Characterization of colloidal silica abrasives with different sizes and their chemical–mechanical polishing performance on 4H-SiC (0 0 0 1)

Applied Surface Science 307 (2014) 414–427

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Characterization of colloidal silica abrasives with different sizes and their chemical–mechanical polishing performance on 4H-SiC (0 0 0 1) Xiaolei Shi a,b , Guoshun Pan a,b,∗ , Yan Zhou a,b , Zhonghua Gu a,b , Hua Gong a,b , Chunli Zou a,b a b

The State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Shenzhen Key Laboratory of Micro/Nano Manufacturing, Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China

a r t i c l e

i n f o

Article history: Received 22 January 2014 Received in revised form 2 April 2014 Accepted 6 April 2014 Available online 15 April 2014 Keywords: Silicon carbide Chemical–mechanical polishing Colloidal silica abrasives Step-terrace structure Material removal mechanism

a b s t r a c t In this paper, a detailed analysis is presented to characterize the performance of colloidal silica abrasives based slurry with different abrasive sizes on CMP of hexagonal 4H-SiC wafer, and indicates that the abrasive size is an important factor to determine the efficiency of CMP and the final planarization quality of wafer surface. The authors also present a detailed hypothesis to describe the material removal mechanism of 4H-SiC by colloidal silica abrasives during CMP process, and design two groups of experiments to demonstrate the rationality of the hypothesis. Furthermore, the authors put forward some suggestions to optimize the CMP efficiency and planarization quality of 4H-SiC wafer. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Hexagonal silicon carbide (SiC) material, include 4H- and 6HSiC, is one of the most significant substrate for semiconductor materials like gallium nitride (GaN) to grow [1–4]. In most cases, the c-plane (0 0 0 1) of 4H-SiC substrate is used for device epitaxy. The quality of substrate wafer surface will play a critical role in electronic integration fabrication, for that the defects on the substrate surface could be replicated into the epilayer and impair the quality of epitaxy [5]. SiC possesses high mechanical hardness and strong stability against chemicals [6,7], it is very difficult to realize ideal planarization with high removal rate and good surface quality [1]. Many studies have been reported to investigate the planarization of SiC include chemical–mechanical polishing (CMP) [7–10], which is introduced as an efficient treatment to produce atomiclevel smooth surface by removing the irregularities and the damages on the surface [11,12]. For 4H-SiC (0 0 0 1) surface, the atomic step-terrace structure is significant, because the formation of atomic step-terrace structure is more persuasive and credible to

∗ Corresponding author at: The State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China. Tel.: +86 0755 26957301; fax: +86 0755 26957008. E-mail addresses: [email protected], [email protected], [email protected] (G. Pan). http://dx.doi.org/10.1016/j.apsusc.2014.04.048 0169-4332/© 2014 Elsevier B.V. All rights reserved.

verify the quality of planarization technique due to that the atomic step-terrace structure is determined by the crystal structure of 4HSiC. For CMP, by using appropriate scanning parameters of atomic force microscope (AFM), we can obtain atomic step-terrace structure with both high definition and high stability, which has been widely recognized and accepted by academic circles [13–22]. Colloidal silica (SiO2 ) abrasive is one of the most widely used polishing abrasive for CMP. Compared to other abrasive like alumina and ceria, SiO2 has a low hardness, low maximum particle size (about 130 nm) and stable chemical properties, it can minimize the amount of scratches and damages of wafer surface by CMP, and realizes both high planarization efficiency and good planarization quality. Many researchers chose SiO2 as their frequently-used abrasives in their researches for CMP [23–39]. SiO2 abrasive has also been preliminarily used in the field of CMP of SiC wafer, and many achievements have been acquired in recent years [39–45]. However, even though an atomic-level smooth surface has been obtained by SiO2 abrasives in previous work, the definition of atomic step-terrace structure is still poor, and the formation mechanism of this structure is also unclear. Some researches jumped to a conclusion that SiO2 abrasives with a small particle size has no effect for material remove, but there was no further detailed explanation. Furthermore, the material removal mechanism of SiO2 abrasives is still not clear, the effect of abrasive size on the quality of atomic step-terrace structure is also blank in this field. Once the questions above are resolved, we can both increase the CMP

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Fig. 1. AFM images of SiO2 abrasives with small size (a), normal size (b) and big size (c). The sizes of images are all 2 ␮m × 2 ␮m, the scale bars are all 40 nm.

efficiency of SiC and obtain a better planarization quality, which has a great research value for industrial production. Based on this, we prepared three types of SiO2 abrasives with different abrasive sizes characterized by AFM and laser granulometer, and compared the CMP performance of these abrasives based on original 4H-SiC wafers in the same CMP environment. During the processes of CMP, we periodically observed the topography of wafer surface to verify the CMP efficiency and the planarization quality of wafer surface. When we obtained global ultra-smooth surface of wafers, we compared the qualities of atomic step-terrace structure from the three wafer surfaces. We also proposed a hypothesis to explain the material removal mechanism of hexagonal materials by SiO2 abrasives, and designed two groups of experiments to confirm the rationality of the hypothesis.

silicon nitride cantilever, for an unprecedented level of high resolution and force control on super smooth surface. The reflective side is Au. The typical resonant frequency is 65 kHz. The typical force constant is 0.35 N/m. The typical aspect ratio is 3:1. The typical tip cone angle is 22.5◦ . The scan speed of AFM was 0.8 Hz, and the scan size was 768 × 768 points. The working temperature was kept at 25 ◦ C. 3. Results and discussions 3.1. Characterization of SiO2 abrasives with different sizes

2. Experimental

Because using SEM could not successfully characterize the topography of SiO2 abrasives with small particle size due to failing to focus, we choose AFM instead to characterize the topography of abrasives and obtain the results as Fig. 1 shows. The size range of small abrasives is from 5 nm to 20 nm, which could be classified as extreme small nano-abrasives. The size range

The SiO2 abrasives were prepared by ion exchange method. We dissolved a certain amount of Na2 SiO3 in deionized water, the Na ions from Na2 SiO3 was exchanged by the hydrogen ions from resin particles and then we obtained SiO2 crystal seeds. When the process conditions were well controlled, the SiO2 crystal seeds started to grow and be controlled as needed particle size and distribution. The product was treated by ultrafiltration and concentration, and then the SiO2 raw materials were obtained. The slurries contain oxidant H2 O2 with 0–0.9 wt.%, base KOH with 0–0.9 wt.% or monoethanolamine (MEA) with 0–1.8 wt.%, and SiO2 abrasives with 0–30 wt.%. The slurries were dispersed in deionized water. Several two-inch wafers of commercial 4H-SiC (0 0 0 1) with a 0◦ -off (±0.5◦ , on-axis oriented) Si-face were used. The wafers were cut from the same 4H-SiC column crystal by sequence. The wafers have an original thickness of 376 ␮m, and had been preliminarily polished by double sides lapping. The processed wafer has a high degree of planar and parallel on the two sides. The wafers were polished by CETR CP-4 machine. A PU polishing pad was used for CMP. The slurry supplying rate was 70 ml/min, the upper plating rotating speed was 120 r/min and the down plating rotating speed was 160 r/min. The process was carried out by slurry recycling flow. After CMP, the wafers were cleaned by liquid cleaner and deionized water, and then dried off by air spray gun for measurements. The processed surfaces were observed by Leica DM2500 optical microscope. The surface topography and roughness (Ra) were evaluated by AFM (Bruker Dimension Icon) and optical interferometry profiler (Zygo New View 7200). For AFM, Si probes (SNL-10) were obtained from Bruker, Inc. The super-sharp AFM probes possess a typical tip radius of only 2 nm, which combine the sharpness of a silicon tip with the low spring constants and high sensitivity of a

Fig. 2. The size distribution curves of SiO2 abrasives with small size (a), normal size (b) and large size (c).

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Fig. 3. The force curves of SiO2 abrasives with small size (a), normal size (b) and large size (c). The comparison of adhesion of SiO2 abrasives with three different sizes by force curves is represented in (d).

of normal abrasives is from 20 nm to 100 nm, and the big abrasives have a size range from 60 nm to 130 nm. Because the tip of AFM probe has a radius of 10 nm, the abrasive topography may exist some extent of error, so the characterization results of abrasive topography are relative. The size distributions of abrasives were characterized as Fig. 2 shows. In Fig. 2, for the abrasives with a small size, most abrasives are within 10 nm, the average size is 3.6 nm tested by laser

granulometer. For the abrasives with normal size and large size, the average size is 30.8 nm and 79.3 nm respectively. For the comparison of adhesion of SiO2 abrasives with different sizes, the result acquired by AFM force curves is showed in Fig. 3. In Fig. 3(d), the adhesion force between tip and small abrasives is about 1.93 V, and the adhesion force of normal abrasives and large abrasives is 0.81 V and 0.71 V respectively, which indicate that in the same experiment environment, smaller abrasives always have a stronger adhesion. The adhesion of abrasives is a significant impact

Fig. 4. Original Si-face of 4H-SiC commercial wafer. Scratches (a) and pit (b) are marked in the figures.

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Fig. 5. The optical interferometry profiler image (a), a corresponding optical microscope image (b) and their section line (c). The roughness Ra tested was 78.361 nm. The unit of scale bar in (a) is ␮m.

factor to effect the material removal mechanism during CMP, which will be discussed in Section 3.3. 3.2. The macroscopic CMP performance on 4H-SiC wafer by using SiO2 abrasives with different sizes After cutting from the column crystal, the surface of 4H-SiC wafer had been polished preliminarily to form an original status. We used optical microscope to characterize the original wafer surface as Fig. 4 shows. From Fig. 4 we can see that, the original wafer surface was very rough, there were also many defects on the wafer surface, described as scratches and pits. We used optical interferometry profiler to obtain further information as Fig. 5 shows. A rough surface is showed in Fig. 5(a) and (b). The section line described in Fig. 5(c) fluctuates drastically and irregularly, and the surface roughness Ra is 78.361 nm. The wafer surface did not meet the requirement of epitaxy, it need to be polished until the wafer surface is global flat and reach an atomic smooth level. In order to compare the planarization results by using SiO2 abrasives with different sizes, we prepared three portions of slurries included abrasives with three different sizes, and make sure that the concentrations of three abrasives were same in the slurries. The content of other ingredients as deionized water, alkali and hydrogen peroxide were also the same to ensure the accuracy of results. Three original 4H-SiC wafers were prepared, which had the same surface status. The roughness Ra of three wafers were all around 78 nm. During CMP processes, we characterized the wafers hourly on fixed point to compare the planarization efficiency and quality, until global smooth wafers were obtained. First, we used normal size SiO2 abrasives to polish the original 4H-SiC wafer by CMP. On the fixed test point with a 20 ␮m × 20 ␮m area, we acquired wafer surface topography hourly by AFM as Fig. 6 shows. It took 12 h to realize global planarization by using normal size abrasives. The whole CMP process could be divided into two stages. During the first 3 h, because the original wafer surface was very rough, the CMP efficiency was high, there were many residual abrasives embedded in the ravines of wafer surface as showed in Fig. 6(a) and (b). After the first stage, the rough irregularities had been polished enough, and there were only several deep ravines remained on wafer surface. However, because the remained ravines were very deep, it cost about 9 h to remove all the ravines and

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obtained a global smooth wafer surface. When the wafer surface achieved global smooth, the material removal rate remained unchanged, which was 0.62 mg/h. We used optical interferometry profiler to acquire surface roughness hourly to quantify the planarization results, which is showed in Fig. 7. During CMP, the change regulation of wafer surface roughness Ra could also be divided into two stages corresponding to Fig. 6. In the first 2 h, the planarization efficiency was high, which led to drastic decrease of wafer surface roughness. Then, the deep ravines started to be removed and the decrease speed of Ra was also reduced slightly. When all ravines were wiped out, the whole wafer surface achieved global smooth, and the wafer surface roughness started to keep unchanged with a low value. When it comes to the large size SiO2 abrasives, on the same fixed test point with a 20 ␮m × 20 ␮m area, we acquired wafer surface topography hourly by AFM as Fig. 8 shows. It took 8 h to realize global planarization by using large size SiO2 abrasives instead. The whole CMP process could also be divided into two stages, which indicated that the change of abrasive size could not affect the change regulation of wafer surface topography. From the comparison of the two curves showed in Fig. 9 we can see that, during CMP, the planarization efficiency by using large size abrasives was always higher than by using normal size abrasives. The change regulations of the two curves are the same, which indicates that the material removal rules of different size abrasives are uniform. According to this rule, the planarization efficiency by using small size SiO2 abrasives should be low. In order to confirm it, we used small size SiO2 abrasives to polish the third original wafer by CMP. On the same fixed test point with a 70 ␮m × 50 ␮m area, we acquired wafer surface topography hourly by optical interferometry profiler as Fig. 10 shows. Because the planarization efficiency by using small size abrasives was very low, after 6 h of CMP, the wafer surface was still rough and beyond the measure limit of AFM, so we chose optical interferometry profiler to characterize the change regulation of wafer surface topography instead. The cost time of obtaining a global smooth wafer surface by using small size abrasives may be rather long by prediction, so it did not meet the requirement for CMP production efficiency. From Fig. 11 we can see that the planarization efficiency of small size abrasives was very low. During the first 6 h, the wafer surface roughness decreased more slowly compared with the other two abrasives, which indicates that the small size abrasives are not suitable for the CMP of original 4H-SiC wafer. We used slurry contained no abrasive as a control to polish the fourth original 4H-SiC wafer by CMP and obtained the results as Fig. 12 shows. The wafer surface roughness kept unchanged by using slurry contained no abrasive during 6 h of CMP, which indicates that the slurry with no abrasive could rarely cause any removal compared with the result of using small size abrasives based slurry. The result illustrates that small size SiO2 abrasives also possess the ability to remove materials, even though the removing efficiency is not ideal.

3.3. The microscopic CMP performance on 4H-SiC wafer by using SiO2 abrasives with different sizes and further research on material removal mechanism Now we attempt to explain the material removal rules by using SiO2 abrasives with different sizes during CMP. Firstly we introduce the definition of atomic step-terrace structure of 4H-SiC in brief. Typical 4H-SiC step-terrace structure scanned by AFM after CMP is showed in Fig. 13.

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Fig. 6. AFM images of 4H-SiC wafer surface by CMP after every 1 h. The average size of abrasives was 30.8 nm. The sizes of images are all 20 ␮m × 20 ␮m. The scale bars are 200 nm from (a) to (b), 150 nm from (c), 30 nm from (d) to (g) and 20 nm from (h) to (l) respectively.

The crystal structure of SiC is formed by periodic stacking sequences of Si-C bilayers that produce tetrahedral sheets. If the c-axis orientation is absolute 0◦ -off, we should obtain an absolute smooth surface which is made up by Si atoms (on Si-face). However, the c-axis orientation always has an off-angle, the possible reasons may be that the SiC wafer is normally slightly mis-cut from the crystal c-plane, or it may due to the dislocations and any other crystal defect, etc. The slight incline of c-axis causes the whole stacking of Si–C bilayers to deflect slightly at a small angle  as showed in Fig. 14. The deflected stacked bilayers terminate at the surface of Si–C wafer, and so we obtain an atomic step-terrace-form structure composed by Si–C bilayers. The parameter “d” represents the width of terraces, and the parameter “h” represents the height of

terraces, which is 0.25 nm, same as the theoretical thickness of Si–C bilayer. In the stage of deep ravines removed by SiO2 abrasives gradually, we have been able to obtain the atomic step-terrace structure by AFM. The change rule of atomic step-terrace structure during CMP was showed in Fig. 15. During the removal of deep ravines on wafer surface, the topography of atomic step-terrace structure changed regularly. When the deep ravines existed, terraces around the deep ravines turned to be distorted, which indicates that during the procedure of preliminary polishing by double sides lapping, the wafer surface was damaged mechanically. The mechanical force destroyed the normal crystal structure of 4H-SiC wafer substrate

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Fig. 9. Curves plotted by surface roughness Ra during CMP tested by every 1 h. The average sizes of abrasives were 30.8 nm and 79.3 nm respectively.

and made the crystal orientation distort, as well as a tiny deflection of c-bias of the whole crystal substrate, which caused the width of terraces become large. However, the distortion and deflection of crystal substrate would have little impact on the

physical and chemical processes of CMP because both of the two are based on atomic-scaled remove. By CMP, the distorted and deflected crystal structure was removed by SiO2 abrasives, the width of terraces became small and finally an ultra-smooth wafer

Fig. 8. AFM images of 4H-SiC wafer surface by CMP after every 1 h. The average size of abrasives was 79.3 nm. The size of images were all 20 ␮m × 20 ␮m. The scale bars were 200 nm from (a), 150 nm from (b), 90 nm from (c), 50 nm from (d), 30 nm from (e) to (f) and 20 nm from (g) to (h).

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Fig. 10. Optical interferometry profiler images of wafer surface by CMP after every 1 h. The average abrasive size was 3.6 nm. The size of images were all 70 ␮m × 50 ␮m. The unit of bar is ␮m.

surface with a non-damaged atomic step-terrace structure could be obtained. In Section 3.2 we had obtained global smooth wafer surfaces by using both normal size abrasives and large size abrasives by CMP. According to the results, both of the two wafers could emerge atomic step-terrace structures as Fig. 16 shows. According to the comparison, the atomic step-terrace structure acquired from the wafer surface polished by normal size SiO2 abrasives had a higher definition, and the edge of terraces was

also straight and clear. There was no pit created in the substrate of terraces. The atomic step-terrace structure acquired from the wafer surface polished by large size SiO2 abrasives had a lower definition, and the edge of terraces was rough and blur. There were many tiny pits created in the terrace. Because the wafers were cut from the same 4H-SiC column crystal by sequence, they had the same terrace width and terrace orientation on the same fixed test area, which is convenient to analyze and compare the results.

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Fig. 13. Typical atomic step-terrace structure of 4H-SiC (a) with a cross section curve (b) by AFM. The size of image is 3 ␮m × 3 ␮m. The scale bar is 1 nm. The average height of terraces “h” tested by AFM is 0.25 nm.

Now we propose a hypothesis based on both chemical reactions and crystallography to analyze the formation rule of step-terrace structure, as well as the fundamental material removal mechanism of 4H-SiC during CMP. In typical CMP environment, between the Si-face of wafer and the polishing pad, there are several chemical reactions occurred as below: H2 O2 → H2 O + O2

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For the Si-face of 4H-SiC wafer, in terms of physical space, a layer of Si atoms should be exposed to the outside of wafer surface as dangling bonds showed in Fig. 14. During CMP, the wafer surface collided with the polishing pad, as well as the SiO2 abrasives in the slurry or embedded in the polishing pad, which may transfer the mechanical energy generated from the friction interface of crystal and polishing pad as well as abrasives into thermal energy, to make the real contact interface be in a high temperature and high pressure condition. In this environment, the H2 O2 could release *OH more easily, and the crystal surface of 4H-SiC is also not stable, the Si atoms as dangling bonds could infiltrate into the slurry and be preferentially reacted with *OH to produce silicon dioxide (SiO2 ), H2 O and carbon dioxide (CO2 ). The new generated

SiO2 layer is not as stable as former SiC because of the broken of Si C bonds. On the one hand, the new generated SiO2 layer could be collided to pieces and removed by SiO2 abrasives by adherence physically in an instant; on the other hand, when the new generated SiO2 layer was created, it could react with OH− and comes out as soluble SiO3 2− ions, which could dissolve in the slurry. However, this chemical reaction is difficult to carry, so the removal process is finished mainly by material removal by SiO2 abrasives physically. In terms of physical space, the outmost part of 4H-SiC wafer is the edges of terraces, which is also the edges of Si–C bilayers. During CMP, in the slurry environment, when a layer of SiO2 is generated, the structure of new generated SiO2 layer was also step-terrace formed, as confirmed in Fig. 17. When a wafer was taken off from the polishing head after CMP, on the wafer surface there was full of slurry and abrasives, as described in Fig. 17(a), and it must be washed by deionized water and surface active agent to remove the slurry and abrasives. However, the using of deionized water and surface active agent could not remove the SiO2 layer absolutely, there was still SiO2 remained on the wafer surface, which was also step-terrace form as described in Fig. 17(b). If the wafer was soaked in lye for enough time to remove all the SiO2 layer and exposed pure SiC step-terrace layers, the structure could become smoother, and the roughness Ra could decrease to 0.0505 nm and kept unchanged. According to the discussion above, after the removal of one layer of SiC substrate, the 4H-SiC substrate with new Si dangling bonds would be exposed and the removal process will

Fig. 14. The model of step-terrace structure of 4H-SiC. Blue atoms represent Si and white atoms represent C.

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Fig. 15. AFM images of 4H-SiC wafer surface by CMP after every 1 h. The average size of abrasives was 30.8 nm. The size of images are all 10 ␮m × 10 ␮m. The scale bars are 5 nm from (a) to (b), 2 nm from (c) to (d) and 1 nm from (e) to (f).

perpetuate the circulation. All the processes analyzed above were dynamic and continuous, not isolated. Because the removal of materials was atomic-level, the resultant of reaction was controlled within a tiny depth of crystal surface, that is why the wafer could be polished from a macroscopic rough surface to a global atomic-level smooth surface. As the model showed in Fig. 18, if the material removal hypothesis analyzed above is reasonable, in case of fixed polishing pressure, when the abrasive size is large, the impact force on terraces by single abrasive is also large, it could collide the new generated SiO2

to pieces easily. Besides, large size abrasives always possess large surface area. When the new generated SiO2 is collided to pieces by abrasives, it can adhere with the large size abrasives with a large volume, so the unit material remove volume by single abrasive would be large. The abrasives prefer to collide and remove SiC substrate from the edge of Si–C bilayers, which may cause an irregular terrace edge. Oppositely, if the abrasive size is small, the impact force on terraces by single abrasive is also low. With a small abrasive surface area but high adhesion tested in Section 3.1, when the new generated SiO2 is collided to pieces by abrasives, the unit

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Fig. 16. The comparison of step-terrace structure on 4H-SiC wafer (0 0 0 1) by using SiO2 abrasives with normal size (a) and large size (b) after obtaining global smooth wafer surface by CMP. The results were acquired from the same fixed area of wafers. The scale bar of images are both 1 nm. The measure areas are both 2 ␮m × 2 ␮m.

Fig. 17. Step-terrace structure on 4H-SiC wafer (0 0 0 1) by using SiO2 abrasives after CMP (a), after washing (b) and after soaking in lye (c). The scale bar of images are all 1 nm. The measure areas are all 2 ␮m × 2 ␮m. The roughness Ra were 0.174 nm, 0.0765 nm and 0.0505 nm respectively.

material remove volume by single abrasive would be small. The abrasives also prefer to collide and remove SiC substrate from the edge of Si–C bilayers, so the removal of material would be more regular and the terrace edge would be more straight and smooth. That is why the planarization efficiency by using big size abrasives is much higher than by using small size abrasives on macro-level. In order to confirm the hypothesis discussed above, we used small size SiO2 abrasives to polish 4H-SiC wafer by CMP to obtain a corresponding atomic step-terrace structure. To consider that

the cost time of obtaining global smooth wafer surface by small size abrasives from original rough wafer is too long, we directly used a wafer with a global smooth surface polished by normal size abrasives to carry the experiment. The CMP process lasted until the topography of atomic step-terrace structure was stable, then we compare the three types of atomic step-terrace structure images as Fig. 19 shows. We may find that the atomic step-terrace structure acquired by using small size SiO2 abrasives possessed the highest definition.

Fig. 18. The model of material removal procedure for atomic step-terrace structure by using SiO2 abrasives with different sizes.

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Fig. 19. The comparison of step-terrace structures on 4H-SiC wafer (0 0 0 1) by using SiO2 abrasives based slurry with normal size (a), large size (b) and small size (c) after obtaining global smooth wafer surface by CMP. The results were acquired from the same fixed area of the wafers. The scale bars are 1 nm from (a) to (b), 0.5 nm from (c). The measure areas are all 2 ␮m × 2 ␮m.

Fig. 20. The model of creating atomic pits in step-terrace structure by SiO2 abrasives.

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The edge of terraces was extreme smooth and straight. The terraces were intact, there was no damage created in the substrate of terraces. Besides, the arrangement of the terraces in Fig. 19(c) is taken the form of a wide–narrow–wide–narrow array, which just indicates that the arrangement of bilayers is b Aa Bb Cc B. This phenomenon shows that small abrasive size abrasives has a very high precision of material removal, which could even disclose the crystal information of wafer surface. Now we discuss the phenomenon of damages and pits created in the terraces. In Fig. 16(b) we may find that the inner substrate of terraces was damaged, and there were many tiny pits in the substrate of terraces. As discussed above, the edge of terrace should be more fragile and easier to be removed by abrasives than the inner substrate of terraces. However, if the impact force by single abrasive is large enough, the abrasives may have sufficient capability to collide and remove the inner substrate of SiO2 terraces to create atomic pits as a result. This process is also dynamic. A model of creating pits in step-terrace structure is showed in Fig. 20. In order to verify the rationality of atomic pits creating hypothesis proposed above, we designed two groups of experiments to acquire further information. For the first experiment, we used a 4H-SiC wafer possessed a c-bias with irregular deflection. The irregular deflection of c-bias would cause different areas of one wafer possess different terrace widths and orientations of atomic

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step-terrace structure. We used normal size SiO2 abrasives to polish this wafer and obtained the results as Fig. 21 shows. In Fig. 21, three types of atomic step-terrace structures from different test areas in one wafer are showed. We can see that the terrace orientations from different test areas are totally different, as well as the terrace widths, which is why we emphasized that the results must be acquired from the same fixed area of one wafer in Figs. 16 and 19. For Fig. 21(b), the terrace width is very large, which indicates that the c-bias deflection of this area is very small, the wafer surface is in close proximity to (0 0 0 1). In this case, the edge of terraces is very rough and irregular, there are also many tiny pits created in the layers, the pits even connect with each other to form a cellular structure. This is because when the terrace width is large, the whole surface of crystal is close to (0 0 0 1), the edge and the inner substrate of terraces may have no obvious differences in height, so the SiO2 abrasives could have more opportunity to collide and remove the atoms from the inner substrate of Si–C bilayers, and create atomic pits in the substrate of Si–C bilayers. Oppositely, if the terrace width is very small, the SiO2 abrasives may have few opportunity to collide and remove the atoms from the inner substrate of Si–C bilayers, almost all the abrasives were used to remove the atoms from the edges of Si–C bilayers, so the edges of terraces are straight and smooth. For the second experiment, we chose a 4H-SiC wafer which a typical atomic step-terrace structure had been obtained by using

Fig. 21. Three types of atomic step-terrace structures acquired from different areas in one 4H-SiC wafer by CMP. The scale bars are all 1 nm. The measure areas are all 3 ␮m × 3 ␮m.

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Fig. 22. The comparison of step-terrace structures on 4H-SiC wafer (0 0 0 1) by using SiO2 abrasives with normal polishing pressure (a) and larger polishing pressure (b) by CMP. The results were acquired from the same fixed area of the wafers. The scales of AFM images are both 1 nm. The measure areas are both 1 ␮m × 1 ␮m.

normal polishing pressure after CMP. Then we continued to polish this wafer by using larger polishing pressure. After a certain time of CMP, we obtained a new step-terrace structure as Fig. 22(b) shows. Compared with Fig. 22(a), the two images possess the same definition, but the edges of terraces in Fig. 22(b) are very rough and zigzag, there are many pits with different sizes created in the substrate of terraces. The reason may be that when the polishing pressure is larger, the wafer surface would contact with the polishing pad more closely, and the impact force on terraces by single abrasive would also be larger, so the abrasives with a higher impact force may have sufficient capability to collide and remove the inner substrate of terraces to create atomic pits more easily. Beside the generation of tiny pits, many pits continue to grow and expand from a small pit into a huge one, some pits even expand their domains until the boundary of pits connect with the edge of terrace, and the edges appear some gaps with an inscribed circle or crescent shape. This is because when a pit created, the brink of pit is also more fragile and easier to be removed by abrasives as well as the edge of terrace, so the pits could grow up gradually and finally joined with the edge of terraces. The material removal mechanism of CMP is very complicated, the studies discussed above may be only a beginning to explore the rules of material removal of CMP. More researches should be carried to dissolve this problem.

4. Conclusion During CMP, the large size SiO2 abrasives could achieve high planarization efficiency but low planarization quality, and the small size SiO2 abrasives could achieve low planarization efficiency but perfect planarization quality. With the removal of deep ravines, the step-terrace structure became regular, the orientations of terraces became uniform and the width of terraces became small. A hypothesis to explain the material removal mechanism during CMP was proposed. Under special experiment conditions, more micro-pits could be observed in the internal substrate of terraces, which also confirm the hypothesis of material removal mechanism proposed. The research of polishing abrasives is significant. For this study, we could comprehensively use the SiO2 abrasives with different abrasive sizes to achieve both high CMP efficiency and perfect planarization quality of wafer surface. Besides the abrasive size, there must be more important factor to affect the CMP efficiency and the planarization quality, more researches are needed to explore the mechanism of CMP.

Acknowledgements The authors would like to thank National Key Basic Research Program of China – 973 Program (no. 2011CB013102) for a grant to this research. The support from National Natural Science Foundation of China (no. 91223202) is also gratefully acknowledged. Meanwhile, this research has been supported by a grant from the International Science & Technology Cooperation Program of China (no. 2011DFA73410).

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