Hydrofluoric acid Free Etching and Nickel Displacement on Aluminum Electrodes for Substitution of Zincate Process

An electroless Ni-P plating on aluminum pad has been mainly used. A zincate method has also mainly used to initiate electroless Ni-P plating. However, the zincate solution is strong alkaline solution. Therefore, other milder solution is requested. Our previous papers have described possibility of direct electroless Ni-P plating without using the zincate method. Moreover, a fluoric acid(HF)is also commonly used to remove the passive films. The fluoric acid is not appropriate from the viewpoint of environmental concern. This paper also reported new etching solution without using fluoric acid. This etching solution was performed by two steps, first investigation was used pure Al and second investigation using actual wafer after conventional semiconductor process. In this investigation, 4 inorganic acids, HCl, H2SO4, HNO3 and H3PO4, and their mixing solutions were investigated. Only by using the mixture of HNO3 and H3PO4 solution, enough adhesion strength between Al and electroless deposited Ni-P film was obtained. The aluminum surface before and etching were observed by scanning electron microscopy(SEM), atomic force microscopy(AFM), and X-ray photoelectron spectroscopy(XPS). The aluminum surface that etched with mixture of HNO3 and H3PO4 solution showed as same as the surface etched by HF. Accordingly, the effect of mixture etching solution was evaluated to Al pad of actual wafer. To obtain a good adhesion, removing fluorine is necessary and this mixing solution shows high ability of removing fluorine from the Al substrate. Therefore, it is concluded that etching by mixture of HNO3 and H3PO4 solution and following the electroless Ni-P plating is environmental friendly method for obtains a good adhesion