Effect of Ion Mass Transfer and Electric Field Distribution on the Formation of Void Defect in Electroformed Nickel Microcolumns

Abstract

The void defect inside the electroformed nickel microcolumns is a common problem affecting the performance of the microsystem. In this study, a simulation model for the dynamic deposition of microcolumns is set up to clarify the changing characteristics of ion mass transfer and electric field distribution inside the microcavities under the electroforming process, with the aim of revealing the formation mechanism of the void effect. The simulation results show that the dynamic aspect ratio (DAR) of the microcavity increases from the initial 2 : 1 to infinity during the electroforming process. As a result, the ion mass transfer within the microcavity decreases, resulting in a gradual deterioration in the uniformity of the nickel ion concentration distribution along the microcavity sidewall. The increased difference in the electrolyte potential along the microcavity sidewall leads to a reduction in the uniformity of the overpotential distribution. These two factors lead to a faster deposition rate at the open side of the microcavities, and consequently to the formation of a void defect inside the microcolumn. Compared to the overpotential distribution, the nickel ion concentration distribution plays a dominant role in the size of the void defect. Aiming to eliminate the void defect, several approaches have been carried out to improve the uniformity of the nickel ion concentration distribution inside the microcavity. The experimental results show that compared to a high current density (1 A/dm 2 ), the low current density (0.25 A/dm 2 ) helps to reduce the ion consumption rate, thus contributing to a uniform distribution of nickel ion concentration, as well as a reduced void defect size. The pulse reverse current proves effective in eliminating the effects of the nonuniform distribution of the ion concentration and the overpotential on the microcavity deposition and realized the defect-free electroforming of the microcolumn with a width of 30 µm and a height of 60 µm.