Plasma cleaning on bond pad surfaces for gold wire bonding

Abstract

Ar plasma was employed to remove/reduce the oxidation of two typical types of bond pad surfaces, namely aluminum (Al) surface and gold (Au)-coated palladium (Pd) surface, prior to Au wire-bonding process. Field emission scanning electron microscopy (FESEM) and Auger electron spectroscopy (AES) were employed to characterize the bond pad surfaces without and with the plasma cleaning. Processability and reliability results were compared, analyzed and discussed. Although the plasma cleaning did not make significant change for either the Al bond pad surface or the Au-coated Pd bond pad surface in terms of surface morphology, it was found that the impacts of the plasma cleaning on the two types of bond pad surfaces were quite different. On the one hand, for the Al bond pad surface, the Al oxide layer on the bond pad was reduced with the plasma cleaning. However, there was no further significant improvement observed in terms of either the ball shear modes or shear values after wire-bonding process and after stress. On the other hand, for the Au-coated Pd bond pad surface, poor bondability and serious non-stick-on-pad (NSOP) were observed in the initial stage without the plasma cleaning. Its bondability has been improved significantly after the plasma cleaning. Both the shear modes and shear values after the wire-bonding process showed significant improvements for the samples with the plasma cleaning. This is mainly because there was an interdiffusion between the Pd and Au layers; the Pd diffused from underneath to the top of bond pad surface and formed Pd oxide, which degraded the bondability of the bond pad. The plasma cleaning improved the top layer surface condition of the Au-coated Pd bond pad by removing/reducing the Pd oxide.