## 3 Results  
Table 1 shows the precursor gases flow (sccm), the IPC power (W), the RF power (W), and the APC pressure (mTorr) used in every recipe along with the average results of various deposited films in DR (nm/min), stress gradient (MPa), roughness, refractive index \(n\) and extinction coefficient \(k\) at 1550 nm for seven investigated recipes. Figure 2: Top view schematic representation of a MEMS-FP cavity. On the left is the bottom electrode. On the right is the complete device with optical window (blue) and top electrode (dark orange). The design goal of the recipe is to achieve an as high as possible refractive index, a \(k\)-value below 0.0124 and a compressive stress inferior to 150 MPa in absolute value, in order to overcompensate the stress of the SiO\({}_{\text{y}}\) quarter wavelength layers of about 130 MPa compressive. These characteristics are necessary to achieve functional MEMS mirrors in mechanical and optical terms. The limit for the extinction factor \(k\) is chosen such that the absorption constant at 1550 nm, \(\alpha=\frac{4\pi}{\lambda_{0}}k\) is below 1000/cm. The roughness should be as low as possible while the deposition rate is secondary. First, we determined approximate process parameters in preliminary work (not shown here). Table 1 shows the final optimisation step. In Table 1, the recipes can be classified in two groups, in the first one (recipes 1 to 3) the flow volume of SiH\({}_{4}\) is of 9.8 sccm, and the second one (recipes 4 to 7) with 9.6 sccm of SiH\({}_{4}\). In both groups, the roughness increases dramatically when RF power is introduced while the refractive index, stress gradient, and the deposition rate increase and the extinction coefficient decreases (recipe 2 and 6). In deposition 3, the APC pressure was reduced by 0.3 mTorr giving as a result a decrease in the refractive index and an increase in stress with respect to deposition 1 and 2. A similar situation occurs in deposition 4 where the APC pressure is decreased by 2 mTorr, compared to recipe 5, in this case both refractive index and stress show a rise. On the other hand, when this pressure is increased (recipe 7) both refractive index and stress are reduced. From all the seven recipes, depositions 2 and 4 show the optical and mechanical characteristics that are best suited for the MEMS. Their characteristics are very similar, both have a high refractive index, a low absorption, and their stress gradient is inside the desired boundaries. Recipe 2 was dismissed due to its high roughness, with recipe 4 as the material selected for the fabrication of the FP-filter reflectors. Figure 3 shows the measured reflectivity of a 6.5-layer pair silicon-rich SiN\({}_{\text{x}}\)/SiO\({}_{\text{y}}\) DBR deposited with the silicon-rich SiN\({}_{\text{x}}\) deposition parameters 4 in Table 1 and the growth conditions described for SiO\({}_{\text{y}}\) in the methods section. This reflection corresponds to the top-DBR of the FP-filters described in this manuscript. The expected total thickness is 2716 nm. The \(\sigma\)-polarisation provides a refection band (\(R>96\%\)) in the range from 1140 to 1620 nm and in the case of \(\pi\)-polarisation this range goes from 1220 to 1480 nm. This figure also presents the theoretical reflectivity at 40\({}^{\circ}\) and 0\({}^{\circ}\) of incidence angle, yet with a slightly higher reflectivity of \(R>98\%\). Spectrally, the theoretical graph matches with negligible differences the measured curve. The simulation of reflectivity at vertical incidence angle offers a reflectivity band from 1280 to 1640 nm with \(R>97\%\). The central wavelength shifts to longer wavelengths compared with the 40\({}^{\circ}\) curves. We remark that at 1700 nm, the structure still shows a reflection of \(R=94\%\).