Fabrication of Oxide-Free Dimple Structure on Germanium via Electrochemical Jet Machining Enhanced by Opposing Laser Irradiation

The electrochemical machining (ECM) of germanium (Ge) wafer can be significantly improved in terms of efficiency and localization by introducing laser irradiation. However, residual oxide particles usually exist when using neutral electrolyte. In this study, the NaNO₃ electrolyte jet is formed via a cathodic nozzle and impacts perpendicularly on the bottom of the Ge wafer, at which position the nanosecond (ns) laser irradiates from opposing side to locally and precisely enhance the electrical conductivity. In this way, the dimple structure without any detectable oxidation products can be efficiently fabricated. Characterization of surface morphology is then amply conducted. The radial lines consisting of regular micro-waving structure are usually noticeable, especially near the dimple edge, which may be attributed to the rapid and radial electrolyte flow along the concave sidewall. In contrast, irregular corrugated pattern can be observed near the dimple center, which may be caused by the electrolyte turbulence after impingement. In addition, parametric experimental study has been carried out, and the effects of the applied voltage, laser power and electrolyte concentration on the dimple profile and surface characteristics have been respectively detailed discussed. Finally, simulation study of the electrolyte flow field is conducted to reveal the distribution regularities of velocity and pressure, to deepen the understanding of formation mechanism of the complex surface morphology.