Impedance Studies of Copper in Alkaline Solutions, Especially During Electroless Plating

Abstract

AbstractAn impedance study of the electrochemical double layer of copper in an electroless copper plating bath was performed. Of the bath components (formaldehyde, ethylenediamine‐tetraacetic acid (EDTA), CuEDTA2−, NaOH, Na2SO4, and NaHCOO) only the influence of hydroxide on the double layer is visible from capacitance data. The estimated surface coverage of hydroxide at the mixed potential of the electroless plating process is about 10%. On copper single crystal surfaces the OH−‐adsorption shows specific characteristics. Due to a surface reconstruction these characteristics, however, disappear with time after the immersion of the electrode. During the deposition process high electrode capacitances are measured (200–500 μF/cm2). They are mainly caused by a surface roughness on the microscopic scale. When the deposition process is terminated, the surface relaxes within about 10 minutes. The addition of cyanide or Gafac (a mixture of organic esters of phosphoric acid) to the electroless plating bath markedly influences the capacitance data. However, always a correlation between the deposition rate and the electrode capacitance is observed. If, on the other hand, at potentiostatic cupric ion reduction formaldehyde in the plating bath is substituted by ethylenediamine, which also catalyzes the CuEDTA2−‐reduction, the capacitance values decrease with increasing deposition current. Presumably, in the first case the nucleation rate dominates the deposit growth, while in the second case the opposite is true.