Void-free upper cladding deposition process for low-loss integrated silicon nitride photonics

Silicon nitride (${\mathrm{Si}\mathrm{N}}_{x}$) is one of the most promising integrated photonics platforms, offering ultralow loss, significant Kerr nonlinearity, and extremely high integration capability. However, ${\mathrm{Si}\mathrm{N}}_{x}$ in the C band exhibits normal material group-velocity dispersion (GVD), while anomalous GVD, essential for most nonlinearity-based applications, can only be achieved in thick waveguides through geometric dispersion. The common challenge in the fabrication of thick waveguides without using the Damascene technique is the uniform filling of the gaps between closely located waveguides. To address this challenge, we elaborated a combined technological approach that includes plasma-enhanced chemical vapor deposition, reactive ion etching, and plasma-enhanced atomic-layer deposition. This approach reliably provides full and uniform filling of the complex etched profile gaps between 1-$\text{\ensuremath{\mu}}\mathrm{m}$-thick silicon nitride waveguides, spaced at distances of the order of hundreds of nanometers. Additionally, using microring resonators, we experimentally demonstrated that voids within the upper cladding layer significantly affect the performance of the photonic integrated components.