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.

These Indium Bonding Bumps were treated by two different versions of the two-step plasma process. The pockmarks on the left bump were caused by using greater-than-optimum plasma-generating power in the second step of the process. The right bump was processed at optimum power.
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.

In the first step of the plasma process, a device on which indium bonding bumps has been formed is exposed for a suitable amount of time (typically, 20 minutes) to a plasma generated in a gaseous mixture of 1/3 argon + 1/3 methane + 1/3 hydrogen. During this step, the oxygen in the indium oxide is removed through incorporation into CO and CO2 gas molecules, while the indium in the indium oxide is removed through incorporation into In(CH3), which is volatile. Following this step, a carbonaceous surface film is also formed on device surfaces that are not covered by indium.

A second step for removing the carbonaceous film is as follows. The device is exposed to a plasma generated in a gaseous mixture comprising 72 percent of argon and 28 percent of hydrogen. This step greatly reduces the carbon content without exerting any significant adverse effect on the indium. The power used to generate the plasma in this step must be chosen carefully: the power should be high enough to ensure effective removal of the carbonaceous film, but not so high as to melt or otherwise damage the indium bumps (see figure).

This work was done by Harold F. Greer, Richard P. Vasquez, Todd J. Jones, Michael E. Hoenk, Matthew R. Dickie, and Shouleh Nikzad of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management


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Refer to NPO-45911, volume and number of this NASA Tech Briefs issue, and the page number.

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This article first appeared in the June, 2009 issue of NASA Tech Briefs Magazine.

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