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Preparation of flower-shaped silica abrasives by double system template method and its effect on polishing performance of sapphire wafers
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Preparation of flower-shaped silica abrasives by double system template method and its effect on polishing performance of sapphire wafers Lei Xua,b, Hong Leia,b,∗, Tianxian Wanga, Yue Dongb, Sanwei Daib a b
Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flower-shaped silica abrasive Double system template method Chemical mechanical polishing Sapphire wafers
As one of the most important factors of chemical mechanical polishing, abrasives have their unique mechanical grinding action and chemical action, providing the most critical support for the nanometer surface smoothness of workpieces. In this paper, the irregularly flower-shaped silica abrasives were prepared by a novel double-system microemulsion template method to replace conventional spherical silica abrasives. The synthesis principle was mainly based on heterogeneous polycondensation growth on irregular silica seeds. The morphology of the flower-shaped silica was verified by scanning electron microscopy. Due to the particularity of the synthesis method, a new environmentally friendly experimental device was designed. Chemical mechanical polishing experiments showed that the material removal rate of the flower-shaped silica with the solid content of only 6% is 217.4% of the spherical silica. The Ambios Xi-100 surface profiler showed that the surface roughness of flowershaped silica was comparable to that of spherical silica. By establishing a contact wear model, it is analyzed that the actual contact area between the flower-shaped silica abrasives and the sapphire wafer is larger and the chemical solid phase reaction occurs more. This study provides a novel and feasible method for preparing irregularly shaped silica.
1. Introduction Sapphire, α-Al2O3 in single crystal form has a series of excellent physical and chemical properties such as high strength, high hardness, high temperature resistance, corrosion resistance, friction resistance, good light transmission performance, and good electrical insulation properties [1,2]. Therefore, sapphire is widely used in various fields, and is often used as a glass device (such as optical instrument lenses [3], high-end watches [4], mobile phone screens [5], etc.) and semiconductor workpieces (such as large-scale integrated circuits [6] and substrate materials for superconducting nanostructures [7], etc.). With the changes of the times and the rapid development of sapphire equipment, people are increasingly pursuing the surface quality of sapphire workpieces. Sapphire, on the other hand, has a huge challenge for its surface polishing due to its high hardness and chemical inertness. At present, only the chemical mechanical polishing technology capable of achieving global nanoscale planarization of the sapphire surface is available [8]. Chemical mechanical polishing (CMP) is a global planarization technique in which mechanical grinding and chemical etching are combined with each other to achieve an equilibrium state [9]. As the
∗
core part of the CMP technology, the nanometer abrasive particles and chemical reagents in the polishing liquid have a decisive influence on the surface quality of the workpiece being polished, therefore they have been explored by many researchers. Nano-abrasives are a hot topic in recent years, and their unique mechanical grinding and chemical reactions with workpieces are increasingly respected by researchers. Nowadays, the Nano-abrasives commonly used for the polishing of sapphire wafers are mainly aluminum oxide (Al2O3) [10] and silicon oxide (SiO2) [1,11–16]. Although the alumina abrasives have a high material removal rate (MRR), due to their greater hardness, they can easily leave deep scratches on the surface of the sapphire, which seriously affects the surface quality of the polished sapphire pieces. The material removal rate of the silica abrasives is generally low, but the average surface roughness (Ra) of the polished sapphire wafer is small because of its soft material. Therefore, many researchers are working on improving the removal rate of material by silica abrasive. There have been many reports on the synthesis methods of silica particles, from the original Stöber method [17] to various modern improved sol-gel methods [18,19], but the most synthesized silica finally has a spherical shape. Due to the particle morphology is an important parameter during polishing process, therefore, the physical
Corresponding author. Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China. E-mail address: [email protected] (H. Lei).
https://doi.org/10.1016/j.ceramint.2019.01.158 Received 26 September 2018; Accepted 21 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xu, L., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.158
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2.2. Preparation method of different shape silica sol
properties of the non-spherical particles with spherical particles have significant differences [20]. Non-spherical particles as the building blocks can not only reflect the inherent properties of the material itself, but also their novel particle accumulation types can improve the performance of the material, give it more application potential [21]. In order to improve the physical and chemical properties of silica particles, numerous researchers have conducted extensive research on nonspherical silica particles. Lei and Dong et al. [11] induced the preparation of peanut-type silica abrasives by doping rare-earth element lanthanum ions. The MRR value was increased by 32.6% (TSC = 10%) compared to spherical silica; Liang et al. [22] prepared chains-type silica by cation induction method. The MRR value of silica abrasives was increased by 65.7% compared to spherical silica (TSC = 15%); Lee et al. [23] prepared non-spherical silica abrasives by multi-stepping method, and the MRR value was increased by 110% (TSC = 15%) compared with spherical silica. The non-spherical silica prepared by the above researchers was all obtained by combining two or more spherical silica particles, mostly dumbbell-shaped or chain-shaped, and was also currently used in the mainstream of chemical mechanical polishing. Murphy et al. [24] prepared silica particles with different size and morphology by microemulsion template method and studied the conditions of their formation; Kuijk et al. [25] synthesized a rod-shaped silica colloid with adjustable size by unidirectional growth induced by sodium citrate; Wang et al. [26] prepared non-spherical colloidal particles by seed emulsion polymerization. However, most of the studies on the non-spherical shape of the individual particles have only stayed in the research of morphology, and they are rarely used in chemical mechanical polishing and other applications. Accordingly, the study of non-spherical silica can be divided into two categories. One is a plurality of spherical silica connected and the other is a single particle being non-spherical silica. The former has been studied more and is suitable for chemical mechanical polishing, but the particles as a whole still have a certain regular morphology, and the particles tend to agglomerate during the preparation process. Compared with spherical silica, the material removal rate of non-spherical silica increases slightly. The latter single particles have extremely irregular surface morphology, but the surface modification is still in preliminary research, and the preparation method is complicated and has not been used in practical applications. In order to further increase the polishing rate of the material, reduce the surface roughness of the material, we have made a new synthetic method for the preparation of individual particle bodies, namely nonspherical flower-shaped silica abrasives, through a water-in-oil and oilin-water double-system microemulsion template method. At the same time, its formation principle and polishing performance are further analyzed.
The two-step microemulsion template method was used to prepare flower-shaped silica (FSS) abrasives, which were carried out in two steps. In the first step, a flower-shaped silica seed crystal was prepared. First, 9 g of PVP was added to a three-necked flask containing 300 ml of n-pentanol. After stirring for 20 min until PVP was completely dissolved, 25 ml of anhydrous ethanol and 10 ml of 0.68 M sodium citrate solution were added. After stirring for 20 min, the solution became turbid, and then 6.5 ml of 25 wt % aqueous ammonia was added, stirring and heating the mixed solution to 70 °C followed by the slow dropwise addition of 10 ml of TEOS. After the dropwise addition, the reaction was continued at 70 °C for 2 h, then poured into a beaker and sealed and cooled overnight. In the second step, a flower-shaped silica sol is prepared through the seed crystals prepared in the first step. First, a certain amount of deionized water was poured into a beaker containing seed crystals, and the emulsion was transferred to a 5 L fournecked flask, stirred and heated until boiling. Subsequently, 1600 ml of fresh silicic acid (prepared by a cationic resin) and 72 ml of the 3 wt % NaOH solution were added dropwise thereto, the speed of the dripping of the silicic acid was kept consistent with the evaporation rate of the water, the pH was maintained at approximately 10, and the temperature was maintained at the boiling point of the solution. The n-pentanol was separated by a water-oil separator, and the whole dropwise addition reaction was carried out for about 2 h. After the reaction was completed, the emulsion was transferred to a beaker and allowed to cool overnight for polishing experiments. Referring to the preparation process of the flower-shaped silica sol, the spherical silica sol is also carried out in two steps. The experimental steps are almost same, and some of the added raw materials are modified. In the first step, spherical silica seed crystals were prepared, and the initial solution was changed to a mixed solution of 150 ml of deionized water and 150 ml of ethanol without adding a sodium citrate solution, and finally a spherical silica seed crystal was obtained. In the second step, a spherical silica sol is prepared by the seed crystal prepared in the first step. The method is similar to the first step, but the step of removing the oil phase is removed, and the reaction is carried out for 2 h to obtain a final spherical silica sol.
2.3. Design of experimental device Since the finally prepared silica sol is required for chemical mechanical polishing experiments, the organic solvents such as ethanol and pentanol used in the reaction have a great influence on the polishing experiment, which seriously reduces the polishing effect, and pentanol has intense irritating odor. Therefore, it is necessary to design a reasonable experimental device to gradually remove organic reagents such as pentanol and ethanol and reduce irritating odors when the experimental requirements are satisfied. For this purpose, we designed a completely new experimental device for the second step system. The oil-water separator and the exhaust gas absorption device are introduced under the condition that the original feed is not affected. The separation of n-pentanol can be achieved according to the inherent physical properties of the material with different boiling points and densities. The specific experimental setup is shown in Fig. 1. Because the second step requires heating, ethanol will preferentially separate because of the low boiling point, and then the solution will reach the boiling point. The solvent such as pentanol and water will evaporate and return to the oil-water separator under the action of condensation, and be layered to achieve the effect of separation. And the upper end of the condenser tube was introduced into the beaker filled with ethanol to reduce the odor emission. The final isolated pentanol can be reused for further purification, resulting in cost savings.
2. Experimental sections 2.1. Chemicals The chemical reagents used in the experiments in this paper include: Polyvinylpyrrolidone PVP ((C6H9NO)n, Sinopharm Chemical Reagent Co., LTD, China), n-pentanol (C5H11OH, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), anhydrous ethanol(C2H5OH, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), sodium citrate(Na3C6H5O7·2H2O, Sinopharm Chemical Reagent Co., LTD, China), ammonia, tetraethyl orthosilicate TEOS (C8H20O4Si, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), Sodium silicate (Na2SiO3, JiNan DeWang Chemical Industry Co., LTD, China), NaOH(Sinopharm Chemical Reagent Co., LTD, China), cation exchange resin (Sinopharm Chemical Reagent Co., LTD, China), hydrochloric acid (HCl, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), deionized water (DI).
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With regard to the first step, the related research [24–27] on the waterin-oil (W/O) system has been very thorough. First, PVP, sodium citrate, ethanol, ammonia, and water will form stable small droplets in n-amyl alcohol, followed by TEOS. It dissolves rapidly in n-pentanol and gradually contacts with the water droplets therein to hydrolyze and form silica. In addition, heating during the hydrolysis can make the water molecules move violently, resulting in the asymmetry of the hydrolysis and condensation of TEOS, and finally breaking to obtain flower-shaped silica seeds. Among them, PVP plays a role of dispersion and stabilization, and sodium citrate acts as an end-group inducing factor. The second step, the oil-in-water (O/W) system, provides a more suitable environment for crystal growth. In the first step, the seed crystals formed in the n-pentanol first become small oil droplets dispersed in water, and then the freshly added silicic acid is first dissolved in water to be further reacted. Based on the Ostwald ripening theory [28,29], silicic acid preferentially condenses on the flower-shaped silica seeds in the pentanol droplets to grow further, and because of the heating the oil-water interface is unstable, and the condensation reaction of silicic acid occurs asymmetrically, resulting in more irregular silica abrasives. In addition, in the second step system, since the surface of the silica particles has a lot of active hydroxyl groups, the particles have a certain affinity for n-pentanol and water, so the dispersion and growth conditions of the silica are more complicated and can be divided into the following three situations. Firstly, the seeds are located in n-pentanol droplets, and the condensation reaction of silicic acid occurs at the unstable oil-water interface. Secondly, the seed crystals are located at the oil-water interface, and the unidirectional condensation reaction of silicic acid occurs on the silica on the aqueous solution side. Thirdly, part of the seed crystals are in aqueous solution, and the silicic acid is condensed directly on the seeds. These three cases can be seen from the actual diagram of the oil-water dual system, and Fig. 3 shows the distribution of the solution at different stages. It can be clearly seen that in the presence of silica, the solution assumes an emulsion state, and the oil and water blend with each other due to the formation of hydrogen bonds between the hydroxyl groups on the surface of the silica seed crystal in n-pentanol and the water molecules. Therefore, the system is relatively stable. In such an environment, some silica crystals in the npentanol tend to move toward the water droplets, resulting in the dispersion of the silica crystals in the above three cases.
Fig. 1. Second step reaction experimental device.
2.4. Chemical-mechanical polishing (CMP) tests Chemical mechanical polishing experiments were conducted using a silica sol containing spherical and flower-shaped silica with the same solid content as the polishing solution. The polishing machine was a UNIPOL-1502 CMP machine (Shenyang Kejing Instrument Co., Ltd., China) and the polishing pad was a Rodel porous polyurethane pad. The parameters settings for the polishing were: the feed rate of the polishing liquid was 180 ml/min, the load pressure was 6 Kg, the turntable speed was 70 r/min, and the polishing time was 2 h. After the polishing is finished, the sapphire wafer is washed many times with washing liquid, water, amyl alcohol, ethanol, etc., and finally dried and cooled. 2.5. Characterizations The morphologies of flower-shaped abrasives were characterized by scanning electron microscopy (SEM), and the average surface roughness (Ra) of sapphire wafers was measured by Ambios Xi-100 surface profiler. The morphology of flower-shaped silica abrasives were characterized by scanning electron microscopy (SEM, JSM-7500F) with 15 kV. The average surface roughness of the polished and unpolished sapphire wafers was measured by Ambios Xi-100 surface profiler (Ambios Technology Corp., USA) with a resolution of 0.1 Å, a focal depth of 3.0 μm, and a working distance. It is 7.4 mm and the measurement area is 500 μm × 500 μm.
3.3. Chemical mechanical polishing performance of different abrasives The polishing performance of silica abrasives is generally judged by material removal rate (MRR) and surface roughness (Ra). MRR can be calculated by equation (1):
3. Results and discussion
MRR = 3.1. Morphology of the abrasives
Δm*106 ρπR2T
(1)
Where, MRR: material removal rate (μm/h), Δm : mass difference of sapphire wafer before and after polishing, ρ: sapphire density (g·cm−3, ρsapphire = 3.98 g·cm−3), R: The radius of sapphire (nm), T: polishing time (h). Fig. 4 shows the MRR of different morphology abrasives. It can be seen that the FSS abrasives have higher MRR values under the same polishing conditions. The MRR of FSS abrasives increased by 117.2% compared to SS abrasives. This is because the combination of chemical corrosion and mechanical wear of FSS abrasives is superior to SS abrasives. Fig. 5a shows the surface roughness of sapphire wafers polished with different morphology abrasives. It can be seen that, whether it is SS or FSS, the surface of the sapphire wafers after polishing is smoother than the unpolished one. In addition, the Ra between the FSS and SS abrasives is little difference, which means that the FSS can ensure the maximum degree of planarization of the surface to be polished while improving the material removal rate. Fig. 5(b ∼ d) shows the surface profile of unpolished and polished sapphire wafers. Through these figures, the
Fig. 2a shows the morphology of the spherical silica (SS) abrasives prepared by the two-step method provided in this paper, and Fig. 2b shows the flower-shaped silica (FSS) abrasives prepared by the twosystem template method. As can be seen from the figure, the original morphology of the silica is greatly changed by the two-system template method. Therefore, it can be confirmed that the flower-shaped silica having a very irregular morphology is prepared. 3.2. Synthesis mechanism analysis In this paper, the flower-shaped silica is an improvement and innovation based on some existing irregular silica research. On the basis of changing a series of reaction conditions, the required irregular silica seed crystals were prepared by water-in-oil system, and a new oil-inwater system was proposed for the irregular growth of the crystal seed so as to obtain the silica sol used for chemical mechanical polishing of CMP. The specific experimental principle flow is shown in Scheme 1. 3
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Fig. 2. SEM images of different abrasives (a) SS abrasives (b) FSS abrasives.
polishing effect of FSS abrasives can be more visually perceived.
3.4. Analysis and discussion of polishing mechanism Chemical mechanical polishing is a method of achieving nanoscale planarization by the combination of chemical corrosion and mechanical grinding. In order to study the polishing mechanism of the FSS abrasives, as shown in Fig. 6, a contact wear model between the silica abrasives and the sapphire substrate was established. For chemical mechanical polishing, mechanical grinding can be based on the MRR equation (2) proposed by Luo et al. [30] MRRthickness = C1[(1-Ø(3-C2P01/3)]P01/2υ
Fig. 3. Solution images of different stages (a) seed crystal obtained in W/O system (b) pure pentanol and water (c) pentanol - silica seed – water in O/W system (d) the FSS sol.
(2)
Scheme 1. Preparation principle of flower-shaped silica abrasive. 4
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Fig. 4. The MRR values of SS abrasives and FSS abrasives (The solid content is 6.0 wt %). Fig. 6. Contact wear model of silica abrasives and sapphire wafer.
Ep 2 2 ds k 2ρs ms − a DSUM al , C2 C1 = ρa πx avg (b1 Hw )2/3 0.25 × =
2 4 3 (x avg 3
()
+ 3σ )
σ
(
1 Hp
+
2 Hw
area is the sum of the effective contact points [31,32]. As can be clearly seen in Fig. 5, the SS abrasives are in contact with the plane only at one point, whereas the FSS abrasives need multiple points to make contact with the plane to balance. Therefore, the effective contact point of the abrasive per unit particle FSS is more than the effective contact point of the SS abrasive, and the actual contact area of the FSS abrasive is larger. It was further confirmed that the polishing rate of the FSS abrasives was higher than that of the SS abrasives. In terms of chemical corrosion, SiO2 and Al2O3 can undergo solid-phase chemical reactions. The principle of the reaction has been discussed by many researchers [11,12]. There are three main chemical reaction equations:
)E
2/3 p
b1
Where, MRRthickness: material removal rate, ds : abrasive volume fraction, k : constant, ρs : density of silica sol, ms − a : abrasive mass fraction, DSUM : rough peak density per unit area, a : average contact area at the time of abrasive contact, l : the average height of the abrasive, ρa : abrasive density, x avg : the average particle size of the abrasive, Ep: young's modulus of elasticity of polishing pad, (b1 Hw )2/3 : constant, Hw : the hardness of the polished parts, Hp : the hardness of polishing pad, σ : standard deviation of the average particle size of the abrasive, P0: polishing pad loading pressure, υ: relative speed of polishing pad and workpiece, Ø: normal distribution function. In the polishing experiment, the main influencing factor is the actual contact area between the abrasives and the substrate. The greater the actual contact, the better the grinding effect, and the actual contact
Al2O3 + H2O = 2AlOOH
(3)
2AlOOH + SiO2 = Al2SiO7·H2O
(4)
Al2O3 + H2O + SiO2 = Al2SiO7·H2O
(5)
Similarly, due to the increase of the number of effective contact
Fig. 5. (a) Surface roughness of sapphire wafers unpolished and after polishing with different abrasives; and surface profile of sapphire wafers: (b) unpolished, (c) polished by SS abrasives and (d) polished by FSS abrasives. 5
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points of FSS, the probability and degree of solid-phase chemical reaction of flower-shaped silicon dioxide and Al2O3 are greater, further improving the polishing performance.
(2016) 713–720. [9] M. Krishnan, J.W. Nalaskowski, L.M. Cook, Chemical mechanical planarization: slurry chemistry, materials, and mechanisms, J. Chem. Rev. 110 (2010) 178. [10] X. Wang, H. Lei, R. Chen, CMP behavior of alumina/metatitanic acid core-shell abrasives on sapphire substrates, J. Precis. Eng. 50 (2017) 263–268. [11] Y. Dong, H. Lei, W. Liu, T. Wang, L. Xu, Preparation of non-spherical silica composite abrasives by lanthanum ion-induced effect and its chemical–mechanical polishing properties on sapphire substrates, J. Mater. Sci. 53 (2018) 10732–10742. [12] T. Liu, H. Lei, Nd3+-doped colloidal SiO2, composite abrasives: synthesis and the effects on chemical mechanical polishing (CMP) performances of sapphire wafers, J. Appl. Surf. Sci. 413 (2017) 16–26. [13] L. Zazzera, B. Mader, M. Ellefson, J. Eldridge, S. Loper, J. Zabasajja, J. Qian, Comparison of ceria nanoparticle concentrations in effluent from chemical mechanical polishing of silicon dioxide, J. Environ. Sci. Technol. 48 (2014) 13427–13433. [14] G. Chen, Z. Ni, L. Xu, Q. Li, Y. Zhao, Performance of colloidal silica and ceria based slurries on CMP of Si-face 6H-SiC substrates, J. Appl. Surf. Sci. 359 (2015) 664–668. [15] L. Peedikakkandy, L. Kalita, P. Kavle, A. Kadam, V. Gujar, Preparation of spherical ceria coated silica nanoparticle abrasives for CMP application, J. Appl. Surf. Sci. 357 (2015) 1306–1312. [16] S. Salleh, I. Sudin, A. Awang, Effects of non-spherical colloidal silica slurry on AlNiP hard disk substrate CMP application, J. Appl. Surf. Sci. 360 (2016) 59–68. [17] W. Stöber, A. Fink, Ernst Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69. [18] H. Nakamura, Y. Matsui, Silica gel nanotubes obtained by the sol-gel method, J. Am. Chem. Soc. 117 (1995) 2651–2652. [19] R. Brambilla, G.P. Pires, J.H. dos Santos, M.S. Lacerda Miranda, Octadecylsilane hybrid silicas prepared by the sol-gel method: morphological and textural aspects, J. Colloid Interface Sci. 312 (2007) 326–332. [20] H. Tao, W. Zhong, Y. Zhang, P. Yang, Effects of particle properties on separation behavior of differently-sized non-spherical particle mixtures, J. Chin. Soc. Power Eng. 35 (2015) 652–658. [21] M.L. Saladino, T.E. Motaung, A.S. Luyt, A. Spinella, G. Nasillo, E. Caponetti, The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA, J. Polym. Degrad. Stabil. 73 (2012) 34–39. [22] C. Liang, L. Wang, W. Liu, Z. Song, Non-spherical colloidal silica particles—preparation, application and model, J. Colloid. Surface. A 457 (2014) 67–72. [23] H. Lee, M. Kim, H. Jeong, Effect of non-spherical colloidal silica particles on removal rate in oxide CMP, Int. J. Precis. Eng. Manuf. 16 (2015) 2611–2616. [24] R.P. Murphy, K. Hong, N.J. Wagner, Synthetic control of the size, shape, and polydispersity of anisotropic silica colloids, J. Colloid Interface Sci. 501 (2017) 45–53. [25] A. Kuijk, A.V. Blaaderen, A. Imhof, Synthesis of monodisperse, rodlike silica colloids with tunable aspect ratio, J. Am. Chem. Soc. 133 (2011) 2346–2349. [26] J. Wang, Y. Lu, Facile synthesis of asymmetrical flower-like silica, J. Mater. Des. 111 (2016) 206–212. [27] C. Lu, M. Urban, Rationally designed gibbous stimuli-responsive colloidal nanoparticles, J. Acs Nano. 9 (2015) 3119–3124. [28] P. Kokhanenko, K. Brown, M. Jermy, Silica aquasols of incipient instability: synthesis, growth kinetics and long term stability, J. Colloid. Surface. A 493 (2016) 18–31. [29] A. Lazaro, M.C. van de Griend, H.J.H. Brouwers, J.W. Geus, The influence of process conditions and Ostwald ripening on the specific surface area of olivine nano-silica, J. Micropor. Mesopor. Mat. 181 (2013) 254–261. [30] J. Luo, D.A. Dornfeld, Material removal mechanism in chemical mechanical polishing: theory and modeling, J. Ieee. T. Semiconduct. M 14 (2001) 112–133. [31] Y. Kimura, Estimation of the number and the mean area of real contact points on the basis of surface profiles, J. Wear. 15 (1970) 47–55. [32] V.A. Yastrebov, G. Anciaux, J.F. Molinari, On the accurate computation of the true contact-area in mechanical contact of random rough surfaces, J. Tribol. Int. 114 (2017) 161–171.
4. Conclusions For the first time, we propose to prepare irregular flower-shaped silica abrasives by double system (W/O-O/W) template method for the polishing of sapphire wafers. The reaction mechanism of this twosystem template method for synthesizing flower-shaped silica was analyzed. By changing the chemical reaction environment, the morphology of silica was greatly changed. The polishing experiment was carried out using the flower-shaped silica sol, and the MRR value thereof was increased by 117.4% compared with the spherical silica sol. The surface flatness of the sapphire wafer is also improved, and the Ra value is low. The main action mechanism of flower-shaped silica abrasives lies in the effective multi-point contact between abrasives and sapphire wafers. While increasing the contact area between abrasives and sapphire wafers, the solid-phase chemical reaction between them is also increased, thus enhancing the overall MRR and improving the surface morphology of sapphire chips. Thus, the irregularly flowershaped silica abrasives are promising in place of conventional spherical silica abrasives. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Number 51475279). References [1] D. Yin, X. Niu, K. Zhang, J. Wang, Y. Cui, Preparation of MgO doped colloidal SiO2, abrasive and their chemical mechanical polishing performance on c-, r- and a-plane sapphire substrate, J. Ceram. Int. 44 (2018) 14631–14637. [2] H. Aida, T. Doi, H. Takeda, H. Katakura, S. Kim, K. Koyama, T. Yamazaki, M. Uneda, Ultraprecision CMP for sapphire, GaN, and SiC for advanced optoelectronics materials, J. Curr. Appl. Phys. 12 (2012) S41–S46. [3] R. Osellame, N. Chiodo, V. Maselli, A. Yin, M. Zavelani-Rossi, G. Cerullo, P. Laporta, L. Aiello, S. De Nicola, P. Ferraro, A. Finizio, G. Pierattini, Optical properties of waveguides written by a 26 MHz stretched cavity Ti: sapphire femtosecond oscillator, J. Opt. Express. 13 (2005) 612–620. [4] D. Hayes, Sapphire watch glass machining with diamond, J. Ind. Diamond Rev. 58 (1998) 55–56. [5] Y. Gurbuz, O. Esame, I. Tekin, W.P. Kang, J.L. Davidson, Diamond semiconductor technology for RF device applications, J. Solid State Electron. 49 (2005) 1055–1070. [6] N. Kumar, M.G. Fissel, K. Pourrezaei, B. Lee, E.C. Douglas, Growth and properties of TiN and TiOxNy, diffusion barriers in silicon on sapphire integrated circuits, J. Thin Solid Films 153 (1987) 287–301. [7] S. Thakoor, D.M. Strayer, G.J. Dick, J.E. Mercereau, A lead‐on‐sapphire superconducting cavity of superior quality, J. Appl. Phys. 59 (1986) 854–858. [8] Y. Xu, J. Lu, X. Xu, Study on planarization machining of sapphire wafer with softhard mixed abrasive through mechanical chemical polishing, J. Appl. Surf. Sci. 389
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Preparation of flower-shaped silica abrasives by double system template method and its effect on polishing performance of sapphire wafers Lei Xua,b, Hong Leia,b,∗, Tianxian Wanga, Yue Dongb, Sanwei Daib a b
Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flower-shaped silica abrasive Double system template method Chemical mechanical polishing Sapphire wafers
As one of the most important factors of chemical mechanical polishing, abrasives have their unique mechanical grinding action and chemical action, providing the most critical support for the nanometer surface smoothness of workpieces. In this paper, the irregularly flower-shaped silica abrasives were prepared by a novel double-system microemulsion template method to replace conventional spherical silica abrasives. The synthesis principle was mainly based on heterogeneous polycondensation growth on irregular silica seeds. The morphology of the flower-shaped silica was verified by scanning electron microscopy. Due to the particularity of the synthesis method, a new environmentally friendly experimental device was designed. Chemical mechanical polishing experiments showed that the material removal rate of the flower-shaped silica with the solid content of only 6% is 217.4% of the spherical silica. The Ambios Xi-100 surface profiler showed that the surface roughness of flowershaped silica was comparable to that of spherical silica. By establishing a contact wear model, it is analyzed that the actual contact area between the flower-shaped silica abrasives and the sapphire wafer is larger and the chemical solid phase reaction occurs more. This study provides a novel and feasible method for preparing irregularly shaped silica.
1. Introduction Sapphire, α-Al2O3 in single crystal form has a series of excellent physical and chemical properties such as high strength, high hardness, high temperature resistance, corrosion resistance, friction resistance, good light transmission performance, and good electrical insulation properties [1,2]. Therefore, sapphire is widely used in various fields, and is often used as a glass device (such as optical instrument lenses [3], high-end watches [4], mobile phone screens [5], etc.) and semiconductor workpieces (such as large-scale integrated circuits [6] and substrate materials for superconducting nanostructures [7], etc.). With the changes of the times and the rapid development of sapphire equipment, people are increasingly pursuing the surface quality of sapphire workpieces. Sapphire, on the other hand, has a huge challenge for its surface polishing due to its high hardness and chemical inertness. At present, only the chemical mechanical polishing technology capable of achieving global nanoscale planarization of the sapphire surface is available [8]. Chemical mechanical polishing (CMP) is a global planarization technique in which mechanical grinding and chemical etching are combined with each other to achieve an equilibrium state [9]. As the
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core part of the CMP technology, the nanometer abrasive particles and chemical reagents in the polishing liquid have a decisive influence on the surface quality of the workpiece being polished, therefore they have been explored by many researchers. Nano-abrasives are a hot topic in recent years, and their unique mechanical grinding and chemical reactions with workpieces are increasingly respected by researchers. Nowadays, the Nano-abrasives commonly used for the polishing of sapphire wafers are mainly aluminum oxide (Al2O3) [10] and silicon oxide (SiO2) [1,11–16]. Although the alumina abrasives have a high material removal rate (MRR), due to their greater hardness, they can easily leave deep scratches on the surface of the sapphire, which seriously affects the surface quality of the polished sapphire pieces. The material removal rate of the silica abrasives is generally low, but the average surface roughness (Ra) of the polished sapphire wafer is small because of its soft material. Therefore, many researchers are working on improving the removal rate of material by silica abrasive. There have been many reports on the synthesis methods of silica particles, from the original Stöber method [17] to various modern improved sol-gel methods [18,19], but the most synthesized silica finally has a spherical shape. Due to the particle morphology is an important parameter during polishing process, therefore, the physical
Corresponding author. Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China. E-mail address: [email protected] (H. Lei).
https://doi.org/10.1016/j.ceramint.2019.01.158 Received 26 September 2018; Accepted 21 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xu, L., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.158
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2.2. Preparation method of different shape silica sol
properties of the non-spherical particles with spherical particles have significant differences [20]. Non-spherical particles as the building blocks can not only reflect the inherent properties of the material itself, but also their novel particle accumulation types can improve the performance of the material, give it more application potential [21]. In order to improve the physical and chemical properties of silica particles, numerous researchers have conducted extensive research on nonspherical silica particles. Lei and Dong et al. [11] induced the preparation of peanut-type silica abrasives by doping rare-earth element lanthanum ions. The MRR value was increased by 32.6% (TSC = 10%) compared to spherical silica; Liang et al. [22] prepared chains-type silica by cation induction method. The MRR value of silica abrasives was increased by 65.7% compared to spherical silica (TSC = 15%); Lee et al. [23] prepared non-spherical silica abrasives by multi-stepping method, and the MRR value was increased by 110% (TSC = 15%) compared with spherical silica. The non-spherical silica prepared by the above researchers was all obtained by combining two or more spherical silica particles, mostly dumbbell-shaped or chain-shaped, and was also currently used in the mainstream of chemical mechanical polishing. Murphy et al. [24] prepared silica particles with different size and morphology by microemulsion template method and studied the conditions of their formation; Kuijk et al. [25] synthesized a rod-shaped silica colloid with adjustable size by unidirectional growth induced by sodium citrate; Wang et al. [26] prepared non-spherical colloidal particles by seed emulsion polymerization. However, most of the studies on the non-spherical shape of the individual particles have only stayed in the research of morphology, and they are rarely used in chemical mechanical polishing and other applications. Accordingly, the study of non-spherical silica can be divided into two categories. One is a plurality of spherical silica connected and the other is a single particle being non-spherical silica. The former has been studied more and is suitable for chemical mechanical polishing, but the particles as a whole still have a certain regular morphology, and the particles tend to agglomerate during the preparation process. Compared with spherical silica, the material removal rate of non-spherical silica increases slightly. The latter single particles have extremely irregular surface morphology, but the surface modification is still in preliminary research, and the preparation method is complicated and has not been used in practical applications. In order to further increase the polishing rate of the material, reduce the surface roughness of the material, we have made a new synthetic method for the preparation of individual particle bodies, namely nonspherical flower-shaped silica abrasives, through a water-in-oil and oilin-water double-system microemulsion template method. At the same time, its formation principle and polishing performance are further analyzed.
The two-step microemulsion template method was used to prepare flower-shaped silica (FSS) abrasives, which were carried out in two steps. In the first step, a flower-shaped silica seed crystal was prepared. First, 9 g of PVP was added to a three-necked flask containing 300 ml of n-pentanol. After stirring for 20 min until PVP was completely dissolved, 25 ml of anhydrous ethanol and 10 ml of 0.68 M sodium citrate solution were added. After stirring for 20 min, the solution became turbid, and then 6.5 ml of 25 wt % aqueous ammonia was added, stirring and heating the mixed solution to 70 °C followed by the slow dropwise addition of 10 ml of TEOS. After the dropwise addition, the reaction was continued at 70 °C for 2 h, then poured into a beaker and sealed and cooled overnight. In the second step, a flower-shaped silica sol is prepared through the seed crystals prepared in the first step. First, a certain amount of deionized water was poured into a beaker containing seed crystals, and the emulsion was transferred to a 5 L fournecked flask, stirred and heated until boiling. Subsequently, 1600 ml of fresh silicic acid (prepared by a cationic resin) and 72 ml of the 3 wt % NaOH solution were added dropwise thereto, the speed of the dripping of the silicic acid was kept consistent with the evaporation rate of the water, the pH was maintained at approximately 10, and the temperature was maintained at the boiling point of the solution. The n-pentanol was separated by a water-oil separator, and the whole dropwise addition reaction was carried out for about 2 h. After the reaction was completed, the emulsion was transferred to a beaker and allowed to cool overnight for polishing experiments. Referring to the preparation process of the flower-shaped silica sol, the spherical silica sol is also carried out in two steps. The experimental steps are almost same, and some of the added raw materials are modified. In the first step, spherical silica seed crystals were prepared, and the initial solution was changed to a mixed solution of 150 ml of deionized water and 150 ml of ethanol without adding a sodium citrate solution, and finally a spherical silica seed crystal was obtained. In the second step, a spherical silica sol is prepared by the seed crystal prepared in the first step. The method is similar to the first step, but the step of removing the oil phase is removed, and the reaction is carried out for 2 h to obtain a final spherical silica sol.
2.3. Design of experimental device Since the finally prepared silica sol is required for chemical mechanical polishing experiments, the organic solvents such as ethanol and pentanol used in the reaction have a great influence on the polishing experiment, which seriously reduces the polishing effect, and pentanol has intense irritating odor. Therefore, it is necessary to design a reasonable experimental device to gradually remove organic reagents such as pentanol and ethanol and reduce irritating odors when the experimental requirements are satisfied. For this purpose, we designed a completely new experimental device for the second step system. The oil-water separator and the exhaust gas absorption device are introduced under the condition that the original feed is not affected. The separation of n-pentanol can be achieved according to the inherent physical properties of the material with different boiling points and densities. The specific experimental setup is shown in Fig. 1. Because the second step requires heating, ethanol will preferentially separate because of the low boiling point, and then the solution will reach the boiling point. The solvent such as pentanol and water will evaporate and return to the oil-water separator under the action of condensation, and be layered to achieve the effect of separation. And the upper end of the condenser tube was introduced into the beaker filled with ethanol to reduce the odor emission. The final isolated pentanol can be reused for further purification, resulting in cost savings.
2. Experimental sections 2.1. Chemicals The chemical reagents used in the experiments in this paper include: Polyvinylpyrrolidone PVP ((C6H9NO)n, Sinopharm Chemical Reagent Co., LTD, China), n-pentanol (C5H11OH, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), anhydrous ethanol(C2H5OH, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), sodium citrate(Na3C6H5O7·2H2O, Sinopharm Chemical Reagent Co., LTD, China), ammonia, tetraethyl orthosilicate TEOS (C8H20O4Si, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), Sodium silicate (Na2SiO3, JiNan DeWang Chemical Industry Co., LTD, China), NaOH(Sinopharm Chemical Reagent Co., LTD, China), cation exchange resin (Sinopharm Chemical Reagent Co., LTD, China), hydrochloric acid (HCl, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), deionized water (DI).
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With regard to the first step, the related research [24–27] on the waterin-oil (W/O) system has been very thorough. First, PVP, sodium citrate, ethanol, ammonia, and water will form stable small droplets in n-amyl alcohol, followed by TEOS. It dissolves rapidly in n-pentanol and gradually contacts with the water droplets therein to hydrolyze and form silica. In addition, heating during the hydrolysis can make the water molecules move violently, resulting in the asymmetry of the hydrolysis and condensation of TEOS, and finally breaking to obtain flower-shaped silica seeds. Among them, PVP plays a role of dispersion and stabilization, and sodium citrate acts as an end-group inducing factor. The second step, the oil-in-water (O/W) system, provides a more suitable environment for crystal growth. In the first step, the seed crystals formed in the n-pentanol first become small oil droplets dispersed in water, and then the freshly added silicic acid is first dissolved in water to be further reacted. Based on the Ostwald ripening theory [28,29], silicic acid preferentially condenses on the flower-shaped silica seeds in the pentanol droplets to grow further, and because of the heating the oil-water interface is unstable, and the condensation reaction of silicic acid occurs asymmetrically, resulting in more irregular silica abrasives. In addition, in the second step system, since the surface of the silica particles has a lot of active hydroxyl groups, the particles have a certain affinity for n-pentanol and water, so the dispersion and growth conditions of the silica are more complicated and can be divided into the following three situations. Firstly, the seeds are located in n-pentanol droplets, and the condensation reaction of silicic acid occurs at the unstable oil-water interface. Secondly, the seed crystals are located at the oil-water interface, and the unidirectional condensation reaction of silicic acid occurs on the silica on the aqueous solution side. Thirdly, part of the seed crystals are in aqueous solution, and the silicic acid is condensed directly on the seeds. These three cases can be seen from the actual diagram of the oil-water dual system, and Fig. 3 shows the distribution of the solution at different stages. It can be clearly seen that in the presence of silica, the solution assumes an emulsion state, and the oil and water blend with each other due to the formation of hydrogen bonds between the hydroxyl groups on the surface of the silica seed crystal in n-pentanol and the water molecules. Therefore, the system is relatively stable. In such an environment, some silica crystals in the npentanol tend to move toward the water droplets, resulting in the dispersion of the silica crystals in the above three cases.
Fig. 1. Second step reaction experimental device.
2.4. Chemical-mechanical polishing (CMP) tests Chemical mechanical polishing experiments were conducted using a silica sol containing spherical and flower-shaped silica with the same solid content as the polishing solution. The polishing machine was a UNIPOL-1502 CMP machine (Shenyang Kejing Instrument Co., Ltd., China) and the polishing pad was a Rodel porous polyurethane pad. The parameters settings for the polishing were: the feed rate of the polishing liquid was 180 ml/min, the load pressure was 6 Kg, the turntable speed was 70 r/min, and the polishing time was 2 h. After the polishing is finished, the sapphire wafer is washed many times with washing liquid, water, amyl alcohol, ethanol, etc., and finally dried and cooled. 2.5. Characterizations The morphologies of flower-shaped abrasives were characterized by scanning electron microscopy (SEM), and the average surface roughness (Ra) of sapphire wafers was measured by Ambios Xi-100 surface profiler. The morphology of flower-shaped silica abrasives were characterized by scanning electron microscopy (SEM, JSM-7500F) with 15 kV. The average surface roughness of the polished and unpolished sapphire wafers was measured by Ambios Xi-100 surface profiler (Ambios Technology Corp., USA) with a resolution of 0.1 Å, a focal depth of 3.0 μm, and a working distance. It is 7.4 mm and the measurement area is 500 μm × 500 μm.
3.3. Chemical mechanical polishing performance of different abrasives The polishing performance of silica abrasives is generally judged by material removal rate (MRR) and surface roughness (Ra). MRR can be calculated by equation (1):
3. Results and discussion
MRR = 3.1. Morphology of the abrasives
Δm*106 ρπR2T
(1)
Where, MRR: material removal rate (μm/h), Δm : mass difference of sapphire wafer before and after polishing, ρ: sapphire density (g·cm−3, ρsapphire = 3.98 g·cm−3), R: The radius of sapphire (nm), T: polishing time (h). Fig. 4 shows the MRR of different morphology abrasives. It can be seen that the FSS abrasives have higher MRR values under the same polishing conditions. The MRR of FSS abrasives increased by 117.2% compared to SS abrasives. This is because the combination of chemical corrosion and mechanical wear of FSS abrasives is superior to SS abrasives. Fig. 5a shows the surface roughness of sapphire wafers polished with different morphology abrasives. It can be seen that, whether it is SS or FSS, the surface of the sapphire wafers after polishing is smoother than the unpolished one. In addition, the Ra between the FSS and SS abrasives is little difference, which means that the FSS can ensure the maximum degree of planarization of the surface to be polished while improving the material removal rate. Fig. 5(b ∼ d) shows the surface profile of unpolished and polished sapphire wafers. Through these figures, the
Fig. 2a shows the morphology of the spherical silica (SS) abrasives prepared by the two-step method provided in this paper, and Fig. 2b shows the flower-shaped silica (FSS) abrasives prepared by the twosystem template method. As can be seen from the figure, the original morphology of the silica is greatly changed by the two-system template method. Therefore, it can be confirmed that the flower-shaped silica having a very irregular morphology is prepared. 3.2. Synthesis mechanism analysis In this paper, the flower-shaped silica is an improvement and innovation based on some existing irregular silica research. On the basis of changing a series of reaction conditions, the required irregular silica seed crystals were prepared by water-in-oil system, and a new oil-inwater system was proposed for the irregular growth of the crystal seed so as to obtain the silica sol used for chemical mechanical polishing of CMP. The specific experimental principle flow is shown in Scheme 1. 3
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Fig. 2. SEM images of different abrasives (a) SS abrasives (b) FSS abrasives.
polishing effect of FSS abrasives can be more visually perceived.
3.4. Analysis and discussion of polishing mechanism Chemical mechanical polishing is a method of achieving nanoscale planarization by the combination of chemical corrosion and mechanical grinding. In order to study the polishing mechanism of the FSS abrasives, as shown in Fig. 6, a contact wear model between the silica abrasives and the sapphire substrate was established. For chemical mechanical polishing, mechanical grinding can be based on the MRR equation (2) proposed by Luo et al. [30] MRRthickness = C1[(1-Ø(3-C2P01/3)]P01/2υ
Fig. 3. Solution images of different stages (a) seed crystal obtained in W/O system (b) pure pentanol and water (c) pentanol - silica seed – water in O/W system (d) the FSS sol.
(2)
Scheme 1. Preparation principle of flower-shaped silica abrasive. 4
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Fig. 4. The MRR values of SS abrasives and FSS abrasives (The solid content is 6.0 wt %). Fig. 6. Contact wear model of silica abrasives and sapphire wafer.
Ep 2 2 ds k 2ρs ms − a DSUM al , C2 C1 = ρa πx avg (b1 Hw )2/3 0.25 × =
2 4 3 (x avg 3
()
+ 3σ )
σ
(
1 Hp
+
2 Hw
area is the sum of the effective contact points [31,32]. As can be clearly seen in Fig. 5, the SS abrasives are in contact with the plane only at one point, whereas the FSS abrasives need multiple points to make contact with the plane to balance. Therefore, the effective contact point of the abrasive per unit particle FSS is more than the effective contact point of the SS abrasive, and the actual contact area of the FSS abrasive is larger. It was further confirmed that the polishing rate of the FSS abrasives was higher than that of the SS abrasives. In terms of chemical corrosion, SiO2 and Al2O3 can undergo solid-phase chemical reactions. The principle of the reaction has been discussed by many researchers [11,12]. There are three main chemical reaction equations:
)E
2/3 p
b1
Where, MRRthickness: material removal rate, ds : abrasive volume fraction, k : constant, ρs : density of silica sol, ms − a : abrasive mass fraction, DSUM : rough peak density per unit area, a : average contact area at the time of abrasive contact, l : the average height of the abrasive, ρa : abrasive density, x avg : the average particle size of the abrasive, Ep: young's modulus of elasticity of polishing pad, (b1 Hw )2/3 : constant, Hw : the hardness of the polished parts, Hp : the hardness of polishing pad, σ : standard deviation of the average particle size of the abrasive, P0: polishing pad loading pressure, υ: relative speed of polishing pad and workpiece, Ø: normal distribution function. In the polishing experiment, the main influencing factor is the actual contact area between the abrasives and the substrate. The greater the actual contact, the better the grinding effect, and the actual contact
Al2O3 + H2O = 2AlOOH
(3)
2AlOOH + SiO2 = Al2SiO7·H2O
(4)
Al2O3 + H2O + SiO2 = Al2SiO7·H2O
(5)
Similarly, due to the increase of the number of effective contact
Fig. 5. (a) Surface roughness of sapphire wafers unpolished and after polishing with different abrasives; and surface profile of sapphire wafers: (b) unpolished, (c) polished by SS abrasives and (d) polished by FSS abrasives. 5
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points of FSS, the probability and degree of solid-phase chemical reaction of flower-shaped silicon dioxide and Al2O3 are greater, further improving the polishing performance.
(2016) 713–720. [9] M. Krishnan, J.W. Nalaskowski, L.M. Cook, Chemical mechanical planarization: slurry chemistry, materials, and mechanisms, J. Chem. Rev. 110 (2010) 178. [10] X. Wang, H. Lei, R. Chen, CMP behavior of alumina/metatitanic acid core-shell abrasives on sapphire substrates, J. Precis. Eng. 50 (2017) 263–268. [11] Y. Dong, H. Lei, W. Liu, T. Wang, L. Xu, Preparation of non-spherical silica composite abrasives by lanthanum ion-induced effect and its chemical–mechanical polishing properties on sapphire substrates, J. Mater. Sci. 53 (2018) 10732–10742. [12] T. Liu, H. Lei, Nd3+-doped colloidal SiO2, composite abrasives: synthesis and the effects on chemical mechanical polishing (CMP) performances of sapphire wafers, J. Appl. Surf. Sci. 413 (2017) 16–26. [13] L. Zazzera, B. Mader, M. Ellefson, J. Eldridge, S. Loper, J. Zabasajja, J. Qian, Comparison of ceria nanoparticle concentrations in effluent from chemical mechanical polishing of silicon dioxide, J. Environ. Sci. Technol. 48 (2014) 13427–13433. [14] G. Chen, Z. Ni, L. Xu, Q. Li, Y. Zhao, Performance of colloidal silica and ceria based slurries on CMP of Si-face 6H-SiC substrates, J. Appl. Surf. Sci. 359 (2015) 664–668. [15] L. Peedikakkandy, L. Kalita, P. Kavle, A. Kadam, V. Gujar, Preparation of spherical ceria coated silica nanoparticle abrasives for CMP application, J. Appl. Surf. Sci. 357 (2015) 1306–1312. [16] S. Salleh, I. Sudin, A. Awang, Effects of non-spherical colloidal silica slurry on AlNiP hard disk substrate CMP application, J. Appl. Surf. Sci. 360 (2016) 59–68. [17] W. Stöber, A. Fink, Ernst Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69. [18] H. Nakamura, Y. Matsui, Silica gel nanotubes obtained by the sol-gel method, J. Am. Chem. Soc. 117 (1995) 2651–2652. [19] R. Brambilla, G.P. Pires, J.H. dos Santos, M.S. Lacerda Miranda, Octadecylsilane hybrid silicas prepared by the sol-gel method: morphological and textural aspects, J. Colloid Interface Sci. 312 (2007) 326–332. [20] H. Tao, W. Zhong, Y. Zhang, P. Yang, Effects of particle properties on separation behavior of differently-sized non-spherical particle mixtures, J. Chin. Soc. Power Eng. 35 (2015) 652–658. [21] M.L. Saladino, T.E. Motaung, A.S. Luyt, A. Spinella, G. Nasillo, E. Caponetti, The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA, J. Polym. Degrad. Stabil. 73 (2012) 34–39. [22] C. Liang, L. Wang, W. Liu, Z. Song, Non-spherical colloidal silica particles—preparation, application and model, J. Colloid. Surface. A 457 (2014) 67–72. [23] H. Lee, M. Kim, H. Jeong, Effect of non-spherical colloidal silica particles on removal rate in oxide CMP, Int. J. Precis. Eng. Manuf. 16 (2015) 2611–2616. [24] R.P. Murphy, K. Hong, N.J. Wagner, Synthetic control of the size, shape, and polydispersity of anisotropic silica colloids, J. Colloid Interface Sci. 501 (2017) 45–53. [25] A. Kuijk, A.V. Blaaderen, A. Imhof, Synthesis of monodisperse, rodlike silica colloids with tunable aspect ratio, J. Am. Chem. Soc. 133 (2011) 2346–2349. [26] J. Wang, Y. Lu, Facile synthesis of asymmetrical flower-like silica, J. Mater. Des. 111 (2016) 206–212. [27] C. Lu, M. Urban, Rationally designed gibbous stimuli-responsive colloidal nanoparticles, J. Acs Nano. 9 (2015) 3119–3124. [28] P. Kokhanenko, K. Brown, M. Jermy, Silica aquasols of incipient instability: synthesis, growth kinetics and long term stability, J. Colloid. Surface. A 493 (2016) 18–31. [29] A. Lazaro, M.C. van de Griend, H.J.H. Brouwers, J.W. Geus, The influence of process conditions and Ostwald ripening on the specific surface area of olivine nano-silica, J. Micropor. Mesopor. Mat. 181 (2013) 254–261. [30] J. Luo, D.A. Dornfeld, Material removal mechanism in chemical mechanical polishing: theory and modeling, J. Ieee. T. Semiconduct. M 14 (2001) 112–133. [31] Y. Kimura, Estimation of the number and the mean area of real contact points on the basis of surface profiles, J. Wear. 15 (1970) 47–55. [32] V.A. Yastrebov, G. Anciaux, J.F. Molinari, On the accurate computation of the true contact-area in mechanical contact of random rough surfaces, J. Tribol. Int. 114 (2017) 161–171.
4. Conclusions For the first time, we propose to prepare irregular flower-shaped silica abrasives by double system (W/O-O/W) template method for the polishing of sapphire wafers. The reaction mechanism of this twosystem template method for synthesizing flower-shaped silica was analyzed. By changing the chemical reaction environment, the morphology of silica was greatly changed. The polishing experiment was carried out using the flower-shaped silica sol, and the MRR value thereof was increased by 117.4% compared with the spherical silica sol. The surface flatness of the sapphire wafer is also improved, and the Ra value is low. The main action mechanism of flower-shaped silica abrasives lies in the effective multi-point contact between abrasives and sapphire wafers. While increasing the contact area between abrasives and sapphire wafers, the solid-phase chemical reaction between them is also increased, thus enhancing the overall MRR and improving the surface morphology of sapphire chips. Thus, the irregularly flowershaped silica abrasives are promising in place of conventional spherical silica abrasives. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Number 51475279). References [1] D. Yin, X. Niu, K. Zhang, J. Wang, Y. Cui, Preparation of MgO doped colloidal SiO2, abrasive and their chemical mechanical polishing performance on c-, r- and a-plane sapphire substrate, J. Ceram. Int. 44 (2018) 14631–14637. [2] H. Aida, T. Doi, H. Takeda, H. Katakura, S. Kim, K. Koyama, T. Yamazaki, M. Uneda, Ultraprecision CMP for sapphire, GaN, and SiC for advanced optoelectronics materials, J. Curr. Appl. Phys. 12 (2012) S41–S46. [3] R. Osellame, N. Chiodo, V. Maselli, A. Yin, M. Zavelani-Rossi, G. Cerullo, P. Laporta, L. Aiello, S. De Nicola, P. Ferraro, A. Finizio, G. Pierattini, Optical properties of waveguides written by a 26 MHz stretched cavity Ti: sapphire femtosecond oscillator, J. Opt. Express. 13 (2005) 612–620. [4] D. Hayes, Sapphire watch glass machining with diamond, J. Ind. Diamond Rev. 58 (1998) 55–56. [5] Y. Gurbuz, O. Esame, I. Tekin, W.P. Kang, J.L. Davidson, Diamond semiconductor technology for RF device applications, J. Solid State Electron. 49 (2005) 1055–1070. [6] N. Kumar, M.G. Fissel, K. Pourrezaei, B. Lee, E.C. Douglas, Growth and properties of TiN and TiOxNy, diffusion barriers in silicon on sapphire integrated circuits, J. Thin Solid Films 153 (1987) 287–301. [7] S. Thakoor, D.M. Strayer, G.J. Dick, J.E. Mercereau, A lead‐on‐sapphire superconducting cavity of superior quality, J. Appl. Phys. 59 (1986) 854–858. [8] Y. Xu, J. Lu, X. Xu, Study on planarization machining of sapphire wafer with softhard mixed abrasive through mechanical chemical polishing, J. Appl. Surf. Sci. 389
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