Microfabrication of semiconductors is crucial because semiconductor materials are extensively employed in solar cells, microelectronic machinery, optical components, etc. Due to the characteristics of semiconductor materials, including a fast increase in electrical conductivity with temperature, and high hardness and brittleness, traditional mechanical processing has been unable to meet the demands of microfabrication. Simple electrochemical processing has low processing efficiency, severe stray corrosion, environmental pollution from electrolyte solutions, and other challenges. Because the laser has the benefits of high precision and strong domain fixation to produce thermal impact on materials and the integration of electrochemical processing has the benefit to eliminate microcracks and heat-affected zones, the laser and electrochemical machining can be combined to produce good surface processing quality.Thus, the single-crystal germanium is employed as the experimental material, and a neutral and non-polluting NaNO3 electrolyte is employed to perform backward laser-controlled electrolytic processing to confirm the processing mechanism’s feasibility, and on this basis, a auto-coupled laser-electrochemical co-machining approach is employed to perform experimental research on micro grooving of single-crystal germanium materials.
In this research, an experimental investigation of single-crystal germanium through auto-coupled laser-electrochemical co-machining is performed. First, a scanning electron microscope is employed to observe the processing morphological characteristic, an energy spectrometer is employed to detect the elements and their occupancy, a confocal microscope is employed to obtain 3D morphology, and a Raman spectrometer is employed to examine the residue composition on the dimple and record the current changes during processing. To examine the dimple’s depth, entrance diameter, removal volume, and sidewall taper produced by processing in terms of both heat and mass transfer. Based on this, the experimental research of microgrooves is conducted using laser-electrochemical co-machining to investigate the morphological characteristics of microgrooves under the combined laser and laser-electrochemical processing and to examine the trends in microgroove width and depth under the auto-coupled hybrid laser electrochemical machining.
Auto-coupled laser-electrochemical machining is employed to achieve non-ablation on the upper surface and electrolytic micro-dimples on the lower surface to confirm the processing mechanism. The benefit of this approach is that the irradiation position of the incident laser corresponds to the electrolytic dimple’s position, and no special cathode design is needed for the automatic coupling process. The obtained electrolytic dimples are free of microcracks and heat-affected zones, which are typical characteristics of electrolytic processing (Fig. 5). Isolated and non-dense oxide GeO2 attached to the machined surface can hinder the electrolytic processing and influence the surface quality of single-crystal germanium (Figs. 6 and 7). The current variation’s trend demonstrates that the large change in current between the laser beam’s withdrawal and the addition of the processing beam is attributed to the localized increase of the conductivity of single crystal germanium by laser irradiation. The processing results demonstrate that the maximum entrance diameter, depth, removal volume, and sidewall taper are obtained at the offset distance of 7-9 mm. This finding may be related to the impact of offset distance on both aspects of heat and mass transfer. For the small offset distance, the cooling impact is not conducive to material reduction, and is conducive to the discharge of processing products and suppression of concentration polarization.For the large offset distance, the cooling impact is weak, but the product discharge ability is also reduced, both of which contradict each other and jointly determine the processing process and findings (Fig. 8). Based on this, the characteristics of microgroove morphologies processed by various processing approaches are investigated, and it is concluded that auto-coupled laser-electrochemical co-processing can eliminate the defects including recast layer and scatters caused on the surface, and obtain better microgroove morphology (Figs. 11 and 12). Meanwhile, this AHLECM is employed to investigate the impacts of various processing voltages on the processing findings, and it is found that the microgroove’s width and depth gradually widen and deepen with the increase of used voltage (Table 1).
It is proposed that laser irradiation can cause a fixed-domain conductive channel within the material, therefore obtaining the processing of single-crystal germanium through auto-coupled laser-electrochemical co-machining. When a neutral sodium nitrate solution is employed as the electrolyte and a horizontal copper sheet is employed as the cathode, the laser irradiation on the upper surface can generate fixed-domain electrolysis on the backside of single-crystal germanium to produce micro-dimple. The offset distance’s effect on the dimple morphology is studied. The maximum processing current, dimple diameter and depth, sidewall taper, and removal volume all increase first and then decrease with the increase of offset distance. Their turning points occur at the offset distance of 7-9 mm, and the possible effects generated by various offset distances are examined in terms of heat and mass transfer. By linking laser etching and electrochemical dissolution at the same processing position, the auto-coupled laser-electrochemical co-processing can be achieved, and the preliminary experimental research of microgrooves is conducted. The microgrooves’ structural and morphological characteristics are examined, and the processing voltage’s influence rules on the microgroove dimensions are investigated. It is found that the microgroove’s depth and width increase as the processing voltage increases, which demonstrates that electrochemical dissolution is the crucial phenomenon in the process of auto-coupled laser-electrochemical co-machining.
