Metalenses for visible wavelengths: NIL volume manufacturing

Abstract

Building visible wavelength metalenses presents significant challenges for nanofabrication due to the high aspect ratio features and tight tolerances required for good performance. The requisite phase profiles often impart dramatic changes in nanostructure fill fraction, which are challenging to pattern via optical lithography. One metasurface of interest is a spatially-varying array of nanopillars ranging in diameter from 70nm - 180nm, with gaps between pillars ranging from 180nm - 70nm. To manufacture this and other metastructured devices in volume, Nanoimprint Lithography (NIL) becomes a key enabling technology due to its demonstrated scalability and ability to reliably replicate nanostructures with extremely tight tolerances, even with variations in local spacing. Another requirement for building metasurfaces for visible light applications, is the ability to pattern full wafers with good repeatability in high volume. Moxtek has therefore set up a 200 mm diameter manufacturing demonstration, where high aspect ratio nanopillars of varying diameter are etched from high refractive index material in order to make visible wavelength metalenses. In this work, metalenses designed for green light were fabricated with both a square grid arrangement and with a radially periodic arrangement. The metalenses were also given a protective coating and the focusing performance was characterized. The manufacturing process evaluation has three key components: 1) characterize the processing bias (from design dimensions to final nanostructure dimensions) at various stages; 2) monitor process stability and repeatability using metrology test devices distributed over the wafer; 3) characterize and verify functioning optical devices. Collectively, we have demonstrated volume manufacturing of metalenses for the visible regime, which was made possible by high precision NIL and Etch processes.