Embodiments of the disclosure generally relate to methods of forming gratings. The method includes depositing a resist material on a grating material disposed over a substrate, patterning the resist material into a resist layer, projecting a first ion beam to the first device area to form a first plurality of gratings, and projecting a second ion beam to the second device area to form a second plurality of gratings. Using a patterned resist layer allows for projecting an ion beam over a large area, which is often easier than focusing the ion beam in a specific area.