Fabrication and characteristics of high capacitance Al thin films capacitor using a polymer inhibitor bath in electroless plating process

An aluminum (Al) thin film capacitor was fabricated for a high capacitance capacitor using electrochemical etching, barrier-type anodizing, and electroless Ni-P plating. In this study, we focused on the bottom-up filling of Ni-P electrodes on Al₂O₃/Al with etched tunnels. The Al tunnel pits were irregularly distributed on the Al foil, diameters were in the range of about 0.5∼1 μm, the depth of the tunnel pits was approximately 35∼40 μm, and the complex structure was made full filled hard metal. To control the plating rate, the experiment was performed by adding polyethyleneimine (PEI, C₂H₅N), a high molecular substance. PEI forms a cross-link at the etching tunnel inlet, playing the role of delaying the inlet plating. When the PEI solution bath was used after activation, the Ni-P layer was deposited selectively on the bottoms of the tunnels. The characteristics were analyzed by adding the PEI addition quantity rate of 100∼600 mg/L into the DI water. The capacitance of the Ni-P/Al₂O₃ (650∼700 nm)/Al film was measured at 1 kHz using an impedance/gain phase analyzer. For the plane film without etch tunnels the capacitance was 12.5 nF/cm² and for the etch film with Ni-P bottom-up filling the capacitance was 92 nF/cm². These results illustrate a remarkable maximization of capacitance for thin film metal capacitors.