Silicon nitride (SiN) has emerged as a critical material in silicon photonics due to its unique optical properties, including low loss, high refractive index contrast, and broad transparency window spanning visible to mid-infrared wavelengths. Its compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes makes it an attractive candidate for integrated photonic circuits. The integration of SiN with silicon photonics enables low-loss and broadband applications, addressing limitations inherent in pure silicon-based platforms, such as high two-photon absorption and limited transparency beyond the near-infrared range.

Material deposition methods for SiN films play a pivotal role in determining optical performance. Low-pressure chemical vapor deposition (LPCVD) is widely used for stoichiometric SiN, offering high film uniformity and low hydrogen content, which minimizes absorption losses in the near-infrared spectrum. Plasma-enhanced chemical vapor deposition (PECVD) provides a lower-temperature alternative, suitable for back-end-of-line integration, though it often results in higher hydrogen incorporation and increased optical loss. Atomic layer deposition (ALD) has also been explored for ultra-thin SiN layers, enabling precise thickness control at the atomic scale. Each method presents trade-offs between deposition temperature, stress management, and optical quality, necessitating careful selection based on the target application.

Waveguide losses in SiN are a key metric for photonic integration. Propagation losses below 0.1 dB/cm have been achieved in LPCVD-deposited SiN waveguides at telecommunications wavelengths (1550 nm), rivaling the performance of silicon-on-insulator (SOI) waveguides. Scattering losses due to sidewall roughness can be mitigated through optimized etching techniques, such as reactive ion etching with post-processing smoothing steps. Absorption losses, particularly in PECVD films, are often linked to N-H and Si-H bonds, which introduce peaks around 1510 nm and 2200 nm, respectively. Annealing treatments can reduce these bonds, lowering losses to acceptable levels for high-performance devices.

The nonlinear optical properties of SiN further enhance its utility in photonic applications. With a nonlinear refractive index approximately an order of magnitude higher than silica, SiN enables efficient four-wave mixing and Kerr frequency comb generation. The absence of two-photon absorption at telecommunications wavelengths allows high-power handling, making SiN ideal for nonlinear signal processing and frequency comb generation. Dispersion engineering via waveguide geometry tuning permits phase matching for parametric processes, enabling broadband nonlinear interactions across hundreds of nanometers.

Hybrid Si/SiN platforms leverage the strengths of both materials, combining the high confinement of silicon with the low loss and broadband transparency of SiN. Edge coupling between Si and SiN waveguides facilitates efficient light transfer, while vertical integration schemes, such as wafer bonding or monolithic growth, enable compact, multi-functional photonic circuits. These platforms are particularly advantageous for biosensing, where the high sensitivity of SiN to refractive index changes is complemented by silicon’s electro-optic tunability. In quantum optics, SiN’s low autofluorescence and high Kerr nonlinearity support photon pair generation and manipulation. Mid-infrared photonics benefits from SiN’s transparency up to 6 µm, enabling on-chip spectroscopy and chemical sensing applications.

Thermal management remains a challenge in SiN photonics, particularly for high-power applications. The lower thermal conductivity of SiN compared to silicon can lead to localized heating, affecting device stability and performance. Strategies such as incorporating heat-spreading layers or optimizing waveguide geometries to reduce power density are under investigation. Fabrication uniformity is another critical issue, as variations in film stress and thickness can lead to inconsistent optical properties across a wafer. Advanced process control techniques, including in-situ monitoring and feedback loops, are being developed to address these challenges.

The future of SiN in silicon photonics lies in further refining material quality and integration techniques. Advances in deposition methods, such as low-temperature LPCVD or ALD with reduced hydrogen content, could push propagation losses even lower. Novel device architectures, including multi-layer heterostructures and inverse-designed components, promise to unlock new functionalities. As the demand for low-loss, broadband photonic circuits grows, SiN is poised to play an increasingly central role in enabling next-generation applications across communications, sensing, and quantum technologies.