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Synthesis, characterization of ceria-coated silica particles and their chemical mechanical polishing performance on glass substrate
Applied Surface Science 257 (2010) 1750–1755
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis, characterization of ceria-coated silica particles and their chemical mechanical polishing performance on glass substrate Zefang Zhang a,b,c,∗ , Weili Liu a,b , Jingkang Zhu a,b , Zhitang Song a,b a State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China b Shanghai Xinanna Electronic Technology Co., Ltd., Shanghai 201506, China c Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 March 2010 Received in revised form 1 September 2010 Accepted 1 September 2010 Available online 15 September 2010 Keywords: Ceria-coated silica particle Chemical mechanical polishing Poly(vinylpyrrolidone) Glass substrate
a b s t r a c t Nano-sized ceria particles were coated on the silica surface by the precipitation method using ammonium cerium nitrate and urea as precipitant with poly(vinylpyrrolidone) (PVP) as assistant. The structures and compositions of ceria-coated silica particles were characterized using X-ray diffraction (XRD), fieldemission scanning microscopy (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. The results show that nano-size ceria particles were coated uniformly around the surface of silica particles when PVP was used as assistant during coating process, while without PVP, the ceria particles were grown sparsely on the silica particle surface and many ceria particles grow up through independent nucleation in the solution. Then, the chemical mechanical polishing (CMP) behaviors of the as-prepared ceria-coated silica particles on glass substrate were investigated. The CMP test results suggest that the as-prepared ceria-coated silica particles exhibit higher removal rate than pure silica particles without deteriorating the surface quality. In addition, online coefficient of friction (COF) was conducted during the polishing process. The COF data indicate that the COF values of ceria-coated silica particles are larger than those of pure silica particles due to their surface properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Chemical mechanical polishing (CMP) has become an accepted technique in manufacturing of semiconductor and digital CD glass substrate [1,2]. In the CMP process, abrasive as the basic element of slurry is crucial for a desired CMP performance in terms of high material removal rate (MRR) and high surface quality [3,4]. Ceria is one of the key abrasive materials for CMP of dielectric film, shallow trench isolation (STI) and glass due to their high polish rates and high removal selectivity [5]. However, the cost, broad size distribution and irregular shape of the ceria particles limited their widespread commercial acceptance [6]. The early and most commonly used abrasive in CMP slurry is SiO2 . SiO2 abrasive is a suitable candidate because it can be prepared as monodispersed spheres and in narrow size distribution. In addition, its low preparation cost is also an important factor in CMP applications. In spite of these advantages, the polish rate of dielectric film, STI and glass using SiO2 abrasive slurry is the lowest among the single abrasive slurry [7].
∗ Corresponding author at: State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China. E-mail address: [email protected] (Z. Zhang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.009
It was found that just one kind of abrasive used in slurries leads to undesired CMP performance. Therefore, composite particles as abrasive in slurry have been paid much attention [6–14]. To combine the advantages of CeO2 and SiO2 , the ceria-coated silica composite particles have been investigated by several works [6–12]. Lee et al. [6] prepared ceria-coated silica particles and studied the polish effect using slurries of ceria-coated silica powders and compared with slurries containing pure nano-sized ceria or silica. Choi et al. [12] had shown that the polishing of silica glass with porous silica particles coated with ceria is considerably more efficient than when CMP was carried out with only nano-sized ceria. Zhao et al. [8] synthesized ceria@silica nanoparticles by sol–gel method and studied their CMP behavior on oxide. It was found that ceria@silica nanoparticles possess high removal rate on oxide. In the present work, nano-sized ceria particles were coated on the silica surface by the precipitation method using ammonium cerium nitrate and urea as precipitant with poly(vinylpyrrolidone) (PVP) as assistant. Compared with other methods, the method preparing ceria-coated silica particles in this study has its advantages as follows. Firstly, the colloidal silica core was prepared by the methods of particle growth through ion exchange and hydrothermal synthesis, taking uniform particle size and high stability as well as low cost. Secondly, ammonium cerium nitrate and urea were used as reactants, thus, the charge of cerium ion is about +4 which
Z. Zhang et al. / Applied Surface Science 257 (2010) 1750–1755
just suitable for the formation of CeO2 without further oxidation [15]. Finally, the amphiphilic nonionic polymer PVP was used as assistant to enhance the affinity between ceria and silica, producing the relatively complete and uniform ceria coating on the silica surface.
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Table 1 Process parameters used for glass polishing using CP-4 polisher. Pad rotation speed (rpm) Wafer rotation speed (rpm) Down force (psi) Slurry feed rate (ml/min) Polishing time (min)
100 100 5 100 30
2. Experimental methods 2.1. Synthesis and characterization of ceria-coated silica particles In this section, the chemicals, synthesis method and the characterizations were stated as follows. 2.1.1. Chemicals Chemicals used in this synthesis were ammonium cerium nitrate ((NH4 )2 Ce(NO3 )6 ), urea (CO(NH2 )) and poly(vinylpyrrolidone) (PVP). All the chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. 2.1.2. Synthesis process The colloidal silica core was prepared by the methods of particle growth through ion exchange and hydrothermal synthesis as reported by our group previously [16,17]. Ceria-coated silica particles were prepared by the precipitation method in accordance with the following procedure. 10 mmol (NH4 )2 Ce(NO3 )6 and 40 mmol CO(NH2 ) were dissolved in 100 ml deionized (DI) water and stirred magnetically to get good homogenous mixed solution. 0.1 g PVP was added into 100 g 10 wt.% colloidal silica sol under stirring. After PVP dissolved completely, the silica sol was heated to 100 ◦ C and above mixed solution was added quickly under stirring into silica sol at this temperature. The whole mixture was then stirred for 5 h at 100 ◦ C. Finally, the mixture was separated with a centrifuge to obtain ceria-coated silica composite particles. The composite particles were purified by dispersing in DI water and following centrifugal separation for five times.
The pad was conditioned prior to each polishing for 5 min using a 4 inch diamond grit conditioner. The polishing slurry containing 5 wt.% modified ceria-coated silica particles was prepared, and pH value was fixed to about 10. Then, the prepared slurry was filtrated by a filter with 20 m pore. For comparison, the control slurry containing pure silica particles was prepared with the same procedures, and the polishing test was conducted under the same conditions. After polishing, the substrate was rinsed in an ultrasonic bath of DI water and dried by N2 gas. 2.2.2. Examinations of the polished substrates The weight of glass substrate before and after polishing was measured by electron balance to calculate the MRR according to Eq. (1) [19,20]. MRR =
107 × m × 2.542 × × t
(1)
Here, m (g) is the mass variation of glass after polishing, t (min) is the polishing time, is the density of glass substrate, and MRR (nm/min) is the corresponding removal rate. The MRR is the average of 3 individual polishing tests. The polished surface topographical and root mean square (RMS) were measured by a Quesant Q-Scope 250 Atomic force microscopy (AFM) with contacting mode. The RMS roughness was measured on five points of size 10 m × 10 m. The values reported in this study are the average of the RMS roughness calculated at these fifteen points of three polishing runs. 3. Results and discussion
2.1.3. Characterization methods The phases of pure silica, pure ceria and ceria-coated silica composite particles were determined by X-ray diffraction (XRD) using a Rigaku D/max-2200PS diffractometer with graphite monochro˚ in the 2 range of 10–90◦ . matized Cu K␣ radiation ( = 1.54178 A) The morphologies of pure silica particles, ceria-coated silica particles prepared with and without PVP were investigated by Hitachi S-4700 field-emission scanning microscopy (FE-SEM) with Oxford Inca x-sight energy dispersive spectroscopy (EDS) and FEI Tecnai G2 S-TWIN transmission electron microscopy (TEM). Size distributions of pure silica and ceria-coated silica particles were examined by dynamic light scattering (DLS) using Particle Sizing System (PSS) Nicomp 380. 2.2. CMP test To investigate the CMP performance of as-prepared composite particles, the CMP test and examinations of polished glass surfaces were conducted and the details were as follow. 2.2.1. CMP process In this study, 2 inch commercial Soda Lime glass substrates which were widely used in mastering substrates in the optical media were polished using a CMP tester (CETR, CP-4) with a polyurethane (PU) pad (Universal Photonics, LP66). In addition, the CP-4 polisher is designed with the online detector of COF. Chemical composition of Soda Lime glass was SiO2 (70–73 wt.%), Na2 O and K2 O (13–15 wt.%), CaO (7–12 wt.%) [18]. Table 1 shows the CMP process parameters.
3.1. Structures and compositions of ceria-coated silica particles In this section, the results of XRD, SEM, TEM, EDS, and DLS were analyzed to investigate the structures and compositions of as-prepared composite particles. 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of silica particles, ceria particles, and ceria-coated silica particles prepared with PVP. The pattern of pure silica shows its amorphous structure. The diffraction peaks of ceria particles correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0), and (4 2 2) planes, which can be indexed to the face-centered cubic structure for CeO2 (space group Fm3m (2 2 5)) with a lattice constant of ˛ = 0.5411 nm according to JCPDS No. 65-2975. It was found that the as-synthesized composite particles consisted of silica and ceria phase. But the silica peak intensity of as-synthesized composite particles is much weaker than that of the pure silica even though the ceria peaks keep almost the same intensity. This is because the SiO2 particles may be well coated with CeO2 particles and the outer CeO2 particles prevent the diffraction of inner SiO2 particles to some degree. Similar result was also observed in the literature [21]. In addition, it is clearly seen that the reflection peaks of ceria phase are broad, indicating that the ceria crystal size is small and should be in the range of nano-scale. 3.1.2. SEM and EDS analyses To directly investigate the structures of the as-prepared composite particles, the SEM images of silica, ceria-coated silica
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Z. Zhang et al. / Applied Surface Science 257 (2010) 1750–1755
Fig. 1. XRD patterns of silica particles, ceria particles and ceria-coated particles prepared with PVP.
particles prepared with and without PVP are shown in Fig. 2. It was found that the silica particles presented a spherical shape with smooth surface and narrow size distribution. After coating, the silica particles remained spherical particles but their surfaces became very rough. EDS was used to confirm the presence of elements on composite particles surface, which suggests that as-prepared composite material is composed of Si, Ce, O and N elements (Fig. 2 (c)). EDS analysis corroborated the XRD results proved that nanoceria particles coated on silica surface account for the rough surface of composite particles. In image of ceria-coated silica particles prepared without PVP, it was observed that ceria coating is not complete and many ceria particles of homogenous nucleation produced in the coating process did not coat on the silica surface. However, the ceria coating is relatively complete and uniform and almost no homogenous nucleation particles were observed in the image of ceria-coated silica particles prepared with PVP. 3.1.3. TEM analysis To further confirm the structures of ceria-coated silica particles, the TEM was conducted as shown in Fig. 3. Alien form the SEM pictures, many homogenous nucleations of ceria particles were also observed in the TEM image of ceria-coated silica particles prepared with PVP (Fig. 3(b)). On the basis of these observations, one can be concluded that heterogeneous nucleation on the silica surface and homogenous nucleation of ceria particles take place simultaneously in the reaction process with and without PVP, but the relatively complete, uniform, distinctive and crystalline ceria shell was obtained with PVP (as shown in Fig. 3(c)), which suggests that PVP play an important role in controlling nucleation of ceria particles on the silica surface. This may be explained that in the synthesis process with PVP, PVP which is an amphiphilic, nonionic polymer [22–24] was adsorbed onto colloidal silica particles. After this functionalization heterogeneous nucleation and growth of ceria took place on the silica surface by adding the ammonium cerium nitrate and urea mixed solution to the colloidal silica sol since the affinity between ceria and silica was enhanced.
Fig. 2. SEM images of silica particles (a), ceria-coated particles prepared without PVP (b), ceria-coated particles prepared with PVP (c) and their EDS spectra.
3.1.4. DLS measurement Fig. 4 and Table 2 show the Gaussian Intensity/Volume/Number weighted distributions and mean size as well as polydispersion index (PDI) of pure silica particles and ceria-coated particles prepared with PVP, respectively. The former particle size distribution is very narrow and its PDI is very low, while the latter distribution and PDI increases significantly. As can be seen for SEM and TEM images, the thickness of ceria coating is about 7–10 nm, however, the mean size of silica particles increases from 102.3 nm to 179.4 nm, which
Fig. 5 shows the COF as a functional of polishing time for both pure silica particles and ceria-coated silica particles. The COF at the beginning is very high (about 0.434) and quickly decreases with the polishing time in the range of 0–200 s. However, there is no significant change in the COF when the time increases from 200 s to 1800 s. In this study, glass substrates polished were ground glass and had many rough peaks (as shown in Fig. 4(a)) which were firstly removed in the polishing process with the high MRR [2]. The rough surface and high MRR may account for the high COF in
indicates that agglomeration occurred in the ceria-coated silica particles. Furthermore, agglomeration can be also observed in SEM and TEM images. The homogenous nucleation of ceria particles and agglomeration may account for the increase of size distribution and PDI after coating of ceria. 3.2. CMP performance of ceria-coated silica particles
Z. Zhang et al. / Applied Surface Science 257 (2010) 1750–1755
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Fig. 4. Size distributions of silica particles and ceria-coated silica particles.
Table 2 Mean size and PDI of pure silica particles and ceria-coated silica particles. Type of abrasive
Mean size
PDI
Pure silica Ceria-coated silica
102.3 179.4
0.017 0.134
the initial polishing process. With increasing of polishing time, the number of rough peaks decreased, which result in the decrease of RMS roughness and MRR, COF as did. When the rough peaks were completely removed, and consequently the contact area tends to remain constant, stable COF could be observed [25]. In Fig. 5, it was observed that COF of glass CMP using ceriacoated silica particles is higher than that of pure silica particles.
Fig. 3. TEM images of ceria-coated particles prepared without PVP (a), ceria-coated particles prepared with PVP (b) and their HRTEM image (c).
Fig. 5. COF as a function of polishing time for glass substrates polished by both pure silica particles and ceria-coated silica particles.
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Z. Zhang et al. / Applied Surface Science 257 (2010) 1750–1755
Table 3 The MRR and RMS of glass substrate polished by pure silica particles and ceria-coated silica particles. Type of abrasive
MRR (nm/min)
RMS (nm)
Pure silica abrasive Ceria-coated silica abrasive
58.49 126.41
1.024 1.073
The COF is known to be strongly dependent on interfacial chemical interactions, interfacial electrostatic interactions, dynamic surface conditions, properties of the opposing surfaces, and abrasive particles size, which all influence the contact area between the opposing surfaces [21]. Ceria, as a widely used abrasive for glass polishing, is
believed to form the direct bonding to glass, which creates higher shear force [26,27], and as a result, the COF increases. The MRR and RMS of glass substrates polished by pure silica and ceria-coated silica particles were given in Table 3. It can be seen that the MRR of ceria-coated silica particles is two times larger than that of pure silica particles, while the RMS roughness values of the glass polished by both particles are almost identical. It can be concluded that the MRR is strongly dependant on COF in this study. In order to further investigate the difference in polishing performance between pure silica and ceria-coated silica, the topographical micrographs of polished glass substrate surfaces were analyzed by AFM, and the result is shown in Fig. 6. By comparison with pure silica particles, the as-prepared ceria-coated silica particles give lower topographical variations as well as less scratch. Since CeO2 is softer than SiO2 , it inflicts less damage such as scratches on the glass than other particles. The CeO2 particles feature softness and high polishing rate. If mechanical polishing is dominant, soft particles give slower polishing rate, but the fact is reversed. So, there should be a large contribution from chemical reactivity in the polishing mechanism. 4. Conclusions Nano-sized particles ceria were coated on the silica surface by the precipitation method using ammonium cerium nitrate and urea as precipitant with PVP as assistant. Glass substrates were polished with slurries containing either pure silica particles or as-prepared ceria-coated silica particles. The polish rate of the latter was significantly higher than that of the former and the surface quality in terms of RMS was approximately identical after polishing with both silica and ceria-coated silica. Acknowledgements The work is sponsored by National Key Technologies R & D Program of China during the 11th Five-Year Plan Period (No. 2009ZX02030), Shanghai Nano Technology Program (No. 0952nm00200), Shanghai Major State Technology Program (No. 08111100300). References
Fig. 6. AFM morphology of glass substrate (a) before polishing, (b) polished by pure silica particles and (c) polished by ceria-coated silica particles.
[1] H. Lei, H. Lu, P. Zhang, Z. Zhang, B. Xiao, 7th International Conference on Frontiers of Design and Manufacturing, Guangzhou, China, 2006, pp. 445–448. [2] Z.F. Zhang, H. Lei, Microelectron. Eng. 85 (2008) 714–720. [3] H. Lei, H.S. Lu, J.B. Luo, X.C. Lu, Thin Solid Films 516 (2008) 3005–3008. [4] H. Lei, P.Z. Zhang, Appl. Surf. Sci. 253 (2007) 8754–8761. [5] X.D. Feng, D.C. Sayle, Z.L. Wang, M.S. Paras, B. Santora, A.C. Sutorik, T.X.T. Sayle, Y. Yang, Y. Ding, X.D. Wang, Y.S. Her, Science 312 (2006) 1504–1508. [6] S.H. Lee, Z.Y. Lu, S.V. Babu, E. Matijevic, J. Mater. Res. 17 (2002) 2744–2749. [7] X.L. Song, N. Jiang, Y.K. Li, D.Y. Xu, G.Z. Qiu, Mater. Chem. Phys. 110 (2008) 128–135. [8] X. Zhao, R. Long, Y. Chen, Z. Chen, Microelectron. Eng. 87 (2010) 1716–1720. [9] M.-H. Oh, J.-S. Lee, S. Gupta, F.-C. Chang, R.K. Singh, Colloids Surf. Physicochem. Eng. Asp. 355 (2010) 1–6. [10] H. Lei, F. Chu, B. Xiao, X. Tu, H. Xu, H. Qiu, Microelectron. Eng. 87 (2010) 1747–1750. [11] M.-S. Tsai, Mater. Sci. Eng., B 104 (2003) 63–67. [12] W. Choi, S.-M. Lee, a.R.K. Singh, Mater. Res. Soc. Symp. Proc. (2001) M5.1.1–M5.1.6. [13] R. Shen, S.N. Shafrir, C. Miao, M. Wang, J.C. Lambropoulos, S.D. Jacobs, H. Yang, J. Colloid Interface Sci. 342 (2010) 49–56. [14] S.N. Shafrir, H.J. Romanofsky, M. Skarlinski, M. Wang, C. Miao, S. Salzman, T. Chartier, J. Mici, J.C. Lambropoulos, R. Shen, H. Yang, S.D. Jacobs, Appl. Opt. 48 (2009) 6797–6810. [15] M.S. Tsai, Mater. Sci. Eng., B: Solid 110 (2004) 132–134. [16] K.L. Zhang, Z.T. Song, C.L. Lin, S.F. Feng, B. Chen, J. Ceram. Process. Res. 8 (2007) 52–55. [17] K.L. Zhang, Z.T. Song, F. Wang, L.Y. Wang, S.L. Feng, J. Nanosci. Nanotechnol. 9 (2009) 1054–1057. [18] C.H. Lien, Y.H. Guu, Mater. Manuf. Processes 23 (2008) 838–843. [19] Z. Zhang, L. Yu, W. Liu, Z. Song, Appl. Surf. Sci. 256 (2010) 3856–3861. [20] Z. Zhang, W. Liu, Z. Song, X. Hu, J. Electrochem. Soc. 157 (2010) H688–H691.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis, characterization of ceria-coated silica particles and their chemical mechanical polishing performance on glass substrate Zefang Zhang a,b,c,∗ , Weili Liu a,b , Jingkang Zhu a,b , Zhitang Song a,b a State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China b Shanghai Xinanna Electronic Technology Co., Ltd., Shanghai 201506, China c Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 March 2010 Received in revised form 1 September 2010 Accepted 1 September 2010 Available online 15 September 2010 Keywords: Ceria-coated silica particle Chemical mechanical polishing Poly(vinylpyrrolidone) Glass substrate
a b s t r a c t Nano-sized ceria particles were coated on the silica surface by the precipitation method using ammonium cerium nitrate and urea as precipitant with poly(vinylpyrrolidone) (PVP) as assistant. The structures and compositions of ceria-coated silica particles were characterized using X-ray diffraction (XRD), fieldemission scanning microscopy (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. The results show that nano-size ceria particles were coated uniformly around the surface of silica particles when PVP was used as assistant during coating process, while without PVP, the ceria particles were grown sparsely on the silica particle surface and many ceria particles grow up through independent nucleation in the solution. Then, the chemical mechanical polishing (CMP) behaviors of the as-prepared ceria-coated silica particles on glass substrate were investigated. The CMP test results suggest that the as-prepared ceria-coated silica particles exhibit higher removal rate than pure silica particles without deteriorating the surface quality. In addition, online coefficient of friction (COF) was conducted during the polishing process. The COF data indicate that the COF values of ceria-coated silica particles are larger than those of pure silica particles due to their surface properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Chemical mechanical polishing (CMP) has become an accepted technique in manufacturing of semiconductor and digital CD glass substrate [1,2]. In the CMP process, abrasive as the basic element of slurry is crucial for a desired CMP performance in terms of high material removal rate (MRR) and high surface quality [3,4]. Ceria is one of the key abrasive materials for CMP of dielectric film, shallow trench isolation (STI) and glass due to their high polish rates and high removal selectivity [5]. However, the cost, broad size distribution and irregular shape of the ceria particles limited their widespread commercial acceptance [6]. The early and most commonly used abrasive in CMP slurry is SiO2 . SiO2 abrasive is a suitable candidate because it can be prepared as monodispersed spheres and in narrow size distribution. In addition, its low preparation cost is also an important factor in CMP applications. In spite of these advantages, the polish rate of dielectric film, STI and glass using SiO2 abrasive slurry is the lowest among the single abrasive slurry [7].
∗ Corresponding author at: State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China. E-mail address: [email protected] (Z. Zhang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.009
It was found that just one kind of abrasive used in slurries leads to undesired CMP performance. Therefore, composite particles as abrasive in slurry have been paid much attention [6–14]. To combine the advantages of CeO2 and SiO2 , the ceria-coated silica composite particles have been investigated by several works [6–12]. Lee et al. [6] prepared ceria-coated silica particles and studied the polish effect using slurries of ceria-coated silica powders and compared with slurries containing pure nano-sized ceria or silica. Choi et al. [12] had shown that the polishing of silica glass with porous silica particles coated with ceria is considerably more efficient than when CMP was carried out with only nano-sized ceria. Zhao et al. [8] synthesized ceria@silica nanoparticles by sol–gel method and studied their CMP behavior on oxide. It was found that ceria@silica nanoparticles possess high removal rate on oxide. In the present work, nano-sized ceria particles were coated on the silica surface by the precipitation method using ammonium cerium nitrate and urea as precipitant with poly(vinylpyrrolidone) (PVP) as assistant. Compared with other methods, the method preparing ceria-coated silica particles in this study has its advantages as follows. Firstly, the colloidal silica core was prepared by the methods of particle growth through ion exchange and hydrothermal synthesis, taking uniform particle size and high stability as well as low cost. Secondly, ammonium cerium nitrate and urea were used as reactants, thus, the charge of cerium ion is about +4 which
Z. Zhang et al. / Applied Surface Science 257 (2010) 1750–1755
just suitable for the formation of CeO2 without further oxidation [15]. Finally, the amphiphilic nonionic polymer PVP was used as assistant to enhance the affinity between ceria and silica, producing the relatively complete and uniform ceria coating on the silica surface.
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Table 1 Process parameters used for glass polishing using CP-4 polisher. Pad rotation speed (rpm) Wafer rotation speed (rpm) Down force (psi) Slurry feed rate (ml/min) Polishing time (min)
100 100 5 100 30
2. Experimental methods 2.1. Synthesis and characterization of ceria-coated silica particles In this section, the chemicals, synthesis method and the characterizations were stated as follows. 2.1.1. Chemicals Chemicals used in this synthesis were ammonium cerium nitrate ((NH4 )2 Ce(NO3 )6 ), urea (CO(NH2 )) and poly(vinylpyrrolidone) (PVP). All the chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. 2.1.2. Synthesis process The colloidal silica core was prepared by the methods of particle growth through ion exchange and hydrothermal synthesis as reported by our group previously [16,17]. Ceria-coated silica particles were prepared by the precipitation method in accordance with the following procedure. 10 mmol (NH4 )2 Ce(NO3 )6 and 40 mmol CO(NH2 ) were dissolved in 100 ml deionized (DI) water and stirred magnetically to get good homogenous mixed solution. 0.1 g PVP was added into 100 g 10 wt.% colloidal silica sol under stirring. After PVP dissolved completely, the silica sol was heated to 100 ◦ C and above mixed solution was added quickly under stirring into silica sol at this temperature. The whole mixture was then stirred for 5 h at 100 ◦ C. Finally, the mixture was separated with a centrifuge to obtain ceria-coated silica composite particles. The composite particles were purified by dispersing in DI water and following centrifugal separation for five times.
The pad was conditioned prior to each polishing for 5 min using a 4 inch diamond grit conditioner. The polishing slurry containing 5 wt.% modified ceria-coated silica particles was prepared, and pH value was fixed to about 10. Then, the prepared slurry was filtrated by a filter with 20 m pore. For comparison, the control slurry containing pure silica particles was prepared with the same procedures, and the polishing test was conducted under the same conditions. After polishing, the substrate was rinsed in an ultrasonic bath of DI water and dried by N2 gas. 2.2.2. Examinations of the polished substrates The weight of glass substrate before and after polishing was measured by electron balance to calculate the MRR according to Eq. (1) [19,20]. MRR =
107 × m × 2.542 × × t
(1)
Here, m (g) is the mass variation of glass after polishing, t (min) is the polishing time, is the density of glass substrate, and MRR (nm/min) is the corresponding removal rate. The MRR is the average of 3 individual polishing tests. The polished surface topographical and root mean square (RMS) were measured by a Quesant Q-Scope 250 Atomic force microscopy (AFM) with contacting mode. The RMS roughness was measured on five points of size 10 m × 10 m. The values reported in this study are the average of the RMS roughness calculated at these fifteen points of three polishing runs. 3. Results and discussion
2.1.3. Characterization methods The phases of pure silica, pure ceria and ceria-coated silica composite particles were determined by X-ray diffraction (XRD) using a Rigaku D/max-2200PS diffractometer with graphite monochro˚ in the 2 range of 10–90◦ . matized Cu K␣ radiation ( = 1.54178 A) The morphologies of pure silica particles, ceria-coated silica particles prepared with and without PVP were investigated by Hitachi S-4700 field-emission scanning microscopy (FE-SEM) with Oxford Inca x-sight energy dispersive spectroscopy (EDS) and FEI Tecnai G2 S-TWIN transmission electron microscopy (TEM). Size distributions of pure silica and ceria-coated silica particles were examined by dynamic light scattering (DLS) using Particle Sizing System (PSS) Nicomp 380. 2.2. CMP test To investigate the CMP performance of as-prepared composite particles, the CMP test and examinations of polished glass surfaces were conducted and the details were as follow. 2.2.1. CMP process In this study, 2 inch commercial Soda Lime glass substrates which were widely used in mastering substrates in the optical media were polished using a CMP tester (CETR, CP-4) with a polyurethane (PU) pad (Universal Photonics, LP66). In addition, the CP-4 polisher is designed with the online detector of COF. Chemical composition of Soda Lime glass was SiO2 (70–73 wt.%), Na2 O and K2 O (13–15 wt.%), CaO (7–12 wt.%) [18]. Table 1 shows the CMP process parameters.
3.1. Structures and compositions of ceria-coated silica particles In this section, the results of XRD, SEM, TEM, EDS, and DLS were analyzed to investigate the structures and compositions of as-prepared composite particles. 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of silica particles, ceria particles, and ceria-coated silica particles prepared with PVP. The pattern of pure silica shows its amorphous structure. The diffraction peaks of ceria particles correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0), and (4 2 2) planes, which can be indexed to the face-centered cubic structure for CeO2 (space group Fm3m (2 2 5)) with a lattice constant of ˛ = 0.5411 nm according to JCPDS No. 65-2975. It was found that the as-synthesized composite particles consisted of silica and ceria phase. But the silica peak intensity of as-synthesized composite particles is much weaker than that of the pure silica even though the ceria peaks keep almost the same intensity. This is because the SiO2 particles may be well coated with CeO2 particles and the outer CeO2 particles prevent the diffraction of inner SiO2 particles to some degree. Similar result was also observed in the literature [21]. In addition, it is clearly seen that the reflection peaks of ceria phase are broad, indicating that the ceria crystal size is small and should be in the range of nano-scale. 3.1.2. SEM and EDS analyses To directly investigate the structures of the as-prepared composite particles, the SEM images of silica, ceria-coated silica
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Fig. 1. XRD patterns of silica particles, ceria particles and ceria-coated particles prepared with PVP.
particles prepared with and without PVP are shown in Fig. 2. It was found that the silica particles presented a spherical shape with smooth surface and narrow size distribution. After coating, the silica particles remained spherical particles but their surfaces became very rough. EDS was used to confirm the presence of elements on composite particles surface, which suggests that as-prepared composite material is composed of Si, Ce, O and N elements (Fig. 2 (c)). EDS analysis corroborated the XRD results proved that nanoceria particles coated on silica surface account for the rough surface of composite particles. In image of ceria-coated silica particles prepared without PVP, it was observed that ceria coating is not complete and many ceria particles of homogenous nucleation produced in the coating process did not coat on the silica surface. However, the ceria coating is relatively complete and uniform and almost no homogenous nucleation particles were observed in the image of ceria-coated silica particles prepared with PVP. 3.1.3. TEM analysis To further confirm the structures of ceria-coated silica particles, the TEM was conducted as shown in Fig. 3. Alien form the SEM pictures, many homogenous nucleations of ceria particles were also observed in the TEM image of ceria-coated silica particles prepared with PVP (Fig. 3(b)). On the basis of these observations, one can be concluded that heterogeneous nucleation on the silica surface and homogenous nucleation of ceria particles take place simultaneously in the reaction process with and without PVP, but the relatively complete, uniform, distinctive and crystalline ceria shell was obtained with PVP (as shown in Fig. 3(c)), which suggests that PVP play an important role in controlling nucleation of ceria particles on the silica surface. This may be explained that in the synthesis process with PVP, PVP which is an amphiphilic, nonionic polymer [22–24] was adsorbed onto colloidal silica particles. After this functionalization heterogeneous nucleation and growth of ceria took place on the silica surface by adding the ammonium cerium nitrate and urea mixed solution to the colloidal silica sol since the affinity between ceria and silica was enhanced.
Fig. 2. SEM images of silica particles (a), ceria-coated particles prepared without PVP (b), ceria-coated particles prepared with PVP (c) and their EDS spectra.
3.1.4. DLS measurement Fig. 4 and Table 2 show the Gaussian Intensity/Volume/Number weighted distributions and mean size as well as polydispersion index (PDI) of pure silica particles and ceria-coated particles prepared with PVP, respectively. The former particle size distribution is very narrow and its PDI is very low, while the latter distribution and PDI increases significantly. As can be seen for SEM and TEM images, the thickness of ceria coating is about 7–10 nm, however, the mean size of silica particles increases from 102.3 nm to 179.4 nm, which
Fig. 5 shows the COF as a functional of polishing time for both pure silica particles and ceria-coated silica particles. The COF at the beginning is very high (about 0.434) and quickly decreases with the polishing time in the range of 0–200 s. However, there is no significant change in the COF when the time increases from 200 s to 1800 s. In this study, glass substrates polished were ground glass and had many rough peaks (as shown in Fig. 4(a)) which were firstly removed in the polishing process with the high MRR [2]. The rough surface and high MRR may account for the high COF in
indicates that agglomeration occurred in the ceria-coated silica particles. Furthermore, agglomeration can be also observed in SEM and TEM images. The homogenous nucleation of ceria particles and agglomeration may account for the increase of size distribution and PDI after coating of ceria. 3.2. CMP performance of ceria-coated silica particles
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Fig. 4. Size distributions of silica particles and ceria-coated silica particles.
Table 2 Mean size and PDI of pure silica particles and ceria-coated silica particles. Type of abrasive
Mean size
PDI
Pure silica Ceria-coated silica
102.3 179.4
0.017 0.134
the initial polishing process. With increasing of polishing time, the number of rough peaks decreased, which result in the decrease of RMS roughness and MRR, COF as did. When the rough peaks were completely removed, and consequently the contact area tends to remain constant, stable COF could be observed [25]. In Fig. 5, it was observed that COF of glass CMP using ceriacoated silica particles is higher than that of pure silica particles.
Fig. 3. TEM images of ceria-coated particles prepared without PVP (a), ceria-coated particles prepared with PVP (b) and their HRTEM image (c).
Fig. 5. COF as a function of polishing time for glass substrates polished by both pure silica particles and ceria-coated silica particles.
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Table 3 The MRR and RMS of glass substrate polished by pure silica particles and ceria-coated silica particles. Type of abrasive
MRR (nm/min)
RMS (nm)
Pure silica abrasive Ceria-coated silica abrasive
58.49 126.41
1.024 1.073
The COF is known to be strongly dependent on interfacial chemical interactions, interfacial electrostatic interactions, dynamic surface conditions, properties of the opposing surfaces, and abrasive particles size, which all influence the contact area between the opposing surfaces [21]. Ceria, as a widely used abrasive for glass polishing, is
believed to form the direct bonding to glass, which creates higher shear force [26,27], and as a result, the COF increases. The MRR and RMS of glass substrates polished by pure silica and ceria-coated silica particles were given in Table 3. It can be seen that the MRR of ceria-coated silica particles is two times larger than that of pure silica particles, while the RMS roughness values of the glass polished by both particles are almost identical. It can be concluded that the MRR is strongly dependant on COF in this study. In order to further investigate the difference in polishing performance between pure silica and ceria-coated silica, the topographical micrographs of polished glass substrate surfaces were analyzed by AFM, and the result is shown in Fig. 6. By comparison with pure silica particles, the as-prepared ceria-coated silica particles give lower topographical variations as well as less scratch. Since CeO2 is softer than SiO2 , it inflicts less damage such as scratches on the glass than other particles. The CeO2 particles feature softness and high polishing rate. If mechanical polishing is dominant, soft particles give slower polishing rate, but the fact is reversed. So, there should be a large contribution from chemical reactivity in the polishing mechanism. 4. Conclusions Nano-sized particles ceria were coated on the silica surface by the precipitation method using ammonium cerium nitrate and urea as precipitant with PVP as assistant. Glass substrates were polished with slurries containing either pure silica particles or as-prepared ceria-coated silica particles. The polish rate of the latter was significantly higher than that of the former and the surface quality in terms of RMS was approximately identical after polishing with both silica and ceria-coated silica. Acknowledgements The work is sponsored by National Key Technologies R & D Program of China during the 11th Five-Year Plan Period (No. 2009ZX02030), Shanghai Nano Technology Program (No. 0952nm00200), Shanghai Major State Technology Program (No. 08111100300). References
Fig. 6. AFM morphology of glass substrate (a) before polishing, (b) polished by pure silica particles and (c) polished by ceria-coated silica particles.
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