During plasma cleaning and activation, high-energy species in the plasma gas phase are projected onto the wafer surface to be treated. These high-energy species exert physical bombardment or chemical reactions on the wafer surface, causing the decomposition of surface molecules and adsorbed contaminants through reactions with high-energy radicals, forming gaseous products that desorb from the surface. This process exposes dangling bonds on the wafer surface, which then adsorb free –OH groups from the air. The presence of –OH groups on the activated wafer surface significantly improves its hydrophilicity.

Effect of Plasma Cleaning and Activation on Surface Hydrophilicity of SiC Wafers

Figure 1 shows the variation of the water contact angle on the carbon face of SiC wafers with oxygen plasma cleaning and activation time. It can be seen that plasma activation significantly improves the hydrophilicity of the wafer surface. As activation time increases, the contact angle decreases and tends to stabilize after 30 seconds. Beyond this activation time, the hydrophilicity of the SiC wafer surface remains relatively stable, fluctuating around a contact angle of 14°. Experimental results show that dry oxygen plasma activation achieves a greater improvement in the surface hydrophilicity of SiC wafers than wet chemical activation. In addition, although plasma activation greatly improves surface hydrophilicity, it cannot fully render the SiC wafer surface completely wettable.

Figure 1 Variation of water contact angle on SiC wafer surface with oxygen plasma activation time: (a) as-received C-face, (b) 10 s, (c) 20 s, (d) 30 s, (e) 40 s, (f) 60 s

To compare the enhancement of surface hydrophilicity by oxygen plasma activation, dry surface activation experiments were conducted using a nitrogen–oxygen gas mixture (molar ratio 3:1) in a plasma cleaning system. Figure 2 shows the variation of the water contact angle on the carbon face of SiC wafers with N₂/O₂ mixed plasma activation time. It can be observed that the effect of N₂/O₂ mixed plasma activation on the hydrophilicity of the SiC C-face follows a similar trend to that of pure oxygen plasma.

When activation time is less than 20 seconds, the mixed plasma shows slightly inferior performance compared with pure oxygen plasma, resulting in weaker hydrophilicity improvement. However, as activation time increases, the effect of N₂/O₂ mixed plasma activation gradually surpasses that of pure oxygen plasma, leading to slightly stronger surface hydrophilicity. These results indicate that N₂/O₂ mixed plasma activation improves the hydrophilicity of the SiC C-face within a certain time range and may exhibit superior activation performance under specific conditions.

Figure 2 Variation of water contact angle on SiC wafer surface with N₂/O₂ mixed plasma activation time: (a) as-received C-face, (b) 10 s, (c) 20 s, (d) 30 s, (e) 40 s, (f) 60 s

XPS Analysis of SiC Wafer Surfaces After Plasma Cleaning and Activation

Figure 3 Comparison of XPS Results of SiC Wafers Before and After Oxygen Plasma Dry Activation and Cleaning

(a) Full spectrum of the as-received C-face(b) Full spectrum of the C-face after dry cleaning(c) C 1s characteristic spectrum of the as-received C-face(d) C 1s characteristic spectrum of the C-face after dry cleaning

By comparing the C 1s characteristic spectra, it is found that after oxygen plasma activation, the C–C bond attributed to carbon contamination increases, while the content of COₓSiᵧ species from the native oxide layer decreases, and the C–Si content in the substrate also declines slightly. This indicates that plasma activation can not only remove the native oxide layer on the SiC wafer surface but also possibly damage the SiC substrate, break C–Si chemical bonds, and thus form free carbon impurities.

In summary:In experiments on the effect of plasma cleaning and activation on the surface chemical state of SiC wafers, it is found that oxygen plasma removes the native oxide layer from the wafer surface, damages the wafer substrate, and generates free C–C impurities. Meanwhile, a new chemical structure Si–O–Si is formed on the wafer. It is clarified that the improved hydrophilicity of the SiC wafer surface originates from a significant increase in C–OH and Si–OH bonds.