Backside ultraviolet illumination enhanced metal-assisted chemical etching for high-aspect-ratio silicon microstructures

High-aspect-ratio silicon microstructures have been widely used in micro-electro-mechanical systems, microelectronics cooling and electronic packaging. Recently, metal-assisted chemical etching (MACE) was proved to be an effective alternative to achieve various microstructures. In traditional MACE, holes are generated by the metal catalyst induced reduction of the etchant. However, when the aspect ratio of the fabricated microstructure increases, the mobility of the reactants and the resultant are correspondingly impeded, resulting in insufficient holes, thus, the etching rate decreases significantly. In this study, back-side ultraviolet (UV) illumination in MACE was proposed to provide additional holes. A novel experimental setup was designed and developed. Compared with the traditional MACE, the back-side UV illumination enhanced MACE can significantly enhance the etching rate and fabricate root-like Si microstructures. It is revealed that the holes which are generated by the back-side UV illuminating area can diffuse through bulk Si, accumulate in the etchant side because of the trapping of the pore tip, and participate in the etching reaction. By tuning the applied UV power, root-like microstructures with its width varying from 4.9 to 63.8 μm and depth varying from 13.7 to 47.4 μm were achieved. This study can provide useful insights for developing novel Si processing techniques and fabricating various microstructures.