Chemical–Mechanical Polishing of 4H Silicon Carbide Wafers

4H silicon carbide (4H‐SiC) holds great promise for high‐power and high‐frequency electronics, in which high‐quality 4H‐SiC wafers with both global and local planarization are cornerstones. Chemical–mechanical polishing (CMP) is the key processing technology in the planarization of 4H‐SiC wafers. Enhancing the performance of CMP is critical to improving the surface quality and reducing the processing cost of 4H‐SiC wafers. In this review, the superior properties of 4H‐SiC and the processing of 4H‐SiC wafers are introduced. The development of CMP with chemical, mechanical, and chemical–mechanical synergistic approaches to improve the performance of CMP is systematically reviewed. The basic principle and processing system of each improvement approach are presented. By comparing the material removal rate of CMP and the surface roughness of CMP‐treated 4H‐SiC wafers, the prospect on the chemical, mechanical, and chemical–mechanical synergistic improvement approaches is finally provided.


Introduction
4H silicon carbide (4H-SiC) is emerging as a dominant semiconductor material to enable the ever-increasing deployment of high-power, high-frequency, high-temperature and radiation-resistant devices. [1,2] Compared with silicon (Si)-based the highpower devices based on 4H-SiC have the advantages of higher power density, lower power consumption, and smaller size. [3] As shown in Figure 1, the devices using 4H-SiC have been successfully utilized in a series of fields such as electric vehicles, railways, uninterruptible power supplies (UPS), high-voltage grids, and 5G communications. [4] For example, the powerconversion efficiency of the G2020 series 4H-SiC-based UPS developed by Toshiba has reached 98.2%. [1] Toyota has replaced Si-based PIN diodes with 4H-SiC-based Schottky-barrier diodes in the DC converters of hybrid electric vehicles, leading to 30% reduction of energy loss and 0.5% increase of conversion efficiency. [5] Mitsubishi Electric Corporation has equipped a 4H-SiC power module in the train traction inverter of the N700S Shinkansen subway, enabling the switching loss and volume of the system to be reduced by 55% and 40-65%, respectively. [6] The promising applications of 4H-SiC have boosted the requirements for high-quality 4H-SiC wafers. Since the surface quality of 4H-SiC wafers significantly affects the quality of epitaxial 4H-SiC (homoepitaxy) or GaN (heteroepitaxy) and thus the quality of final devices, the processing of the surface of 4H-SiC wafers is crucial to the success of 4H-SiC in semiconductor industry. [7] Chemical-mechanical polishing (CMP) has been developed as a state-of-the-art technology to achieve global planarization of the surface of 4H-SiC wafers. [8] During CMP, the surface of a 4H-SiC wafer undergoes cycles of chemical oxidation and mechanical removal under the interaction among the 4H-SiC wafer, polishing slurry, and polishing pad. [9] Given the high hardness and strong chemical inertness of 4H-SiC, the CMP of 4H-SiC wafers faces the challenges of long processing time, high cost, and low technical flexibility. [10,11] In order to improve the surface quality and processing efficiency of 4H-SiC wafers, researchers have been investigating various approaches.
In this review, we discuss recent progress on the CMP of 4H-SiC wafers after a brief overview of the basic properties of 4H-SiC. Chemical, mechanical, and chemical-mechanical 4H silicon carbide (4H-SiC) holds great promise for high-power and highfrequency electronics, in which high-quality 4H-SiC wafers with both global and local planarization are cornerstones. Chemical-mechanical polishing (CMP) is the key processing technology in the planarization of 4H-SiC wafers. Enhancing the performance of CMP is critical to improving the surface quality and reducing the processing cost of 4H-SiC wafers. In this review, the superior properties of 4H-SiC and the processing of 4H-SiC wafers are introduced. The development of CMP with chemical, mechanical, and chemicalmechanical synergistic approaches to improve the performance of CMP is systematically reviewed. The basic principle and processing system of each improvement approach are presented. By comparing the material removal rate of CMP and the surface roughness of CMP-treated 4H-SiC wafers, the prospect on the chemical, mechanical, and chemical-mechanical synergistic improvement approaches is finally provided.
The (1120) plane and (1100) plane are often denoted as the a face and m face, respectively. For 4H-SiC, the electron mobility along the c-axis is higher than that perpendicular to the c-axis. Furthermore, the step-flow growth mode and the p-type doping are both enhanced on the Si face of 4H-SiC substrates. [15,16] Therefore, the homoepitaxy of the 4H-SiC substrate is often grown on the Si-face. In view of this, the CMP technologies of the Si face of 4H-SiC wafers have attracted great attention.
Given the same bonding between Si and C, 3C-, 4H-, and 6H-SiC have similar mechanical and chemical properties. However, due to the different stacking sequences of SiC bilayers, the electrical properties of 3C-, 4H-, and 6H-SiC are different from each other. [17] As shown in Figure 3, the critical electric field, bandgap energy, saturated electron velocity and electron mobility of 4H-SiC all prevail over those of 3C-and 6H-SiC, which makes 4H-SiC being the ideal polymorph of SiC for the applications of high-power, high-frequency, and high-temperature electronics. [18] Therefore, we focus on the improvement of the CMP of 4H-SiC in this review. It should be noted that early work was mainly related to the CMP of 6H-SiC. Because of the similar mechanical and chemical properties of 4H-and 6H-SiC, the mechanical and chemical improvements for the CMP of 6H-SiC wafers are conducive to the improvements of the CMP of 4H-SiC wafers.

Manufacturing of 4H-SiC Wafers
As shown in Figure 4, the processing procedure of a 4H-SiC wafer mainly includes wire sawing, edge rounding, grinding, lapping, and CMP. [7,19] Firstly, a 4H-SiC boule is sliced into wafers during multi-wire sawing. Each 4H-SiC wafer is then mounted on a grinding chuck. A profile rounding wheel rounds the wafer edge according to the product's specifications. Grinding is subsequently carried out to rapidly reduce the thickness of the wafer. The wafer further undergoes a series of lapping processes to increase the parallelism of the surfaces and remove surface/sub-surface damage induced by the wire-sawing. Finally, CMP is used to improve the flatness and achieve an atomically smooth surface of the wafer.
The CMP process is a crucial step to control the surface quality of 4H-SiC wafers, while other steps mainly aim to remove the wire-sawing-induced damages and rapidly reduce  www.advmatinterfaces.de the thickness of 4H SiC wafers. [20] The cost of CMP accounts for 30-40% of the total processing cost of 4H-SiC wafers. The CMP of the Si face or C face of 4H-SiC wafers takes 3-5 hours. Both the cost and efficiency of the CMP of 4H-SiC wafers deserve further optimization although existing CMP approaches can be employed to produce qualified 4H-SiC wafers. [21]

Traditional CMP Methods
CMP appeared in the mid-20th century, which had attracted great attention owing to its capability of planarizing both glass and metal. [22] When IBM corporation first introduced CMP to the manufacturing of integrated circuits in 1988, CMP began to be widely used in the processing of semiconductor wafers because both the global and local planarization can be achieved simultaneously. [23] As shown in Figure 5, a CMP system mainly consists of three parts: the polishing slurry, the polishing pad, and the polishing machine containing components such as the polishing head and polishing plate. [24] In a typical CMP process, a wafer is bonded to the bottom of a polishing head. The polishing slurry with specific functions is continuously dripped to the surface of the polishing pad. The wafer, polishing slurry, and polishing pad are in close contact under certain pressure and rotated with the polishing head and polishing plate. During the relative motion, the surface of the wafer and the polishing slurry chemically react. The reaction product is removed by the mechanical interaction among abrasives in the polishing slurry, the polishing pad and the wafer. The global and local planarization of the wafer is ultimately realized by the cycling of the chemical reaction and mechanical removal processes.
In 1997, CMP was first employed in the planarization of SiC wafers. [25] Diamond particles and SiO 2 particles were used as the abrasives of the mechanical polishing and CMP of 4H-SiC wafers, which yielded that the material removal rate (MRR) of the mechanical polishing and CMP were 2-2.25 µm h −1 and lower than 200 nm h −1 , respectively. The surface roughness after mechanical polishing and CMP were 70 and 0.5 nm, respectively. Processing parameters such as the rotation speed, pressure, and temperature are key factors affecting the CMP performance of 4H-SiC wafers. It was found that the maximum MRR of 250 nm h −1 for CMP could be realized at the temperature of 23 °C, the pressure of 5 psi, the polishing head rotation speed of 180 rpm, and the pH of 9.9. [26] With an alkaline-based polishing slurry, the MRR of 4H-SiC reached 0.1-0.5 µm h −1 . The surface roughness of 4H-SiC was less than 5 nm. [27] It is observed that the MRR of 6H-SiC increased by 20% after adding 0.6 wt% KOH to the slurry. [28] A series of more in-depth analyses shed light on the formation mechanism of atomic step-terrace structure on the surface of 6H-and 4H-SiC wafers after CMP. [11,[29][30][31] As shown in Figure 6a, when the surface roughness of a 4H-SiC wafer is extremely low (Ra [the arithmetic mean deviation of contours] < 0.06 nm), the surface of the 4H-SiC wafer features a step-terrace structure. The atomic   Figure 6b shows that the step height of h about 0.25 nm. Combined with the crystal structure model of 4H-SiC in Figure 6c, the step height (h) of a bimolecular layer of 4H-SiC is calculated from the lattice parameters to be 0.25 nm. Therefore, the mutual corroboration of experiments and theory indicates that CMP can realize the processing requirements of atomically smooth surface of 4H-SiC wafer.
The removal mechanism of the CMP of 4H-SiC wafers is shown in Figure 7. [32] The oxidant in the polishing slurry firstly oxidizes 4H-SiC, forming the transition oxidation state of SiCO on the surface of 4H-SiC. The oxide has a lower hardness than that of 4H-SiC, which ensures that it can be removed by the abrasives in the slurry through mechanical effect. Based on the cycling of the oxidation and mechanical removal, the planarization of a 4H-SiC wafer is realized (Figure 7). [30] It has been proposed that the oxidation is the rate-limiting step in the whole CMP process. [33] H 2 O 2 and colloidal silica were initially used as the oxidant and abrasive, respectively. A low MRR (<200 nm h −1 ) was obtained. Before CMP, the depth of the surface damage of a 4H-SiC wafer is usually 2-5 µm. [34] The low MRR means that the CMP step requires significant processing time and costs.
Researchers have started improving the efficiency of CMP by accelerating oxidation. Potassium permanganate (KMnO 4 ) is of considerable interest because of its excellent capability of oxidation. By using KMnO 4 as the oxidant and colloidal silica as the abrasives, the MRR of 185 nm h −1 and Ra of 0.254 nm for the CMP of 6H-SiC were obtained. [35] The MRR might be further Figure 5. Schematic diagram of a typical CMP system, including polishing head, polishing plate, polishing slurry, and polishing pad. During CMP process, the 4H-SiC wafer, polishing slurry, and polishing pad are in close contact under certain pressure and rotated with the polishing head and polishing plate. Figure 6. a) AFM image showing the atomic step structure on the surface of 4H-SiC wafers, b) dimension of the atomic steps measured from X 1 to X 2 labeled in (a), and c) formation mechanism of atomic step structure on the surface of 4H-SiC, where d and h denote the width and height of the steps. Reproduced with permission. [11] Copyright 2014, Elsevier.

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increased to 308 nm h −1 in a strong acid environment. [36] However, insufficient mechanical action limited the timely removal of the oxide layer and thus the efficiency of CMP in these studies. It was found that increasing the size of SiO 2 abrasives could increase the MRR. But the surface quality was scarified. [29] Therefore, the CMP efficiency must be enhanced by balancing the chemical oxidation and mechanical removal.

Fenton-Like-Reaction
The standard reduction potential of hydroxyl radicals ( • OH) and H 2 O 2 are 1.90-2.80 V and 1.77 V, respectively. [37] Therefore, the hydroxyl radicals ( • OH) generated by Fenton reagents are more oxidizing than H 2 O 2 , which can be adopted to accelerate the oxidation process during the CMP of 4H-SiC. Taking the Fe catalyst as an example, the Fenton reaction can be described as: It is clear that the main factors affecting the Fenton reaction are the pH value, Fe 2 + dosage, and the concentration of H 2 O 2 . Some researchers used the rotation friction of a Fe rod to introduce Fe 2 + to the CMP of 4H-SiC wafers, increasing the removal volume to ≈2300 µm 3 h −1 . [38] The overall removal mechanism of the CMP adopting Fenton-like reaction is: It has been found that the catalytic effect of Fe-based catalysts decreases in the order of Fe 3 O 4 , FeO, Fe, and Fe 2 O 3 . [39][40][41][42] More significantly, the concentration ratio of Fe 2+ and H 2 O 2 plays an important role in the effective concentration of • OH in the slurry. High Fe 2+ concentration leads to flocculent precipitates in the slurry of CMP under acidic conditions, while high H 2 O 2 concentration leads to partial quenching of the concentration of • OH.
The performance of CMP improved by the Fenton-like reaction is limited by the pH value of the slurry. Both the catalytic efficiency of the Fenton reaction and the stability of the slurry are highly dependent on the pH environment. When the pH value of slurry is higher than 3, Fe 2+ converts to Fe (OH) 3 precipitation, resulting in a decrease in the efficiency of the Fenton reaction. The Fenton reaction experiments in these studies were mainly carried out under strong acid conditions (pH < 3), thus avoiding the problem of Fe (OH) 3 precipitation. On the other hand, the abrasives in the slurry tend to agglomerate under acid conditions, which degrades the performance of mechanical removal and thus the surface quality of 4H-SiC wafers. That is to say, when the pH value of the slurry is in the range of 3-7, the slurry suffers from both problems of Fe (OH) 3 precipitation and abrasive agglomeration. This agglomeration problem may not be severe at the stage of experiment research, because the execution time of a single CMP experiment is usually 1 h and the polishing slurry is not recycled. However, in the mass production of 4H-SiC wafers, the execution time of a single CMP process is 3-5 h, and the polishing slurry circulation time is usually more than 10 h. Therefore, when the technology moves from the lab to the fab, the agglomeration of colloidal silica will become a growing problem that cannot be ignored under strong acid conditions.

Special-Gas Atmosphere
The CMP process may be carried out in an ultra-clean chamber in an air atmosphere. Due to the strong chemical inertness of 4H-SiC, researchers proposed to change the gas atmosphere to increase the rate of oxidation by placing the polishing device in a closed chamber and changing its gas atmosphere by delivering certain gas inward (Figure 8). [43] By changing the gas atmosphere of CMP from air to O 2 , the MRR of 4H-SiC wafers increased from 35 to 60 nm h −1 . [44] The MRR during the CMP of 4H-SiC wafers significantly increased from 25 to 200 nm h −1 by adding bubbles of ozone gas to the polishing slurry. [45] In these studies, the MRR of the CMP of 4H-SiC wafers in the atmosphere of O 2 or ozone is higher than that in an air environment. However, this method requires the polishing equipment to be sealed. The special gas atmosphere poses additional safety issue on the CMP of 4H-SiC wafers. The three diagrams are unfolded sequentially with the direction of the arrows, representing the dynamic process of the formation and removal of the oxide layer. Among them, the green polygon represents the 4H-SiC wafer, the blue polygon represents the polishing slurry, and the brown hexagon represents the abrasive grains in the polishing liquid, and the yellow polygon represents the polishing pad. Reproduced with permission. [32] Copyright 2021, Elsevier. www.advmatinterfaces.de

Pre-Processing
Pre-processing of 4H-SiC wafers includes chemical softening and activation (e.g., thermal oxidation and laser irradiation) before CMP. [46][47][48][49][50][51][52][53][54][55] With the pre-processing thermal oxidation at the temperature of 1100 °C for 10 h the formation rate of the oxide layer at the surface of a 4H-SiC wafer reaches 29 nm h −1 . [48] The laser irradiation method refers to the irradiation of a 4H-SiC wafer by an ultra-fast laser, including femtosecond laser, nanosecond laser, and picosecond laser. [56,57] Due to its high pulse frequency and high energy density, the ultrafast laser introduces micro-pits or deep grooves at the surface of 4H-SiC wafers, which increases the effective contact area between the surface of 4H-SiC wafer and the polishing slurry during CMP. Hence, the efficiency of CMP may be improved. It was shown that the pretreatment of 6H-SiC wafer by a nanosecond fiber laser increased the MRR of CMP from 370 to 1700 nm h −1 . [57] By optimizing the laser processing parameters such as the laser power, repetition frequency, and scanning times through finite-element simulations and experimental research, the minimum width error of the laser-induced microgrooves reached 2 µm and provided insights on the mechanism of the laser ablation of SiC. [53] Among the pre-processing methods, the laser irradiation pretreatment can obtain a higher MRR without sacrificing the surface quality of a 4H-SiC wafer. This endows the laser irradiation pretreatment great potential in the improvement of the CMP of 4H-SiC wafers. As the diameter of 4H-SiC wafers is increasing to 200 mm, controlling the uniformity of laser ablation of the wafer without introducing new damages becomes the main challenge of the laser irradiation pre-processing. Moreover, the portability and controllability of the pre-processing approaches are common challenges for high-volume manufacturing of 4H-SiC wafers.

Photocatalysis-Assisted CMP
The photocatalysis-assisted CMP (PCMP) is based on the photo sensitized oxidation reaction. As shown in Figure 9a, three factors, including the light source, photocatalyst, and electron capture agent, are involved in the photocatalysis reaction. [32] The ultraviolet (UV) or mercury lamp is used as the light source. The metal oxides such as nano-TiO 2 or nano-ZnO are selected as the photocatalyst. H 2 O 2 or K 2 FeO 4 is used as the electron capture agent. [58] Taking the slurry containing TiO 2 and H 2 O 2 as an example, we show the reactions involved in a photocatalysis reaction as follows: [59] hv VB CB During PCMP, electrons and holes are first excited into the conduction band (CB) and valance band (VB) of TiO 2 under UVlight illumination. [60] The photo-generated electrons and holes then diffuse to the surface of TiO 2 . The electron capture agent Figure 8. Schematic diagram of a special-gas-atmosphere improved CMP system, including auxiliary devices such as pressure chamber and gas cylinder. A gas cylinder is added to a closed CMP system, which guarantees the special-gas-atmosphere during CMP. Reproduced with permission. [43] Copyright 2012, IOP Publishing.  [59] Copyright 2020, Elsevier. b) Image of the PCMP system with the UV lamp illuminated vertically above the polishing pad. . Please note that the photo-generated electrons and holes at the surface of TiO 2 are not stable. The photocatalytic reaction ends with the recombination of photo-generated carriers. Combined with Equation (5), enhancing the efficiency of the ROS generation is beneficial to improving the oxidation of 4H-SiC and thus enhancing the efficiency of CMP. [61] In 2017, the PCMP method was introduced into the polishing of 4H-SiC wafers. [62] 100 W UV LED was used as the light source. Nano-TiO 2 particles with an average diameter of 25 nm were used as the photocatalyst. H 2 O 2 are the electron capture agent. The MRR for the CMP of 4H-SiC wafers significantly increased from 200 to 352.8 nm h −1 . The Ra of PCMP-treated 4H-SiC was as low as 0.0586 nm. The catalytic effect was further improved by directly irradiating the polishing slurry, rather than the polishing plate. [63] Most notably, a diamond-based slurry containing TiO 2 with an average diameter of 7 nm and cathilon dye was developed for polishing 4H-SiC wafers under mercury lamp illumination, which resulted in the MRR and Ra of 18.78 µm h −1 and 1.99 nm, respectively. [64] The redox potential of the sulfate radical ( − SO 4 • ) (2.60-3.10 V) is higher than that of OH  [65,66] Hence, the sulfate radical-based advanced oxidation processes (SR-AOPs) are promising to improve the efficiency of PCMP. By activating potassium persulfate (K 2 S 2 O 8 ) with the synergy of UV and TiO 2 , the MRR and surface roughness (Sq [the root mean square deviation]) were 608 nm h −1 and 0.521 nm, respectively. The mechanism for the enhancement of the CMP efficiency based on SR-AOPs is summarized as: [32] SiC 2SO The PCMP method can improve MRR and maintain excellent surface quality. [58,59,64,[67][68][69][70] Furthermore, PCMP only requires adding a UV light source, which requires neither structural modifications to existing CMP equipments nor additional complex equipments. Therefore, PCMP holds great promise for the use with existing CMP procedures. However, there exist several challenges. Firstly, the dispersity of photocatalysts in the slurry needs to be guaranteed. PCMP processes often use nano-particles as photocatalysts. But such nanoparticles are highly prone to agglomeration in the aqueous phase, leading to decrease in the photocatalytic efficiency. This requires further improvement of the dispersion stability of the photocatalytic nanoparticles in the slurry. Secondly, the performance degradation of polishing pads undergoing prolonged UV illumination needs to be solved. Finally, the heat generated by prolonged UV-lamp illumination may affect the performance of the polishing slurry, which requires further investigation.

Catalyst-Referred Etching
The catalyst-referred etching (CARE) method belongs to an abrasive-free planarization method. As shown in Figure 10, the CARE device consists of a polishing pad with catalytic properties and a wafer holder, which are immersed in an etchant-filled vessel. [71,72] 2 , suggesting that HF molecules and fluorine (Fand − HF 2 ) ions were the active components during the CARE process. [74] In general, the highly ordered surfaces of 4H-SiC wafers can be observed after polishing using the CARE method. But the toxicity of HF cannot be ignored. Therefore, replacing HF with other environmentally friendly etchants should be considered in the future development of the CARE method.

Single-Abrasive Slurry
The polishing slurry usually contains only one kind of abrasives. Table 1 lists the Mohs hardness and Knoop hardness of common abrasive particles. [75] Various abrasives, such as cerium dioxide (CeO 2 ), alumina (Al 2 O 3 ), boron carbide (B 4 C), SiC, and nano-diamonds, have been employed to enhance mechanical action during the CMP of 4H-SiC wafers. In addition to the SiO 2 abrasives described above, Al 2 O 3 abrasives are also widely used for polishing 4H-SiC wafers given their hardness between that of 4H-SiC and SiO 2 . [76][77][78][79][80] After decades of development, the MRR and Ra of polishing 4H-SiC based on Figure 10. Schematic diagram of the CMP system equipped with CARE, which is composed of the polishing pad with catalytic properties, etching solution and wafer holder. Reproduced with permission. [71] Copyright 2014, Transtec Publications.  [80] CeO 2 abrasives were initially used in the CMP of Si 3 N 4 or SiO 2 materials. Although the hardness of CeO 2 is lower than that of SiO 2 , the surface of CeO 2 particles tends to form oxygen vacancies, which enables Ce ions to achieve an oxidation state (Ce 3+ /Ce 4+ ) transition. [81] This valence change reaction imparts CeO 2 particles with a unique chemical activity, enhancing the removal of oxidized 4H-SiC. Additionally, the SiOCe chemical tooth can increase the friction coefficient between the slurry and the 4H-SiC wafer during the CMP. Therefore, higher MRR (1089 nm h −1 ) and lower Ra (0.11 nm) were obtained by using CeO 2 as the abrasive and KMnO 4 as the oxidizing agent. [35] According to Yamamura et al.'s investigations, the effect of mechanical friction prevails over that of the chemical activity of CeO 2 abrasives during the CMP. [82] In particular, researchers have also attempted to polish SiC with nano-scale diamond abrasives to achieve elastic interaction and high MRR of 4H-SiC wafers, due to the superior hardness of the diamond material. [83][84][85][86] However, the higher MRR is usually achieved by sacrificing the surface quality, giving rise to the mechanical polishing of 4H-SiC wafers. Huang et al.'s recent work showed that the MRR increased with the increase of the size of diamond particles (from 100 nm to 1 µm), while the surface quality deteriorated. [86] When the diamond particle were 1 µm large, the MRR and Ra were 1.1 µm h −1 and 1.2 nm, respectively.
Since it is difficult to obtain high MRR by using SiO 2 -based slurries, researchers have tried to use Al 2 O 3 or diamond particles with higher hardness as abrasive particles. It has been found that a single nano-diamond slurry introduces new scratches or subsurface damage at the wafer surface, although it renders extremely high MRR. In this context Al 2 O 3 -based polishing slurry with hardness higher than SiO 2 but lower than 4H-SiC becomes an ideal choice. The preparation of alumina suspensions has been a research hotspot in the field of powder processing. [87] Especially, when considering the application in the CMP field, we should be aware that the dispersion stability is a more serious problem under the combined action of strong acid and strong oxidant. [88] In addition to hard abrasive particles, a single CeO 2 slurry with stronger chemical activity but lower hardness has also been used to polish 4H-SiC, which can achieve higher MRR (>1 µm h −1 ) without sacrificing surface quality. However, CeO 2 and Si atoms will combine to form SiOCe chemical teeth with strong binding energy, which presents challenges to the post-cleaning technology of residual CeO 2 particles at the surface of 4H-SiC wafer after the CMP. [89,90] In the meantime, the preparation of the rareearth material of CeO 2 requires complex purification processes, giving rise to the high price of commercial CeO 2 -based polishing slurries.

Mixed-Abrasive Slurry
A mixed-abrasive slurry (MAS) contains two or more types of solid-phase abrasives, [91] including ZrO 2 /SiO 2 , nano-diamond/ SiO 2 , polymeric magnesium iron aluminum zinc silicate/ SiO 2 , SiO 2 /CeO 2 , and ZrP/Al 2 O 3 . [92][93][94][95][96] Compared with the single-abrasive slurry, a MAS exhibits better CMP performance. It has been found that adding 10 wt% diamond abrasives to SiO 2 slurry could increase the MRR from 60 to 600 nm h −1 and reduce the Ra from 68.3 to 0.55 nm. [97,98] The CMP performance (MRR: 920 nm h −1 , Ra: 0.52 nm) was further improved after adding 10 vol% NaClO to the preceding MAS. It was proposed that the added diamond particles not only enhanced the grinding action of the slurry, but also increased the local pressure at the 6H-SiC wafer surface. Hence, the tribo-chemistry reaction at the SiC/slurry interface was promoted.
Recently, a novel MAS system based on graphene oxide (GO)-based aqueous nanosuspensions was developed to polish 4H-SiC wafers. [86] As shown in Figure 11a,b, monocrystalline diamond particles (particle size: 1 µm) and multi-layered GO nanosheets were mixed into a slurry with a mass ratio of 1:8. The MAS resulted in the MRR of 0.8-0.9 µm h −1 and the surface roughness is 0.6-0.7 nm. The synergistic mechanism is proposed in Figure 11c. In general, GO nanosheets were reduced by friction action during the CMP. The generated ⋅ OH and • COOH radicals promoted the oxidation reaction at the 4H-SiC surface. Meanwhile, the reduced GO nanosheets could be deposited on the surface of SiC, forming a lubricating film and reducing the formation of scratches by diamond abrasives. In addition, Wang et al. designed a MAS system containing ZrO 2 particles with the size of 50-100 nm and Al 2 O 3 particles with the size of 300-400 nm by high-energy ball milling (Figure 12a). [99] The resulting MRR and Ra were 694 nm h −1 and 0.489 nm, respectively. As shown in Figure 12b, ZrO 2 with photocatalytic activity could enhance the chemical action of the slurry, and the small-sized ZrO 2 particles and large-sized Al 2 O 3 particles played a mechanically complementary role in the material removal process.
In summary, MAS technologies have potential applications in the CMP of 4H-SiC wafers. [21,100] But technical issues including the MAS preparation and storage and the post-CMP cleaning need to be further investigated.

Core-Shell Abrasive
As shown in Figure 13, a hard particle is commonly used as the core material, which is coated with the shell material with low hardness or better chemical activity. [101] It is promising that the resulting core-shell particle synthesized with a specific process has the advantages of both the core and shell materials. Typical core-shell abrasives and their CMP performance for SiC wafers are summarized in Table 2. [101][102][103][104][105][106] It is clear that the structures of core-shell abrasives are designed for specific polishing processes. Especially, in the PCMP process, TiO 2 is used as the shell material due to its photocatalytic activity. In the electrochemical mechanical polishing (ECMP) process, CeO 2 is selected as the shell material because of valence change. It is interesting that the MRR of ≈2.3 µm h −1 and Ra of 0.449 nm were achieved using polystyrene/CeO 2 core-shell abrasives in the ECMP. [105] When polyurethane or polystyrene is used as the core particle, the abrasive particles have a better elastic effect, making the material removal mechanism shift from the ploughing regime to the adhering regime. This is conducive to obtaining a good surface quality after polishing.

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Comprehensive literature analysis shows that designing corresponding core-shell abrasive particles for a specific polishing process may enhance the efficiency of CMP of 4H-SiC wafers. However, current core-shell abrasive particles have the shortcomings of a complicated preparation process and low uniformity of shell coating, which makes the method difficult to do under the industrial scale. [107]

Fixed or Semi-Fixed Abrasive
As shown in Figure 14, fixed-abrasive polishing (FAP) can improve the utilization efficiency of the abrasives by fixing the abrasives on the polishing pad through hot-pressing consolidation, UV curing, or sol-gel technology. [108] The resinbonded ultra-fine diamond abrasive polishing pad was prepared Figure 11. Mixed-abrasive slurry containing diamond particles and graphene oxide nanosheets: a) SEM image of diamond particles, b) SEM image of graphene oxide nanosheets with oxidation degree of 45%, and c) schematic diagram of improvement mechanism of mixed abrasive slurry, in which the graphene oxide nanosheet promotes free radical generation and reduces diamond grinding induced scratches. Reproduced with permission. [86] Copyright 2021, Elsevier. Figure 12. a) SEM image of mixed-abrasive slurry containing alumina particles (300-500 nm) and zirconium dioxide particles (50-80 nm). b) Synergistic mechanism of mixed abrasive particles during CMP, because the small-sized ZrO 2 particles and large-sized Al 2 O 3 particles play a mechanically complementary role in the material-removal process. Reproduced with permission. [99] Copyright 2022, Elsevier.

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by electrophoretic co-deposition process, with which the Ra dropped from 90.3 to 4.3 nm after polishing the Si-face of SiC wafers in 1 h. [109] In another study, the MRR increased from 450 nm h −1 to 1.08 µm h −1 when replacing the polyurethane pad with a fixed-diamond-abrasive pad. [110] The FAP process can also achieve chemical efficiency enhancement during the CMP of SiC wafers by immobilizing various materials. A polishing pad with fixed Fe powder and Al 2 O 3 abrasives was designed, and then used together with an abrasive-free polishing slurry containing 20 wt% H 2 O 2 to polish 4H-SiC wafers, leading to an MRR of 740 nm h −1 and Sa (the mean difference in height from the mean plane) of 0.159 nm. [111] Furthermore, semi-fixed abrasive polishing (Semi-FAP) has been proposed, in which the abrasive can be moved up and down within a limited range of the pad surface by the action of soft binding agents, such as unsaturated resins and sodium alginate salts. [109,112,113] Compared with the FAP process, it is easier for Semi-FAP to obtain an atomically smooth surface of a 6H-SiC wafer. [114] To date, FAP and Semi-FAP have been shown to significantly improve the MRR of 4H-SiC materials, while simultaneously solving the stability problem of free abrasive particles in the slurry. However, the fixed diamond particles generate fragments due to the abrasion of 4H-SiC wafers during the CMP process. These fragments can introduce severe scratches and subsurface damage if not removed from the polishing pad in time.

Electro-Chemical Mechanical Polishing
Conventional CMP technologies use chemical reagents such as H 2 O 2 , KMnO 4 , and K 2 S 2 O 8 as oxidizing agents. An alternative strategy to improve the efficiency of CMP relies on the introduction of anodization based on electrochemical reactions, which is referred to as ECMP. [105,106,[115][116][117][118][119][120][121] A common setup for the ECMP is schematically displayed in Figure 15. [117] A constant voltage or current is applied to the conductive path formed by the electrolyte (i.e., slurry), 4H-SiC wafer and copper plate by using a potentiostat. In the applied electric field the surface of the 4H-SiC wafer (i.e., anode) rapidly reacts to form an oxide layer. Then the oxide layer is removed by abrasives in the electrolyte. It can be seen from Figure 16 that the transition oxide SiO x C y was rapidly formed on the surface of the 4H-SiC wafer after 2 min polishing. This softened layer is mechanically removed by the abrasive particles in the slurry. Subsequently, Figure 13. Transmission electron microscopy image of SiO 2 coating on diamond abrasives, with the thickness of the SiO 2 coating being 5 nm. The dark area in the lower right corner of the figure represents the diamond particles, and the light area near the diagonal area of the picture represents the SiO 2 film. Reproduced with permission. [101] Copyright 2017, Elsevier.  Figure 14. Schematic diagram of the fixed abrasive polishing, where the abrasive particles are fixed on the surface of the polishing pad. Reproduced with permission. [108] Copyright 2022, Springer Nature.

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the new surface exposed on the wafer surface is oxidized again. Ultimately, the dynamic cycle of "oxidation-removal" results in the planarization of 4H-SiC wafers. Since the occurrence of the cycle of "oxidation-removal" is dynamic, the appearance and removal of SiC x O y is also transient. Therefore, this method can effectively improve the oxidation rate of SiC material. [122] The type of electrolyte has an important influence on the ECMP results. Using KNO 3 and SiO 2 as the electrolyte, and a continuously applied constant current of 20 mA cm −2 , Li et al. obtained that the MRR and Sq were 400-500 nm h −1 and 10.52 nm, respectively. [123] By contrast, if NaCl solution was used as the electrolyte, the most outstanding MRR (Sq) was 23 µm h −1 (1.352 nm) at the current of 10 mA cm −2 . [117]

Plasma-Assisted Polishing
Plasma-assisted polishing (PAP) is a novel polishing technology combining the irradiation of atmospheric pressure plasma and soft abrasive polishing. [124][125][126][127][128][129] Figure 17a shows that PAP requires the addition of a plasma-assisted module consisting of a radio-frequency electric power, a helium-based water vapor generator, and an ultrapure water bubbling machine to the CMP equipment. [130] In 2010, some researchers employed highfrequency electricity to produce plasma from water vapor containing helium, which was then delivered to the wafer surface by the bubbling method. [131] The surface of a 4H-SiC wafer was oxidized under the plasma. The resulting oxide layer was then mechanically removed by CeO 2 abrasives. The hardness of the 4H-SiC surface was reduced from 37.38 to 4.53 GPa. The root mean squared roughness of the 4H-SiC wafer after the PAP was 0.28 nm. Further work indicated that the presence of ⋅ OH played a dominant role in the oxidation of the 4H-SiC wafer. [132] Figure 17b shows the cross-sectional transmission electron microscope image of a 4H-SiC wafer after 1 h of plasma irradia-tion. The middle layer was an 80 nm thick oxide layer. [133] It is interesting that the SiO 2 /SiC boundary was atomically smooth. Although the formation of the atomically smooth boundary was not fully understood, it was considered to be critical to the planarization of the PAP.

Other Synergistic Approaches
The improvement of the performance of CMP results from the synergy of the chemical oxidation and mechanical removal process. When the mechanical removal is weaker than the chemical oxidation, the oxide layer at the surface of 4H-SiC wafers cannot be timely removed. This hinders the oxidation and ultimately lowers the MRR. On the contrary, if the mechanical removal is too strong, scratches may be introduced at the surface of a 4H-SiC wafer during CMP. Therefore, researchers have investigated synergistic efficiency enhancement to achieve a balance between mechanical and chemical interactions. As summarized in Table 3, several efficiency-enhancing methods concerning Fenton-like reaction, core/shell abrasive particles, FAP, PCMP, and MAS have been combined to form a hybrid polishing system. [43,54,99,102,105,111,[134][135][136][137][138][139] It can be seen that the synergistic approaches including Fenton-like reaction combined with FAP, ECMP combined with core-shell abrasives, and PCMP combined with MAS help simultaneously achieve high MRR and good surface quality (Ra < 0.5 nm).
Since the efficiency of CMP is limited by the oxidation of 4H-SiC, researchers have developed synergistic oxidation approaches such as the plasma-assisted electrochemical oxidation, the ultrasonic-assisted electrochemical oxidation, and the electro-assisted photocatalysis oxidation. [140][141][142][143] Among these approaches, the highest oxidation rate of SiC reaches 57.1 nm min −1 when the anodic oxidation is employed. This provides a clue for the future improvement of the CMP for 4H-SiC wafers. [144] The synergistic enhancement methods are capable of balancing the chemical and mechanical interactions during CMP. They are the most attractive for the improvement of the CMP of 4H-SiC wafers. In the meantime, we should bear in mind that reducing the cost of CMP is one of the most important issues for the commercialization of synergistic enhancement methods.

Conclusions and Prospects
In summary, we have reviewed the CMP of 4H-SiC wafers. Although CMP has achieved great success on the processing of 4H-SiC wafers, [145,146] chemical, mechanical, and chemicalmechanical synergistic improvement approaches have been proposed to enhance the MRR of CMP, and improve the surface quality of 4H-SiC wafers.
For chemical improvement approaches, the Fenton-like reaction may improve the efficiency of CMP with low cost. But residual Fe ions after CMP pose a challenge for post-CMP cleaning. For the emerging PCMP, the long-term stability of the photocatalyst in the slurry is an important issue. Furthermore, the temperature fluctuation induced by Figure 15. Schematic diagram of electro-chemical mechanical polishing device, where the SiC wafer, electrolyte, and copper plate form an electronically conductive path. Reproduced with permission. [117] Copyright 2019, Elsevier.
www.advmatinterfaces.de long-time UV irradiation during PCMP should be monitored and controlled.
For mechanical improvement approaches, alumina-or ceriabased MAS and core-shell abrasive particles have been regarded as excellent components for novel polishing slurries. However, the dispersion stability of the nanoparticles needs to be optimized to increase the reliability and reproducibility of the highvolume fabrication of MAS or core-shell abrasive slurry.
For chemical-mechanical synergistic improvement approaches, a variety of new synergistic enhancement methods have been developed to improve the balance of chemical and mechanical interactions during the CMP of 4H-SiC wafers. It is suggested that future research could be extended to combine ECMP or PAP with traditional CMP methods to develop new processes for two-step polishing. In addition, understanding on the oxidation kinetics is crucial to Figure 16. Comparison of XPS results after 2 min ECMP: (a) and (c) are the comparison of Si 2p and C 1s spectra before and after ECMP treatment; (b) and (d) correspond to Si 2p peak separation before and after treatment, respectively. Obvious oxide peaks of SiC x O y appeared in the XPS spectrum of 4H-SiC wafers after ECMP treatment. Reproduced with permission. [122] Copyright 2018, Elsevier. Figure 17. a) Schematic diagram of the PAP device, which consists of an independent external plasma generator and a conventional CMP system. Reproduced with permission. [130] Copyright 2012, Transtec Publications. b) Cross-sectional TEM image of the 4H-SiC wafer after PAP treatment for 1 h, showing a smooth SiC/SiO 2 boundary formed on the surface of SiC. Reproduced with permission. [133] Copyright 2013, Elsevier. www.advmatinterfaces.de the improvement of PAP and ECMP. The composite polishing pad that associates photocatalysts with FAP is another key topic to discuss. The challenges of these approaches include the compatibility of compounding processes, technical fitting to customized wafer processing (e.g., different requirements of surface quality, thickness, and flatness), and fluctuation in processing cost.
To date, the emerging CMP efficiency enhancement technologies are still in the industrial validation stage of moving from the lab to the fab. Currently, PCMP and MAS technologies are showing potential in the industrial standard. In terms of leading on a research level, PAP and ECMP are considered to be more forward-looking. Xie et al. [54] a) Pre-processing b) Special gas