Investigation of mass transfer inside micro structures and its effect on replication accuracy in precision micro electroforming

Abstract

Micro electroforming is a promising electrodeposition-based precision replication technique for the fabrication of microstructured moulds. In this process, a poor mass transfer inside the micro structure with a high aspect ratio significantly affects the replication accuracy of the mould. In this paper, a novel star pattern with a continuously changing line width in the range of 20–320 μm, corresponding to a variable aspect ratio of 0.16–2.5, was firstly proposed for the assessment of mass transfer capability and microstructural replication accuracy. Based on the designed patterns, the effects of different flow fields (cathode rotating/jetting agitation) on the ion concentration distribution and thickness of the diffusion layer were investigated theoretically. Our simulation indicated that nickel ion transportation was determined by convection and diffusion, depending on the width and aspect ratio of the micro structure. When the aspect ratio was higher than 1, the diffusion of nickel ions dominated the mass transfer. When a hybrid agitation combining cathode rotating and jetting flow was applied, the mass transfer of nickel ions inside a high-aspect-ratio micro structure achieved a 50% decrease in the thickness of the diffusion layer compared with individual rotating or jetting agitation. This will significantly affect the replication accuracy. Star-pattern micro electroforming experiments with the hybrid agitation were conducted to validate the effect of mass transfer on the pattern replication accuracy. The results indicated that the maximum replication relative error of the height and aspect ratio was ~16% and ~10%, respectively, with a designed high aspect ratio of 2.5 at a feature width of 20 μm. Both experiments and simulations consistently indicated that the thickness of the diffusion layer determined the replication accuracy of high-aspect-ratio features, in which the limiting current density was constrained by the thickness of the diffusion layer because of the ion transportation efficiency. Hybrid agitation can effectively reduce the thickness of the diffusion layer inside high-aspect-ratio feature, thus increasing the limiting current density. This can effectively increase the replication accuracy of high-aspect-ratio micro structures using micro electroforming.