Anisotropic Chemical Etching of Copper Foil: I . Electrochemical Studies in Acidic 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, 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, , , and , and that the dominant cuprous species was . Based on rotating disk measurements it was concluded that anisotropic chemical etching was related to the presence of a sparingly soluble surface film of . 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.