Single-crystal SiC is a typical third-generation semiconductor power-device material because of its excellent electronic and thermal properties. An ultrasmooth surface with atomic surface roughness that is scratch free and subsurface damage (SSD) free is indispensable before its application. As the last process to reduce the surface roughness and remove surface defects, precision polishing of single-crystal SiC is essential. In this paper, precision polishing technologies for 4H-SiC and 6H-SiC, which are the most commonly used polytypes of single-crystal SiC, such as chemical mechanical polishing (CMP), photocatalytic chemical mechanical polishing (PCMP), plasma-assisted polishing (PAP), electrochemical mechanical polishing (ECMP), and catalyst-referred etching (CARE), were reviewed and compared with emphasis on the experimental setup, polishing mechanism, material removal rate (MRR), and surface roughness. An atomically smooth surface without SSD can be obtained by CMP, PCMP, PAP, and CARE for single-crystal SiC. However, their MRRs are meager, and the waste treatment after CMP is difficult and expensive. Moreover, PAP’s operation is poor due to the complex polishing system, plasma generation, and irradiation devices. A high MRR can be achieved by ECMP. In addition, it is an environmentally friendly precision polishing process for single-crystal SiC since the neutral salt solution is generally used as the electrolyte in ECMP. However, the formation of the egglike protrusions at the oxide/SiC interface during anodic oxidation would lead to a bigger surface roughness after ECMP than that after PAP is processed. The HF solution used in CARE was toxic, and Pt was particularly expensive. Ultrasonic vibration-assisted single-crystal SiC polishing and electrolyte plasma polishing (EPP) were discussed; furthermore, the research direction of further improving the surface quality and MRR of single-crystal SiC was prospected.

In this paper, theoretical models of the critical indentation depth and critical force on brittle materials using cleavage strength and contact theory are proposed. A Berkovich indenter is adopted for nanoindentation tests on a 4H-SiC single crystal sample to evaluate its mechanical behaviors. The stages of brittle material deformation (elastic, plastic, and brittle) can be characterized by the load versus indentation depth curves through the nanoindentation test. The curve of the elastic deformation stage follows the Hertz contact theory, and plastic deformation occurs at an indentation depth of up to 10 nm. The mechanism of 4H-SiC single crystal cracking is discussed, and the critical indentation depth and critical force for the plastic-brittle transition are obtained through the occurrence of the pop-in point. This shows that the theoretical results have good coherence with the test results. Both the values of the elastic modulus and hardness decrease as the crack length increases. In order to obtain more accurate mechanical property values in the nanoindentation test for brittle materials such as SiC, GaN, and sapphire, an appropriate load that avoids surface cracks should be adopted.

Silicon carbide (SiC) surface modification is an essential step in chemical mechanical polishing. For high quality and high efficiency surface modification of SiC, a green and promising surface modification method named plasma electrochemical oxidation (PECO) is proposed.

For high-quality and effective polishing of SiC, a novel polishing technique that combines anodic oxidation and mechanical polishing (AOMP) is proposed herein. To clarify the SiC surface anodic oxidation mechanism, AOMP experiments were conducted. The results show that as a result of surface oxidation, the main elements of the modified surface were Si and O by X-ray diffraction (XRD), indicating that the SiC surface was modified and formed a SiO2 oxide layer. Micro Vickers hardness tests revealed that the hardness of the modified surface greatly decreased to 1/9 of that of the as-received surface, which was easy to remove. Considering the experimental results, an anodic oxidation process model is proposed herein based on the inner-outer double-directional diffusion theory. During the oxidation process, a transition layer containing silicon oxycarbide (Si-CO) was formed between the SiO2 and SiC, the amount of which varied with the thickness of the oxide. Based on the Deal-Grove model, the relationship between the oxide layer thickness and oxidation time was determined, and the initial oxidation rate was 44.81 nm/min. The surface roughness after chemical mechanical polishing (CMP) was determined for different oxidation time and polishing time, and it was clear that when the anodic plasma oxidation rate matched the CMP rate, a just-polished surface was obtained.

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