*4.3. Effect of H2O<sup>2</sup> Concentrations on the Polishing Performance of Tungsten*

*4.3. Effect of H2O2 Concentrations on the Polishing Performance of Tungsten* During the polishing process, a passivation film is formed on the tungsten surface due to oxidizing agents. Because of the low hardness of oxide and the weak interface that exists between tungsten and oxide, the passive film is easily removed by the mechanical action of abrasives [28]. The concentrations of H2O2 can significantly affect the generation rate of the passivation film on the tungsten surface, which in turn affects the polishing performance of C‐SDP [29]. The effects of H2O2 concentrations on material removal rate and surface roughness of tungsten are shown in Figure 8. In the experiments, the pH val‐ ues of polishing slurries with different H2O2 concentrations were all 9. As shown in Figure 8a, with the increase of H2O2 concentration, the *MRR* of tungsten increased continuously, from 9.71 μm/h to 34.95 μm/h. After the concentration of H2O2 increased, the chemical corrosion effect of polishing slurry on the tungsten surface material was enhanced, which could generate faster or thicker soft passivation films. As shown in Figure 8b, the surface roughness of tungsten decreased firstly and then increased, and its surface roughness was the lowest when H2O2 concentration was 1 vol.%. As the H2O2 concentration increased from 0 to 1 vol.%, *R*<sup>a</sup> decreased from 2.35 nm to 1.87 nm. *R*<sup>a</sup> began to increase, reaching During the polishing process, a passivation film is formed on the tungsten surface due to oxidizing agents. Because of the low hardness of oxide and the weak interface that exists between tungsten and oxide, the passive film is easily removed by the mechanical action of abrasives [28]. The concentrations of H2O<sup>2</sup> can significantly affect the generation rate of the passivation film on the tungsten surface, which in turn affects the polishing performance of C-SDP [29]. The effects of H2O<sup>2</sup> concentrations on material removal rate and surface roughness of tungsten are shown in Figure 8. In the experiments, the pH values of polishing slurries with different H2O<sup>2</sup> concentrations were all 9. As shown in Figure 8a, with the increase of H2O<sup>2</sup> concentration, the *MRR* of tungsten increased continuously, from 9.71 µm/h to 34.95 µm/h. After the concentration of H2O<sup>2</sup> increased, the chemical corrosion effect of polishing slurry on the tungsten surface material was enhanced, which could generate faster or thicker soft passivation films. As shown in Figure 8b, the surface roughness of tungsten decreased firstly and then increased, and its surface roughness was the lowest when H2O<sup>2</sup> concentration was 1 vol.%. As the H2O<sup>2</sup> concentration increased from 0 to 1 vol.%, *R*<sup>a</sup> decreased from 2.35 nm to 1.87 nm. *R*<sup>a</sup> began to increase, reaching 4.14 nm when the H2O<sup>2</sup> concentration was 2 vol.%, since the H2O<sup>2</sup> concentration continued to increase. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 9 of 17

**Figure 8.** Effect of H2O2 concentrations on removal rate and surface roughness: (**a**) material removal rate; (**b**) surface roughness. **Figure 8.** Effect of H2O<sup>2</sup> concentrations on removal rate and surface roughness: (**a**) material removal rate; (**b**) surface roughness.

Figure 9 shows the surface morphologies of tungsten after polishing with different H2O2 concentrations. Compared with the different polishing slurries without H2O2, the tungsten surface became smoother after adding 1 vol.% H2O2. However, when the con‐ centration of H2O2 was higher than 1 vol.%, the surface quality of tungsten became slightly deteriorated due to excessive corrosion. Figure 9 shows the surface morphologies of tungsten after polishing with different H2O<sup>2</sup> concentrations. Compared with the different polishing slurries without H2O2, the tungsten surface became smoother after adding 1 vol.% H2O2. However, when the concentration of H2O<sup>2</sup> was higher than 1 vol.%, the surface quality of tungsten became slightly deteriorated due to excessive corrosion.

**Figure 9.** Surface morphologies of polished tungsten under different H2O2 concentrations: (**a**) 0

Figure 10 shows the surface morphologies of polished tungsten under different H2O2 concentrations. As shown in Figure 10c, when the H2O2 concentration was 2 vol.%, a slight orange peel phenomenon and obvious micropores appeared on the tungsten surface. This phenomenon may be related to the fact that the oxidizing property of polishing slurry was too powerful, so that the mechanical removal of abrasives could not keep up with the formation rate of the passivation film. Moreover, the micropores existing in the tungsten were further enlarged. Tungsten samples before and after polishing are shown in Figure 11. Under the optimum parameters of pH = 9 and 1 vol.% H2O2 concentration, the surface

vol.%; (**b**) 0.1 vol.%; (**c**) 0.5 vol.%; (**d**) 1.0 vol.%; (**e**) 1.5 vol.%; (**f**) 2.0 vol.%.

rate; (**b**) surface roughness.

deteriorated due to excessive corrosion.

**Figure 9.** Surface morphologies of polished tungsten under different H2O2 concentrations: (**a**) 0 vol.%; (**b**) 0.1 vol.%; (**c**) 0.5 vol.%; (**d**) 1.0 vol.%; (**e**) 1.5 vol.%; (**f**) 2.0 vol.%. **Figure 9.** Surface morphologies of polished tungsten under different H2O<sup>2</sup> concentrations: (**a**) 0 vol.%; (**b**) 0.1 vol.%; (**c**) 0.5 vol.%; (**d**) 1.0 vol.%; (**e**) 1.5 vol.%; (**f**) 2.0 vol.%.

**Figure 8.** Effect of H2O2 concentrations on removal rate and surface roughness: (**a**) material removal

Figure 9 shows the surface morphologies of tungsten after polishing with different H2O2 concentrations. Compared with the different polishing slurries without H2O2, the tungsten surface became smoother after adding 1 vol.% H2O2. However, when the con‐ centration of H2O2 was higher than 1 vol.%, the surface quality of tungsten became slightly

Figure 10 shows the surface morphologies of polished tungsten under different H2O2 concentrations. As shown in Figure 10c, when the H2O2 concentration was 2 vol.%, a slight orange peel phenomenon and obvious micropores appeared on the tungsten surface. This phenomenon may be related to the fact that the oxidizing property of polishing slurry was too powerful, so that the mechanical removal of abrasives could not keep up with the formation rate of the passivation film. Moreover, the micropores existing in the tungsten were further enlarged. Tungsten samples before and after polishing are shown in Figure 11. Under the optimum parameters of pH = 9 and 1 vol.% H2O2 concentration, the surface Figure 10 shows the surface morphologies of polished tungsten under different H2O<sup>2</sup> concentrations. As shown in Figure 10c, when the H2O<sup>2</sup> concentration was 2 vol.%, a slight orange peel phenomenon and obvious micropores appeared on the tungsten surface. This phenomenon may be related to the fact that the oxidizing property of polishing slurry was too powerful, so that the mechanical removal of abrasives could not keep up with the formation rate of the passivation film. Moreover, the micropores existing in the tungsten were further enlarged. Tungsten samples before and after polishing are shown in Figure 11. Under the optimum parameters of pH = 9 and 1 vol.% H2O<sup>2</sup> concentration, the surface of the tungsten sample after C-SDP achieved a mirror effect without obvious scratches, pits, or other defects. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 10 of 17 of the tungsten sample after C‐SDP achieved a mirror effect without obvious scratches, pits, or other defects.

**Figure 10.** Surface morphologies of polished tungsten under different H2O2 concentrations: (**a**) 1.0 vol.% H2O2; (**b**) 1.5 vol.% H2O2; (**c**) 2.0 vol.% H2O2. **Figure 10.** Surface morphologies of polished tungsten under different H2O<sup>2</sup> concentrations: (**a**) 1.0 vol.% H2O<sup>2</sup> ; (**b**) 1.5 vol.% H2O<sup>2</sup> ; (**c**) 2.0 vol.% H2O<sup>2</sup> .
