Anisotropic Chemical Etching of Copper Foil I. Electrochemical Studies in Acidic CuCl₂ Solutions

Etching of thick copper foil masked except for a single rectangular opening having an aspect ratio (width:depth) of 5:1 or 1:1 was investigated in the presence of controlled laminar flow across the width of the pattern opening. In a solution of 3.5M CuCl₂+ 0.5M HC1 + 0.5M KC1, copper was found to form a surface film spontaneously at the “corrosion potential,” to dissolve at a rate that depended upon convective mass-transfer conditions, and also to exhibit anisotropic wet chemical etching. Based on thermodynamic calculations, it was estimated that the dominant cupric species were Cu⁺², CuCl⁺, CuCl₂, and CuCl₃^-, and that the dominant cuprous species was Cucl₃^-2. Based on rotating disk measurements it was concluded that anisotropic chemical etching was related to the presence of a sparingly soluble surface film of CuCl. Under operating conditions for which anisotropic etching was observed, current transients were found to exhibit a characteristic “overshoot” response that may represent a useful diagnostic signal for process control.