Physical and electrical characterization of thin nickel films obtained from electroless plating onto aluminum

In this work, nickel was deposited onto smooth aluminum surfaces using the electroless plating technique, which is selective, does not consume surface and does not need an external current source. Before electroless nickel deposition, the aluminum surface was previously covered using a nickel displacement solution. Physical and electrical characterization of the Ni/Al structure was performed with the aid of Rutherford Backscattering Spectrometry (RBS), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and C-V measurements. For the C-V measurements, MOS capacitors were fabricated using basic microelectronics steps. During the displacement reaction in solution 0.1 M NiCl2/0.1 M HF/0.108 M NH4F, aluminum was partially consumed and the surface RMS microroughness gradually increased from 16.8 to 129.8 nm after immersion from 0 to 45 S, respectively. The surface RMS microroughness slightly increased to 133.1 run after electroless plating for 180 s in solution 0.063 M NiCl2/0.09 M Na2H2PO2/0.2 M (NH4)HC6H5O7/0.1 M NH4OH at 85 degreesC. As a result, a final nickel thickness of 307 nm and small grains with diameter of approximately 0.5 mum were obtained. In conclusion, electroless plating onto aluminum with a good control can be used as an alternative Ni bumping base for Al bondpads without substantial lateral corrosion. Also, the electrical characteristics of the Ni/Al/SiO2/Si structure did not substantially deteriorate (especially the flat band voltage shift was lower than 0.15 V).