Nickel and Gold Coatings of Germanium and Kovar Substrates

The integration of germanium to other materials such as Kovar ® (registered trademark of Carpenter Technology Corporation), an Fe-Ni-Co alloy, by soldering requires a surface preparation due to poor wetting characteristics of germanium. Coating (metallization) by successive electroplating of 3 mm wide nickel and gold frames well adhered to the outer edge of a circular germanium window and kovar body was applied to attain successful joining of germanium to kovar. Surface cleaning of parts to be joined or plated plays an important role in the quality of the depositions. HF and HCl solutions were used for germanium surface cleaning and the results showed that HF solution was more effective than HCl in terms of removing oxide capability. The desired small grain sized solderable nickel layer was achieved by pulse plating from Watts solution. Likewise, non-porous very thin gold was deposited by pulse plating again and it prevented the diffusion of oxygen to nickel and diffusion of nickel to the free surface. Characterization of the coatings were performed in terms of microstructure, roughness and elemental analysis by scanning electron microscopy (SEM), white light interferometry and x-ray photoelectron spectroscopy (XPS), respectively.