Hydantoin derivatives were investigated as the complexing agents for gold plating. 1-Methylhydantoin (MH), 5,5-dimethylhydantoin (DMH) and 1,5,5-trimethylhydantoin (TMH) were tested. HAuCl4 was used as the gold source and phosphate salt was contained as the pH buffer. Thallium ion was added to refine the deposited grains. These complexing agents shifted the deposition potential to a less noble side, which suggested that these agents had a function of stabilizing the gold complex. The process with DMH and TMH deposits a uniform and dense film. The deposition efficiency of the bath containing DMH and TMH was approximately 40.8 mg/A·min, which is 100% of the current efficiency for the trivalent state of gold, while that for the bath with MH was even larger. From the result of measuring the limiting current using a rotating disc electrode, the number of electrons transferred for the plating in the MH bath was conspicuously smaller than that of the DMH bath. Therefore, it was suggested that a monovalent gold complex was formed in the MH bath.
