Large-aperture Fabry-Perot filters based on silicon/silicon carbonitride distributed Bragg reflectors for the near-infrared
This contribution presents silicon carbonitride as a new low refractive index material for integration together with silicon in suspended distributed Bragg reflector membranes. We show how its benign mechanical stress properties can be used to fabricate inherently flat mirror membranes of up to 5 mm diameter. Due to the large refractive index contrast to silicon such mirror membranes exhibit high reflectance over a wide spectral range. Using these mirrors, we demonstrate a proof of concept for a large aperture Fabry-Perot filter with narrow bandwidth that could help miniaturize hyperspectral imaging sensors.
Plasma-enhanced chemical vapor deposition of amorphous silicon carbonitride: Deposition temperature dependence of bonding structure, refractive index, mechanical stress and their aging under ambient air
Abstract Amorphous silicon carbonitride deposited at temperatures between 200 °C and 395 °C by plasma-enhanced chemical vapor deposition has been characterized by Fourier transfrom infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, ultraviolet–visible–near-infrared reflectance-transmittance spectroscopy, residual stress measurements and etch tests in hydrofluoric acid (HF). The optical and mechanical properties have been measured both directly after deposition and after storage under ambient air. The bonding structure derived from FTIR spectra indicates that higher deposition temperatures lead to a denser material with a higher degree of crosslinking due to an increased dissociation of hydrogen molecules, thereby reducing the amount of hydrogen-terminated bonds. This results in an increased refractive index and a reduced compressive stress level as well as a better resistance against HF etching of the as-deposited layers. Diffusion of ambient moisture into the thin-films is identified as the main aging mechanism which affects layers deposited at lower temperatures more pronouncedly due to their higher degree of porosity. The resulting adsorbed water molecules cause an increase of layer thickness accompanied by a decrease of refractive index and an increase of compressive stress.
Surface-micromachined silicon air-gap Bragg reflector for thermal infrared
We present a MEMS-based distributed Bragg reflector for thermal infrared (TIR) wavelengths 7 µm < λ < 12 µm. Surface micro-machining process flow was developed for [poly-Si/air-gap/poly-Si] λ/4-mirror using low-pressure chemical-vapour deposited SiO2 as intra-mirror and mirror-to-substrate sacrificial layers. Circular 3 mm diameter mirrors with theoretical reflectance exceeding 99% were designed. Poly-Si layers of the mirror were anchored for retaining constant mutual distance. Anchoring density and mirror-to-substrate gap were varied among samples. We utilized scanning-electron microscope (SEM) imaging for qualitative estimation of processing result success as well as for layer-thickness measurements. We characterized the mirror topography and mechanical response under local point loading by scanning with a stylus profilometer. Fourier-transform IR (FT-IR) spectroscopy was utilized in studies of a reflectance spectrum. A one-dimensional computer simulation was allowed to fit model parameters to FT-IR data. Best-fit thicknesses of air gaps and poly-Si layers were compared with design parameters and with SEM measurements in order to verify the final structure corresponding to the design.