KINETICS OF THE DISSOLUTION OF COPPER IN IRON(III) CHLORIDE SOLUTIONS

Wet etching is commonly used to produce patterned copper films for printed circuit boards and semiconductor devices. The investigation of copper etching by FeCl3-HCl solutions in a batch stirred-tank reactor was undertaken to determine the intrinsic reaction kinetics. The variation of the etch rate with FeCl3 concentration was linear at low concentrations and gave first-order rate constants of 0.2039, 0.3083, and 0.3362 mg Cu.kg H2O/cm2.s.mol FeCl3 at 30, 40, and 50-degrees-C, respectively. At approximately 2 mol/kg the etch rate reached a maximum. Etch rates obtained with 0.85 and 2 mol/L HCl over the range of FeCl3 concentration were similar. The etch rate was substantially reduced with the use of Fe(NO3)3-HNO3 solutions and increased uniformly as the chloride content of the solution was increased. Concentrations of the chloro complexes of the iron(III) ion were computed and showed that FeCl2+ and FeCl3(0) are the principal species present in the etchants studied. The etch rate was found to vary linearly with FeCl2+ concentration at low (<1 mol/kg) concentrations. The maximum in the etch rate is attributed to the inhibition by FeCl3(0), the concentration of which becomes significant in etchants of greater than 2 mol/kg FeCl3.