# Low-Temperature and Low-Pressure Silicon Nitride Deposition by ECR-PECVD for Optical Waveguides
## 2 Fabrication  
### Silicon Nitride Thin-Film Deposition  
We deposited silicon nitride thin films on various substrates using an ECR-PECVD system. The system is divided into three parts, allowing for a separate load-lock, main chamber and ECR plasma generation sections. The remotely generated ECR plasma allows for a high fractional ion density at low process pressure, limiting the damage to the film during growth. A forward microwave power of 500 W is applied through a quartz window near an 875 G electromagnet with N\({}_{2}\), O\({}_{2}\) and Ar inlets for ECR plasma generation during deposition. In the ECR plasma generation section of the chamber, the strong electromagnet is used to minimize ionized plasma species sidewall reactions and confine electron motion to field lines concentrated towards the sample surface inside the main chamber. Due to the relatively small main chamber size of roughly 10 cm radius, deposition pressures in the 1-2 mTorr range are possible using ECR-assisted confinement. A more detailed description of the system and the orientation of the gas shower heads, ECR plasma section and sample holder can be found in Figure 22 and Figure 27 in [31]. These features are aimed at lowering the incorporation of interstitial hydrogen, an unwanted contamination which leads to strong absorption in the S and C telecommunication wavelength windows [2]. The system's low deposition pressure with operating conditions shown here of 1-2 mTorr also ensures a low concentration of Si-H bonds to form during deposition while using SiH\({}_{4}\) precursors, limiting interstitial hydrogen incorporation during deposition, as is a common drawback in PECVD systems. Another expected source of hydrogen is invasive H\({}_{2}\)O in the form of water vapour likely entering the chamber through the \(\sim\)5 m-long gas transit lines and potentially residual from the sample transfer process. This also explains the presence of oxygen in the films which has been quantified with RBS. The deposition system uses dedicated turbopumps for both the main chamber and the separate load-lock chamber, keeping the main chamber isolated during venting for transfer. The system is also equipped with in situ sputtering [32] and metal organic delivery lines which provide in situ incorporation of rare earths for the development of solid-state lighting devices. Additionally, it has been used for the formation of silicon nanoclusters embedded in Si\({}_{x}\)N\({}_{y}\)[33]. We deposited SiO\({}_{x}\)N\({}_{y}\):H\({}_{x}\) films on different substrate materials mounted to a stainless-steel stage with tungsten clips to secure the samples. The sample stage sits inside the main chamber, mounted on a motor-controlled arm for rotation, holding the sample face normal to the precursor gas showerhead, and ECR-generated plasma. A heating element in contact with the back of the sample stage allows for evenly distributed heating, which provides additional surface energy for the reaction. As mentioned, the strong ECR plasma acts as an auxiliary power source for the reaction, allowing the lower deposition temperatures when compared to ICP depositions. Sample stage rotation was maintained at a constant 0.5 Hz during all of the depositions and the substrates were heated to 140 \({}^{\circ}\)C. The partial pressure from each introduced gas species was allowed to vary throughout the deposition whilst maintaining a constant gas flow rate. This resulted in the partial pressure from each gas being on average 5% higher by the end of the deposition. The partial pressures imparted by each of the gases at the start of depositions are shown in Table 1, with an assumed associated error of \(\pm\)0.01. Across the film recipes, SiH\({}_{4}\)/Ar flow was held constant while the N\({}_{2}\)/Ar flow was varied. This was due to the higher range of gas flow choices associated with higher flow rates from the mass flow controller (MFC) with N\({}_{2}\)/Ar gas flow. Any change in the silane flow would result in a drastic stoichiometry variation even across 0.1 sccm of gas flow change.