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Research ArticleReceived: 18 October 2007 Revised: 12 December 2007 Accepted: 10 January 2008 Published online in Wiley Interscience: 7 March 2008

(www.interscience.com) DOI 10.1002/sia.2804

Catalyst-referred etching of 4H–SiC substrateutilizing hydroxyl radicals generated fromhydrogen peroxide moleculesKeita Yagi,a∗ Junji Murata,b Akihisa Kubota,c Yasuhisa Sano,b

Hideyuki Hara,b Takeshi Okamoto,b Kenta Arima,b Hidekazu Mimurab

and Kazuto Yamauchia,b

We describe a new environmentally friendly planarization technique for 4H-silicon carbide (SiC) substrates. The method useshydroxyl (OH) radicals generated from hydrogen peroxide (H2O2) molecules. The surface morphology and the removal rateshow strong dependencies on the plane direction of the substrate. This is attributed to the oxidation mechanism whereinoxidation progresses through the exchange of the surface C-sites with O atoms. A difference in the number of C–Si bondsaround the surface C-sites causes a difference in the removal rate between the opposite faces. On the (0001) face, the oxidationprogresses preferentially at the step edge, where the C-sites are locally exposed. As a result, a step-terrace structure is formed.Copyright c© 2008 John Wiley & Sons, Ltd.

Keywords: planarization; silicon carbide; hydroxyl radical; hydrogen peroxide; catalyst

Introduction

Atomically flat and defect-free SiC surfaces are required for powerdevice applications, since surface roughness and residual damagegenerated during preparation processes introduce defects inthe subsequent epitaxial growth stage.[1] However, such highlyaccurate surfaces are difficult to produce by conventionalplanarization techniques because of the high mechanical hardnessand remarkable chemical inertness of SiC. Catalyst-referred etchinghas been developed as a novel damage-free planarizationtechnique for the SiC substrate.[2,3] In the method, fluorineatoms generated by the catalytic effect of a platinum plateimmersed in hydrofluoric acid are used. The fluorine atoms are veryreactive, but they immediately return to hydrofluoric moleculesby recombination and lose their reactivity once they depart fromthe platinum plate; hence etching occurs only where the SiCsubstrate and the platinum plate are in contact with each other.Consequently, atomically flat and defect-free SiC surfaces can beproduced.

In addition, in recent years, waste products have introduced sig-nificant environmental problems in device fabrication processes.In our previous work, a new environmentally friendly planariza-tion method of 4H–SiC(0001) substrates was examined using OHradicals.[4] The OH radicals were generated from H2O2 moleculeson an iron plate by its catalytic effect. The molecules of H2O2 canalso be readily dissociated into water and oxygen, thus gener-ating only small amounts of waste products during the process.The resulting planarized surface was very flat and smooth, and noscratches or etch pits were observed. In this study, the dependenceof the processing characteristics on the plane direction was deter-mined. Then, the transition of the surface state during the etchingprocess was investigated using XPS measurements. Furthermore,

the processing mechanism of the method was considered referringto the results.

Experimental

Orientation dependence of processing characteristics

The dependence of the processing characteristics on the facedirection was determined. The samples used in this work werecommercially available 2-in 4H–SiC(0001) substrates with theplane directions of (0001) on-axis (on-axis Si-face), (0001) 8◦-off(8◦-off Si-face), (000 1) on-axis (on-axis C-face), and (000 1) 8◦-off(8◦-off C-face), having the following specifications: conductiontype n (dopant: nitrogen), resistivity 0.01–0.05 � · cm, micropipedensity <100 cm−2.

Each substrate was processed as follows. The substrate wasplaced on an iron plate with a pressure of 0.06 MPa. The ironplate and the substrate were rotated at speeds of 10.00 and10.01 rpm, respectively, in the same plane with a center distanceof 95 mm. After 6 h of planarizing, the substrate was treated

∗ Correspondence to: Keita Yagi, Research Center for Ultra-Precision Science andTechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan. E-mail: yagi@upst.eng.osaka-u.ac.jp

a Research Center for Ultra-Precision Science and Technology, Graduate Schoolof Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871,Japan

b Department of Precision Science and Technology, Graduate School ofEngineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

c Department of Mechanical System Engineering; Kumamoto University, 2-39-1Kurokami, Kumamoto City, Kumamoto 860-8555, Japan

Surf. Interface Anal. 2008, 40, 998–1001 Copyright c© 2008 John Wiley & Sons, Ltd.

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New planarization method for 4H–SiC substrate utilizing hydroxyl radicals

with SPM (H2SO4 + H2O2) solution and deionized water in orderto remove organic impurities from the surface of the substrate.The morphologies of the planarized surface were observed byatomic force microscopy (AFM) (SII Nanotechnology, Inc., SPA400+ SPI3800N), and the removal rate was estimated from thedecrease in mass. In addition, to investigate the surface stateof the processed surface, the Si 2p photoelectron spectrum wasmeasured by XPS (ULVAC-PHI, PHI Quantum 2000) using an AlKα X-ray source. The spectrum was decomposed into the Si 2p1/2

and Si 2p3/2 spin-orbit partner lines before curve fitting. In thisdecomposition, it is assumed that the spin-orbit splitting of the Si2p photoelectron spectrum is 0.60 eV, and the Si 2p1/2 to Si 2p3/2

intensity ratio is 0.5.[5]

Transition of surface state during etching process

To investigate the processing mechanism in detail, the transitionof the surface state during the etching process was observedby XPS analysis. Generally, etching of semiconductor materials isperformed through the oxidation and the subsequent dissolutionof the oxide. In the case of SiC, oxidation cannot occur by reactionwith a commonly used oxidizer such as H2O2 solution and ozonewater. However, when the OH radicals attack, the SiC surface isoxidized in accordance with the following formula.[6]

SiC + 4OH + O2 → SiO2 + 2H2O + CO2 (1)

In the case of Si etching, the surface oxide is dissolved in thealkaline solution,[7,8] so the resultant silicon oxide will dissolve as asilicate product if it is reacted with hydroxyl ions (OH−), as follows:

SiO2 + 2OH− → [Si(OH)2O2]2− (2)

In this study, to observe the surface state at each step ofetching, the following treatments were conducted. First, an SiCsample, which was cut to 10×10 mm2, was oxidized by immersioninto a mixture of 0.01 mol/l of iron(II) sulfate (FeSO4) and 30%H2O2 solution for 3 h (oxidation process). Here, OH radicals wereproduced from the H2O2 molecules through the reaction withthe Fe2+ ions, which is known as the Fenton reaction.[9] Next,the sample was immersed into 0.02 mol/l of potassium hydroxide(KOH) solution with a pH of about 12.1 for 3 h to dissolve theoxide (dissolution process). The SiC sample was an (0001) on-axissubstrate having the same specifications as described above. XPSmeasurements were conducted on both the Si- and C- faces beforeand after immersion.

Results and Discussion

Orientation dependence of processing characteristics

Figure 1 is an AFM image of the processed on-axis Si-face. Astep-terrace structure is seen, indicating that step flow removal isthe main reaction of the planarizing process. In contrast, on theprocessed on-axis C-face, such a step-terrace structure is not seen.This suggests that the mechanism of step flow removal is basedon the polarization of the processed face.

Figure 2 shows the rates of removal from the substrateswith different plane directions. It indicates strong orientationdependence. As the plane direction changes from (0001) on-axisto (000 1) on-axis, the etching rate increases. The removal rate fromthe 8◦-off Si-face is about 10 times higher than that from on-axis

Figure 1. AFM image of processed on-axis Si-face.

Figure 2. Removal rates from SiC substrates with different plane directions.

Si-face, and the removal rate from the on-axis C-face is more than250 times of that from the on-axis Si-face.

Figure 3 shows the XPS spectrum for Si 2p3/2 of the processedon-axis Si-face. It consists of two main peaks: One is an Si–Cbulk component (100.6 eV) and the other is a Si1+ component(101.3 eV); no peak corresponding to an Si–O2 component(102.9 eV) is seen. As mentioned above, the etching of SiC isperformed through oxidation and the subsequent dissolution ofthe oxide. This implies that a mechanism for the removal of theoxide exists, and that the oxide is removed as soon as it is formed.These results also indicate that the oxidation reaction is ratelimiting, thus the processing characteristics primarily depend onthe oxidation process.

Transition of surface state during etching process

Figure 4(a) shows the XPS spectra for Si 2p3/2 of the on-axis Si-face.The spectrum of the initial surface (upper figure) has two main

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Figure 3. XPS spectrum for Si 2p3/2 of processed on-axis Si-face. Dashedlines represent curve-fitting results.

peaks, corresponding to the Si–C bulk component and the Si1+

component. On the surface after the oxidation process (middlefigure), a small peak of the Si4+ component is also seen. TheSi4+ component corresponds to the Si–O2 bond. Conversely, theintensity of the Si–C bulk component is slightly decreased. Notethat the total area of the Si 2p3/2 spectrum remains the same. Thisindicates that OH radicals break the Si–C bonds, and Si atomsremain as oxide. Figure 4(b) shows the XPS spectra for C 1s ofthe on-axis Si-face. On both the initial surface (upper figure) andthe oxidized surface (middle figure), two peaks correspondingto the C–Si and C–H components are seen. No additional peakcorresponding to the C–O bond is seen. Note also that the peakintensity for the C–Si component on the oxidized surface isdecreased. This indicates that C atoms are removed during theoxidation process. As shown in Fig. 4(c), the O 1s peak intensityincreases during the oxidation process. These results imply thatthe oxidation progresses through the exchange of C-sites with Oatoms.

Figure 5 shows the XPS spectra of the on-axis C-face. The sametrends as those of the on-axis Si-face are seen, but the amounts ofchanges become significantly larger. This means that the C-face

is oxidized more readily than the Si-face. This is probably relatedto the number of C–Si bonds around the surface C-site. On theC-face, the surface C-sites are combined with three C–Si bonds.In contrast, on the Si-face, the surface C-sites are combined withfour C–Si bonds. Only at the step edge are the C-sites exposedto the solution, and the number of C–Si bonds becomes two orthree. Thus, the oxidation occurs preferentially at the step edge,resulting in the removal of the step flow. Also, the number ofsurface C-sites causes the difference in the removal rate. As thenumber of the step edges on the 8◦-off face is larger than that onthe on-axis face, the removal rate from the 8◦-off face becomeshigher than that from the on-axis face.

The bottom figures in Figs 4 and 5 show the XPS spectra ofthe surface after the dissolution process. The oxides producedin the oxidation process are removed, and the surface generallyreturns to its initial state. Although the results are not shownhere, the dissolution of the oxide is not observed in the 0.02 mol/lhydrochloric (HCl) solution. In the H2O2 solution, the surface ofthe iron plate is oxidized to Fe(OH)2, and its isoelectric pointis known to be about 12.0. This means that the surface of theiron plate is basic in the H2O2 solution, so the oxide should beremoved by reaction with the OH− ions generated on the ironplate surface.

Conclusion

The orientation dependencies of the processing characteristicswere investigated. A step-terrace structure was seen on the on-axis Si-face, but not on the C-face. The removal rate increased asthe plane direction changed from (0001) on-axis to (000 1) on-axis.XPS measurement showed that the C-face is more readily oxidizedthan the Si-face. These results were attributed to the numberof C–Si bonds around the C-site. These insights concerning theprocessing mechanism discussed here should contribute to therealization of highly accurate SiC surfaces.

Figure 4. XPS spectra of on-axis Si-face: (a) Si 2p3/2 core level, (b) C 1s core level, and (c) O 1s core level. All spectra are charge compensated to the C–Sicomponent at 282.6 eV in (b). Dashed lines represent curve-fitting results. From the top, the results for the initial surface after the oxidation process andafter the dissolution process are shown, respectively.

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Figure 5. XPS spectra of on-axis C-face: (a) Si 2p3/2 core level, (b) C 1s core level, and (c) O 1s core level. All spectra are charge compensated to the C–Sicomponent at 282.6 eV in (b). Dashed lines represent curve-fitting results. From the top, the results for the initial surface after the oxidation process andafter the dissolution process are shown, respectively.

Acknowledgements

The authors gratefully acknowledge the support provided by agrant for the 21st century COE program, ‘Center for AtomisticFabrication Technology’, from the Ministry of Education, Culture,Sports, Science and Technology of Japan, and by the IndustrialTechnology Research grant program in 2005 from the New Energyand Industrial Technology Development Organization (NEDO) ofJapan.

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[4] Yagi K, Murata J, Kubota A, Sano Y, Hara H, Arima K, Okamoto T,Mimura H, Yamauchi K. Jpn. J. Appl. Phys., Part 1 2008; 47: 107.

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Surf. Interface Anal. 2008, 40, 998–1001 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia