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We present a FPI with a (TiO2/SiO2)3 reflector stack with a reflectance of 97 % and TiO2 as high refractive index layer for the use in the VIS-range of 555 nm to 585 nm. Main achievements of TiO2 instead of Si3N4 are a higher reflectance and a minimized reflector complexity. Furthermore, we introduce a dry etch process which is compatible and integrated in the manufacturing process chain of the MEMS FPI.
Manufacturing of the 7.5 mm x 7.5 mm chip size FPI is done on 6" wafers consisting of a moveable reflector on a 210 nm thin and 5 mm in diameter LP-Si3N4 membrane and a fixed reflector with an aperture of 2 mm in diameter. The measured peak transmittance is between 28 % and 37 % with a FWHM bandwidth between 1.5 nm and 1.8 nm. It could be shown that the FPIs are tunable over the spectral range from 555 nm to 585 nm with a maximum control voltage of 45 V using the 18th interference order.
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 %.
Tunable Fabry-Pérot interferometer with subwavelength grating reflectors for MWIR microspectrometers
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