Two-Step Plasma Process for Cleaning Indium Bonding Bumps

This process could increase yields in the manufacture of consumer electronic products.

A two-step plasma process has been developed as a means of removing surface oxide layers from indium bumps used in flip-chip hybridization (bump bonding) of integrated circuits. This process has considerable commercial potential in that flip-chip hybridization is used in the manufacture of cellular telephones and other compact, portable electronic products.

The need for this or another, similar cleaning process arises as follows: Indium bonding bumps tend to oxidize during exposure to air. As the duration of exposure and the level of oxidation increase, the electrical resistances of the bonds subsequently formed via the bumps also increase. In some cases, the resistances can become so large that the bump bonds may act as open circuits, preventing proper functioning of the bump-bonded devices.

There is a patented process for removal of surface indium oxide layers by etching with hydrochloric acid. Unfortunately, once the oxide is removed, the acid can continue to attack the indium, reducing the size of the bumps and even undercutting them. The acid can also attack metal layers on and under the bond pads, potentially creating open circuits and thus negating the benefit of removing the oxide. In contrast, the two-step plasma process makes it possible to remove surface indium oxide, without incurring the adverse effects of the acid etching process.