Kinetics of the dissolution of copper in iron(III) chloride solutions

Abstract

Wet etching is commonly used to produce patterned copper films for printed circuit boards and semiconductor devices. The investigation of copper etching by FeCl 3 -HCl solutions in a batch stirred-tank reactor was undertaken to determine the intrinsic reaction kinetics. The variation of the etch rate with FeCl 3 concentration was linear at low concentrations and gave first-order rate constants of 0.2039, 0.3083, and 0.3362 mg Cu-kg H 2 O/cm 2 s.mol FeCl 3 at 30, 40, and 50 o C, respectively. At approximately 2 mol/kg the etch rate reached a maximum. Etch rates obtained with 0.85 and 2 mol/L HCI over the range of FeCl 3 concentration were similar. The etch rate was substantially reduced with the use of Fe(NO 3 ) 3 -HNO 3 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 FeCl 2 + and FeCl 3 0 are the principal species present in the etchants studied. The etch rate was found to vary linearly with FeCl 2 + concentration at low (<1 mol/kg) concentrations. The maximum in the etch rate is attributed to the inhibition by FeCl 3 0 , the concentration of which becomes significant in etchants of greater than 2 mol/kg FeCl 3