Study on controllable thickness and flatness of wafer-scale nickel shim in precision electroforming process: simulation and validation

A new approach to precision electroforming of a wafer-scale nickel shim with rotating cathode using an auxiliary cathode mask is developed to improve thickness uniformity and flatness. The effects of critical process parameters, including cathode rotating speed, cathode mask size, and current density, on the thickness uniformity and flatness of electroformed nickel shim are systematically studied based on experiments and numerical simulations. The results show that the thickness uniformity of deposits is highly dependent on the current density distribution, where a cathode mask can effectively tune electrical field lines and induce a more uniform current density distribution. The simulations and experimental results consistently agree that a minimum thickness nonuniformity of 8% and below 1% on the wafer with a diameter of 80 mm and 40 mm, respectively, can be achieved using a mask with a 70 mm opening size. However, for flatness, cathode rotating speed influences the surface warpage due to the intrinsic stress, which results from the electrocrystallization process and uneven thickness caused by the edge effect. It is also found that the gradient current density can significantly reduce the intrinsic stress with better flatness. The best flatness is controlled below 47 µm and 32 µm on the wafer with diameters of 80 mm and 40 mm, respectively, under the synergistic effect of optimal cathode rotating speed (30 rpm) and gradient current density.