Abstract
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. We have conducted a systematic investigation of the influence of the deposition parameters on the material and optical properties of NH3-free PECVD SiNx layers fabricated at 350 °C using a design of experiments methodology. In particular, this paper discusses the effect of the SiH4 flow, RF power, chamber pressure and substrate on the structure, uniformity, roughness, deposition rate, refractive index, chemical composition, bond structure and H content of NH3-free PECVD SiNx layers. The results 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.
Highlights
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 (250nm-7μm) [?, ?]
We generated different central composite designs (CCD) with Minitab to study the effect of the SiH4 flow and the radio frequency generator (RF) power on the material and optical properties of the films following a similar approach to the one described by Tien et al [?], in order to have enough information to estimate how the properties of the layers would change if these deposition parameters were altered in any direction
The structure of the deposited films is important for propagation losses because grains, pores, defects and rough interfaces are a direct cause of scattering losses [?]
Summary
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 (250nm-7μm) [?, ?] It has the advantage of exhibiting low non-linear losses, which are useful for non-linear applications [?]. 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 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 [?, ?, ?]. Processing temperatures above 400oC are incompatible with the back-end-ofline (BEOL) integration needed to fabricate photonic devices in multilayer platforms
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