*3.2. Experimental Measurements for the SWR of SOI Waveguide Sample*

In this work, we selected the commercial CLSM tool–LSM710 that was produced by the ZEISS company. Then, the device sample for the CLSM measurements was selected by selecting an SOI-waveguide sample having a BOX layer of 2.0 and 1.5 μm, where the waveguide ribs having a width of 4.0 μm and a height of 0.5 μm were etched with the advanced ion etching—the inductively coupled plasma (ICP) etching technique. The CLSM measurements only showed the existing isotropic roughness, then we obtained the reconstructed image of the waveguide SWR, as shown in Figure 3a, and by scanning the measured area along a vertical direction at the *x* = 100 nm position, a line-scanning roughness was obtained as *Pz* = 20 nm from the data display. Then, in the same manner, we measured the other five lines with every 100 nm position change along the vertical direction for two reconstructed images (the total height of the etched SOI rib was 500 nm), and then the total 6 measured values are depicted in Figure 3b, which were in the range of 16–23 nm roughness and gave rise to an average isotropic SWR of 20.33 nm. First, we needed to clarify that the *x* coordinate in Figure 3 was the traveling direction *z* of the optical beam, the *z* and *y* coordinates of Figure 3 were the coordinates *x* and *z* of Figure 1, respectively. Then, we also noticed that the SWR values of the SOI waveguides were only in the range of a few tens of nanometers, which were much smaller than the CLSM-measured values of the silica waveguide etched by the traditional reactive ion etching (RIE) technique owing to the advanced etching technique ICP and the etched material of silicon [16,17]. Accordingly, the disparities of SWR distributions between the *x* and *y* coordinates existed.

Here, what needed to be clarified was that the measurement accuracy was 10 nm in the above measurements, but for the roughness smaller than 10 nm, the accuracy needed to be improved, which would be realized with the improvement of the laser FWHM values in both the lateral and axial directions. In addition, as shown in Figure 2a, in one CLSM measurement, we scanned the device sample having four equal size waveguides to acquire the data, but as shown in Figure 3a, only one waveguide was selected from the reconstructed CLSM image to analyze and measure the roughness. In order to analyze the SWR uniformity of one channel, several sections could be selected along the waveguide to obtain the average SWR value of each section, and then all the roughness results of all the sections could be compared. In the same manner, in order to analyze the SWR uniformity among all the four measured waveguides, the CLSM measured results of SWR for all the four waveguides could be carried out, and then all the roughness results could be compared.

**Figure 3.** CLSM Measurements and data process of SOI waveguide SWR: (**a**) is the reconstructed image of CLSM measurement, (**b**) is the isotropic roughness distribution of five measured spots.
