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Silicon photonics is a very promising solution to achieve high-speed and energy-efficient optical interconnects at reduced cost of production. For several years now, Silicon photonic platforms have offered high performance passive and active compact components, with first 100 Gb/s silicon photonics commercial transceivers on the market. However, interferometric devices still remain sensitive to temperature changes. Silicon nitride appears as an appealing material for CMOS compatible, energy-efficient and cost-effective photonics. Its low optical index contrast with the SiO2 cladding provides low loss waveguide and a better tolerance to fabrication imperfections while its optical index is much less sensitive to temperature variations. In particular, the monolithic integration of multiple Si and SiN layers on the same platform provides a promising solution for Coarse Wavelength Division Multiplexing (CWDM) transceiver applications for which thermal stability is essential. We report here on a 200mm-CMOS-compatible platform where SiN is co-integrated with Si to benefit from the advantages of both materials. We present the fabrication flow of this enhanced platform and we show the wafer scale characterization of its passive Si, SiN and hybrid components, assessing their performances in term of 4-wavelength CWDM application in the O-band.
Building on the CEA-LETI silicon photonics fabrication line, we present the monolithic integration of a low stress PECVD SiN deposited at 300°C, which make it compatible with the standard SOI platform with doped active devices. The success of the integration is confirmed by the preserved performances of Si components (waveguide, bend and fiber grating coupler losses) as well as propagation losses as low as 0.8dB/cm for a single mode SiN waveguide. Furthermore, characterizing micro-ring resonators, the resonance shift with temperature allows us to extract a 10-fold reduction of the SiN thermo-optic coefficient compared to Si.
An essential building block of the Si-SiN platform is the Si-SiN interlayer transition. We present a complete study of such transitions with both simulation and experimental data of various geometries, and we demonstrate state-of-the-art insertion losses of 0.09 ± 0.01 dB over the O-band for the TE mode at the wafer scale. We show as well the realization of hybrid Si-SiN grating couplers, compatible with a standard packaging fiber angle of 8°. In such couplers, the main SiN grating is combined with a Si grating placed underneath and a longitudinal shift between gratings allows the tuning of the coupler’s directionality. Consequently, we achieve a dramatic improvement in terms of bandwidth with respect to the standard all Si fiber couplers, going from a 23nm -1dB bandwidth for Si couplers to more than 50nm for hybrid couplers. These wideband hybrid couplers are key for CWDM where a flat transmission spectrum is needed over the O-band. Finally, to complete the CWDM components review, we present SiN Echelles grating (de)multiplexer for 4-channels CWDM in the O-band showing quasi-absolute thermal insensitiveness and low insertion losses.
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