3.1.

Theoretical Analysis

The combined process includes the water vapor plasma oxidation of RS-SiC and ceria slurry polishing of the oxide layer. Therefore, in order to investigate the plasma oxidation process and the polishing process, we had analyzed the plasma oxidation of RS-SiC in accordance with the Deal–Grove model¹⁰ and the removal of the oxide by ceria slurry polishing using the Preston equation,¹¹ respectively.

First, we used an atomic force microscope (AFM) to investigate the difference of oxidation rates between SiC and Si grains, and the measurement results when the oxidation time is 60 min and 120 min are shown in Figs. 4(a) and 4(b), respectively. From the two transversals, we found that the oxidation rates of Si grains are slightly higher than that of SiC grains, which can be judged from the difference of the oxide height between SiC and Si grains. However, the height difference is not more than 10 nm after plasma oxidation of RS-SiC for 120 min. Taking into account that the rate of volume expansion in the oxidation of Si to ${\mathrm{SiO}}_2$ is slightly higher than that in oxidation of SiC to ${\mathrm{SiO}}_2$, the interface between the RS-SiC substrate and oxide layer is smooth. Therefore, an ultrasmooth RS-SiC surface is expected to be obtained after polishing of the oxide layer generated by water vapor plasma.

Fig. 4

Analysis of oxidized surface by atomic force microscope (AFM): (a) oxidation 60 min and (b) oxidation 120 min.

Schematic diagram of the plasma oxidation of RS-SiC and the polishing of the oxide is shown in Fig. 5. The relationship between oxidation depth $d$ (nm) and oxidation time $t_1$ (min) can be calculated based on the classic Deal–Grove model, as shown in Eq. (1), where $A$ and $B$ are the parameters in the Deal–Grove model:

Fig. 5

Schematic diagram of the water vapor plasma oxidation of RS-SiC and removal of the oxide layer by ceria slurry polishing.

The thickness of the oxide layer $D$ (nm) is the product of the oxidation depth $d$ and the rate of volume expansion $r$, which is shown in Eq. (2):

In order to obtain an ultrasmooth RS-SiC surface without any remaining oxide, the polishing depth should be equal to the thickness of the oxide layer. Therefore, the relationship between $D$ and the polishing time $t_2$ (min) can be expressed by Eq. (3) according to the Preston equation, where $k$, $p$, and $v$ are the Preston’s coefficient, the downward pressure, and the relative velocity between the polishing pad and the sample, respectively.

For better control of the plasma oxidation time and corresponding ceria polishing time, in future research we will confirm the exact Deal–Grove model in water vapor plasma oxidation of RS-SiC by measuring the oxidation depth corresponding to a certain oxidation time. Meanwhile, through experiments on the ceria abrasive polishing of silicon oxide, we can verify the polishing rate with the aim of keeping the polishing depth equal to the thickness of the oxide layer.

3.2.

Evaluation of Surface Morphology

The original surface, shown in Fig. 2(a), was obtained by diamond lapping. After water vapor plasma oxidation of RS-SiC for 90 min, the root-means-square (rms) and Ra surface roughnesses measured by SWLI are 1.090 and 0.864 nm, respectively, as shown in Fig. 6(a). SWLI measurements were conducted to evaluate the surface roughness after polishing for a certain period, as shown in Figs. 6(b) to 6(k). The evolutions of surface roughnesses corresponding to polishing time were summarized in Fig. 6(l). From the SWLI measurements, it could be concluded that the surface roughness reached its optimum after polishing for 40 min, where the corresponding rms and Ra roughnesses are 0.626 and 0.480 nm, respectively.

Fig. 6

Surface roughness measured and evaluated by SWLI: (a) after plasma oxidation 90 min, (b) after polishing 10 min, (c) after polishing 20 min, (e) after polishing 40 min, (e) after polishing 50 min, (f) after polishing 60 min, (g) after polishing 80 min, (h) after polishing 100 min, (i) after polishing 120 min, (j) after polishing 140 min, (k) after polishing 160 min, (l) evolution of surface roughnesses rms and Ra corresponding to polishing time.

SEM observation was conducted to evaluate the surface morphologies before oxidation, after oxidation, after optimal polishing, and after over polishing at the same position, as shown in Figs. 7(a) to 7(h), which was aimed to investigate the plasma oxidation behavior of RS-SiC and the polishing property of the oxide layer.

Fig. 7

Morphologies observed by SEM: (a), (b), (c), and (d) are morphologies before oxidation, after oxidation, after optimal polishing, and after over polishing at the same position. (e), (f), (g), and (h) are those at another position.

The elements and their distributions of the oxidized surface and the optimal polished surface were investigated by SEM-EDX, as shown in Figs. 8(a) to 8(f). In contrast to the original RS-SiC surface in Fig. 1, after water vapor plasma oxidation, the surface was completely oxidized since there were few carbon elements on the oxidized surface and oxygen elements were distributed uniformly.

Fig. 8

Elements and their distributions investigated by SEM-EDX: (a) oxidized RS-SiC surface, (b) element analysis corresponding to (a), (c) oxygen element distribution corresponding to (a), (d) optimal polished surface of plasma oxidized RS-SiC substrate, (e) element analysis corresponding to (d), (f) oxygen element distribution corresponding to (d).

3.3.

Discussion of Experimental Data

From the evolutions of surface roughnesses in Figs. 6(a) to 6(k), it can be observed that the rms and Ra are improved by ceria slurry polishing during the former 40 min. That is because the polishing depth is in the oxide layer and it is easy for the ceria slurry polishing of ${\mathrm{SiO}}_2$ to obtain an ultrasmooth surface. Along with the increase of polishing time, the surface roughnesses are deteriorated since the oxide layer has already been removed as can be judged from the oxygen element distribution of the optimal polished surface (polishing for 40 min) in Fig. 8(f). The Vickers hardnesses (GPa) of SiC, ceria, ${\mathrm{SiO}}_2$, and Si are 24–28,¹² 5–7.5,¹³ 7.6,¹⁴ and 7–9,¹⁵ respectively. As the hardness of a ceria particle is much less than that of SiC and is slightly smaller than those of silica and Si, the ceria slurry polishing of RS-SiC whose major components are SiC and Si would result in worse surface roughness in further polishing. Thus, the polishing depth should be kept within the oxide layer in order to obtain a smooth surface.

It is interesting to note that there are few scratches on the oxidized RS-SiC surfaces by water vapor plasma, as shown in Figs. 7(b), 7(f), and 8(a), although the initial surfaces, which are prepared by diamond lapping, have many scratches, as shown in Figs. 2(a), 2(b), 7(a), and 7(e). 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 zone 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 bulgy oxide, scratches are eliminated on the oxidized RS-SiC surface.¹⁶ Thus, after plasma oxidation the surface roughness is reduced, which can be validated by comparing the original surface in Fig. 2(a) and the oxidized surface in Fig. 6(a).

As shown in Figs. 8(a) and 8(b), the entire RS-SiC surface is completely oxidized by plasma and the distribution of the oxygen element is uniform. It is difficult to estimate the difference of the oxidation rate between SiC and Si grains in RS-SiC, which are different from the oxidation behaviors of RS-SiC in anodic oxidation and thermal oxidation. In the anodic oxidation of RS-SiC, the oxidation rates of the SiC grains are higher than that of Si grains; in contrast, in thermal oxidation of RS-SiC, the oxidation rates of the SiC grains are lower than that of the Si grains.¹⁶ The achievement of an ultrasmooth surface by ceria slurry polishing requires that the removal depth should be not more than the thickness of the oxide layer; thus, the uniform oxide layer in the plasma oxidation of RS-SiC is desirable in order to obtain a better surface.

Figures 8(d) to 8(f) reveal that there is little remaining oxide on the optimal polished surface, which means that we can obtain ultrasmooth RS-SiC surface and simultaneously keep the outstanding properties of the RS-SiC substrate. Otherwise, the remaining oxide will reduce the excellent mechanical and chemical properties of RS-SiC. In the thermal oxidation of RS-SiC followed by ceria slurry polishing, although an ultrasmooth surface with an Ra surface roughness of 0.274 nm in $5\mu \mathrm{m}\times 5\mu \mathrm{m}$ region measured by AFM can also be obtained by polishing for an appropriate time,¹⁶ there is remaining oxide on the surface, since the oxidation rates of SiC and Si grains differ in thermal oxidation. Furthermore, plasma oxidation can be applied in the process of a large-scale RS-SiC substrate by scanning the oxidation spot through the entire surface. Therefore, plasma oxidation has an obvious superiority as a postprocessing method in finishing of RS-SiC.