# Process Development of Low-Loss LPCVD Silicon Nitride Waveguides on 8-Inch Wafer
## 4 Device Characterization and Discussion  
After the fabrication, we characterized the basic performance of the waveguide, including propagation loss and bending loss. The test platform is shown in Figure 12c; the light source was a tunable laser covering the whole C-band (model: Keysight 8164B). A lensed single-mode fiber was used to couple the light from the laser into the waveguide, and another lensed fiber was used to couple the light out of the waveguide to a power detector. A cutback structure, as shown in Figure 12d, was used to measure the propagation loss. Considering the low loss of the Si\({}_{3}\)N\({}_{4}\) waveguide, we increased the length of  
Figure 11: (**a**) XSEM image of the waveguide with measured dimensions, (**b**) XSEM image of two adjacent waveguides observed after cutting through with FIB, (**c**) SEM image of the tip after CMP, (**d**) SEM image of the waveguide after CMP. Figure 10: SEM images of the waveguide structure after HM removal. (**a**) partially removed HM, (**b**) fully removed HM. each waveguide in the cutback structures; the parameters needed to accumulate enough propagation loss are shown in Table 2. Due to similar considerations, a set of waveguides with a different number of bends are used to measure the bending loss; parameters are shown in Table 3.  
For measurement of the propagation loss and bending of the Si\({}_{3}\)N\({}_{4}\) waveguide, the transmission loss of the Si\({}_{3}\)N\({}_{4}\) waveguides was scanned in the wavelength range 1490-1580 nm. The wavelength response of the cutback structures is shown in Figure 13a. Figure 12: (**a**) Finished 8-inch wafers, (**b**) silicon nitride chips, (**c**) semi-automatic test platform, (**d**) layout design of the spiral and bending waveguides for the test. \begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Length** & **Radius of Bending Waveguide** \\ \hline waveguide 1 & 12.72 cm & R = 100 \(\mu\)m \\ waveguide 2 & 17.62 cm & R = 100 \(\mu\)m \\ waveguide 3 & 22.52 cm & R = 100 \(\mu\)m \\ waveguide 4 & 27.42 cm & R = 100 \(\mu\)m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Structural parameters of the cutback structure. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Radius of Bending Waveguide** & **No.** & **Number of 90\({}^{\circ}\) Arcs** \\ \hline  & bend21 & 24 \\
50 \(\mu\)m & bend2 & 72 \\  & bend3 & 120 \\  & bend4 & 168 \\ \hline  & bend1 & 24 \\
80 \(\mu\)m & bend2 & 72 \\  & bend3 & 120 \\  & bend4 & 168 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Structural parameters of the bending waveguides. Through linear fitting of the data, the propagation loss in the same wavelength range can be calculated as the results shown in Figure 13b. It can be seen that the propagation loss of the waveguide has a peak of 1.33 dB/cm at a wavelength of around 1500 nm, which is due to the absorption loss of the Si-H bonding in the Si\({}_{3}\)N\({}_{4}\). To reduce this loss, annealing with a higher temperature and longer time is needed to further repel the H element in the film. As the wavelength moves away from the absorption band, the propagation loss reduces quickly to transmission of 0.157 dB/cm at 1550 nm and 0.06 dB/cm at 1580 nm. This is a remarkable value compared to other reported works, as summarized in Table 4.  
We further measured the bending loss by fitting the transmission spectrum of waveguides with a different number of bends. The results are shown in Figure 13c,d for two different bending radii; the bending loss is 0.0095 dB per 90\({}^{\circ}\) bending waveguide for a radius of 80 \(\upmu\)m and 0.013 dB even for a radius of 50 \(\upmu\)m, which is very close to the previous simulations.