Comparative analysis on surface property in anodic oxidation polishing of reaction-sintered silicon carbide and single-crystal 4H silicon carbide

For effective machining of difficult-to-machine materials, such as reaction-sintered silicon carbide (RSSiC) and single-crystal 4H silicon carbide (4H-SiC), a novel polishing technique named anodic oxidation polishing was proposed, which combined with the anodic oxidation of substrate and slurry polishing of oxide. By scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDX) observation and atomic force microscopy analysis, both the anodic oxidation behaviors of RS-SiC and 4H-SiC were investigated. Through comparison of the surfaces before and after hydrofluoric acid etching of the oxidized samples by the scanning white light interferometry (SWLI) measurement, the relationships between oxidation depth and oxidation time were obtained, and the calculated oxidation rate for RS-SiC was 5.3 nm/s and that for 4H-SiC was 5.8 nm/s based on the linear Deal– Grove model. Through anodic oxidation polishing of RSSiC substrate and 4H-SiC substrate, respectively, the surface roughness rms obtained by SWLI was improved to 2.103 nm for RS-SiC and to 0.892 nm for 4H-SiC. Experimental results indicate that anodic oxidation polishing is an effective method for the machining of RS-SiC and 4H-SiC samples, which would improve the process level of SiC substrates and promote the application of SiC products in the fields of optics, ceramics, semiconductors, electronics, and so on.


Introduction
Silicon carbide (SiC) is a promising material for semiconductors, ceramics, and optics [1,2], since it has remarkable excellent mechanical and chemical properties [3,4]. In the SiC family, reaction-sintered silicon carbide (RS-SiC) and single-crystal 4H silicon carbide (4H-SiC) have attracted widely interests for their extraordinary capabilities. Besides the common excellences, RS-SiC has the unique advantage of a low manufacturing cost, which makes it an ideal mirror material for space telescope systems [5,6]. Meanwhile, 4H-SiC is one of the most attractive semiconductor materials for the next-generation power device applications, as it has outstanding electronic properties [7].
However, SiC is a difficult-to-machine (DTM) material because of its high hardness and chemical inertness [8]. Although ultrasmooth 4H-SiC surface can be obtained in chemical mechanical polishing (CMP) [9], plasma-assisted polishing (PAP) [10], and CAtalyst-Referred Etching (CARE) [11], the material removal rates (MRR) in these techniques are low. As the composition and structure of RS-SiC are not uniform [12], few of the techniques which have been developed to process RS-SiC substrates can yield smooth surface and obtain high material removal rate simultaneously [13].
Anodic oxidation has been developed for the oxidation of silicon, and we applied it to process SiC [14]. By applying a mixture of hydrogen peroxide (H 2 O 2 ), hydrochloric acid (HCl), and ultrapure water (H 2 O) as the electrolyte, both the RS-SiC and 4H-SiC were oxidized. Afterward, the scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDX) observation and atomic force microscopy (AFM) analysis were conducted to investigate the oxidation behaviors of RS-SiC and 4H-SiC, respectively. Through comparing the surfaces before and after hydrofluoric acid (HF) etching of the oxidized samples, the relationships between oxidation depth and oxidation time for RS-SiC and 4H-SiC were obtained. By applying anodic oxidation polishing, the RS-SiC and 4H-SiC surfaces in four scenarios which are before oxidation, after oxidation, after simultaneous oxidation-assisted polishing (SOAP), and after divided oxidation-assisted polishing (DOAP) were analyzed comparatively. By calculation of oxidation rate based on the Deal-Grove model [15] and comparison of surface quality, the superiority of anodic oxidation polishing of SiC substrates was revealed.

Experimental apparatus and parameters
Schematic diagrams of the anodic oxidation system and the anodic oxidation polishing system are shown in Fig. 1a The power supply was a three-phase potentiostat, and the oxidation potential was 9.8 V. The cathode, anode, and reference electrode were platinum (Pt), SiC, and Ag/AgCl, respectively. For the anodic oxidation polishing of SiC samples, the experimental parameters are summarized in Table 1. The electrical source was consisted of a digital function generator and a bipolar power supply, and a digital storage oscilloscope was used to observe the status of the oxidation circuitry. The ceria (CeO 2 ) slurry was used as the electrolyte. The SiC substrate and polishing pad were both immersed into the slurry, and the specimen surface was abraded by the polishing pad. The experiments were divided into four groups: (1) DOAP of RS-SiC for 6 h (oxidation 3 h followed by polishing 3 h), (2) SOAP of RS-SiC for 3 h, (3) DOAP of 4H-SiC for 2 h (oxidation 1 h followed by polishing 1 h), and (4) SOAP of 4H-SiC for 1 h.

Analysis of oxidation behaviors
The oxidation behavior of RS-SiC observed by SEM-EDX is shown in Fig. 2a-d. Figure 2a, b shows the comparison of surface morphologies before and after anodic oxidation at the same position. Figure 2c, d shows the detailed surface morphology and the corresponding oxygen element distribution. It is interesting to note that there are few scratches on the anodic oxidized surfaces, although the initial surfaces, which are prepared by diamond lapping, have many scratches, as shown in Fig. 2a. We hypothesize that there are two reasons for the disappearance of scratches on the oxidized RS-SiC surface. First, the area with scratches has a larger surface area than the other areas, and the oxidation of RS-SiC increases its volume; thus, the scratches are closed by the expanded oxide. Second, the area surrounding that with scratches has a higher oxidation rate because of lattice strain; thus, the rapidly expanding oxide will fill the scratches. Through the closing and filling of scratches by anodic oxidation, scratches are eliminated on the oxidized RS-SiC surface. From the observation results in Fig. 2a, b, it could be found that the SiC domains have a higher oxidation rate than the Si domains, which can be clearly concluded from the oxygen element distribution in Fig. 2d. We had also investigated the anodic oxidation of single-crystal Si and 4H-SiC in our system. Under the same experimental conditions, it was observed that the oxidation rate of 4H-SiC was slightly higher, whereas that of Si was much lower than that of RS-SiC. Moreover, for most anodic oxidized SiC grains, the boundary area has a higher oxidation rate than the central area. Meanwhile, for all anodic oxidized Si domains, the oxide surface is smooth. We hypothesize that it is because the electric properties and crystal lattices of SiC and Si differ. Furthermore, there were many cracks and projections on the anodic oxidized RS-SiC surface. We hypothesize that it is because the oxidation of Si/SiC to SiO 2 is a volume expansion process, and the swelling pressure is generated in both the thickness direction and the plane direction. The distribution of this pressure generated by an oxidized grain is determined by the oxidation rate distribution of the grain. Therefore, in the thickness direction, the pressure ejects the oxide, resulting in the formation of projections; in the plane direction, the pressure presses the neighboring grains, introducing cracks in the weak areas.  The oxidation behavior of 4H-SiC obtained by SEM and AFM observation is shown in Figs. 3 and 4. From the SEM and AFM images, it was observed that there were many oxide protuberances on the oxidized surface, and the scratches on the original surface which are introduced by diamond lapping change into linear swellings. That's because the oxidation of SiC is a volume expansion process; thus, the scratches are filled by the bulgy oxides, and the oxidized 4H-SiC surface is covered with bulgy oxides. Comparing with the anodic oxidation behaviors of RS-SiC, there are few cracks or projections on the anodic oxidized 4H-SiC surface. Although the swelling pressure is also generated by the volume expansion in the anodic oxidation of 4H-SiC, it is limited in the thickness direction. Meanwhile, the oxidation behaviors of some SiC grains in the anodic oxidation of RS-SiC are different from that of 4H-SiC, because the crystal directions of each SiC grains in RS-SiC differ.

Calculation of oxidation rate
Through the scanning white light interferometry (SWLI) measurements and comparing the surfaces before and after HF etching of the anodic oxidized SiC samples, the oxidation depth corresponding to a certain oxidation time for RS-SiC and 4H-SiC was obtained, respectively, as shown in Figs. 5a-e and 6a-e, respectively. Moreover, the summarized data for RS-SiC are shown in Fig. 5f and that for 4H-SiC are shown in Fig. 6f, respectively. It can be obviously observed that the oxidation depth is larger with increasing in oxidation time.
From the equations, it can be concluded that the oxidation rate can be 5.3 nm/s for RS-SiC and 5.8 nm/s for 4H-SiC in the linear Deal-Grove model [15]. Therefore, if we conduct SOAP of SiC, the MRR would be 318 nm/min for RS-SiC and 348 nm/min for 4H-SiC, which were hundred times of those in CMP (\500 nm/h) [9], PAP (\200 nm/h) [10], and CARE (100-200 nm/h) [11].

Comparative analysis of polishing property
We conducted the anodic oxidation polishing of RS-SiC and 4H-SiC by the system shown in Fig. 1b. The surface roughnesses rms of RS-SiC substrate after the conducted four scenarios were measured by SWLI and are summarized in Fig. 7a-d, and those of 4H-SiC are summarized in Fig. 8a-d. The original surfaces which were prepared by diamond lapping had many scratches, as shown in Figs. 7a and 8a, because the hardness of diamond is larger than that of SiC substrates. After anodic oxidation, the rms roughness of surface was deteriorated, as shown in Figs. 7b and 8b, since there were undulations and fluctuations resulting from the bulgy oxide.
The original SiC samples which were prepared by diamond lapping had many scratches, since the hardness of diamond is larger than that of SiC. After anodic oxidation, both the RS-SiC and 4H-SiC surfaces were deteriorated, because there are undulations and projections on the oxidized surfaces. The rms roughnesses of RS-SiC surface after SOAP were improved to 2.103 nm and that of 4H-SiC were improved to 0.892 nm, as shown in Fig. 7c, d, which were better than those after DOAP, as shown in Figs. 7d and 8d. The experiment results indicated that anodic oxidation polishing of SiC substrates can obtain high MRR and fine roughness simultaneously by SOAP.

Summaries
In this study, we conducted a comparative analysis of anodic oxidation polishing of RS-SiC and 4H-SiC. The following conclusions were obtained. Anodic oxidation polishing would improve the machining level of SiC substrates and promote the application of SiC products in semiconductors, optics, and ceramics fields. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.