  Silicon-nitride-on-insulator photonic circuits have seen tremendous advances
in many applications, such as on-chip frequency combs, Lidar,
telecommunications, and spectroscopy. So far, the best film quality has been
achieved with low pressure chemical vapor deposition (LPCVD) and
high-temperature annealing (1200 {\deg}C). However, high processing temperature
poses challenges to the cointegration of Si3N4 with pre-processed silicon
electronic and photonic devices, lithium niobate on insulator (LNOI), and
Ge-on-Si photodiodes. This limits LPCVD as a front-end-of-line process. Here,
we demonstrate ultralow-loss Silicon nitride photonics based on
room-temperature reactive sputtering. Propagation losses as low as 5.4 dB/m
after 400 {\deg}C annealing and 3.5 dB/m after 800 {\deg}C annealing are
achieved, enabling ring resonators with more than 10 million optical quality
factors. To the best of our knowledge, these are the lowest propagation losses
achieved with low temperature silicon nitride. This ultralow loss enables
threshold powers for optical parametric oscillations to 1.1 mW and enables the
generation of bright soliton frequency combs at 1.3 and 1.5 {\mu}m. Our work
features a full complementary metal oxide semiconductor (CMOS) compatibility
with front-end silicon electronics and photonics, and has the potential for
hybrid 3D monolithic integration with III-V-on-Si integrated lasers, and LNOI.
