Silicon nitride (Si₃N₄) high-Q resonators have emerged as a leading integrated photonics platform for precision and quantum-enabled photonic systems. Their high quality factor extends photon lifetime and narrows the resonance linewidth, thereby enhancing light–matter interactions, enabling self-injection locking and frequency stabilization of semiconductor lasers, and supporting nonlinear processes such as Kerr frequency comb and photon-pair generation. Owing to its wide transparency window, ultra-low propagation loss, and CMOS-compatible fabrication, Si₃N₄ is particularly attractive for chip-scale optical clocks, long on-chip delay lines, and compact laser filtering systems. Achieving ultra-low-loss resonators, however, requires stringent control of film stress, hydrogen content, sidewall roughness, and high-temperature annealing in order to minimize scattering and absorption losses.