## 2 Fabrication Process and Waveguide Performances
### SiN Waveguide Optical Characterization  
We characterized the fabricated SiN structures in terms of propagation losses, bend losses and thermal properties. In addition, we obtained state-of-the-art performances for Si-SiN interlayer transition, making it possible to fully exploit the potential of the multilayer Si-SiN platform. SiN waveguides and structures are characterized on a semi-automatic 12-inch probe station at the wafer level with a tunable laser source (Tunics T100S, Yenista, Lannion, France) and its complementary photodetector (CT400, Yenista, Lannion, France) with pm resolution. A fiber-to-fiber setup is used for propagation loss measurements. Unless mentioned otherwise, devices are characterized through surface grating couplers with a scattering angle of 11.5\({}^{\circ}\) (8\({}^{\circ}\) in the SiO\({}_{2}\) cladding), which is compatible with packaging standards. * SiN waveguide propagation and bend losses The propagation and bend losses of Si and SiN single-mode waveguides are summarized in Table 1. Propagation losses are extracted from the transmission of 20-, 40- and 60-mm-long spiraled waveguides, de-embedded from the grating coupler transmission, on up to 45 dies for each wafer. Notably, thanks to a measurement conducted at the 200 mm wafer level, we are able to extract statistical data which is reported in Table 1. In Si, both strip and rib losses (3.6 and 1.5 dB/cm respectively) are consistent with the previously reported silicon waveguides characteristics [2], and we can conclude that adding the SiN layer does not affect the performance of the Si waveguides. As expected, propagation losses are much lower in SiN thanks to a lower sensitivity to waveguide roughness, itself resulting from a lower optical index contrast with the SiO\({}_{2}\) cladding compared to Si. We obtain 0.8 dB/cm in the O-band for the PECVD SiNx, 1.5 dB/cm in the C-band for 750 nm thick LPCVD Si\({}_{3}\)N\({}_{4}\) and 1 (1.5) dB/cm at 940 nm for 300-nm-thick Si\({}_{3}\)N\({}_{4}\) (SiNx respectively). The higher propagation loss measured in the C-band for LPCVD Si\({}_{3}\)N\({}_{4}\) is probably due to remaining N-H bonds causing absorption, despite a deposition process tailored to minimize the amount of residual hydrogen atoms. Due to a low mode confinement (itself due to the low optical index), SiN bend waveguides require a higher bend radius compared to Si to minimize the bend losses. For the 600-nm-thick PECVD SiNx (the lowest optical index), we measure 0.06, 0.03 and 0.015 dB/90\({}^{\circ}\) bend for radii of 20, 30 and 40 \(\upmu\)m, respectively. In the 300-nm-thick LPCVD Si\({}_{3}\)N\({}_{4}\), thanks to a higher optical index, losses are below 0.01 dB for a 15 \(\upmu\)m radius bend. As a comparison, in Si, we measure 90\({}^{\circ}\) bend losses below 0.03 dB for both 30 \(\upmu\)m radius bend rib waveguide and 9 \(\upmu\)m radius bend strip waveguide. * PECVD SiN thermo-optic coefficient The thermo-optic coefficient of SiNx is estimated by measuring the wavelength resonance thermal shift of a ring resonator. The resonance spectra acquired at temperatures from 25 \({}^{\circ}\)C to 80 \({}^{\circ}\)C of a 30 \(\upmu\)m radius SiNx ring coupled to an adjacent bus waveguide are shown in Figure 2a. The resonance spectral shift is 14 pm/K, which translates into a thermo-optic coefficient of 1.7 \(\times\) 10\({}^{-5}\) K\({}^{-1}\). This value is indeed more than one order of magnitude lower than for silicon in telecommunication bands, which confirms the relevance of SiN for applications where thermal stability is essential, as for the (de-)multiplexer. * Si-SiN interlayer transition with PECVD SiN To fully exploit a multi-layer Si-SiN platform, there is a need for a compact, efficient and broadband interlayer transition between Si and SiN [20, 35, 36, 37]. These requirements are well satisfied using adiabatic taper interlayer transitions. The principle of the interlayer transition is to reduce the