Optical frequency comb generation using low stress CMOS compatible reactive sputtered silicon nitride waveguides

Photonic chip based Kerr frequency combs are transforming diverse applications including spectroscopy, telecommunication, signal processing and metrology among others. Integrated silicon nitride (SiN) waveguides with anomalous dispersion have the potential to bring practical nonlinear optics to mainstream photonic integrated circuits; however, high stress and high processing temperatures for SiN deposited by low pressure chemical vapour deposition (LPCVD) remain an obstacle to mass adoption. We successfully demonstrate fully CMOS-compatible high confinement SiN microring resonators based on reactive sputtering thin-films at a maximum processing temperature of 400°C. We deposit 0.85 µm thick SiN thin-films with a low stress value of 41.5 MPa and bulk material losses of 0.3 dB/cm. Linear waveguides losses of 0.7 dB/cm (Q_int= 4.9 × 10⁵) and 0.5 dB/cm (Q_int= 6.6 × 10⁵) have been achieved at 1560 nm and 1580 nm, respectively. We characterised the nonlinear properties of the waveguides and measured a nonlinear coefficient of γ = 2.1 W^-1 m^-1 and a nonlinear refractive index n₂ of 5.6 × 10^-19 m² W^-1. Modulation-instability (MI) optical frequency combs are observed by pumping a 120 µm radius microring resonator at 1560 nm with an estimated on-chip pump power of 850 mW, showing a native FSR spaced frequency comb covering a >250 nm wide spectral range.