Study of ion transportation and electrodeposition under hybrid agitation for electroforming of variable aspect ratios micro structures

Abstract

Microstructural replication accuracy in precision micro electroforming process is related to ions transportation inside the micro structure and microstructural feature width and aspect ratio. In this study, a novel star pattern having a varying feature width of 20–320 μm with a corresponding aspect ratio of 5–0.3125 is proposed to characterize ion transportation and electrodeposition for electroforming of micro structures under a hybrid agitation of cathode rotation and jetting. Using designed patterns, simulations and electroforming experiments are conducted to systematically investigate the effect of flow fields on the diffusion layer thickness and current density on the microstructural filling precision. The results demonstrate that a hybrid flow combining cathode rotation and jetting can significantly enhance mass transfer with a ~33% reduction of diffusion layer thickness inside the micro structure with a high aspect ratio of 5, compared to individual rotating or jetting flow. The diffusion-based mass transfer is dominated inside the micro structure with an aspect ratio is greater than 1, where maximum relative replication error of depth is up to ~21.5% with an aspect ratio of 5. With the aspect ratio less than 1, the convection-dominated mass transfer can be significantly enhanced, where the minimum relative replication error of depth decreases to ~0.1%. The hybrid agitation is proved to be effective to supply ions for electrodeposition of micro structures. Additionally, the results also indicate that a low current density of 18 A/m2 should be used for defect-free electroforming of high-aspect-ratio micro structures based on hybrid agitation.