## 4 Optical Frequency Comb Generation in Annealing- and Crack-Free Si\({}_{3}\)N\({}_{4}\)
Here, we report a comb generated by a micro-resonator fabricated by an intrinsically CMOS-compatible annealing-free silicon nitride, following a tailored deposition method which minimizes the hydrogen content instead of hour-long 1200 \({}^{\circ}\)C post-annealing. Our annealing-free and crack-free fabrication process provides our devices with the right specifications (i.e., micro-ring group velocity dispersion and characteristics) to underpin Kerr frequency combs, thus representing a significant step toward the full compatibility of Si\({}_{3}\)N\({}_{4}\)-based Kerr comb sources with the thermal budgets of Si photonics processing. To control the strain and to prevent cracks from appearing, the silicon nitride layer is deposited on a (non-patterned) substrate via low-pressure chemical vapor deposition (LPCVD) in two steps of 370-nm-thick layers each. The deposition is performed with a tailored ultra-low deposition rate (\(\sim\)2 nm/min) to yield a very high quality film [33]. Under such low deposition rates, the thermal activation energy enables silicon and nitrogen to be disposed at the silicon nitride film surface via atomic surface migration phenomena, while compelling hydrogen to escape the film. Furthermore, between the two deposition stages, the wafer is rotated by 45\({}^{\circ}\) in order to distribute the uniaxial strain along the overall film thickness, thus avoiding film cracks upon subsequent subtractive patterning. Each deposition run is carried out at 780 \({}^{\circ}\)C with post-deposition cooling to around 630 \({}^{\circ}\)C for 20 min. Controlled ramp-ups and -downs from/to 780 \({}^{\circ}\)C at 10 \({}^{\circ}\)C/minute to/from 630 \({}^{\circ}\)C are used prior to each deposition, which is carried out under a 112 mTorr pressure using NH\({}_{3}\) (200 sccm) and SiH\({}_{2}\)Cl\({}_{2}\) (80 sccm) as precursor gases. By measuring the wafer bow, before and after removing the silicon nitride from the wafer back side, the material morphological characterization revealed a tensile strain around +1200 MPa, highlighting the as-deposited stoichiometry of the material and minimization of residual hydrogen content [41]. The optical spectrum out of an annealing-free silicon-nitride-on-insulator micro-ring with a 56 \(\upmu\)m radius is shown in Figure 5a. An optical picture of the ring is shown in Figure 5b. The cross-section dimensions (1.5 \(\upmu\)m-wide \(\times\) 740 nm-thick) of the ring waveguide ensure that GVD is anomalous at the 1569 nm pump wavelength. The loaded quality factor of the microring--which is separated by a 350 nm gap from the bus waveguide--exceeds 580,000. A relatively smooth native line spacing (\(\sim\)406 GHz) frequency comb spanning about 800 nm between 1300 nm and 2100 nm is obtained for \(\sim\)1 W continuous-wave power (\(P_{\text{in}}\)) coupled in the bus waveguide (i.e., a value comparable to that required for similar rings in high-temperature annealed Si\({}_{3}\)N\({}_{4}\)[43]. The Optical parametric Oscillator (OPO) threshold power was measured to be around 83 mW (see Figure 5c). In conclusion, these results [32] show that it is possible to generate a wideband comb at telecom wavelengths using annealing-free silicon nitride nonlinear circuits featuring a full FEOL process compatibility with Si photonics. Via such demonstration, we claim the first-time realization of annealing-free silicon nitride frequency comb microresonators, following a tailored deposition method, minimizing the hydrogen content. Our annealing-free and crack-free fabrication process provides our devices with the right specification (microring group velocity dispersion and characteristics) to underpin Kerr frequency combs, thus representing a significant step toward the full compatibility of Si\({}_{3}\)N\({}_{4}\)-based Kerr-comb sources monolithic integration with standard CMOS and Si photonics processing.