We investigated effect of Ar/H2 plasma cleaning on electroless Ni under-bump metallurgy (UBM) layer formation on small-area Al pads. When 5 × 5 μm2 pads are cleaned with the plasma, electroless Ni grows successfully on them, whereas no growth occurs on pads cleaned with conventional wet chemicals. In the Ni-UBM layer formation process, Zn is deposited by displacement plating on the Al pads before the electroless Ni plating. Microscopic observations, however, reveal that Zn is not successfully deposited on the wet-cleaned Al-pad surface. X-ray chemical analysis indicates that the plasma cleaning effectively removes C contaminations even for small pad sizes.
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