The micro electroforming of precision mold inserts with high flatness via internal stress reduction and interfacial bonding enhancement

Abstract

The internal stress significantly affects the flatness of electrodeposits. Excessive internal stress even induces the delamination of electroformed components from the substrate, particularly on mirror-finished silicon wafers. In this study, precision nickel mold inserts with a high flatness were electroformed via internal stress reduction and interfacial bonding enhancement. Firstly, the temperature and pH of electrolytes were optimized to reduce internal stress. Dislocation density and hydrogen evolution were characterized to explain the influence of temperature and pH on the internal stress raised from nickel sulfamate-based electrolytes. In addition, various interlayers (Ni, Cr and Ti) with different thicknesses were provided to enhance the interfacial bonding strength between deposited nickel and silicon wafers. Compared to interlayer-free samples, a 20 nm thick titanium interlayer significantly increased the critical load from 5.7 N to 43.27 N. The presence of a titanium layer dramatically decreased the deformation of nickel deposits. To balance the internal stress and deposition rate, a segmented electroforming process consisting of initial low current deposition (1 A/dm²) and subsequent high current deposition (5 A/dm²) was provided. Combined with the titanium interlayer and optimized processing parameters, nickel mold inserts with a high flatness and precise micro features were successfully electroformed.

Keywords

precision electroforming / internal stress / interfacial bonding strength / flatness / mold inserts

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Central South University