Citation: Behl, C.; Behlert, R.; Seiler, J.; Helke, C.; Shaporin, A.; Hiller, K. Characterization of Thin AlN/Ag/ AlN-Reflector Stacks on Glass Substrates for MEMS Applications. Micro 2024, 4, 142–156. https:// doi.org/10.3390/micro4010010 Academic Editor: Hiroshi Furuta Received: 22 December 2023 Revised: 13 February 2024 Accepted: 26 February 2024 Published: 29 February 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micro Article Characterization of Thin AlN/Ag/AlN-Reflector Stacks on Glass Substrates for MEMS Applications Christian Behl 1, *, Regine Behlert 1,† , Jan Seiler 1,2 , Christian Helke 1,2 , Alexey Shaporin 1 and Karla Hiller 1,2 1 Fraunhofer Institute for Electronic Nano Systems (ENAS), Technologie-Campus 3, 09126 Chemnitz, Germany; jan.seiler@enas.fraunhofer.de (J.S.); christian.helke@enas.fraunhofer.de (C.H.); karla.hiller@enas.fraunhofer.de (K.H.) 2 Center for Microtechnologies (ZfM), TU Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz, Germany * Correspondence: christian.behl@enas.fraunhofer.de; Tel.: +49-45001-610 † Current address: Carl Zeiss, Carl-Zeiss-Promenade 10, 07745 Jena, Germany. Abstract: Thin metal layers such as silver (Ag) are being utilized for various optical and plasmonic applications as well as for electrical purposes, e.g., as transparent electrodes in display devices or solar cells. This paper focuses on optical MEMS applications such as the Fabry–Pérot interferometer (FPI). Within such filters, reflector materials such as distributed Bragg reflectors (DBRs) or subwavelength gratings (SWGs) have been widely used so far, whereas metallic thin films (MTFs) were limited in application due to their comparatively higher absorption. In this paper, thin sputtered Ag layers with thicknesses of 20, 40 and 60 nm on glass substrates have been investigated, and it is shown that the absorption is very low in the visible spectral range (VIS) and increases only in near-infrared (NIR) with increasing wavelength. Thus, we consider Ag-thin layers to be an interesting reflector material at least for the VIS range, which can be easily fabricated and integrated. However, Ag is not inert and stable when exposed to the atmosphere. Hence, it needs a passivation material. For this purpose, AlN has been chosen in this contribution, which can be deposited by sputtering as well. In this contribution, we have chosen thin AlN layers for this purpose, which can also be deposited by sputtering. Thus, various AlN/Ag/AlN-reflector stacks were created and patterned by lift-off technology preferably. The fabricated reflectors were characterized with respect to adhesion, stress, cohesion, homogeneity, and most importantly, their optical properties. It was found that the thickness of the AlN can be used to adjust the reflectance–transmittance ratio in the VIS range, and influences the adsorption in the NIR range as well. Based on the measured values of the reflectors with 40 nm Ag, an exemplary transmission filter characteristics has been predicted for a wavelength range from 400 to 800 nm. Both the maximum transmittance and the full width at half maximum (FWHM) can be tuned by variation of the AlN thickness from 20 to 60 nm. Keywords: metallic thin film; Fabry–Pérot interferometer; visible spectral range; near-infrared; subwavelength gratings; distributed Bragg reflector 1. Introduction Originally optical spectrometers have been large and very cost intensive devices. In the past couple of years, handheld instruments have been designed to make them portable at lower production costs by the use of a micro machined spectrometer, e.g., MEMS FPI [1–5]. Basically, the principle of a MEMS FPI can be described with two-plane, parallel, and semi-transparent reflectors with a gap d, as shown in Figure 1. The incident light I 0 can transmit through the semi-transparent mirror. For resonant wavelengths, it is reflected several times in the cavity and constructive interference will occur (see Equation (1)), with m being the order or the filter. Finally, the transmitted light intensity I t is the output signal of the FPI, which is a periodic signal with respect to the spectrum. Furthermore, Figure 1 shows also the Free Spectral Range (FSR, described by Equation (2)), which is the distance Micro 2024, 4, 142–156. https://doi.org/10.3390/micro4010010 https://www.mdpi.com/journal/micro