Obtaining Atomically Smooth 4H–SiC (0001) Surface by Controlling Balance Between Anodizing and Polishing in Electrochemical Mechanical Polishing
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Single-crystal 4H–SiC is a promising next-generation semiconductor material for high-power and low-loss power devices. Electrochemical mechanical polishing (ECMP) is a very promising polishing technique for the manufacture of SiC wafers owing to its high efficiency and low cost. In this study, the effects of the balance between the anodic oxidation rate and the polishing rate of the oxide layer on the polishing performance of slurryless ECMP were studied in an attempt to obtain an atomically smooth surface efficiently. The polishing performance of ECMP was evaluated from the viewpoints of surface roughness, residual oxide, and material removal rate. It was found that the balance between the anodic oxidation rate and the polishing rate of the oxide layer strongly affects the surface roughness; the polishing rate of the oxide layer should be greater than the anodic oxidation rate to obtain an atomically smooth surface. By controlling this balance at a current density of 10 mA/cm2, we were able to decrease the surface roughness of a diamond-lapped 4H–SiC (0001) surface from Sq 4.290 to 0.577 nm and obtained a high material removal rate of about 10 μm/h. This study provides a promising way of obtaining atomically flat surfaces by slurryless ECMP.
KeywordsSiC Anodic oxidation Electrochemical mechanical polishing Slurryless Anodizing and polishing balance High efficiency
Single-crystal 4H–SiC is a promising next-generation semiconductor material that can improve the energy conversion efficiency and reliability of electronic devices owing to its excellent electronic and thermal properties. Therefore, it is widely used in the fields of photovoltaics , hybrid electric vehicles [2, 3], high-power applications [4, 5], and so forth. However, the difficulty of manufacturing SiC wafers has limited the spread of use of SiC electronic devices. Many polishing techniques, such as chemical mechanical polishing (CMP) , plasma-assisted polishing (PAP) , catalyst-referred etching (CARE) [8, 9], mechanical chemical polishing (MCP) , and UV-assisted polishing , have been developed to polish SiC wafers, but their material removal rate (MRR) is unsatisfactory for the industrial production of SiC wafers. Furthermore, the cost and environmental problems associated with currently industrially used CMP limit its application .
Electrochemical mechanical polishing (ECMP), which combines surface anodic oxidation and mechanical polishing of the oxide layer, has been proposed for the highly efficient polishing of SiC wafers [13, 14, 15]. The application of ECMP to 4H–SiC wafers was first attempted by Li et al.  using a KNO3 and H2O2 solution as an electrolyte and a silica slurry as a polishing medium. Large-area flattening of a SiC (0001) surface was obtained by both two-step and simultaneous ECMP, but many etch pits were generated on the polished surface that significantly increased the surface roughness, and additional hydrogen etching was applied to finish the surface to atomic-scale roughness. In our previous study , a ceria slurry was used as both an electrolyte and a polishing medium. A damage-free surface was obtained, but not an atomically flat surface. It has been reported that the balance between chemical removal and mechanical removal has a significant effect on the surface roughness in the CMP of electronic materials . Since the mechanism of ECMP is similar to that of CMP, the quality of the surface obtained by ECMP is very likely to be affected by the balance between the anodic oxidation rate and the polishing rate of the oxide layer.
In this study, slurryless ECMP, in which fixed soft abrasives are used as the polishing medium, was conducted on diamond-lapped 4H–SiC (0001) surfaces. During the ECMP process, the balance between the anodic oxidation and the mechanical removal of the oxide layer was controlled by the feeding rate of the SiC wafer. The effects of this balance on the surface quality were investigated.
2 Experimental Section
Current density (mA/cm2)
Spindle rotation speed (rpm)
Feeding rate (mm/s)
Reciprocating distance (mm)
Polishing time (min)
Flow rate (mL/min)
3 Results and Discussion
It was also observed that the overall cross-sectional views of the polishing spots were different, and the flatness of the polishing areas became worse with increasing feeding rate, as shown in Fig. 5. The differences in overall cross-sectional views of the polishing spots were caused by the warp of the thin SiC wafers inevitably induced by their manufacturing process. On the other hand, the changes in the flatness of the polishing areas are attributed to the polishing motion in the ECMP process. During ECMP, the ceria grinding stone was rotated with the spindle and the SiC wafer underwent reciprocating motion along the x-axis at a set feeding rate, and a ring-like ceria grinding stone was applied, as shown in Fig. 1. The grinding stone momentarily stopped at both left and right sides of the polishing area owing to the reciprocating motion of the X stage, and thus, a relatively higher removal amount with a ring-like shape would be obtained at both left and right sides of the polishing area owing to a longer polishing time. Therefore, the center of the cross-sectional views would be a little higher than the two sides. With the increase in feeding rate, the moving time between the two stopping sites decreased, which increased the difference in the removal amount between the two stopping sites and other areas, deteriorating the flatness. Furthermore, the contact status between the SiC surface and the grinding stone also affects the polishing result. These problems are solved when the ECMP machine will be improved to polish the entire wafer.
The results in Figs. 8 and 9 verify the proposed model in Fig. 7. A smooth surface was obtained when the oxide layer was removed immediately before the interface between the bulk SiC and oxide layer became rough. This suggests that the removal rate of the oxide layer should be greater than the anodic oxidation rate of SiC to obtain a smooth surface by ECMP.
In this study, the effects of the balance between the anodic oxidation rate and the removal rate of the oxide layer on the polishing performance of slurryless ECMP were investigated. This balance was controlled by the feeding rate in the ECMP of SiC wafers using the prototype apparatus developed by us, and a higher feeding rate resulted in the oxide layer being removed more rapidly. The roughness of the SiC surface after ECMP was found to decrease with increasing feeding rate, and the pits that were generated in ECMP at low feeding rates disappeared at a high feeding rate. A smooth surface with an Sq surface roughness of 0.577 nm and an Sz surface roughness of 3.876 nm was obtained at an MRR of about 10 μm/h. By performing anodic oxidation experiments with different oxidation times, we found that the interface between the bulk SiC and the oxide layer became rough with the thickening of the oxide layer. The results of this study show that the surface roughness obtained by ECMP is mainly determined by the balance between the anodic oxidation rate and the removal rate of the oxide layer. A smooth surface can be obtained if the oxide layer can be removed immediately after its generation. It is concluded that the removal rate of the oxide layer should be greater than the anodic oxidation rate to obtain a smooth surface in the ECMP of SiC wafers.
This work was partially supported by a Grant-in-Aid for Challenging Research (Exploratory) (18K18810) from the MEXT, Japan, a research grant from the Mitsutoyo Association for Science, and a research grant from the Technology and Machine Tool Engineering Foundation.
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