# An Exploration into Damage Repair and Manufacturing Technology of Photomask Glass Substrates
## 4 Damage Detection and Repair of Photomask Quartz Glass Substrates  
### Damage Detection of Photomask Quartz Glass Substrates  
In this experiment, the photomask quartz glass substrates were three-inch glass substrates purchased from Shenzhen Precise Trading Co., Ltd., (Shenzhen, China) as shown in Figure 8. These photomask quartz glass substrates were treated by CMP at first. An ultra-smooth surface laser scattering quality detection device was used to measure the surface damage and defects, as shown in Figure 9a. It can be seen in the dark field test picture that the surface was basically free of defects, due to the fact that the surface hydrolysis layer completely covered the damage. Subsequently, the surface of photomask quartz glass substrates was fully etched by the hydrofluoric (HF) acid etching method, so that the surface hydrolysis layer can be completely dissolved. Then, the dark field test was performed on the surface, as shown in Figure 9b. It can be seen that there were many random scratches on the surface, which would seriously affect the imaging quality of photomask glass substrates. Therefore, it was required to remove scratches and damage on the surface after CMP. Figure 8: Three-inch photomask glass substrate. Figure 9: Dark field test results of surface damage and defects measured by an ultra-smooth surface laser scattering quality detection device. (**a**) Initial Surface Dark Field Test Results of Components after CMP; (**b**) Surface Dark Field Test Results after Sufficient Surface Hydrolysis. In addition, a white light interferometer was adopted to test the surface roughness before and after pickling and magnetorheological polishing, as shown in Figure 10. Figure 10a presents the surface roughness before pickling, Figure 10b presents the surface roughness of the non-damaged area after pickling, Figure 10c presents the surface roughness after magnetorheological uniform scanning, and Figure 10d presents the surface roughness of the scratched area after pickling. The scratch morphology after removing the hydrolysis layer can be seen from the results of Figure 10d. The scratch depth was about 0.05 \(\upmu\)m, which would seriously affect the imaging quality of photomask glass substrates. Hence, it is necessary to introduce magnetorheological processing technology to remove surface damage. Repair and Manufacturing Process of Photomask Quartz Glass Substrates Based on Array-Type Magnetorheological Polishing Technology  
Firstly, a vertical interferometer was employed to measure the surface shape of the photomask glass substrate, as shown in Figure 11a. Then, an array-type magnetorheological device was utilized to modify the surface shape. The removal function for surface shape modification is shown in Figure 12. The removal efficiency was approximately doubled  
Figure 10: Test results of surface roughness before and after pickling and magnetorheological polishing. (**a**) Surface Roughness of Initial Components; (**b**) Surface Roughness of Undamaged Area after Pickling; (**c**) Surface Roughness after Magnetorheological Uniform Scanning; (**d**) Surface Roughness of Scratched Area after Pickling. compared with the original magnetorheological polishing efficiency based on a single ribbon. The repaired surface shape is shown in Figure 11b. It can be seen that the array-type magnetorheological repair can quickly and effectively reduce the surface-shape error of photomask glass substrates. However, there would be mid-frequency ripple errors caused by magnetorheological polishing, as shown in Figure 13 (the sampling frequency band was 0.8-1.6 mm\({}^{-1}\)). Thus, it was required to use the removal function rotation angle to uniformly remove the sub-surface damage and the mid-frequency ripple error caused by surface shape modification. Figure 11: Surface-shape error results before and after magnetorheological polishing repair. (**a**) Surface-shape Error of Initial Components; (**b**) Surface-shape Error of Components after Magne-torheological Repair.