Publication
Silicon nitride (Si3N4) photonic integrated circuits (PICs) offer significant advantages over traditional silicon photonics, including low loss and superior power handling at optical communication wavelength bands. To facilitate high-density integration and effective nonlinearity, the use of thick, stoichiometric Si3N4 films is crucial. However, when using low-pressure chemical vapor deposition (LPCVD) to achieve high optical material transparency, Si3N4 films exhibit large tensile stress on the order of GPa, leading to wafer cracking that challenges mass production. Methods for crack prevention are therefore essential. The photonic Damascene process has addressed this issue, attaining record low-loss Si3N4 PICs, but it lacks control of the waveguide height, leading to large random variations of waveguide dispersion and unpredictable spectrum responses of critical functional devices such as optical couplers. Conversely, subtractive processes achieve better dimension control but rely on techniques unsuitable for large-scale production. To date, an outstanding challenge is to attain both lithographic precision and ultra-low loss in high-confinement Si3N4 PICs that are compatible with large-scale foundry manufacturing. Here, we present a single-step deposited, DUV-based subtractive method for producing wafer-scale ultra-low-loss Si3N4 PICs that harmonize these necessities. By employing deep etching of densely distributed, interconnected trenches into the substrate, we effectively mitigate the tensile stress in the Si3N4 layer, enabling direct deposition of thick films without cracking and substantially prolonged storage duration. A secondary ion mass spectrometry (SIMS) analysis reveals that these deep trenches simultaneously serve as gettering centers for metal impurities, in particular copper, thereby reducing the absorption loss in Si3N4 waveguides. Lastly, we identify ultraviolet (UV)-radiation-induced damage that can be remedied through a rapid thermal annealing. Collectively, we develop ultra-low-loss Si3N4 microresonators and 0.5-m-long spiral waveguides with losses down to 1.4 dB/m at 1550 nm with high production yield. This work addresses the long-standing challenges toward scalable and cost-effective production of tightly confined, low-loss Si3N4 PICs as used for quantum photonics, large-scale linear and nonlinear photonics, photonic computing, and narrow-linewidth lasers.