## 3 Results and Discussion
### Removal of Subsurface Impurity Defects by HF Acid Etching
#### 3.3.2 Surface Quality and Impurity Content of Fused Silica Etched by HF Acid  
As the HF acid etching proceeds, not only the impurity defects included in the re-deposition layers are removed, but the subsurface cracks and scratches are blunted [26; 27]. The optical quality of etched fused silica surfaces would have great changes and the surface roughness value is an important index in evaluating the quality of optical surface. Thus, the evolution of surface roughness value as the HF acid etching proceeds is investigated using the white light interferometer and profilometer to check the effect of acid etching on the surface quality. Figure 11 shows the tested evolution of surface roughness with respect to the etching time (or etching depth) for fused silica etched with HF-based solvent (5% HF and 10% NH\({}_{4}\)F). It is shown that the surface roughness value Ra experiences three changing phases as the HF etching proceeds. During phase 1 with the etching depth smaller than 500 nm, the Ra ascends from 2.6 nm to 4.2 nm. The decrease of surface quality (rise of roughness) arises from the appearance of subsurface plastic scratches and cracks as the topmost re-deposition layers are removed. During phase 2 with the etching depth ranging from 0.5 \(\upmu\)m to 2.15 \(\upmu\)m, the Ra value descends from 4.2 nm to 3.3 nm due to the passivation of subsurface damage under the assistance of acid etchants. The subsurface cracks and scratches are etched and blunted, resulting in the improvement of surface quality. The surface roughness increases again during phase 3 for etching depth larger than 2.15 \(\upmu\)m. The reproduction of scratches and opening of cracks with the increase of etching depth should be responsible for this results. Figure 12 presents the evolution of surface morphology of etched fused silica with various etching periods, which are tested by a 3D stereo microscope and white light interfer  
Figure 11: The evolution of surface roughness of etched fused silica optics as the acid etching proceeds and its corresponding etching depths. that with the increase of etching period, the subsurface defects (e.g., scratches, cracks, pits and so on) emerge and the defect population become larger. This is the further evidence for the great decrease of surface quality during phase 3 as shown in Figure 11. It should be noted that with a mixed etchant of 5% HF and 10% NH\({}_{4}\)F, as applied in Figure 12, the re-deposition layer is totally removed when the etching period is 3 min. Thus, all of the tested surface morphology in Figure 12 corresponds to phase 3 shown in Figure 11.  
To quantitatively evaluate the effectiveness of HF-based acid etching in removing the subsurface impurity defects, the comparison of defect contents on fused silica surfaces before and after acid etching is investigated using energy dispersive X-ray spectrometer (EDS). Figure 13 shows the comparison of defect content and the evolution of Ce content with the increase of etching time. For the defect content before acid etching as shown in Figure 13a, one can see that the contents of Si and O are two types of elements most distributed on the polished silica surface. They originate from the body material of fused silica (SiO\({}_{2}\)) and satisfy their proportional relationship of mass fraction. Besides Si and O, other impurity defects (e.g., Ce, Fe, Cu and Se) are also observed with the maximum defect content of 7.45 wt % for Ce impurity. The results obtained in Section 3.1 and 3.2 indicate that the CeO\({}_{2}\) impurity defect plays the dominant role in lowering the laser damage resistance of polished fused silica optics. Hence, we take Ce defect as an example to investigate its elimination when the SSD layers of silica optics are etched with HF etchant. The tested impurity contents on etched silica surfaces are presented in Figure 13b. It is shown that the contents of most of the impurity defects are effectively lowered by acid etching, especially for the Ce impurity defect, whose content largely descends from 7.45 wt % to 3.24 wt %.