Surface Modification and Microstructuring of 4H-SiC(0001) by Anodic

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Surfaces, Interfaces, and Applications

Surface Modification and Microstructuring of 4H-SiC (0001) by Anodic Oxidation with Sodium Chloride Aqueous Solution Xu Yang, Rongyan Sun, Kentaro Kawai, Kenta Arima, and Kazuya Yamamura ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19557 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

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ACS Applied Materials & Interfaces

Surface Modification and Microstructuring of 4HSiC (0001) by Anodic Oxidation with Sodium Chloride Aqueous Solution Xu Yang,† Rongyan Sun,† Kentaro Kawai,† Kenta Arima,† and Kazuya Yamamura*,†

Division of Precision Science & Technology and Applied Physics, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

KEYWORDS. Silicon carbide, anodic oxidation, surface modification, porous SiC, SiC fiber.

ABSTRACT. Anodic oxidation is a promising surface modification technique for the manufacture of SiC wafers owing to its high oxidation rate. It is also possible to fabricate porous SiC by anodic oxidation and etching owing to the material properties of SiC. In this study, the anodic oxidation of a 4H-SiC (0001) surface was investigated by performing repeated anodic oxidation and hydrofluoric acid etching on a 4H-SiC (0001) 1 ACS Paragon Plus Environment

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surface, during which the formation of porous SiC was observed and studied. Anodic oxidation is very effective for removing the surface damage formed by mechanical polishing, and the surface after removing the surface damage can be oxidized uniformly and has a higher oxidation rate than a surface newly finished by chemical mechanical polishing (CMP). We proposed a model based on the electrochemical impedance method to explain the difference in the oxidation between an as-CMP-finished surface and an oxidized/etched surface. Porous SiC was obtained in this study, which was due to the anisotropy of the SiC crystal. The structure of the porous SiC was significantly dependent on the etch pits generated at the beginning of anodic oxidation and can be controlled via anodic oxidation parameters. Anodic oxidation and hydrofluoric acid etching cannot remove porous SiC owing to the anisotropic oxidation of the SiC surface and the difficulty of anodizing SiC fibers. The study shows that anodic oxidation is a promising technique for the modification of SiC surfaces and the fabrication of porous SiC.

1. INTRODUCTION

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With the development of electric devices, silicon is reaching its limits owing to its narrow bandgap and relatively poor thermal properties. SiC is an ideal material to replace silicon for power-device applications in harsh environments owing to its wide bandgap, high breakdown field, and excellent thermal properties 1. SiC is also widely used as baseplates for the epitaxial growth of other functional materials for electronic sensors, catalytic electrodes, and microelectromechanical systems (MEMS) 2-4. A Smooth and damage-free surface is essential in these applications of SiC because surface roughness strongly affects the performance of these devices 5,6. However, SiC is difficult to machine owing to its high hardness and chemical inertness, and polishing using hard abrasives always causeds surface damage. Therefore, many techniques have been proposed to modify the SiC surface to a soft material, which is then removed using soft abrasive particles. These techniques include chemical mechanical polishing (CMP) 7, plasma oxidation 8,9, thermal oxidation 10, ultraviolet oxidation 11,12, photoelectrochemical oxidation13, and anodic oxidation 14-17. At present, CMP is industrially used for the manufacture of SiC substrates; however, its material removal rate (MRR) is very low and generally less than 0.5 μm/h. The polishing techniques 3 ACS Paragon Plus Environment


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applying plasma oxidation, thermal oxidation, and ultraviolet oxidation are still under development. Although the MRRs of these techniques (approximately 1 μm/h) are greater than that of CMP, the MRRs and technical maturity are still not satisfactory for practical industrial applications. On the other hand, the anodic oxidation rate of SiC is very high and SiC can be continuously oxidized to a depth of micrometer order owing to the porosity of the oxide layer 18. The MRRs of polishing techniques using anodic oxidation can reach 3－4 μm/h 16,17, making anodic oxidation a very promising surface modification method for SiC. Investigation of the anodic oxidation properties of SiC is very important for its future applications. On the other hand, porous SiC surfaces are an attractive research focus owing to their large internal area and high chemical activity, and there are used in energy storage 19,

electromagnetic interference shielding 20, dielectrically isolated electronic devices 21,

and so forth. Because of its excellent optical properties, porous SiC can be used in the field of optoelectronics 22 as well as in optical mirrors 23 in harsh environments owing to its excellent thermal properties and stability. At present, porous SiC is mainly fabricated by metal-assisted photochemical etching (MAPCE) 23,24 and reaction bonding 4 ACS Paragon Plus Environment

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techniques 25,26. In MAPCE, platinum is deposited on the surface of SiC to serve as a catalyst during the porosification process, and the fabrication process is complicated and expensive. In reaction bonding techniques, porous SiC is fabricated by subjecting SiC nanoparticles to cold isostatic pressing under different conditions followed by sintering at a high temperature, then the sinter material is dipped in appropriate solutions to remove the sintering agent, leaving porous SiC. In this process, the orientation of SiC cannot be controlled and the residual sintering agent reduces its electronic performance and degree of interconnectivity. Therefore, a new technique for the fabrication of porous SiC is required. In this study, we investigated the anodic oxidation of a 4H-SiC (0001) surface and obtained porous SiC by repeated anodic oxidation in sodium chloride (NaCl) aqueous solution and hydrofluoric acid (HF) etching. Firstly, the effects of surface damage on the anodic oxidation properties of SiC were studied and the anodic oxidation process was modeled on the basis of the electrochemical impedance method. Then, the generation of porous SiC was observed and its formation mechanism was clarified. It was found that SiC surfaces without damage can be relatively uniformly oxidized and that the 5 ACS Paragon Plus Environment


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oxidized area plays an important role in determining the oxidation rate. Moreover, porous SiC can be fabricated by anodic oxidation and etching owing to the anisotropy of the SiC crystal and the difficulty of anodizing SiC fibers. The results of this study are expected to promote the application of anodic oxidation to the surface modification of SiC and they confirm the applicability of anodic oxidation to the fabrication of porous SiC.

2. EXPERIMENTAL SETUP A CMP-finished single-crystal 4H-SiC substrate (on-axis, n-type) supplied by TankeBlue Semiconductor Co. Ltd. with a thickness of 350 μm and a specific resistance of in the range of 0.015 – 0.028 Ω·cm was used. Before the experiment, the substrate was prepared by wet chemical cleaning. First, the substrate was dipped in a mixed solution of sulfuric acid (H2SO4) (97 wt%) and hydrogen peroxide (H2O2) (34 wt%) with a volume ratio of 4 : 1 for 10 min to remove contaminants. Then, it was dipped in concentrated HF solution (50 wt%) for 10 min to remove native oxides. Finally, it was rinsed with deionized water for 10 min and blow-dried with pure nitrogen (N2) gas. 6 ACS Paragon Plus Environment

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Oxidation and etching experiments were conducted on the (0001) face, which is the most commonly used face in electric power devices. The electrolyte used in this study was NaCl aqueous solution, and its concentration and electric conductivity were 1 wt% and 1.8 S/m, respectively. Before the oxidation and etching experiments, the substrate was

Figure 1. X-ray diffraction (XRD) spectra of as-received 4H-SiC substrate. The (0001) surface was irradiated with X-rays and the Bragg angle was varied from 30° to 40°.

observed by X-ray diffraction (XRD) to confirm its purity. Figure 1 shows the XRD spectrum; only an intense X-ray peak at 35.6°corresponding to hexagonal 4H–SiC was observed, confirming the purity of the 4H-SiC substrate. 7 ACS Paragon Plus Environment


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Figure 2 shows the experimental setup (Model K0235 Flat Cell supplied by Princeton Applied Research). The sample was mounted on a working electrode in a conventional three-electrode cell with a platinum mesh as the counter electrode, and a constant potential of 7 V referred to a Ag|AgCl reference electrode was applied to induce anodic oxidation over an area of 1 cm2. Two sets of experiments were conducted to investigate the repeatability of oxidation/etching and the effects of oxidation time on the surface topography of the oxidized/etched surface. The experimental procedure is shown in Figure 3. In both sets of experiments the SiC substrate was first oxidized for 10 min, then it was dipped in HF solution for 60 min to remove the surface damage formed in the wafer manufacturing process. Then, in the first set, after removing the surface damage, the surface was alternately processed by anodic oxidation (1 min) and HF


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Figure 2. Anodic oxidation setup. A constant potential of 7 V referred to a Ag|AgCl reference electrode was applied to induce anodic oxidation over an area of 1 cm2.

Figure 3. Experimental procedure. Two sets of experiments were conducted. The oxidized/etched surfaces are denoted as surfaces 1-i to 1-v in the first set of experiments and surfaces 2-i to 2-v in the second set of experiments.

etching (30 min) four times. In the second set, the surface was alternately processed by a combination of anodic oxidation (1 min) and HF etching (30 min) twice followed by a combination of anodic oxidation (10 min) and HF etching (60 min) twice. The oxidized/etched surfaces are denoted as surfaces 1-i to 1-v in the first set of experiments and surfaces 2-i to 2-v in the second set of experiments. The surface



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morphologies were observed by atomic force microscopy (AFM, SPM 9700) and scanning electron microscopy (SEM, Hitachi S-4800) after every anodic oxidation or HF etching, and the distribution of oxygen on the oxidized surfaces was observed by energy dispersive X-ray spectrometry (EDX).

3. RESULTS AND DISCUSSION 3.1 Effects of surface damage on the oxidation performance of SiC Figure 4 shows the results of the first two oxidation/etching steps. From Figure 4(a), although the as-received surface used in this study had a clear terrace step structure, the terraces were not straight and had many defects, and a subsurface damage layer existed on them. These structural defects are preferentially oxidized in the anodic oxidation process, leading to the generation of oxide protrusions 27,28. The SiC surface was fully covered by oxide protrusions after oxidation for 10 min, as shown in Figure 4(b), suggesting that the structural defects on the surface were fully oxidized. Figure 5 shows the oxidation depth after removing the oxide layer; an oxidation depth of about 70 nm was obtained. The depth of the damage layer generated during mechanical 10 ACS Paragon Plus Environment

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polishing using diamond abrasives is about 50 nm, whereas there was almost no surface damage on the CMP-processed surface except some slight crystallographic damage 29-31. Therefore, surface 1-i, as shown in Figure 4(c), was considered to be ‘damage-free’ even though there were many etch pits on it. Surface 1-i was oxidized again for 1 min. As shown in Figure 4(d), many small oxide protrusions were generated, which are distributed uniformly on the oxidized surface.

Figure 4. AFM images of SiC surfaces. (a) As-received surface. (b) Surface oxidized for 10 min. (c) Surface 1-i. (d) Surface 1-i oxidized for 1 min.



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Figure 5. Oxidation depth of as-received surface after oxidation for 10 min. The surface after removing the oxide was observed by a scanning white light interferometer (SWLI). The oxidation depth was about 70 nm according to the cross-sectional view along A-B.

Figure 6 shows the EDX measurement results for the oxidized surfaces obtained by the oxidation of an as-received surface and surface 1-i at 7 V for 1 min. As shown in Figure 6(a) and (b), the oxidation of the as-received surface was not uniform owing to the generation of large oxide protrusions, and the distribution of oxygen on the oxidized surface was not uniform and was concentrated at the sites of the oxide protrusions and the lines corresponding to subsurface damage. In contrast, as shown in Figure 6(d) and (e), the oxidation of surface 1-i was relatively uniform, the oxygen on the surface was


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uniformly distributed, and no preferential oxidation occurred even in the regions of scratches. This result indicates that the surface was uniformly oxidized after removing the surface damage.

Figure 6. EDX measurement results of as-received surface and surface 1-i after oxidation for 1 min. (a) SEM image of as-received surface after oxidation, its (b) distribution map of oxygen and (c) element spectrum. (d) SEM image of surface 1-i after oxidation, its (e) distribution map of oxygen and (f) element spectrum. The oxidation of the as-received surface was not uniform whereas the oxidation of surface 1-i was uniform.



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Surface damage, such as scratches, on a SiC surface is always accompanied by a series of edge dislocations, screw dislocations 32, deformations, microcracks 33, and stacking faults 34. These defects induce residual strain and change the material properties of SiC. Research conducted by Jia et al. shows that the atomic bonding at dislocation sites is weaker than that in a perfect SiC crystal and that the activation energies of dislocations are lower than those in a perfect SiC crystal, resulting in a higher etching rate at dislocation sites 35. The oxidation rate at defect sites is also higher than that at defect-free sites in the dry thermal oxidation of SiC 36, and the difference is greater in wet thermal oxidation using a water vapor and oxygen flow owing to the weaker atomic bonding and local polarity inversion at the sites of defects 37. Defects weaken the bonds between atoms and affect the material properties of SiC, resulting in defect sites having higher chemical activity than defect-free areas. These effects also occurred in the anodic oxidation of SiC, which resulted in the preferential oxidation of defects, as shown in Figure 6(a) and (b). Figure 7 shows the changes in current during the oxidation of the as-received surface and surface 1-i. The current during the oxidation of surface 1-i was greater than that 14 ACS Paragon Plus Environment

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during the oxidation of the as-received surface in the initial oxidation stage, and the trends of the current during the two oxidations were different. The spectra shown in Figure 6(c) and (f) suggest that the amount of oxide on the as-received surface after oxidation was less than that on surface 1-i after oxidation because a smaller oxygen peak was observed. In the oxidation of the as-received surface, the change in the current can be divided into three stages: an initial decrease followed by an increase and a second decrease. Combining this with the EDX measurement result shown in Figure 6(b), it was deduced that the region of subsurface damage was preferentially oxidized, and the initial decrease in the current is considered to correspond to the oxidation of the subsurface damage and the passivation of the region without subsurface damage 27. The resistance increased after the region of subsurface damage was covered with an oxide layer, and the current flowed to the region without subsurface damage, leading to the breakdown of the passivation film and the subsequent increase in the current. When the entire surface was covered



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Figure 7. Changes in current during anodic oxidation of (a) as-received surface; the inset shows an enlarged image of the changes in the current during the initial oxidation stage, and (b) surface 1-i. Both anodic oxidations were started at 10 s. The trends of the current during the two oxidations are different.

with an oxide layer, the current decreased for the second time. During the oxidation of surface 1-i, the current decreased monotonically with oxidation time. As there was no subsurface damage on surface 1-i, there was only one stage in the oxidation of the


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surface; the oxidation of the whole surface occurred simultaneously and the current decreased monotonically with the growth of the oxide layer. The difference in oxidation rate between the two oxidations was studied using the electrochemical impedance method, as shown in Figure 8. The impedance of the anodic oxidation system of SiC can be expressed as equation (1), which includes three parts: those of the electrolyte ZE, the SiC surface ZSS, and the bulk SiC ZBS. (1) The impedance of the SiC surface ZSS can be divided into three parts, the impedance of the oxide layer, the impedance of the interface between the oxide layer and the bulk SiC, and the impedance of the space charge layer,

.

(2)

Here, Rox is the resistance of the oxide layer, which can be expressed as equation (3),

Cscl is the capacitance of the space charge layer, which can be expressed as equation (4), Rct is the resistance of the space charge layer, and RIF is the resistance of the interface between the oxide layer and the bulk SiC. Since 4H-SiC is an anisotropic



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crystal, the oxidation rate changes with the orientation, and after the generation of etch pits, not only the (0001) face but also the other faces were exposed to the electrolyte. Therefore, with the increase in the surface area, the area of the other faces exposed to the electrolyte also increased, which resulted in a change in RIF. Thus, RIF can be expressed as a function of surface area by equation (5),

,

(3)

,

(4)

,

(5)

where S is the oxidized area of the surface, dox is the thickness of the oxide layer, ρ is the resistivity of the oxide layer, ε0 is the vacuum dielectric constant, ε is the dielectric constant of the space charge layer, dscl is the thickness of the space charge layer, RIF0 is the initial resistance of the interface, and α is the scaling factor. Therefore, the impedance of the anodic oxidation system can be expressed as

,

(6)


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,

(7) where RSiC and Rsol are the resistances of the bulk SiC and the electrolyte, respectively, which can be considered as constants in the anodic oxidation process. ω is the angular frequency of the alternating current, which is 0 in our experiment because a direct current was applied in this study. Therefore, the impedance can be expressed as

.

(8)



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Figure 8. Model of anodic oxidation of SiC using the electrochemical impedance method.

Figure 9. Relationship between surface area and oxidation current density. (a) First set. (b) Second set. The surface area was calculated from the AFM images using AFM data processing software, where the observation area was 1 μm × 1 μm.


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From equation (8), it is clear that the impedance of the anodic oxidation system decreased with increasing surface area. On the basis of Ohm’s law, the current density also decreased with increasing surface area. In the initial oxidation stage of the asreceived surface, the current was very small owing to the limited damage on the CMPprocessed surface, and as the oxidation area expanded to the whole surface, the current increased. In the oxidation of surface 1-i, as the oxidation area increased, as shown in Figure 8, the impedance of the SiC surface ZSS2 decreased, resulting in a much higher oxidation rate. The relationship between surface area and current density was investigated in all the oxidation experiments. The area of the oxidized/etched surface was considered as the oxidation area at the end of the final oxidation step, which was calculated using the AFM data processing software over the observation area of 1 μm × 1 μm. Figure 9 shows the relationship between surface area and oxidation current density at the end of each oxidation step. Both sets of experiments showed the same trend, that is, the current density changed with the oxidation area, suggesting that the oxidation area has a dominant effect on the oxidation rate.



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3.2 Formation of porous SiC in the oxidation/etching process Figure 10 shows the changes in the surface morphology with the repeated anodic oxidation and HF etching. Many etch pits and fibers were observed on the surface. With the progress of oxidation and etching, the fibers became thinner and small etch pits generated in the original etch pits caused the generation of new fibers. Figure 11 shows the EDX measurement results of surface 1-i shown in Figure 10(a), which clearly indicates that there was no oxide on the surface. Therefore, the fibers among the etch pits and the protrusions at the centers of the etch pits were formed of SiC; these etch pits and SiC fibers constituted the porous SiC. This suggests that the pit size and fiber size of porous SiC can be controlled via the anodic oxidation parameters.


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Figure 10. SEM images of oxidized/etched surfaces. (a) Surface 1-i. (b) Surface 1-ii. (c) Surface 1-iii. (d) Surface 1-iv. (e) Surface 1-v. Porous SiC was observed.



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Figure 11. EDX measurement results of surface 1-i. (a) SEM image of surface 1-i. (b) Distribution map of silicon element. (c) Distribution of carbon element. (d) Distribution of oxygen element. (e) Element spectrum of surface 1-i. Oxygen was not observed on surface 1-i. Figure 12 shows a SEM image of surface 1-ii. It was found that small etch pits were mainly generated along (blue arrows) or in the opposite direction (red arrows) to the , , and directions on the SiC protrusions located in the etch pits. 4H-SiC is an anisotropic crystal and has a higher oxidation rate along these three directions, resulting in the generation of hexagonal etch pits in the oxidation/etching process 38-40. After the first oxidation/etching process, (11-20), (-2110), and (1-210) faces were exposed to the electrolyte owing to the generation of etch pits. These faces were rapidly oxidized in the second oxidation process, resulting the expansion of the etch pits and the generation of small etch pits on the SiC protrusions. The generation of small oxide protrusions during the oxidation of surface 1-i, as shown in Figure 4(d), is considered to be the result of preferential oxidation on these faces. This mechanism dominated all the oxidation/etching processes. As a result, porous SiC was still 24 ACS Paragon Plus Environment

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observed after oxidation/etching five times as shown in Figure 10(e). This suggests that the structure of porous SiC is significantly dependent on the etch pits generated at the beginning of the oxidation process and that an atomically smooth surface cannot be obtained by repeated anodic oxidation and etching. Figure 13 shows the changes in the surface roughness Sz with the progress of oxidation and etching observed by AFM. It was found that Sz increased during the entire experiment owing to a few SiC fibers remaining unchanged with the repeated anodic oxidation and etching. Figure 14 shows SEM images of surfaces 2-iv and 2-v. Although these surfaces were oxidized for a longer time of 10 min, it is clear that many SiC fibers remained on the surface, and the surface roughness Rz was greater than that of surfaces 1-iv and 1-v, which were oxidized for only 1 min. These results suggest that the SiC fibers were difficult to oxidize.



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Figure 12. SEM image of surface 1-ii. Small etch pits were mainly generated along (green arrows) or in the opposite direction (red arrows) to the , , and directions on the SiC protrusions.

The difficulty of dissolving nanometer-thick SiC fibers was attributed to the Fermi level pinning that occurred on the fibers 41. As soon as each SiC fiber became thin, the space charge layer on both sides of the SiC fiber merged and formed a space charge layer passing through the bottom of the SiC fiber, which caused the electrical isolation of the


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main part of the SiC fiber from the crystal. Therefore, further fiber oxidation was impossible and the thin SiC fiber remained stable. Thus, the porous SiC is considered to have formed as a result of the anisotropic oxidation of SiC and the difficulty of dissolving nanometer-thick fibers.

Figure 13. Changes in surface roughness with the progress of oxidation and etching obtained by AFM over an observation area of 1 μm × 1 μm. The surface roughness increased with the progress of oxidation and etching.



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Figure 14. SEM images of oxidized/etched surfaces in the second set of experiments. (a) Surface 2-iv. (b) Surface 2-v. Many SiC fibers remained on the surface and were difficult to oxidize.

4. CONCLUSIONS Repeated anodic oxidation and HF etching were conducted on a 4H-SiC (0001) face, and the effects of surface damage on the anodic oxidation properties were investigated. Anodic oxidation is very effective for removing the surface damage formed by mechanical polishing. However, the surface was easier to oxidize after removing the surface damage by oxidation/etching and the oxidation was relatively uniform. This is favorable for obtaining a SiC substrate with a smooth surface by polishing with the assistance of anodic oxidation. We proposed a model based on the electrochemical impedance method to study the differences in the oxidation of surfaces with and without


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damage, which showed that the oxidation area plays a dominant role in determining the oxidation rate of the SiC surface. Porous SiC was generated owing to the anisotropy of the SiC crystal and the Fermi level pinning that occurred on the nanometer-thick SiC fibers. The structure of the porous SiC was significantly dependent on the etch pits generated at the beginning of the oxidation and can be controlled via the anodic oxidation parameters. These results are expected to promote the use of anodic oxidation in the polishing of SiC surfaces and the fabrication of porous SiC.

AUTHOR INFORMATION

Corresponding Author * Kazuya Yamamura

E-mail: [email protected]

Present Addresses †Division of Precision Science & Technology and Applied Physics, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 29 ACS Paragon Plus Environment


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ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Challenging Research (Exploratory) (18K18810) from the MEXT, Japan, a research grant from the Mitsutoyo Association for Science and a research grant from the Technology and Machine Tool Engineering Foundation.

ABBREVIATIONS CMP, Chemical mechanical polishing; XRD, X-ray diffraction; AFM, Atomic force microscope; SEM, Scanning electron microscope; EDX, Energy dispersive X-ray spectrometry.

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SYNOPSIS.


Surface Modification and Microstructuring of 4H-SiC(0001) by Anodic

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