Effect of Annealing Treatment on the Optical Properties of Silicon Nitride Waveguides

Abstract

Introduction. Silicon nitride is a highly promising material for fabrication of photonic integrated circuits (PICs). Plasma-enhanced chemical vapor deposition is a prospective method for large-scale industrial production of silicon nitride-based PICs. The disadvantage of this method, which limits its practical application, consists in high insertion losses in the telecommunication frequency band due to absorption on the Si–H and N–H bonds remaining from the film growth process. Thermal annealing is the most common method for breaking these bonds and reducing losses. Therefore, investigation of the impact of annealing on the optical properties of photonic integrated waveguides is an important research task.Aim. To investigate the effect of annealing treatment on the optical properties of PICs based on the silicon nitride films with different thicknesses obtained by plasma-enhanced chemical vapor deposition.Materials and methods. The work investigates the effect of annealing treatment on the optical properties of PICs based on the silicon nitride films with thicknesses of 200, 400 and 700 nm. To that end, the transmission characteristics of a set of test elements were measured using a high-definition component analyzer in the frequency range of 185…196 THz.Results. Frequency dependencies of loss and coupling coefficients, as well as the group index before and after annealing were extracted from the measured transmission characteristics of the test elements. It was found that waveguides on a 200-nm-thick film exhibited higher losses in comparison with the waveguides on thicker films. The waveguides with cross sections of 900 × 400 and 900 × 700 nm2 demonstrate the losses below 5 dB in the frequency range of 185…190 THz. A rapid increase in losses due to absorption on the N–H bonds was observed at the frequencies above 190 THz. The work shows that thermal annealing reduces insertion losses across the frequency range from 185 to 196 THz. The adequacy of extracted optical parameters is confirmed by comparing theoretical and experimental transmission characteristics of the ring resonator.Conclusion. The obtained results demonstrate that silicon nitride waveguides fabricated by the method of plasma-enhanced chemical vapor deposition require the stage of thermal annealing. Vacuum annealing at 600 °C for 30 min reduces insertion losses in the waveguides with cross sections of 900 × 400 and 900 × 700 nm2 down to 4 dB/cm in the frequency band from 185 to 196 THz.