  Silicon nitride (Si$_3$N$_4$) photonic integrated circuits (PICs) have shown
low linear loss, negligible nonlinear loss, and high power handling over
traditional silicon photonics. To achieve high-density photonic integration and
high effective nonlinearity through tight optical confinement, thick
stoichiometric Si$_3$N$_4$ films are indispensable. However, when using
low-pressure chemical vapor deposition (LPCVD) to achieve high optical material
transparency, Si$_3$N$_4$ films exhibit large tensile stress on the order of
GPa. Methods for crack prevention are therefore essential. The photonic
Damascene process has addressed this issue, attaining record low loss
Si$_3$N$_4$ PICs, but it lacks control of the waveguide height. Conversely,
precise waveguide dimension and ultra-low loss have been achieved with
subtractive processing, but this method is not compatible with mass production
due to the use of electron beam lithography. To date, an outstanding challenge
is to attain both lithographic precision and ultra-low loss in high confinement
Si$_3$N$_4$ 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 Si$_3$N$_4$ 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
Si$_3$N$_4$ layer, enabling direct deposition of thick films without cracking
and substantially prolonged storage duration. Lastly, we identify ultraviolet
(UV) radiation-induced damage that can be remedied through rapid thermal
annealing. Collectively, we develop ultra-low loss Si$_3$N$_4$ microresonators
and 0.5 m-long spiral waveguides with losses down to 1.4 dB/m at 1550 nm with
high production yield.
