Femtosecond laser-induced periodic oxidation mask for inductively coupled plasma etching: Defect-free ultra-high aspect ratio grating structures with adjustable depth

Laser-induced periodic surface structures (LIPSSs) enable the flexible fabrication of subwavelength patterns on most solid surfaces. However, their applications are constrained by inadequate control over feature depth and the emergence of defect layers within the crystalline material. This paper proposes a hybrid method that combines LIPSSs with inductively coupled plasma (ICP) etching to fabricate defect-free, ultra-high aspect ratio grating structures with adjustable depths. The formation conditions of laser-induced periodic oxidation were determined to involve the generation of ablated silicon LIPSSs, which subsequently become coated with a layer of SiO₂. The thickness of the oxide layer is systematically controlled, varying from 1.03 to 2.63 µm, corresponding to increases in pulse energy and scanning speed. Subsequent ICP etching experiments reveal that the as-formed periodic oxide layer acts as an effective mask. Nevertheless, the periodic oxide layer is also consumed during the silicon ICP etching process. Consequently, the maximum achievable depth of the LIPSSs reaches 1730 nm, corresponding to a maximum aspect ratio of 7.86. Furthermore, ICP etching effectively removes the laser-induced defect layer. This hybrid method offers a viable approach for fabricating silicon nanostructures.