Design optimization of silicon nitride-based micro-ring resonator systems in a CMOS mass production environment

Micro-ring resonators (MRR) are basic photonic components, which serve as crucial building blocks for a variety of devices, e.g. integrated sensors, external cavity lasers, and high speed photonic data transmitters. Silicon nitride photonic platforms are particularly appealing in this field o f a pplication, s ince t his w aveguide material enables on-chip photonic circuitry with (ultra-) low losses in the NIR as well as across the whole visible spectral range. In this contribution we investigate key performance properties of MRRs in the wavelength range around 850 nm, such as free spectral range (FSR), quality factor (Q factor) and extinction ratio. We systematically investigate a large parameter space given by the MRR radii, coupling gaps between ring and bus waveguide, as well as waveguide width. Furthermore, we compare key properties such as the Q factor between low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) Si₃N₄ platforms and find enhanced values for LPCVD ring resonators reaching nearly a Q factor of 10⁶.The fabrication is carried out with standard CMOS foundry equipment, utilizing photolithography and reactive ion etching on 250 nm thick silicon nitride films. A s c ladding m aterial, h igh d ensity P ECVD s ilicon o xide i s d eposited p rior t o the waveguide onto bare silicon and a sputtered oxide serves as upper cladding. With this process toolbox full CMOS backend compatibility is achieved when considering only PECVD Si₃N₄ waveguide material. In terms of manufacturability, special focus is put on the die-to-die as well as on wafer-to-wafer variability of the performance parameters, which is crucial when considering mass production of MRR devices. Finally, the experimental findings are compared to finite difference time domain (FDTD) simulations of the MRR circuits revealing excellent agreement when considering the manufacturing variability.