High aspect ratio slanted grating replication process with low residual layer thickness

Google, 1600 Amphitheatre Parkway, Mountain View, CA 94043 USA

Published online: 18 October 2023

Abstract

Slanted subwavelength gra2ngs are used today in various products, especially as TIR couplers in Augmented Reality (AR) waveguides. Nano-Imprint Lithography (NIL) has been successfully used as a produc2on technique for such gra2ngs with periods down to 250nm, slants up to 45 degrees and aspect ra2o up to 2x. In order to propagate larger Field of View (FOV) in AR headsets, the refrac2ve index of the waveguide substrate needs to be large (1.9 to 2.0). NIL technologies typically produce a Residual Layer Thickness (RLT) of 100nm or more, requiring the index of the gra2ng resin to match the index of the substrate to prevent FOV clipping. This brings up many challenges such as TiO2 nanofiller migra2on during NIL process, scaRering and absorp2on in blue, and hard master / soT stamp combined degrada2ons. These challenges triggered a need for non-NIL techniques to mass produce such nanostructures, also at a lower cost. Avoiding mechanical contact between the master and the gra2ng material is desired, as costly masters would not deteriorate with volume produc2on. First experimental results are presented with aspect ra2o of 10x at 45deg slant and RLT below 3nm.