In this communication, we report on the design, fabrication, and testing of silicon-on-insulator (SOI) and silicon-nitrideon- insulator (SiNOI) photonic circuits for nonlinear and quantum optics applications. As recently demonstrated, the generation of correlated photons on Si platforms can be used for quantum cryptography and quantum computing. Concerning SiNOI waveguides, Kerr frequency combs have been proposed in many applications, such as atomic clocks, on-chip spectroscopy, and terabit coherent communications. Silicon is an attractive platforms for correlated photons sources because of its high nonlinearity, they can have several modes in telecom band with sharp line widths (tens of μeV) and its inherent complementary metal-oxide-semiconductor (CMOS) compatibility. Moreover, the SiNOI is an attractive platform for Kerr comb generation due to their large bandgap and consequently the low two-photon absorption in the telecommunication band. Furthermore, in all the previous SiNOI-based frequency combs, the silicon nitride film undergoes long and high-temperature annealing to reduce the absorption in the telecommunication band caused by the dangling N-H bonds, thus making such annealed Si3N4 films non-CMOS compatible. However, both in the case of correlated photons pairs generation and Kerr frequency combs, the source efficiency is related to the quality factor (Q), so that a high-Q resonator is required to get highly-efficient sources. Authors report here about the fabrication and the characterization of annealing-free CMOS-compatible SiNOI- and hydrogen-annealed silicon-based waveguides and microresonators featuring ultra-low losses (e.g., 0.6 dB/cm for single-mode Si waveguides) that can be used, respectively, as efficient sources for Kerr combs and correlated photon pairs sources.
The superior confinement of light provided by the high refractive index contrast in Si/SiO2 waveguides allows the use of sub-micron photonic waveguides. However, when downscaling waveguides to sub-micron dimensions, propagation losses become dominated by sidewall roughness scattering. In a previous study, we have shown that hydrogen annealing after waveguide patterning yielded smooth silicon sidewalls. Our optimized silicon patterning process flow allowed us to reduce the sidewall roughness down to 0.25 nm (1σ) while maintaining rectangular Strip waveguides. As a result, record low optical losses of less than 1 dB/cm were measured at telecom wavelengths for waveguides with dimensions larger than 350 nm. With Rib waveguides, losses are expected to be even lower. However, in this case the Si reflow during the H2 anneal leads to the formation of a foot at the bottom of the structure and to a rounding of its top. A compromise is thus to be found between low losses and conservation of the rectangular shape of the Rib waveguide. This work proposes to investigate the impact of temperature and duration of the H2 anneal on the Rib profile, sidewalls roughness and optical performances. The impact of a Si/SiO2 interface is also studied. The introduction of H2 thermal annealing allows to obtain very low losses of 0.5 dB/cm at 1310 nm wavelength for waveguide dimensions of 300-400 nm, but it comes along an increase of the pattern bottom width of 41%, with a final bottom width of 502 nm.
We report the successful fabrication of low-loss sub-micrometric Silicon-On-Insulator strip waveguides for on-chips links. Several strategies including post-lithography treatment, and post-Silicon smoothening techniques such as thermal oxidation and hydrogen annealing have been investigated to smoothen the waveguide sidewalls, as roughness is the major source of transmission losses. An extremely low silicon line edge roughness of 0.75nm is obtained with the optimized process flow combining resist mask Si patterning and hydrogen annealing at 850°C. As a result, record low optical losses of less than 0.5dB/cm are measured at 1310nm for waveguide dimensions superior to 500nm. They range from 2dB/cm to 0.8dB/cm for 300-400nm wide waveguides. Those results are to our knowledge the best ever published for a 1310nm wavelength.
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