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Functional Nanostructured Materials (including low-D carbon)

Fabrication of Uniform Wrinkled Silica Nanoparticles and their Application to Abrasives in Chemical Mechanical Planarization Jaehoon Ryu, Wookhwan Kim, Juyoung Yun, Kisu Lee, Jungsup Lee, Haejun Yu, Jae Hyun Kim, Jae Jeong Kim, and Jyongsik Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15952 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Materials & Interfaces

Fabrication of Uniform Wrinkled Silica Nanoparticles and their Application to Abrasives in Chemical Mechanical Planarization Jaehoon Ryua‡, Wookhwan Kimb, c‡, Juyoung Yuna, Kisu Leea, Jungsup Lee,a Haejun Yu,a Jae Hyun Kimd, Jae Jeong Kimb,*, and Jyongsik Janga,* a

World Class University (WCU) Program of Chemical Convergence for Energy &

Environment (C2E2), School of Chemical and Biological Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea b

School of Chemical and Biological Engineering, Institute of Chemical Process, Seoul

National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea c

Foundry CMP Technology Team, Samsung Electronics, 1, Samsung-ro, Giheung-gu, Yongin-

si, Gyeonggi-do 17113 Korea d

Foundry Material Technology Group, Semiconductor Div. Samsung Electronics, 1,

Samsung-ro, Giheung-gu, Yongin-si, Gyeonggi-do 17113 Korea ‡These authors contributed equally to this work.

Corresponding Author: *E-mail: [email protected] (J. Jang), aTel: 82-2-880-8348; Fax: +82-2-888-7295 *E-mail: [email protected] (J.J. Kim)

KEYWORDS Mesoporous silica nanoparticles; Wrinkled silica nanoparticles; Cooling process; Abrasives; Chemical mechanical planarization

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ABSTRACT A simple one-pot method is reported for the fabrication of uniform wrinkled silica nanoparticles (WSNs). Rapid cooling of reactants at the appropriate moment during synthesis allowed the separation of nucleation and growth stages, resulting in uniform particles. The factors affecting particle size and inter-wrinkle distance were also investigated. WSNs with particle sizes of 65 – 400 nm, inter-wrinkle distances of 10 – 33 nm, and surface areas up to 617 m2g-1 were fabricated. Furthermore, our results demonstrate the advantages of WSNs over comparable non-porous silica nanospheres and fumed silica-based products as an abrasive material in chemical mechanical planarization (CMP) processes.


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Introduction There has been a great deal of interest in mesoporous silica nanoparticles (NPs) due to their unique characteristics such as high specific surface area, low density, and unique morphology.1-3 The properties of mesoporous silica NPs have contributed to the development of packing materials for chromatography, drug delivery systems, cosmetics, adsorption substrates, and hard templates for nanocomposites.4-9 Among the various structures of mesoporous silica NPs,10 wrinkled silica nanoparticles (WSNs) have recently grown in popularity because they provide excellent pore accessibility and smooth molecular diffusion.11 The high accessibility of silica NPs with fibrous morphologies is particularly useful for catalyst applications.12 Therefore, various methods to fabricate WSNs have been reported.12-15 Note that the size and morphology of these particles affect their physical and chemical properties. In particular, sub-100 nm particles are especially applicable to the biomedical and pharmaceutical fields.14, 16, 17 Several investigations have focused on the control of particle size and pore diameter. Mesoporous silica NPs have also been used as a slurry abrasive material in chemical mechanical planarization (CMP) processes.18 There are two types of silica abrasive in CMP: fumed silica and colloidal silica. Initially, the use of fumed silica was dominant, as it is relatively inexpensive and offered high purity and relatively simple synthesis. However, careful control of defects and planarity has become increasingly necessary as device dimensions have decreased. Therefore, colloidal silica, which boasts a specified particle size and uniform particle distribution, has been used increasingly in CMP applications that require a polished film with high surface quality. In particular, mesoporous silica with a low hardness and Young’s modulus could better meet the requirements of advanced CMP slurry abrasives for high quality wafer surfaces.19 For this reason, mesoporous silica NPs are



attracting attention as abrasives in CMP. However, most of the particles prepared to date have been large with a broad size distribution, a critical vulnerability for CMP abrasives.20 Therefore, a method of fabricating monodisperse, mesoporous silica NPs of suitable size is required for improved CMP slurries. The mechanism of NP formation is complex but can be broadly divided into nucleation and growth stages. In most cases, nucleation and growth occur simultaneously during particle formation, which results in a size distribution in the final product. To improve particle uniformity, it is important to separate the nucleation stage and the growth stage during the fabrication process.21 New particle formation during the growth stage results in further non-uniformity.22 For these reasons, a number of studies have adopted seeded growth methods and attempted to control factors that affect size distribution by controlling reaction conditions such as temperature and the type and amount of silica precursors.16, 23, 24 However, these methods are limited in their range of conditions and often require complex processes that limit their practical application. Herein, a simple, one-pot method for fabricating monodisperse WSNs is reported. This method is based on rapidly cooling the reactants and then resuming the reaction without adding any material. Rapid cooling of the reactants at the appropriate moment during synthesis allowed complete separation of the nucleation and growth stages, resulting in uniform WSNs. The factors affecting the particle size and inter-wrinkle distance of these WSNs were also investigated. Furthermore, the resulting uniform WSNs were used in a slurry abrasive in CMP. For comparison, non-porous SiO2 nanospheres of the same size, and a fumed silica-based commercial oxide slurry, were also tested as slurry materials under the same conditions. Our results demonstrate that the mesoporous structure of colloidal WSNs contributes to improved CMP performance.


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RESULTS AND DISCUSSION Fabrication of uniform WSNs with cooling process

Figure 1. TEM images of WSNs fabricated (a) with and (b) without cooling process. (c) Size distribution profiles of WSNs fabricated with and without cooling process.



WSNs were fabricated from the bicontinuous microemulsion phase of a Winsor III system, which results in a wrinkled NP structure.25 Fast mechanical stirring of the Winsor III system yielded an oil-in-water type macroemulsion. In this system, WSNs were synthesized by continuous hydrolysis and condensation of tetraethyl orthosilicate (TEOS), a silica precursor.15 At the beginning of the reaction, TEOS is dissolved in the oil layer of the bicontinuous phase. Upon contact with the water layer of the bicontinuous phase, TEOS undergoes condensation and hydrolysis, resulting in a closed, spherical surfactant-silicate system.26 These surfactant-silicate systems were demulsified and gathered into repetitive mesophases, which serve as the seeds of WSNs.27 Newly formed seeds of WSN grew via the continuous hydrolysis and condensation of TEOS in the water layer. Eventually, this process yields hierarchical, wrinkled WSNs due to the mixed distribution of water and oil within the bicontinuous phase. Figures 1a and b show transmission electron microscopy (TEM) images of WSNs prepared with and without cooling during synthesis, respectively. Timing of the reaction was started when the temperature reached 70 °C. In both cases, the total reaction time at 70 °C, which affects particle size, was 7 h. In the former case, immediately after 1 h at 70 oC, the reaction was cooled to 10 °C and held for 2 h. After cooling, the reaction was brought back to 70°C and allowed to proceed for an additional 6 h. The diameters of WSNs fabricated with and without the cooling process were 90 ± 5 nm and 150 ± 25 nm, respectively (Figure 1c). Although both reactions were run for the same amount of time at 70 oC, the addition of the cooling process yielded smaller and more uniform WSNs. The production of NPs with a wide size distribution is mostly the result of uncontrolled primary particle (or seed) aggregation and simultaneous nucleation and growth.11, 21, 28 Secondary particle formation during seed particle growth can also result in a broad size distribution.22, 29 Our results suggest that the


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cooling process prevented one or both of these mechanisms. The effect of reaction time before cooling (hereafter referred to as the ‘pre-reaction’ time) was also investigated (Figure S1). All of the samples were allowed 6 h at 70°C after cooling, since this phase of the reaction is related to seed growth. The size and size distributions of WSNs fabricated with 0 h and 0.5 h of pre-reaction time were similar to those of the 7-h reaction without cooling. However, the incorporation of cooling after heating for 1 or 2 h resulted in smaller, more uniform WSNs. This suggests that 1–2 h from the start of the reaction is an important timeframe for the fabrication of uniform WSNs. As such, we consider this period to be a burst nucleation stage. Initiating the cooling process after a pre-reaction time of 1 h was deemed optimal for WSN uniformity. Nucleation time affects the particle size distribution; the longer nucleation time induces different growth times of each formed particle. Thus, shorter nucleation times contribute to narrower particle-size distributions.16, 30 Primary particles (or seeds), consisting of uniform NPs about 20 nm in diameter, were observed in the middle layer of the reaction system, which formed a bicontinuous emulsion structure, after a pre-reaction time of 1 h (Figure S2).15 Rapid cooling of the reaction system during the nucleation stage stopped the formation and growth of primary particles. In addition, because stabilization of the particles reduces aggregation in the subsequent reaction, our results suggest that rapid cooling of the reactants stabilized the unstable primary particles.31, 32

When the reaction was resumed, the particles stabilized in the cooling process proceeded to

grow rather than form new particles, because this is more thermodynamically favorable.33 Thus, rapid cooling initiated a mechanism similar to that of ‘seeded growth’.34-36 Figure S3 shows TEM micrographs and size distributions of WSNs that underwent a slow cooling process at 25°C after a 1-h pre-reaction. The reaction was resumed at 70°C for 6 h after cooling. WSNs fabricated with a cooling step to room temperature for 2 h and 24 h were



140 ± 36 nm and 95 ± 11 nm in diameter, respectively. Compared to WSNs prepared without a cooling step, those fabricated with a room-temperature cooling step for 2 h were slightly smaller, with a larger size distribution. This indicates that the room temperature processing did not completely stabilize the primary particles. In addition, the heat retained by the slow cooling of reactants caused some irregular aggregation of the primary particles, which may explain the observed reduction in average diameter and the increase in size distribution. However, prolonged reaction at 25°C resulted in somewhat more uniform WSNs. Thus, primary particles may be stabilized at higher temperatures with sufficient cooling time, which allows complete separation of the nucleation and growth stages. In contrast, rapid cooling of the reactants may prevent unwanted heat-induced reactions enabling the formation of more uniform particles. Rapid cooling has the additional advantage of shortening the time required for the stabilization of primary particles, thereby reducing the total fabrication time. A 2-h pre-reaction at 70 °C resulted in WSNs that were smaller (70 nm) than those produced with a 1h pre-reaction stage (90 nm) because more nuclei were formed during nucleation.37 This indicates that growth proceeded from a uniform dispersion of TEOS over all nuclei. In addition, WSNs fabricated with a 1h pre-reaction at 80°C had an average diameter of 70 nm, smaller than those produced with the same pre-reaction time at 70°C (Figure S4). It can therefore be inferred that increased reaction temperatures increase the hydrolysis rate of TEOS, leading to the formation of more seeds.38 Furthermore, our results show that particle formation dominates during the nucleation stage. Figure S5 shows the particle distribution of WSNs fabricated with a cooling process applied after the first 3 h of heating at 70°C. These data show that disordered aggregation and rapid growth of the newly formed nuclei proceeded after around 3 h of reaction, and that the cooling process had no effect at this stage. Based on these results, a formation mechanism for uniform WSNs via the


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added cooling process is proposed as shown in Figure S6. Nucleation proceeded from about 1 h of reaction and unstable seeds were aggregated irregularly from about 3 h. As a result, the seeds present in the reaction mixture after 3 h were fewer and more uneven than those present after 1 h. This leads to the formation of larger and uneven WSNs than when the cooling process is added at the appropriate time.



Characterization of size control variables of WSNs

Figure 2. Effect of reaction time after cooling process; TEM images of the WSNs reacted at 70 °C for (a) 2 h, (b) 3 h, (c) 6 h, and (d) 12 h after 1 h pre-reaction and cooling process. All samples reacted before cooling process for 1 h at 70 °C. (e) Variation of WSN size and WSN yield relative to addition amount of TEOS as function of the reaction time.


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Effect of growth time. The effect of reaction duration after cooling (hereafter referred to as the ‘post-reaction’) was also investigated. Figure 2 shows TEM micrographs of WSNs made with various post-reaction times after a 1-h pre-reaction and cooling process. All reactions proceeded at 70 °C. WSNs of various diameter, including 50 (± 4), 65 (± 4), 90 (± 5), and 100 (± 5) nm, were fabricated with post-reaction times of 2, 3, 6, and 12 h, respectively. Pore size remained unchanged.25 The average diameter of the WSNs increased with increasing post-reaction time, but with decreasing growth rates. No growth was observed after 12 h of post-reaction time (Figure S7). This is consistent with our previous results.25 The decrease in growth rate with prolonged reaction time is due to a pH change in the reaction mixture and the presence of urea, which was used as a basic catalyst for TEOS hydrolysis. The pH of the mixture changes from 7 at the beginning of the reaction to 10 after a total of 13 h of heating due to the thermal decomposition of urea. The yield of fabricated WSNs was also investigated for a range of post-reaction times. The variation in WSN yield revealed a similar tendency to that of growth rate. After 12 h of post-reaction, the relative mass ratio of WSN yield to TEOS addition was about 28.4%, comparable to the yield obtained using the Stöber method.39 Because a post-reaction time of more than 12 h provided a high WSN yield, a 12-h post-reaction time was applied as the basic experimental condition for further investigations of WSN diameter and pore size. Scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analyses of WSNs confirmed the structure and composition of the particles. Dot maps of silicon and oxygen are presented in Figure S8. The dots of the two elements (Si and O) represent the entire distribution of the particles.



Figure 3. Effect of reaction temperature after cooling process; TEM images, SEM images and the corresponding BJH plots of the WSNs reacted at (a, b, c) 70, (d, e, f) 75 °C, and (g, h, i) 80 °C for 12 h after cooling process. All samples reacted before cooling process for 1 h at 70 °C.

Effect of growth temperature. WSN diameter and their inter-wrinkle distance were investigated for a range of post-reaction temperatures. All experiments were carried out with a 1-h pre-reaction at 70 °C and a cooling step before post-reaction. Figure 3 shows TEM and scanning electron micrographs (SEM) and Barrett–Joyner–Halenda (BJH) pore distribution plots of WSNs prepared with 70, 75, and 80 °C post-reactions, respectively. WSNs could not be obtained with a post-reaction at 60 °C. WSN diameter increased with increasing post-


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reaction temperatures.33, 40, 41 This was attributed to increasing condensation rates of silicic acid at higher temperatures.42 Both samples, however, exhibited the same peaks in their BJH plots, at about 3 nm (corresponding to mesopores) and 15 nm (corresponding to the interwrinkle distance). High magnification SEM micrograph is presented in Figure S9. This indicates that temperature affected the size of WSNs but did not affect their morphology. WSNs could be grown using a bicontinuous template under typical reaction conditions with a fixed average distance between layers.15 Nitrogen adsorption–desorption isotherms were used to determine Brunauer–Emmett–Teller (BET) surface areas, as shown in Figure S10. The isotherms displayed IV- and H3-type hysteresis loops, indicating that the WSNs had mesoporous features with slit-like shapes.43-45 The BET surface area of WSNs prepared using a post-reaction at 80 °C (= 617 m2g−1) was the highest, followed by those at 75 °C (583 m2g−1) and 70 °C (533 m2g−1). The surface area of WSNs of the same morphology tended to increase with increasing size. This was attributed to an increase in the overall volume of 3-nm mesopores as the size of the WSNs increased, as shown in the BJH plots. Mesopores 3 nm in diameter formed in the WSN cores. The increase in WSN cores with increasing WSN size was confirmed in TEM micrographs. In addition, as the WSNs became larger, especially above 200 nm in diameter, the shape of the N2 isotherm changed gradually from a H3 hysteresis loop to a H1 hysteresis loop. These results were attributed to an increase in the core portion of the WSNs, with a core structure based on ordered three-dimensional pore networks.43



Figure 4. Effect of oil-to-water ratio; TEM images and the corresponding pore volume distribution plots of the WSNs fabricated with different oil-to-water ratio, (a, b) = 0.2, (c, d) = 5, and (e, f) = 1.

Effect of oil-to-water ratio (o/w). The size and morphology of WSNs were investigated as a function of the oil-to-water ratio of the reaction mixture. Figure 4 presents TEM micrographs and BJH plots of WSNs fabricated in reaction mixtures with oil-to-water ratios of 0.2, 0.5, and 1, respectively, under the same fabrication conditions. The average diameters of WSNs


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prepared in reaction mixtures with oil-to-water ratios of 0.2, 0.5, and 1 were 65 ± 5, 100 ± 5 nm, and 250 ± 6 nm, respectively, and their inter-wrinkle distances were about 10, 15, and 21 nm. Both the particle diameter and the wrinkle distance increased as the oil-to-water ratio increased. As the amount of oil in the emulsion system increased, a swollen micelle formed, increasing the number of surfactant molecules required per micelle.46 This reduced the number of seeds formed during the nucleation stage and increased both the size and the pore size of the final fabricated WSNs. The reduced number of seeds is also related to a decrease in the hydrolysis rate of TEOS.16, 47 The hydrolysis rate of silanes decreases with increasing proportions of oil phase or hydrophobicity of the system.48 At slow hydrolysis rates, particle growth dominates nucleation. This indicates that silica species tend to react with the preexisting nuclei or surfactant species, rather than form new nuclei.17 Therefore, as the amount of oil in the system increases, smaller numbers of larger primary particles are formed.49 This contributed to the observed improvements in the particle size of the final fabricated WSNs. The surface areas of WSNs prepared in reaction mixtures containing oil-towater ratios of 0.2, 0.5, and 1 were 514, 533, and 584 m2g−1, respectively (Figure S11).



Figure 5. Effect of amount of added iso-propanol (iPA); TEM images and the corresponding pore volume distribution plots of the WSNs fabricated with different amounts of iPA, (a, b) 0.92 mL, (c, d) 1.84 mL, and (e, f) 2.76 mL iPA

Effect of amount of added co-solvent. Only the diameter of WSNs was affected by the amount of 2-propanol (IPA) co-solvent employed in the reaction mixture. Figure 5 shows TEM micrographs and BJH plots of WSNs fabricated with different amounts of IPA. ‘*1 iPA’ denotes the addition of 1.84 mL IPA as the basic condition. ‘*0.5 IPA’ and ‘*1.5 IPA’ indicate the addition of 0.5 times and 1.5 times the basic amount of IPA, respectively, corresponding to 0.92 mL and 2.76 mL. The diameters of WSNs made in the presence of *0.5, *1, and *1.5


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IPA were about 70 ± 5, 100 ± 5, and 200 ± 6 nm, respectively, with inter-wrinkle distances of

ca. 15 nm in all cases. While the average diameter of WSNs increased with the amount of IPA, the morphology of the WSNs was unchanged. Alkyl alcohols exhibit amphiphilic characteristics as co-solvents. Thus, the oil-to-water ratio in the emulsion phase remains constant regardless of the amount of co-solvent used. As a result, the inter-wrinkle distance of the WSN remained unchanged. However, the hydrophobicity of the reactant increases with the amount of alcohol. This decreases the hydrolysis rate of the silane species, leading to the formation of a small number of large primary particles. As a result, the diameter of the fabricated WSNs increased, as did the total volume of 3-nm mesopores.17, 48 The BET surface areas of WSNs with the *0.5, *1, and *1.5 IPA in the reaction mixture were 509, 533, and 575 m2g−1, respectively (Figure S12). WSNs with the same morphology tended to exhibit increasing surface areas with increasing size, consistent with previous results.



Figure 6. Effect of cosolvent; TEM images, SEM images, and the corresponding pore volume distribution plots of the WSNs fabricated with different cosolvent, (a, b, c) 1.84 ml iPA, (d, e, f) 2.2 ml butanol, and (g, h, i) 1.1 ml butanol.


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Table 1. Summary of the characteristics of the WSNs according to all experimental conditions in the manuscript. Experimental conditiona Characteristics Co-solvent type, Post-reaction Inter-wrinkle Diameter BET area O/W molar addition temperature distance [nm] [m2 g-1] amount [mol] [°C] [nm] 0.5 0.024 mol iPA 100±5 15 533 70 0.5 0.024 mol iPA 210±5 15 583 75 0.5 0.024 mol iPA 330±7 15 617 80 0.024 mol iPA 70 65±4 10 514 0.2 0.024 mol iPA 70 100±5 15 533 0.5 0.024 mol iPA 70 250±6 21 584 1 0.5 70 70±5 15 509 0.012 mol iPA 0.5 70 100±5 15 533 0.024 mol iPA 0.5 70 200±6 15 575 0.036 mol iPA 0.5 70 100±6 33 514 0.012 mol Butanol 0.5 70 400±7 33 572 0.024 mol Butanol a All samples underwent pre-reaction for 1h at 70 °C and cooling process for 2 h at 10 °C Effect of type of co-solvent. To further assess changes in WSN characteristics as a function of co-solvent, experiments were repeated using 1-butanol as the co-solvent. TEM and SEM micrographs and BJH plots of WSNs fabricated using iPA or 1-butanol as the co-solvent are displayed in Figure 6. Compared with the use of iPA as a co-solvent, WSNs prepared using the same molar amount of 1-butanol showed an increased diameter (400 ± 7 nm) and interwrinkle distance (33 nm).24 1-Butanol has a longer alkyl chain, and therefore greater hydrophobicity, than iPA. This increases the amount of oil available to interact with the particles and also the oil-to-water ratio. In addition, 100-nm WSNs with a 33-nm interwrinkle distance were fabricated by reducing the amount of butanol in the reaction mixture. This is consistent with the discussion above. The BET surface areas of WSNs made with *1 butanol, and *0.5 butanol were 572 and 514 m2g−1, respectively (Figure S13). The characteristics of the WSNs prepared using the experimental conditions described above are summarized in Table 1. The degree of control of particle size and pore size in our WSNs was compared with that reported previously, as summarized in Table S1.



Figure 7. Cross-sectional SEM images and 3D AFM images of the substrates after CMP with corresponding TEM images of (a) monodisperse-SiO2 nanospheres (100 nm), (b) monodisperse-WSNs (100 nm), and (c) STAR4000 as slurry abrasive materials.

Table 2. Summary of MRR and surface roughness of the substrates before and after CMP. Abrasive materials

MRR [nm · sec-1]

Surface roughness [nm]

-

0.226

Non-porous SiO2 nanospheres

1.68

0.195

WSNs

3.18

0.164

STAR4000a

2.37

0.208

Before CMP

a

1 wt%, pH 11 fumed silica-based commercial oxide slurry(Cheil Industry Co.)


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The chemical mechanical polishing performance evaluation of the WSNs as slurry abrasives. The uniformity of CMP slurry abrasives is important for achieving high CMP performance.20 Our method of WSN fabrication provides high uniformity and high yields, both of which are desirable for a slurry abrasives in CMP. Therefore, the fabricated WSNs were evaluated as slurry abrasives. The average diameter of the prepared WSNs was 100 nm, which is suitable for slurry materials.50, 51 For comparison, non-porous SiO2 nanospheres of the same size and a fumed silica-based commercial oxide slurry (STAR4000; Cheil Industry Co.) were also evaluated as slurry materials under the same conditions. The flow rate was 100 mL min−1 and a 1.05-µm-thick SiO2 layer was polished for 1 min (Figure S14). Figure 7 shows crosssectional SEM and atomic force micrographs (AFM) of the substrates after CMP with corresponding TEM micrographs of the abrasive material. The CMP performance is summarized in Table 2. The WSNs exhibited the fastest material removal rate (MRR) among the three materials. Compared with silica nanospheres (42 m2g−1, Figure S15), the significantly higher surface area of the WSNs (533 m2 g−1) allowed for a greater contact area and greater chemical reactivity with the substrate.18 Compared to colloidal silica, fumed silica generally yields relatively high substrate surface roughness due to the presence of large, irregular agglomerates, but shows a faster removal rate.52-54 However, WSNs of colloidal silica provided a 34.2% enhanced removal rate compared with the commercial fumed silica product under the same conditions. The surface of the substrate after CMP processing was roughened in the order WSNs, spheres, and STAR4000. Compared to the fumed silica-based commercial product, the WSNs and spherical colloidal silica provided smoother substrate surfaces after CMP due to their uniformity, softness, and spherical structure.50 The surface roughness of a substrate after CMP with the WSNs was lower than that obtained with SiO2



nanospheres. Surface roughness after CMP is affected by the indentation depth of the abrasive particles.55 Indentation depth depends, in turn, on the mechanical properties of the particles, such as the hardness (H) and Young's modulus (E).56 Porous particles have smaller H and E than nonporous solid particles.57 Compared with non-porous SiO2 nanospheres, porous WSNs were more elastically deformed during CMP, resulting in enhanced contact area with the substrate and a reduced indentation depth. Therefore, the WSNs produced a higher quality substrate surface after CMP than the SiO2 nanospheres. Furthermore, the WSNs did not structurally collapse the CMP process (Figure S16), indicating stable mechanical properties under the pressures applied during CMP.


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CONCLUSIONS Uniform WSNs were fabricated by applying a cooling stage to the reaction mixture. Cooling at an appropriate point in the reaction allowed complete separation of the nucleation and growth stages. Rapid cooling stabilized seeds formed during nucleation and allowed those seeds to grow rather than forming new particles when the reaction was resumed. This enabled the fabrication of uniform NPs. This uniformity was maintained even when the experimental conditions were changed, enabling accurate analyses of the factors affecting the diameter and inter-wrinkle distance of the WSNs. Our method allows the fabrication of uniform WSNs with controlled particle diameter and pore size. This novel method, incorporating a rapid cooling step during the reaction, provides a basis for the development of monodisperse NP fabrication methods. In addition, the CMP performance of our WSNs, compared to that of non-porous silica nanospheres or a fumed silica-based commercial product under the same conditions, demonstrated the superiority of a mesoporous structure in this application. Our results suggest that WSNs may afford performance enhancement in applications where non-porous silica nanospheres are currently used.



MATERIALS AND METHODS Materials: Cetylpyridinium bromide (CPB) (98 %), tetraethyl orthosilicate (TEOS, 98%), cyclohexane, iso-propanol, and 1-butanol were purchased from Aldrich Chemical Co. and used without further purification. Urea was purchased from Samchun Chemical. Fabrication of wrinkled silica nanoparticles: 2 g (5.2 mmol) of cetylpyridinium bromide and 1.2 g (20.0 mmol) of urea were dissolved in 60 mL of water. And then, 30 mL of cyclohexane and 1.84 mL (24 mmol) of iso-propanol were added to the solution. With vigorous stirring, 5 g (24 mmol) of TEOS was dropwised to the mixed solution. After vigorous stirring for 30 min at room temperature, the reaction was proceeded at 70 °C for 1h. After the reaction, the reator was immediately transfer to the water bath for the cooling precess at 10 °C for 2 h. Then, the reaction at 70 °C was resumed, and this state was maintained for 12 h. The reaction mixture washed with ethanol 3 times through centrifugation. The isolated WSNs by centrifugation were dried in the 70 °C. Finally, the dried WSNs were calcined at 550 °C for six hours in air. CMP condition: Si coupon wafers (2.0 cm × 2.0 cm) were used for the estimation of the effectiveness of each kind of abrasive. The Si coupon wafers prepared by thermal Si oxide (1050 nm) and Si substrate. The Oxide CMP slurry included 1wt% colloidal silica (100 nm). The pH of solution was fixed at 10 by adding potassium hydroxide (KOH). To meet the target layer, pH 10 was chosen. Si coupon wafers were polished using a CMP planarizer (POLI-400, G&P Tech. inc., Korea) with an industrial standard CMP pad (IC 1000/Suba IV, Rohm and Haas Electronic Materials, U.S.A). After CMP, the wafers were cleaned using an ammonia peroxide mixture solution (NH4OH:H2O2:H2O = 1:1:10) at 80°C to eliminate residual particles. The polishing test conditions are summarized in Table 3. below.


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Table 3. Experimental conditions Slurry/ Concentration

silica slurry/ 1wt%

Pad

IC1000/SubaIV

Spindle/Table/Conditioner speed

75/80/60 rpm

Pressure

400 g cm-2 (5.7psi)

Flow rate

100 ml min-1

Polishing time

60 sec

Characterization: Images of transmission electron microscopy (TEM) were obtained from LIBRA 120 (Carl Zeiss, Germany) operating at 120 kV. The mean and standard deviation of particle size were determined by measuring at least 50 particles in TEM images. FE-SEM images were obtain from JSM-7800F Prime (JEOL Ltd, Japan) operated at an acceleration voltage of 3−10 kV, which are installed at the National Center for Inter-university Research Facilities (NCIRF) at Seoul National University. Brunauer−Emmett−Teller (BET) surface areas were measured using a Micromeritics analyzer (ASAP 2000; Micromeritics Co., Norcross, GA) at -196 °C. Sample pretreatment for BET measurement was carried out at 110 °C for 24 h. Atomic force microscope (AFM) images with 0.5 µm × 0.5 µm were acquired using an Innova SPM (Vecco, USA) and analyzed using SPMLabAnalysis software.

Supporting Information Available: The contents of the Supporting Information include the following: (1) TEM images of the WSNs; (2) Proposed the WSNs forming mechanism; (3) N2 adsorption–desorption isotherms of the WSNs; (4) Cross-sectional SEM images and 3D AFM images of the substrates before CMP; (5) TEM image of WSNs after CMP process. This material is available free of charge via the Internet at http://pubs.acs.org.



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SYNOPSIS TOC


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