Silicon nitride (SiNx) is a CMOS-compatible material that has a wide transparency window that makes it viable for applications covering the ultra-violet to the mid-infrared (250 nm–7 μm). It also has the advantage of exhibiting low non-linear losses, which are useful for non-linear applications . In recent years, it has drawn attention as a potential alternative for a variety of photonic devices mostly because its physical, chemical and optical properties can be tailored through the deposition conditions to fulfil the requirements of different applications. Amongst these applications, SiNx films have proved particularly attractive for passive waveguides due to the refractive index of the material (1.7–2.8) capable of providing tight optical confinement with low propagation losses over a wide wavelength range. SiNx layers intended for photonic applications are typically fabricated using LPCVD and PECVD. These techniques rely on high-temperature processing (>400 °C) to obtain low propagation losses. An alternative version of PECVD SiNx layers deposited at temperatures below 400 °C with a recipe that does not use ammonia (NH3-free PECVD) was previously demonstrated to be a good option to fabricate strip waveguides with propagation losses <3 dB cm−1.
The results of our work show that the properties and the propagation losses of the studied SiNx layers depend entirely on their compositional N/Si ratio, which is in fact the only parameter that can be directly tuned using the deposition parameters along with the film uniformity and deposition rate. These observations provide the means to optimise the propagation losses of the layers for photonic applications through the deposition parameters. In fact, we have been able to fabricate SiNx waveguides with H content <20%, good uniformity and propagation losses of 1.5 dB cm−1 at 1550 nm and <1 dB cm−1 at 1310 nm. As a result, this study can potentially help optimise the properties of the studied SiNx layers for different applications.
The SIN platform development was initiated within the EPSRC first grant EP/K02423X/1 HERMES: High dEnsity Silicon GeRManium intEgrated photonicS. The platform is currently developped further through collaboration under the EPSRC : A Platform Grant EP/N013247/1: Electronic-Photonic Convergence, EP/L021129/1 CORNERSTONE: Capability for OptoelectRoNics, mEtamateRialS, nanoTechnOlogy aNd sEnsing and European project H2020 COSMICC,