Large-scale fabrication of LP-CVD Si3N4 photonic crystal structures as freestanding reflectors with 1 mm aperture for Fabry-Pérot interferometers

Abstract

Further developments of miniaturized spectrometry systems require tunable Fabry-Perot-Interferometers (FPI). A main part of the FPI is the reflector, which is usually realized as a stack of alternating dielectric layers with high and low refractive index. To achieve high reflectivity adequate material property homogeneity for each layer is needed. For stacks with larger number of layers not only the integration with MEMS processes is challenging. Particularly stack structuring and the achievement of process compatibility to moveable MEMS structures are important. As an alternative to the alternating layer stack reflector, nanostructured photonic crystal (PhC) reflectors indicate equivalent performance by using only one layer leading to a minimized reflector complexity. This contribution presents a novel PhC reflector consisting of a 400 nm thin moveable nanostructured LP-CVD Si3N4 membrane realizing an aperture of 0.5 mm and 1 mm for reflectivity in the VIS range. Manufacturing of the reflectors is done on 6 wafers. The array of nanostructures is designed as 1 mm circular dies consisting of 436 nm wide holes with 545 nm pitch. The circular dies are arranged in an 8 x 8 matrix on the wafers with 7.5 mm pitch. Manufacturing and integration of the PhC reflectors into MEMS is realized by eBeam and nanoimprint lithography (NIL) nanostructure replication on 50 µm thin pre-etched Si membranes combined with further dry and wet etching processes. The fabricated PhC reflectors showed 424 nm wide holes and a pitch of 549 nm with a measured reflectivity above 90 % in the spectral range from 557 to 589 nm and a maximum reflectivity of 99 %.