Reactive ion beam etch of slanted gratings for augmented reality

Abstract

Slanted gratings show high efficiency in coupling light into waveguides, which is critical for optics such as the diffractive optical elements (DOEs) used in augmented reality glasses. Fabrication of master molds used for producing slanted gratings require high etch selectivity between SiO2 and a metal mask. The industry standard for trench etch, Reactive Ion Etch (RIE), cannot produce slanted trenches. In this work, we demonstrate the fabrication of surface relief grating (SRG) master molds with slant angles of 30, 45 and 60 degrees. Using Reactive Ion Beam Etch (RIBE), etch anisotropy is controlled via the directionality of the ion beam, combined with angle of incidence. The selectivity between metal mask vs SiO2 can be tuned (7:1 to 12:1). An in-house oxygen cleaning increases stability, leading to high repeatability in process and etch rates. High static uniformity across the wafer surface was achieved using a proprietary tunable ion source.