Implementation of gold deposition by pulse currents for optoelectronic devices

Abstract

Optoelectronic devices, which operate at high frequencies, require gold deposits which are highly pure, dense, conductive and smooth. Often devices as thick as 15 μm are needed, which is much thicker than those used in electronic packaging. Cyanide electrolytes cannot be used for gold deposition because positive photo resists have been found to be incompatible with cyanide. In this study is recorded the methodology adopted by the authors to electrodeposit high conductivity gold for optoelectronic devices using a sulphite electrolyte. The aim was to use laboratory scale apparatus to determine if dense, pure and highly conductive gold with low roughness could be deposited. Laboratory scale experiments were used to determine the range of electrochemical process parameters which are useful in practice. These parameters were used to plate up gold devices in a pilot scale operation. A rotating cylinder Hull cell was used in the laboratory and a rectangular wafer plating cell was used in industry. Mass transfer and kinetic conditions were matched by maintaining similar diffusion layers and cell potentials in the two different systems. It was found that dense, uniform and fine grained deposits were obtained by direct currents up to 3·5 mA cm−2. At higher currents, larger grains were formed, leading to higher roughness. Pulse plating experiments showed fine grained deposits at 5·5 mA cm−2. This was attributed to the renucleation of gold deposits during pulse current deposition.