# An Exploration into Damage Repair and Manufacturing Technology of Photomask Glass Substrates
## 5 Discussion  
Chemical mechanical polishing (CMP) is usually used in the manufacture of photomask quartz glass substrates. However, this technology is prone to induce random scratches on the surface, which would seriously affect the imaging quality of photomask substrates. However, after the surface after CMP was measured by an ultra-smooth surface laser scattering quality detection device, it can be found that there were basically no scratches on the surface. This may be explained that a hydrolyzed layer formed on the surface to cover these scratches and defects. Therefore, the surface hydrolysis layer was fully pickled by HF acid to obtain a surface with many random scratches, as shown in Figure 9. Moreover, the pickling technology could dissolve the hydrolysis layer and expose the subsurface, which significantly increased the surface roughness, as shown in Figure 10. Therefore, it was required to remove scratches and damage on the surface after CMP. Based on that, magnetorheological polishing technology was introduced to perform non-destructive machining of the photomask substrate surface.  
In order to explore the feasibility of magnetorheological polishing technology, the surface roughness change of photomask glass substrates was measured after magnetorhe  
Figure 16: Scratch and surface roughness evolution results under uniform scanning removal based on the magnetorheological rotation angle. (**a**) Scratch Evolution Results under the Uniform Scanning Removal of 20 nm Based on the Magnetorheological Rotation Angle; (**b**) Scratch Evolution Results under the Uniform Scanning Removal of 50 nm Based on the Magnetorheological Rotation Angle; (**c**) Scratch Evolution Results under the Uniform Scanning Removal of 100 nm Based on the Magnetorheological Rotation Angle.  
ological polishing. As shown in Figure 10, the surface roughness of photomask glass substrates decreased, instead of getting deteriorated. Hence, magnetorheological polishing can meet the surface roughness requirements of photomask substrates. In addition, the depth of surface scratches was also measured after pickling, with the depth ranging from 0 nm to 80 nm. Hence, the surface scratches can be completely removed by magnetorheological uniform scanning repair. Meanwhile, due to the large size of photomask glass substrates and higher demands, it is necessary to improve the removal efficiency of magnetorheological polishing. Thus, an array-type magnetorheological polishing device newly developed by our laboratory was used for processing. Due to the fact that the magnetorheological uniform scanning would produce surface mid-frequency ripple errors, as shown in Figure 13, the newly developed magnetorheological rotation angle polishing method can be used to suppress these errors, thus obtaining the photomask glass substrates without ripples on the surface.  
Firstly, the array-type magnetorheological polishing technology was adopted to repair the initial surface. As shown in Figure 12, the PV value of the surface shape increased from 0.657 wavelengths to 0.336 wavelengths. This demonstrated that the array-type magnetorheological polishing technology was effective in repairing the surface shape of photomask substrates, and this technology could improve the quality of photomask substrates. Secondly, due to the existence of mid-frequency ripple errors on the repaired surface, the uniform removal of 100 nm on the surface was performed based on the removal function rotation angle, and the mid-frequency ripple errors on the surface basically disappeared, as shown in Figure 14. Meanwhile, after the uniform removal of 100 nm on the surface was performed, the scratches on the surface were basically eliminated when observing the evolution of the dark field and scratches on the surface treated by uniform removal based on the magnetorheological rotation angle, thus meeting the manufacturing requirements of photomask glass substrates.