## 3 Results and Discussion
Examining the slurry stability (i.e., no sedimentation of colloidal abrasives in a CMP slurry) among Fenton reaction catalysts such as ferric-sulfate, ferric oxalate, ferric-nitride, PDTA-Fe, ferric-citrate, and ferric cyanide, PDTA-Fe showed good slurry stability and a relatively high Ge\({}_{1}\)Se\({}_{4}\)Te\({}_{5}\) film surface polishing rate, as shown in Figures 2 and S3. Without the Fenton reaction in a CMP slurry (i.e., only using oxidizer H\({}_{2}\)O\({}_{2}\) of 1.0-wt%), the Ge\({}_{1}\)Se\({}_{4}\)Te\({}_{5}\) film surface polishing-rate was \(\sim\)27 nm/min. However, the Ge\({}_{1}\)Se\({}_{4}\)Te\({}_{5}\) film surface polishing rate increased considerably from 27 to 105 nm/min when the PDTA-Fe concentration increased from 0 to 0.05-wt%, as shown in Figure 2, indicating that the Fenton reaction significantly enhanced the polishing rate of the Ge\({}_{1}\)Se\({}_{4}\)Te\({}_{5}\) film surface. In addition, the Si\({}_{3}\)N\({}_{4}\) film surface polishing-rate slightly and exponentially increased from 1.3 to 6.0 nm/min, which should be reduced because the Si\({}_{3}\)N\({}_{4}\) film surface should be utilized as a polishing stopping layer. The dependency of the static corrosion rate (i.e., static etch rate) on the PDTA-Fe concentration was investigated after dipping Ge\({}_{1}\)Se\({}_{4}\)Te\({}_{5}\) films into slurries composed of 1.0-wt%, H\({}_{2}\)O\({}_{2}\) (i.e., oxidizer) of 1.0-wt%, PDFA-Fe (i.e., Fenton reaction catalyst) of 0, 0.01, 0.02, 0.03, 0.04, and 0.05-wt%, and HNO\({}_{3}\) (titrant) for 5 min at 70 \({}^{\circ}\)C, as shown in Figure 3. Note that the pad temperature of the Ge\({}_{1}\)Sb\({}_{4}\)Te\({}_{5}\) film CMP was \(\sim\)70 \({}^{\circ}\)C. As soon as PDTA-Fe 0.01-wt% was added to the slurry, the static corrosion rate rapidly decreased from 60.0 to 7.5 nm/min. Then, it slightly decreased from 8.8 to 3 nm when the PDTA-Fe concentration increased from 0.01 to 0.05-wt%. The SEM images of the Ge\({}_{1}\)Sb\({}_{4}\)Te\({}_{5}\) film surface after dipping into the slurries revealed the presence of corrosion-induced pits. Many of the corrosion pits of 10-50 \(\upmu\)m in size were found without the Fenton reaction (i.e., only using H\({}_{2}\)O\({}_{2}\)), as shown in the SEM image in Figure 3. The number and size of corrosion pits decreased when the PDTA-Fe concentration increased from 0 to 0.02-wt%, as shown in (i)-(iii) SEM images of Figure 3. When further increasing the PDTA-Fe concentration from 0.03 to 0.05-wt%, corrosion pits could not be found, as shown in (iv)-(vi) SEM images of Figure 3.  
Figure 1: Dependency of chemical composition of Ge\({}_{1}\)Sb\({}_{4}\)Te\({}_{5}\) film surface on oxidant (i.e., H\({}_{2}\)O\({}_{2}\)) concentration. (**a**) Ge 2p spectra, (**b**) Sb 3d spectra, and (**c**) Te 3d spectra. Figure 2: Dependency of Ge\({}_{1}\)Se\({}_{4}\)Te\({}_{5}\)-, and Si\({}_{3}\)N\({}_{4}\) films polishing-rate on ferric–ionic catalyst (i.e., PDTA–Fe) concentration. PDTA–Fe is decomposed of Fe\({}^{3+}\), NH\({}_{4}\)+, and [C\({}_{1}\)H\({}_{14}\)N\({}_{2}\)O\({}_{8}\)]\({}^{4-}\) in the acid (i.e., pH 2.0) slurry. The average diameter of colloidal silica abrasives was 70 nm, as shown in inset SEM image. In addition, the AFM images, having the scanning area of 5 \(\upmu\)m \(\times\) 5 \(\upmu\)m as shown in Figure 4, of the Ge\({}_{1}\)Sb\({}_{4}\)Te\({}_{5}\) film surface after dipping into the slurries were observed as a function of the PDTA-Fe concentration. Without the ferric-ionic catalyst, the surface roughness magnitude (i.e., root-mean-square (Rq)) of the Ge\({}_{1}\)Sb\({}_{4}\)Te\({}_{5}\) film surface was \(\sim\)3.021 nm. The Rq of the Ge\({}_{1}\)Sb\({}_{4}\)Te\({}_{5}\) film surface evidently decreased from 2.772 to 1.230 nm when the PDTA-Fe concentration was increased from 0.01 to 0.05-wt%.