The Role of Mass Transport on Anisotropic Electrochemical Pattern Etching

Undercutting during high-rate electrochemical dissolution of partially masked copper surfaces in the presence of a flowing 1M H₂SO₄solution was studied. The electrode geometry consisted of a rectangular cavity with inert sidewalls and an active base, and with aspect ratios (width/depth) between 0.75 and 10. It was found experimentally that undercutting could be suppressed by the combined effect of surface film precipitation and by proper exploitation of convective flow eddies in corners. Under such conditions, the surface film formed preferentially in the corner regions and served therefore to stifle dissolution rates in the local area where undercutting occurs. For the conditions under study, a finite element method was used to calculate hydrodynamic patterns and concentration fields from which both local and overall convective mass transfer coefficients were calculated. The predicted overall mass transfer coefficient was found to agree within 6% of experimental measurements made by the electrochemical limiting current method. General conditions for achieving anisotropic pattern generation were identified.