# Process Development of Low-Loss LPCVD Silicon Nitride Waveguides on 8-Inch Wafer
## 1 Introduction  
Photonic integration based on Silicon-On-Insulator (SOI) has grown rapidly over the last decade. The main advantages of silicon photonics are the compatibility with complementary metal-oxide-semiconductor (CMOS) infrastructure and the ability to realize monolithic integration. These features enable the fabrication of highly integrated photonic devices in large quantities at low cost. Silicon photonics are theoretically ideal for large-scale complex integrated optical circuits, such as multi-wavelength coherent transceivers, optical interconnection networks, and optical switching networks. However, the narrow silicon bandgap (1.14 eV) and non-negligible multiphoton absorption effects in the near and partial mid-infrared bands increase transmission losses and limit the application field of silicon waveguides [1, 2]. In contrast, silicon nitride (Si\({}_{3}\)N\({}_{4}\)) is also a CMOS-compatible material with a moderate refractive index between silicon dioxide (SiO\({}_{2}\)) and silicon (Si), a large energy band gap (5.0 eV), and a wide transparent optical window (0.25-8.0 \(\upmu\)m), which covers the visible to mid-infrared band [3, 4]. This means that intrinsic silicon nitride material has low absorption properties and can achieve low loss in broadband. Its advantages are also manifested in the following aspects: 1. A lower thermo-optic coefficient: it is one order of magnitude smaller than that of silicon, so that silicon nitride optical components are much less sensitive to temperature than silicon optical components. This makes them more suitable for applications that need to avoid the influence of temperature and have better popularity. 2. Negligible two-photon absorption: two-photon absorption limits the power handling capacity of the silicon waveguide in the near-infrared band; while benefiting from the negligible two-photon absorption, the silicon nitride waveguide can sustain much higher optical power, which makes it a more suitable material for nonlinearoptics compared to silicon even with a lower nonlinear index. 3. A wide range of application scenarios, including artificial intelligence, lasers, optical communication, biomedicine, optical sensors, optical distance measurement, etc. [1; 5; 6; 7; 8; 9]. For silicon nitride, there are two common deposition methods: plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD). Although PECVD can achieve a thick silicon nitride film, the uniformity is poor compared to that of LPCVD. Furthermore, the film deposited by PECVD is not stoichiometric and contains more H-bonds (about 20%), which will generate a large absorption in the communication band and seriously affect the device loss due to the N-H and Si-H bonds [10; 11]. Compared with PECVD, the silicon nitride film deposited by LPCVD has a more uniform thickness and lower H-bond content. However, due to the difference in the thermal expansion coefficient between silicon nitride and substrate silicon (the thermal expansion coefficient of silicon nitride is: \(2.35\times 10^{-6}/^{\circ}\)C, which is smaller than \(2.61\times 10^{-6}/^{\circ}\)C of silicon) [11; 12; 13; 14; 15; 16; 17], the film deposited by LPCVD has larger stress due to higher process temperature, and the stress will increase with the wafer size. Typically, in an 8-inch wafer, only up to 300 nm-thick silicon nitride films can be deposited; otherwise, cracks will occur. Due to the high stress, thick LPCVD silicon nitride waveguide devices are typically developed on a 4-inch wafer and require a two-step deposition method. These factors limit low-loss silicon nitride waveguides from achieving higher levels of integration and higher economic value [2; 4; 18; 19; 20]. Although there are some reports on low-loss waveguides in recent years, such as adjusting the precursor gas of the PECVD process [2], the report used N\({}_{2}\) instead of NH\({}_{3}\) to reduce the H-bond content of the silicon nitride film deposited by PECVD.