Low-Loss Hydrogen-Free SiNx Optical Waveguide Deposited by Reactive Sputtering on a Bulk Si Platform

This study reports a SiN
_x
channel optical waveguide on a SiO
₂
-covered bulk Si platform. The waveguide shows a low propagation loss of ∼0.6 dB/cm at 1550 and 1310 nm wavelengths. The SiN
_x
film is deposited by reactive sputtering and is free from N–H species, which cause optical absorption loss at ∼1520 nm. No post-deposition high-temperature annealing is required as an increase in sputtering temperature, from room temperature to a moderate temperature of 200 °C, is effective for low-loss propagation. Based on chemical and optical analyses, the quality of the sputtered film at 200 °C is comparable to that prepared by low-pressure chemical vapor deposition at a high temperature of 820 °C. Fabricated fundamental passive optical devices (a multi-mode interferometer splitter and a directional coupler) reveal reasonable device function. A ring resonator shows a thermally stable operation because of the small thermo-optic coefficient for the SiN
_x
, on the order of 10
⁻⁵
K
⁻¹
. The results indicate that the SiN
_x
film by reactive sputtering at 200 °C can be favorably applied for photonic integration concerning CMOS back-end processing and hybrid-integration processing with non-CMOS materials. A Si-on-insulator wafer is not necessarily used as the platform, but a low-cost bulk Si wafer is replaced depending on the application. The reduction in the SiO
₂
under-cladding thickness, as thin as 1.0 µm, is examined regarding the potential in multilayered interconnects.