Influences on the Anisotropy in Through-mask Electrochemical Micromachining Processes

This contribution studies through-mask electrochemical micromachining (TMEMM) of copper, namely the interplay between current density, electrolyte flow, mask opening width, etch depth, the presence of a wetting agent, and the resulting anisotropy in sodium nitrate electrolyte. Factors strengthening the anisotropy are: High electrolyte flow rate, low overall etch depth and large feature size (150 µm > 100 µm > 50 µm), absence of wetting agent, and moderate current densities (between 20 and 40 A cm-²). A model aiming to explain all the observed interdependencies is elaborated, based on the presence of a supersaturated surface film and a local change in dissolution mechanism in dependence on local current distribution and surface film thickness. The thickness of the surface film is greatly influenced by the flow velocity, which in turn depends on the geometry of the cavity and, therefore, opening width and etch depth. In the presence of a wetting agent the differences in surface film thickness are leveled out, which results in less anisotropic etching.