Deep gratings, plasma-etched in various materials, can form optical metasurfaces for a wide range of applications. The geometry of such nanostructures can be difficult to determine with enough precision to predict optical performance, and optical performance testing typically requires that the etch mask is removed. In the case where optical testing indicates that the grating is too shallow, a new etch mask needs to be applied to etch it deeper. Here we present a simple method for depositing a new mask on deep gratings by sputter coating with a 3D printed shade mask, which blocks vertical incidence and prevents the deposited material from reaching the bottom of the grating. The method is demonstrated by tuning the depth of a diamond annular groove phase mask to improve performance. • Tuning an etched grating after optical testing needed for optimal performance. • Depositing an etch mask only at the top of a grating allows further etching. • A 3D printed shade mask was used during sputter coating to only coat the top. • The sputtered etch mask protects the grating top during plasma etching. • Increasing the depth of the grating led to improved performance.
Forsberg et al. (Sun,) studied this question.