Localized and efficient machining of germanium based on the auto-coupling between picosecond laser irradiation and electrochemical dissolution: Mechanism validation and surface characterization

Abstract

The auto-coupling effect between picosecond (ps) laser irradiation and electrochemical dissolution is proposed to achieve localized and efficient dimpling of germanium (Ge), in which a highly localized electrical conduction channel is established via laser irradiation. Theoretical clarification and experimental validation are carried out to verify the proposed mechanism, and the machined surface is detailed characterized. It is found that the current increases rapidly to 3– 5 times higher level up to 225 mA once the defocused laser irradiates on top Ge surface, and a dimple with smooth surface and steep sidewall can be obtained on wafer bottom in neutral NaNO3 electrolyte. Meanwhile, the dimple centre precisely corresponds to the laser irradiating location, and micro oxide particles of about 1 μm in diameter have been observed attaching on the newly formed dimple surface. The influences of laser power, applied voltage and electrolyte jet velocity on the dimple shape and surface morphology are experimentally explored and amply discussed. The dimple entrance diameter can be controlled to around 1 mm with a flat copper cathode, while the depth can be easily up to about 480 μm within 200 s. Finally, the atomic removal mechanism of the localized and enhanced electrochemical dissolution is analysed.