## 1 Introduction  
Silicon oxide (SiO\({}_{\text{y}}\)) and silicon nitride (SiN\({}_{\text{x}}\)) thin films are implemented in a vast number of application fields. The tailored mechanical and optical characteristics of SiN\({}_{\text{x}}\) make it a material attractive for antireflection-, isolation- and passivation coatings in solar cells and CMOS [1, 2], and for waveguides in photonic integrated circuits [3, 4]. High quality dielectric isolating materials are further implemented as so-called high-k dielectrics in MOS devices [5]. Key applications in the optical domain are Bragg mirrors composed of a low refractive index material, frequently SiO\({}_{\text{y}}\), and a high refractive index material [6]. An alternating multilayer sequence of quarter wavelength layers results in a reflectivity very close to 100% due to constructive interference in reflection, given by the negligible loss of the materials. These mirrors outperform metal mirrors that scarcely exceed 98% reflection, resulting in considerable losses for systems with many mirrors. The larger the refractive index contrast between both dielectrics, the larger the block bandwidth. As there are hardly any solid, mechanically stable, and low loss materials with a refractive index below that of SiO\({}_{\text{y}}\) (\(n\)\(\sim\)1.45), increasing the refractive index contrast of the high index layer is the only viable option for increasing the block band width. Silicon carbide (SiC\({}_{\text{z}}\)) is a sensible option in the visible domain with a refractive index around 2.6 that decreases with increasing wavelength. In the telecom band, however, non-crystalline SiC\({}_{x}\) as deposited with typical PECVD processes, features non-negligible losses [7], deteriorating the mirror quality. A further physical parameter that can be utilised in PECVD films to achieve functionality is the tunable stress that allows for implementation in microelectromechanical systems (MEMS) [8; 9]. An example of a tunable optical filter is illustrated in Figure 1a. It consists of a MEMS-tuned microcavity where both stress and high refractive index contrast to SiO\({}_{y}\) are essential in order to obtain a functional device [10; 11]. The filter is composed of a Distributed Bragg Reflector (DBR)-based MEMS Fabry-Perot (FP) cavity, where a temperature- or electro-statically tuned concave Bragg reflector MEMS is grown on top of a static, flat Bragg mirror. The air gap (\(L_{\mathrm{cnr}}\)) between the top and bottom mirrors that is achieved by depositing the top Bragg mirror with compressive stress and subsequently releasing it, determines the free spectral range (FSR) of the cavity [12]. For carefully chosen reflectivities of the two Bragg mirrors, the filter structure features a spectrally very sharp, tunable transparent notch within the block band when the effective cavity length \(L_{\mathrm{eff}}\) equals multiples of a half wavelength as illustrated in Figure 1b. By applying heat or an electrostatic force, the MEMS mirror can be deformed, leading to a change in \(L_{\mathrm{cnr}}\) and consequently a widely tunable transmission filter. Also here, a large refractive index contrast between the individual Bragg layers has several advantages: (i) much fewer periods are required to achieve a target block band reflectivity, (ii) a larger block band and free spectral range, (iii) less inertia and thus faster tuning speed in case of electrostatic tuning, and (iv) less thermal mass and thus faster tuning speed in case of thermal tuning. Figure 1: **(a)** Schematic cross-sectional image of a MEMS-FP cavity. Yellow: metal electrodes for electrothermal tuning (top metal) and electrostatic tuning (voltage between top and bottom metal). There is a small height step, \(h_{0}\), where the sacrificial Chromium layer for MEMS release was situated.