A new material platform of Si photonics for implementing architecture of dense wavelength division multiplexing on Si...

A new materials group to implement dense wavelength division multiplexing (DWDM) in Si photonics is proposed. A large thermo-optic (TO) coefficient of Si malfunctions multiplexer/demultiplexer (MUX/DEMUX) on a chip under thermal fluctuation, and thus DWDM implementation, has been one of the most challenging targets in Si photonics. The present study specifies an optical materials group for DWDM by a systematic survey of their TO coefficients and refractive indices. The group is classified as mid-index contrast optics (MiDex) materials, and non-stoichiometric silicon nitride (SiN_x) is chosen to demonstrate its significant thermal stability. The TO coefficient of non-stoichiometric SiN_x is precisely measured in the temperature range 24-76 °C using the SiN_x rings prepared by two methods: chemical vapor deposition (CVD) and physical vapor deposition (PVD). The CVD-SiN_x ring reveals nearly the same TO coefficient reported for stoichiometric CVD-Si₃N₄, while the value for the PVD-SiN_x ring is slightly higher. Both SiN_x rings lock their resonance frequencies within 100 GHz in this temperature range. Since CVD-SiN_x needs a high temperature annealing to reduce N-H bond absorption, it is concluded that PVD-SiN_x is suited as a MiDex material introduced in the CMOS back-end-of-line. Further stabilization is required, considering the crosstalk between two channels; a 'silicone' polymer is employed to compensate for the temperature fluctuation using its negative TO coefficient, called athermalization. This demonstrates that the resonance of these SiN_x rings is locked within 50 GHz at the same temperature range in the wavelength range 1460-1620 nm (the so-called S, C, and L bands in optical fiber communication networks). A further survey on the MiDex materials strongly suggests that Al₂O₃, Ga₂O₃ Ta₂O₅, HfO₂ and their alloys should provide even more stable platforms for DWDM implementation in MiDex photonics. It is discussed that the MiDex photonics will find various applications such as medical and environmental sensing and in-vehicle data-communication.