第44卷第7期
人工晶体学报Vol．44No．72015年7月JOUＲNAL OF SYNTHETIC CＲYSTALS July ，
2015Chemical Mechanical Polishing of 4H-SiC
with Strong Oxidizing Slurry
LIANG Qing-rui 1，2，HU Xiao-bo 1，CHEN Xiu-fang 1，XU Xian-gang 1，ZONG Yan-min 2，WANG Xi-jie 2
（1．State Key Laboratory of Crystal Materials ，Shandong University ，Jinan 250100，China ；2．SICC Materials Co．，Ltd ，Jinan 250111，China ）
（Ｒeceived 10February 2015，accepted 15March 2015）
Abstract ：A novel chemical mechanical planarization （CMP ）technique was demonstrated using hybrid polishing liquid which was composed of strong oxidizer abrasive slurry （SOAS ）with KMnO 4．After double face polishing with different mass concentrations of KMnO 4，4H-SiCsubstrates were assessed．
Surface roughness and wafer removal rate were measured by atomic force microscopy （AFM ）and precision scale ，respectively．The results show that the SOAS with appropriate KMnO 4concentration can greatly raise the material removal rate （MＲＲ）and improve the surface quality of 4H-SiCsubstrate．
Key words ：SiC ；CMP ；KMnO 4；roughness ；remove rate
CLC number ：TN305．2Document code ：A Article ID ：1000-985X （2015）07-1741-07Ｒeceived date ：2015-02-10；accepted date ：2015-03-15
Foundation item ：National Basic Ｒesearch Program of China （2011CB301904）；Natural Science Foundation of China （11134006，51321091）Biography ：LIANG Qing-rui （1989-），Male ，from Shandong Province ，Doctor Graduate．E-mail ：liangqingrui@126．com
Corresponding author ：HU Xiao-bo ，Professor．E-mail ：xbhu@sdu．edu．cn
4H-SiC 的强氧化液化学机械抛光
梁庆瑞1，2，
胡小波1，陈秀芳1，徐现刚1，宗艳民2，王希杰2（1．山东大学晶体材料国家重点实验室，济南250100；2．山东天岳晶体材料有限公司，济南250111）
摘要：研究了一种新型的化学机械抛光方法，使用以KMnO 4作为氧化剂的强氧化性化学机械抛光液（SOAS ）进行
化学机械抛光。研究了在4H-
SiC 硅面和碳面的化学机械抛光过程中，SOAS 溶液中KMnO 4的浓度对抛光质量的影响。使用原子力显微镜（AFM ）和精密电子天平，分别测试了表面粗糙度和去除率。结果表明，适量的KMnO 4可以
大幅度提高4H-
SiC 的化学机械抛光去除率，同时可提高4H-SiC 衬底的表面抛光质量。关键词：碳化硅；化学机械抛光；高锰酸钾；粗糙度；去除率
1Introduction
SiC belongs to the third generation semiconductor material ，which has a wide range of applications in the fields of optoelectronic and high power semiconductor devices because of its outstanding charac
teristics ，such as excellent mechanical properties ，very wide band gap ，high thermal conductivity ，high carrier mobility ，etc ［1-4］．Commercial SiC substrate requires a defect-free surface．For SiC single crystal substrate ，there are two main kinds of defects．One is grown-in defects such as micropipes ，dislocations．The other is post-growth defects caused by post treatment ，for instance ，scratches ，pits and bumps．CMP is a kind of global planarization polishing process
especially for the polishing of hard and brittle materials ［5，
6］．Before CMP process ，there are many damages on the DOI:10.16553/jki.issn1000-985x.2015.07.005
1742人工晶体学报第44卷
surface of SiC substrates produced by sawing，lapping，and mechanical polishing process for surface preparation．The basic principle of CMP is the combination of mechanical grinding and chemical reaction．Chemical oxidation occurs at first on the wafer surface，and then the surface oxide layer is removed by mechanical grinding on a soft abrasive pad．High quality substrate surface could be obtained by the mutual action of oxidation reaction and mechanical grinding．However，the chemical and mechanical properties of SiC greatly increase the difficulty of CMP process［7］．
For CMP process of SiC single crystal substrate，the MＲＲand surface roughness are the main evaluation parameters．The MＲＲpresents the processing efficiency and the surface roughness reflects the processing quality．Su et al．had studied the influences of the polishing slurry composition including the pH value，the abrasive size and its concentration，the dispersant and the oxidants，as well as the rotational speed of the polishing platen and the polishing pressure on the MＲＲof SiC crystal substrate（0001）Si surface［8］．Their results showed that the chemical reaction rate increased with the raise of the oxidant contents．Lee et al．used the mixed abrasive slurry（MAS）of colloidal silica and nano-diamond for CMP of SiC［9］．Experimental results indicated that the MAS can achieve higher MＲＲthan colloidal silica slurry．Chen et al．studied the CMP of different6H-SiC crystal faces and found that the MＲＲand surface quality varied greatly with the different crystal surface orientations［10］．Up to now，the MＲＲof SiC material is only about100nm/h by CMP which is inefficient for industrialization．Although a high MＲＲof SiC material can be achieved by different methods，it is very hard to get a high-quality surface at the same time．The paper demonstrated a novel CMP method using mixed polishing liquid which was composed of strong oxidizer abrasive slurry with KMnO
4
andα-alumina nanoparticles．After the4off-axis4H-SiC wafers were CMP processed in the mixed polishing liquid，a higher removal rate was achieved and a smoother surface was obtained．
2Experimental
The n-type4ʎoff-axis4inch4H-SiC single crystals were grown by the sublimation method．Fig．1shows the AFM image of SiC surface before CMP．The wafers were polished by diamond abrasive slurry from the grinding wafers．There are many interlaced scratches on the surface of SiC wafer，and the roughness is1．13nm．The CMP experiments were conducted by using different slurries on a CMP machine．All the experiments were done in a clean room with Grade1000at the constant temperature of22-25ħ．The MＲＲwas measured by weight loss method using a precision scale（ʃ0．01mg）．Both the roughness and morphology of SiC surface were measured by AFM．
Theα-alumina（Al
2O
3
）and alkaline-based colloidal silica slurries were purchased from a commercial market，
while the SOAS was made by mixing alumina slurry with KMnO
4．The PH of SOAS was adjusted by HNO
3
．The
CMP was conducted under the conditions summarized in Table1．
Table1Specific experimental conditions Parameters Conditions Platen rotation speed/rad·min－160 Carrier rotation speed/rad·min－140 Applied pressure/kg·cm－20．4 Polishing pad type Felt type pad
Slurry type Strong SOAS with（0wt%，0．1wt%，0．3wt%，0．6wt%）KMnO4andα-Alumina（120nm，20wt%and pH3）；
Alkaline-based colloidal silica slurry（120nm，20wt%，H2O2and pH12）
Slurry flow rate/mL·min－1120 3Ｒesults and Discussion
3．1CMP results of SiC-Si face using different slurries
第7期LIANG Qing-rui et al ：Chemical Mechanical Polishing of 4H-SiC with Strong Oxidizing Slurry 1743
Five kinds of slurries were used in this experiment．Among them ，the colloidal silica shows the alkali property ，whereas the other four kinds of polishing liquids exhibited the acid property．As shown in Fig．2，the variation of MＲＲis rather large．The MＲＲof SiC-Si face polished by colloidal silica slurry is only 0．08μm /h ，and it is very inefficient for volume production．According to the study of Zhou et al ［11］，the chemical reaction mechanism of colloidal silica slurry is as follows．
reaction rate2H 2O 2→2H 2O +O 2
（1）H 2O 2→2*OH
（2）SiC +4*OH +O 2→SiO 2+2H 2O +CO 2
（3）SiO 2+2OH －→SiO 2
－3+H 2O （4）
Fig．1AFM image of SiC after MP ，Ｒa ：1．13nm
Fig．2Ｒemoval rate of SiC-Si face polished by different slurries Table 2
Ｒoughness of SiC-Si face after CMP by using different slurries Slurries
Si face Ｒa /nm Colloidal silica slurry
0．0848SOAS 0wt%KMnO 4+Al 2O 3+HNO 3
None MＲＲSOAS 0．1wt%KMnO 4+Al 2O 3+HNO 3
0．176SOAS 0．3wt%KMnO 4+Al 2O 3+HNO 3
0．0946SOAS 0．6wt%KMnO 4+Al 2O 3+HNO 30．235
At first ，a layer of silicon oxide （SiO 2）is generated on Si face as the result of chemical reaction between
H 2O 2and SiC ，as shown in formula （3）．Then ，a part of SiO 2turns into SiO 2
－3
ions under the action of OH －，and the SiO 2－3
ions could be dissolved in the slurry ，as shown in formula （4）．The remaining SiO 2layer is removed by SiO 2abrasives at the same time．Because SiO 2layer is hard and dense ，the removal e
fficiency of SiO 2layer determined by both OH －and SiO 2abrasives is very low．
The MＲＲs of SiC-Si faces polished by SOAS are raised from 0．31μm /h to 0．98μm /h with the increase of the KMnO 4concentration．When the SOAS （0．6wt%KMnO 4）was used ，the MＲＲof Si face could reach 0．98μm /h．In contrast ，the MＲＲof Si face was close to zero when SOAS without KMnO 4was used as polishing liquid．This result shows that even in the acid liquid it is very hard to remove SiC material by means of mechanical grinding．Therefore ，it is the strong oxidation of KMnO 4that dominates the higher removal rate of SiC substrate．In the mixed polishing liquid with the additive of KMnO 4and the H 2O 2solution ，the following chemical reactions occur respectively．
MnO －4+8H ++5e －→Mn 2++4H 2O
E （MnO －4/Mn 2+）=1．51V （5）2H 2O 2→2H 2O +O 2E （O 2/H 2O 2）=0．693V
（6）As shown in formula （5）and formula （6），it is much easier to generate redox reaction in SOAS in the acid environment due to the higher electrode potential value．Thus ，SiO 2layer is generated at a relative faster rate in SOAS than in the colloidal silica slurry．The removal mechanism of SiO 2in acid-based SOAS is quite different from
1744人工晶体学报第44卷that in alkali-based colloidal silica slurry．Because SiO 2is a kind of acidic oxide which could not react with H +，physical removal is the only way to remove the SiO 2layer in SOAS．The SiO 2could be removed easily by Al 2O 3abrasives in SOAS ，because the Moh＇s hardness of Al 2O 3is 9which is much higher than that of SiO 2．
Fig．3AFM images and the cross-sectional analysis images of SiC-Si face after polishing by different slurries
（a ）colloidal silica slurry ；（b ）SOAS with 0．1wt%KMnO 4；（c ）SOAS with 0．3wt%KMnO 4；（d ）SOAS with 0．6wt%KMnO 4；
（e ）cross-sectional image along the lines between A 1and A 2in （a ）；（f ）cross-sectional image along the lines between
B 1and B 2in （b ）；（g ）cross-sectional image along the lines between
C 1and C 2in （c ）；
（h ）cross-
sectional image along the lines between D 1and D 2in （d ）The CMP removal thickness of SiC-Si face for each sample is 1μm．The surface quality of SiC substrate was assessed by AFM．The roughness of Si face after CMP using different slurries is shown in Table 2．The surface with extremely low roughness Ｒa of less than 0．1nm was achieved respectively when colloidal silica and SOAS （0．3wt%KMnO 4）were used as slurry in the experiments．
As shown in Fig．3，the surface morphologies and the cross-sectional image of Si face after CMP by different polishing slurries were measured by AFM．In Fig．3a and Fig．3（e ），uniform step-terraces on the Si face were observed and the step height showed a small variation when colloidal silica slurry was used ，but we could still see a scratch in Fig．3a．As well known ，SiO 2particles are easy to be crystallized in alkaline environment．The crystallized SiO 2particles could cause the damage of SiC surface．
As shown in Fig．3b and Fig．3（f ），although the atomic step-
terrace of Si face could be seen when SOAS （0．1wt%KMnO 4）was used ，the step terrace exhibits irregular and the variation of step height is about 0．4nm which
第7期LIANG Qing-rui et al ：Chemical Mechanical Polishing of 4H-SiC with Strong Oxidizing Slurry 1745is higher than the step height of original Si-C bilayer ，i．e．0．25nm．On the contrary ，in Fig．3c ，the step terrace becomes regular when SOAS （0．3wt%KMnO 4）was used．Fig．3g demonstrates the cross-sectional image of C 1and C 2in Fig．3c ，which indicates the surface step height of about 0．25nm ，in agreement with the theoretical thickness of one Si-C atomic bilayer in SiC crystalline ［11］．As shown in Fig．3d ，scratch could be observed on the Si face when SOAS
（0．6wt%KMnO 4）was used．In order to investigate the formation mechanism of scratch ，we have to analyze the composition of SOAS slurry．By observing the color of used slurries ，we could know whether the content of KMnO 4is enough to meet the requirement of material removal．The purple disappeared when the content of KMnO 4in SOAS was lowered to 0．1wt%，while the SOAS polishing liquids with 0．3wt%and 0．6wt%KMnO 4show lilac and purple respectively．It is indicated that the KMnO 4content of 0．6wt%is superabundant．Excessive high concentration of KMnO 4will result in the deposition of Al 2O 3particles which would further cause surface damage of SiC substrate in the process of mechanical grinding．Fig．3h is the cross-sectional image of D 1and D 2in Fig．3d．From this figure ，we can see that the height of single step terrace is about 0．25nm ，but the altitude intercept is 0．6nm in the cross-sectional line of 4m．Here ，we try to discuss the effect of the oxidant KMnO 4on atomic step morphology．As described in experimental section ，the CMP process was performed in constant temperature of 22-25ħ．When the concentration of KMnO 4was 0．6wt%，the chemical reaction would release heat．It is inferred that as long as concentration distribution of KMnO 4is inhomogeneous the chemical reaction between KMnO 4and SiC will result in a local high temperature．The local high temperature will enlarge the altitude intercept of adjacent step terraces．In general ，uniformity and height of the atomic step-terrace are the indicators of surface quality ，and the final state of step-terrace structure is determined by many factors ，such as t
he CMP parameters ，the chemical composition of slurry ，the size of particles ，etc ［12］．Along with the increase of KMnO 4concentration ，the atomic steps turned clear firstly and then blurry．It is indicated that appropriate concentration of KMnO 4is necessary to achieve a perfect surface．
3．2CMP results of SiC-C face using different
slurries
Fig．4Ｒemoval rate of SiC-C face polished by different SOAS slurries
Fig．4shows the variation of MＲＲof SiC-C face
wafer polished by different slurries．The MＲＲs are close
to zero when colloidal silica slurry and SOAS （0wt%
KMnO 4）were used．The MＲＲof C face is raised when
the KMnO 4concentration in SOAS increases．As shown in
Fig．2and Fig．4，the removal rate of C face was much
higher than that of Si face in the corresponding SOAS．B．
Hornetz ，et al studied the oxidation mechanism of SiC
material．They believed that there was a transitional atom
layer （Si 4C 4－x O 2，x ≤2）between SiC and oxidation
layer which could prevent the material to be oxidized．The transitional atom layer of Si face is thicker than that of C face．It results in a faster removal rate of C face ［13］．
In Fig．5，AFM images of SiC substrate surfaces polished with SOAS are presented．When the concentration of
KMnO 4was 0．1wt%，
the step-terrace structure could be seen clearly as shown in Fig．5a．In this case ，the removal rate was 2．81μm /h ，and the roughness of C face was 0．24nm as shown in Table 3，lower than those polished in the other kinds of SOASs．When the concentration of KMnO 4was increased to 0．3wt%，the MＲＲwas raised to 3．25μm /h．In such a case ，the step-terrace structure disappeared as shown in Fig．5b and the surface roughness increased．When the concentration of KMnO 4was increased to 0．6wt%，MＲＲcontinued to increase and many white dot defects with different sizes emerged as shown in Fig．5c．Many of the white dot defects exhibited the shape of

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