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-C-O) 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.
Keywords: SiC single crystal; anodic oxidation; mechanical polishing; mechanism
SiC is a promising next-generation semiconductor owing to its excellent physical and chemical properties such as a wide band gap, a high breakdown electric field strength, a high electron mobility, and good thermal conductivity; therefore it has widespread applications in high-power, high-efficiency and high-temperature fields such as the information industry and electronic devices [1]. To realize its excellent properties and performance in precision equipment and power devices, a smooth planarized surface without scratches or subsurface damage is essential. However, SiC is hard to polish by conventional methods due to its high hardness and remarkable chemical inertness. During the SiC fabrication process，which involves crystal growth，slicing，lapping，etching, polishing and packaging, polishing is one of the key steps, as the surface roughness greatly affects device performance. Due to the high integration of computer chips, the wafer substrates are increasing in size and decreasing in thickness, which brings additional challenges to wafer polishing processes. Currently, a variety of polishing techniques have been proposed as surface treatment methods for SiC. Mechanical polishing (MP) [2]，as a conventional polishing method, has a high material removal rate (MRR), but a large number of scratches on the wafer surface and substantial subsurface damage inevitably occur due to the use of hard abrasives, thus damaging the surface integrity [3], and making mechanically polished wafers unacceptable for certain applications. Hydrogen etching polishing [4] has been adopted to etch the SiC wafers; however, the surface smoothness is difficult to control, and the surface quality is poor due to the randomness of the etched surface and uneven erosion amount. Also, chemically combined fluorine still remains on the surface, and other procedures may be needed to remove the fluorine. Chemical mechanical polishing (CMP) [5], which combines chemical etching and mechanical polishing, has widespread applications in finishing planarization processes. However, it is difficult to avoid the appearance of certain major defects such as micropipes and planars on the surface of polished SiC wafer [6]; CMP has a high cost because of the use of slurries and other chemicals. Plasma-assisted polishing (PAP) [7-9], which combines surface modification via atmospheric-pressure plasma irradiation and soft abrasive polishing of the modified layer, has complex facility requirements and may cause environmental pollution. Electrochemical mechanical polishing (ECMP) [10-13] oxidizes the surface first and then removes the modified layer by mechanical polishing; however, etch pits are easily generated on the polished surface, increasing the surface roughness. To resolve these problems and obtain a scratch-free, pit-free and atomically flat surface with a high efficiency, a novel approach that combines surface modification and mechanical polishing, named anodic oxidation and mechanical polishing (AOMP), has been proposed. The ion energy in the plasma is approximately several electron volts, which can avoid mechanical damage to the SiC surface by ion bombardment during the plasma exposure period [14]. In this technique, the oxidation of the reactive plasma modifies the SiC surface to form a soft layer; simultaneously, the modified layer is removed by soft abrasive polishing. AOMP is an environmentally friendly polishing technique owing to the use of a green slurry material, and it can also be applied to polish other brittle and hard-to-machine materials, such as SiC, GaN and sapphire. This paper proposes an AOMP approach for SiC polishing and AOMP experiments were conducted. SiC surface anodizing and abrasive polishing characteristics are reported herein. The surface of the as-received SiC wafer was oxidized and formed a SiO2-modified layer that was easy to remove. On the basis of the inner-outer double-directional diffusion theory, a model for the SiC plasma anodic oxidation process was built to explain the material removal mechanism during the AOMP process. By comparing the surface roughness before and after oxidation with SWLI, we obtained the relationship between the oxide layer thickness and oxidation time, and a just-polished surface formed when the anodic plasma oxidation rate matched the CMP rate.
SiC single crystal has high hardness (Mohs hardness 9.5), and the hardness of SiO2 (Mohs hardness 7.0) is much lower than that of SiC. Through a chemical modification approach, a surface layer on the SiC can be modified to form a SiO2 thin film that can be removed by soft CeO2 (Mohs hardness 6) abrasive particles. After the SiO2 is removed by the CeO2 abrasive particles, a fresh SiC surface is exposed and a new round of corrosion begins. At the same time, due to the preferential corrosion and removal mechanism of the surface bumps, a cyclic process of oxidation-modification to mechanicalremoval to reoxidation-modification occurs, which results in a high material removal rate and decreased surface roughness. After several cycles of surface anodic oxidation and simultaneous abrasive polishing, a smooth surface is obtained.
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