposed portion of the semiconductor substrate and partially covering the ring-shaped metal layer, wherein the metal structure comprises at least a seed layer disposed on the ring-shaped metal layer, a metal under-layer disposed on the seed layer and a solder material layer disposed on the metal under-layer, and the seed layer, the metal under-layer and the solder material layer are centrally concave.