I. Introduction
High-aspect-ratio silicon microstructures have been widely used in micro-electro-mechanical system[1], microelectronics cooling[2] and electronic packaging[3]. Recently, metal-assisted chemical etching (MACE) was proved to be an effective alternative to achieve microstructures with high aspect ratios, such as nanowires[4]-[8], deep trenches[9], deep vias[10], [11] and even 3D spiral structure[12]-[14]. In traditional MACE, holes, which are generated by the metal catalysts induced reduction of the etchant, are mainly accumulated underneath the metal catalysts, resulting in controllable directional etching. However, when the aspect ratio of the fabricated microstructure increases extremely, the etching rate decreased significantly, because the high-aspect-ratio microstructure impedes the mobility of the reactants and the resultant. As a result, insufficient holes are supplied, thus, the directional etching rate sharply decreases. In previous studies, researchers had attempted to fabricate microstructure by using front-side or back-side light illumination on Si[15]-[18], GaAs[19], GaN[20]-[22] and SiC[23], which results in randomly distributed porous microstructures.