Roles of overlapped scratching in grating fabrications assisted by selective wet etching

Gratings are core components of high-precision optical system with wide applications. However, the optimal fabrication of master gratings remains highly concerned in terms of the cost and performance. Nanoscratch-induced selective etching, consisting of pre-scratching with diamond tip and post-etching with a given etchant, presents robust ability in the fabrication of gratings; however, the roles of overlapped scratches in the etching were not clear, which hinders further enhancement in grating manufacturing. In this study, it was found that the deformation in the overlapped scratches could dominate the etching process and the formation of gratings. The overlap ratio of neighboring scratches was highly depended on normal load and line space, which can be visualized by selective etching in KOH solution with different etching times. With the decrease in line space, a thicker amorphous Si (a-Si) layer can be generated between neighboring scratches. The formation of a-Si in overlapped scratches was investigated by transmission electron microscopy (TEM) and Raman spectroscopy, and the stress distribution on Si surface was obtained by simulation. As a result, accurate and reproducible morphology of hierarchical grating structures can be obtained through controlling the overlapped scratching and the etching. Furthermore, different structural colors can be achieved through designing the traces for the overlapped scratching.