ANISOTROPIC CHEMICAL ETCHING OF COPPER FOIL .1. 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 HCI + 0.5M KCl, 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.