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Applied Surface Science 257 (2011) 8679– 8685

Contents lists available at ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

olishing behavior of PS/CeO2 hybrid microspheres with controlled shellhickness on silicon dioxide CMP

ang Chena,b,∗, Renwei Longa

School of Materials Science and Engineering, Changzhou university, Changzhou, Jiangsu 213164, ChinaKey Laboratory of Advanced Metallic Materials of Changzhou City, Changzhou university, Changzhou, Jiangsu 213164, China

r t i c l e i n f o

rticle history:eceived 11 February 2011eceived in revised form 11 May 2011ccepted 11 May 2011vailable online 17 May 2011

eywords:S/CeO2 composite abrasiveore–shell structurehell thickness

a b s t r a c t

Organic–inorganic composite microspheres with PS as a core and CeO2 nanoparticles as a shell weresynthesized by in situ decomposition reaction of Ce(NO3)3 on the surfaces of PS microspheres preparedthrough soap-free emulsion polymerization. The shell thickness of the composite microspheres could beturned by varying the concentration of Ce(NO3)3 in the reaction solution. The whole process requiredneither surface treatment for PS microspheres nor additional surfactant or stabilizer. The as-synthesizedPS/CeO2 composite microsphere samples were characterized by X-ray diffraction (XRD), transmissionelectron microscopy (TEM) and thermogravimetric analysis (TGA). Oxide chemical mechanical polishing(CMP) performance of the PS/CeO2 composite abrasives with different shell thickness was character-ized by atomic force microscopy (AFM). The results indicated that the as-prepared core–shell structured

hemical mechanical polishing (CMP) composite microspheres (220–260 nm in diameter) possessed thin shell (10–30 nm) composed of CeO2

nanoparticles (particle diameter of 5–10 nm), and the final CeO2 contents of the composite microspheresranged from 10 to 50 wt%. A possible mechanism for the formation of PS/CeO2 composite microsphereswas discussed also. The CMP test results confirmed that the novel core–shell structured composite abra-sives are useful to improve oxide CMP performance. In addition, there is an obvious effect of shell thicknessof the composite abrasives on oxide CMP performance.

Although CeO2 is a relatively soft material (Mohs hardness isbout 6–7), it has been widely used for polishing harder glass sub-trates and chemical mechanical polishing (CMP) of oxide filmsffectively as one of the key abrasive materials [1]. Compared toonventional SiO2 abrasives, CeO2 abrasives synthesized by exist-ng techniques are irregularly faceted, and they scratch wafersnd increase defect concentrations to generate defectivity. Thiss mainly due to difficult control of their stability, morphology,gglomerate size and size distribution [1]. Furthermore, the sizef CeO2 abrasives is limited to less than 50 nm, leading to a lowaterial removal rate (MRR) during oxide CMP process [2].With an increasing demand of improving the CMP performances

ncluding minimizing roughness and defects, it is found that justne kind of abrasive can hardly exhibit desired CMP performance.n recent years, the strategy to fabricate nanocoating or shell on col-oidal particles is of burgeoning interest, principally because such

omposite particles can display novel and enhanced propertiese.g., mechanical, chemical, electrical, and magnetic) by indepen-

∗ Corresponding author at: School of Materials Science and Engineering,hangzhou university, Changzhou, Jiangsu 213164, China.

E-mail address: cy.jpu@126.com (Y. Chen).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.05.047

© 2011 Elsevier B.V. All rights reserved.

dently altering the composition, dimension, and structure of coresand shells [3].

CeO2-based composite abrasives with core–shell structure usedin CMP have attracted more attentions. To combine the advantagesof CeO2 shell and SiO2 and/or polymer core, the CeO2-based com-posite abrasives have been reported in several works [4–10]. Leeand Zhao et al. [4,5] prepared spherical CeO2-coated SiO2 com-posite abrasives and studied their oxide CMP performance. CMPresults showed that this kind of composite abrasives possess ahigher material removal rate (MRR) than that of pure CeO2 andSiO2 abrasives. Zhang et al. [6] synthesized ceria-coated silica par-ticles, and investigated their CMP behavior on glass substrates. Theresults suggested that the as-prepared ceria-coated silica particlesexhibited a higher MRR than pure silica particles without deterio-rating the surface quality. In addition, Zhang et al. [7] modified ceriananoparticles with �-aminopropyltriethoxysilane (APS) throughsilanization reaction with their surface hydroxyl groups, and inves-tigated their CMP performance on glass substrates. Their CMPresults showed that the modified ceria particles exhibited gooddispensability, much better surface quality but lower MRR than

unmodified ceria particles. Coutinho et al. [8] prepared compositeparticles containing ceria nanoparticles dispersed within cross-linked, polymeric microspheres prepared by copolymerizationof N-isopropylacrylamide (NIPAM) with 3-(trimethoxysilyl)propyl

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on the surfaces of the amorphous PS cores.TEM proves to be a very effective tool for the observation of

composite microspheres with raspberry-like core–shell morpholo-gies [13]. Fig. 2a shows the typical TEM image of PS microspheres

10 20 30 40 50 60 70 80

(e)(d)(c)(b)(a)

(331)(400)(222)

(311)(220)

(200)

(111)

Intensity(cps)

680 Y. Chen, R. Long / Applied Sur

ethacrylate (MPS) and investigated their oxide CMP perfor-ance. The core–shell structure was not clear and the surface after

MP retained some scratches. Armini et al. [9] prepared PMMAcore)–ceria (shell) composites comprising a 300 nm polymer coreoated by 14 nm ceria particles by either using silane-couplinggents or tuning pH in order to form electrostatic attractive inter-ctions between the core and the shell, and investigated their CMPehavior for SiO2 dielectric layers. These experimental results indi-ated that the ceria composites resulted in reduced defectivity afterMP due to the spring like effect coming from the elastic com-onent of the core, compared with conventional slurry material.owever, they could not obtain complete core–shell structuredMMA (core)–ceria (shell) composites using the above-mentionedethods, and the ceria particles were coated non-uniformly on the

urfaces of PMMA cores. Chen et al. [10] reported a simple andfficient route to fabricate well-defined PS/CeO2 core–shell micro-pheres.

For a given polymer core, the organic–inorganic compositeicrospheres show mechanical properties that are highly effected

y the thickness of the inorganic shell, which enhance chemicalction and surface hardness of the composite microspheres. Herein,n the basis of our previous work, we would like to present a sim-le process to control the shell thickness of PS/CeO2 core–shellomposite microspheres. In particular, our study is aimed at inves-igating the effects of CeO2 shell thickness of the PS/CeO2 compositebrasives on oxide CMP performance.

. Experimental

.1. Synthesis of PS/CeO2 composite microspheres

The negatively charged PS microspheres were prepared by soap-ree emulsion polymerization using KPS as initiator, according tour previous work [11]. All other chemical reagents were of analyticrade and used without further purification.

The core–shell structured PS/CeO2 composite microspheresere synthesized by in situ chemical precipitation method asescribed elsewhere [10]. The typical synthesis was introduceds follows. 4 mL PS colloids were dispersed into 200 mL deion-zed water with ultrasonic vibration, then different concentrationsf Ce(NO3)3·6H2O and excessive amounts of hexamethylenete-ramine (HMT) as precipitant were added. The resulting mixtureas treated by ultrasound for 15 min to form homogeneous suspen-

ion. The obtained homogeneous suspension was reacted at 75 ◦Cor 2 h under magnetic stirring. The resulting precipitates were cen-rifuged and washed with deionized water and ethanol, then driedt 80 ◦C in air for 2 h. The samples obtained with Ce(NO3)3·6H2Ooncentrations of 0.0023, 0.0069, 0.0115, 0.0184 and 0.0230 M,ere denoted as S1–S5 respectively.

The crystalline phases of the products were analyzed using Rigaku D/max 2500 X-ray diffractometer (Cu K� radiation

= 0.154056 nm). The TEM images of the resulting samples werebtained via a Philips Tecnai-12 transmission electron microscopesing an accelerating voltage of 120 kV, and the shell thickness waslso estimated from TEM observation. Thermogravimetric analy-is (TGA) was performed in air from room temperature to 873 K at

heating rate of 10 K/min using a SDT Q600TA thermal analyzer.ize distribution curve of the sample was measured by dynamicight scattering (DLS) using a Malvern Nano ZS analyzer.

.2. CMP tests

The SiO2 film layer for CMP testing was a 1200 nm thermal oxidelm on a silicon wafer. Polishing experiments were conductedsing a Denmark Struers TegraForce-1/TrgraPol-15 polisher with

ience 257 (2011) 8679– 8685

a MD-Chem (Struers Company) pad. Before each CMP experiment,the pad was conditioned for 2 min with a diamond pad conditionerusing deionized water. The rotation speeds of the head and platenwere set at 120 and 90 rpm, respectively. The pressure of the carrierwas 4 psi and the polishing time was set at 1 min. The slurry flowrate was 150 mL/min. For the preparation of the slurry, a shakerwas applied for 20 min and the solid content was 1 wt%. The pHvalue of the slurry was adjusted to 10 by 0.1 M NaOH solution. Afterpolished, the wafers were rinsed by deionized water and surfactantsolution. After ultrasonic cleaned, the wafers were placed in ethanoland dried naturally in a super clean room.

The weight of the wafer before and after polishing was measuredby a Mettler Toledo XS105 electron balance (exact to 0.01 mg) tocalculate the MRR (nm/min) as follow: MRR = (m0 − m)/�ts. Here,� is the density of a thermally grown silicon oxide layer, s is thearea of the wafer, t (min) is the polishing time, m0 is the originalmass of the wafer, m is the mass of wafer after being polished.The surface topography, profilogram and root mean square (RMS)roughness after CMP were measured by a Nanoscope IIIa Atomicforce microscopy (AFM) with a silicon nitride tip in tapping mode,and scan area was 5 �m × 5 �m. The MRR and RMS roughness arethe average of 3 individual polishing tests.

For comparison, the control slurry containing commercial CeO2abrasives (mean particle size of 30–50 nm, and obtained fromChangzhou Geoquin Nano New Materials Co., Ltd.) was preparedwith the same procedures, and the polishing tests were conductedunder the same conditions.

2. Results and discussion

2.1. PS/CeO2 composite abrasives

XRD patterns of the PS/CeO2 composite microspheres synthe-sized at different concentrations of Ce(NO3)3 are shown in Fig. 1.The spectra of synthesized composites revealed well-developedreflections of cerium oxide (JCPDS 34-0394). Furthermore, the spec-trum of the composite microspheres presented a little amorphouspeak at around 2� = 20◦, attributed to the introduction of organicPS core [12]. XRD results of the composite microspheres imply thatthe cubic fluorite CeO2 shell with good crystal structure is formed

Two-Theta(de g)

Fig. 1. XRD patterns of PS/CeO2 composite microspheres (a) S1, (b) S2, (c) S3, (d) S4,(e) S5.

Y. Chen, R. Long / Applied Surface Science 257 (2011) 8679– 8685 8681

micro

pcwtttsstto

Fig. 2. TEM images of PS and PS/CeO2 composite

repared by soap-free emulsion polymerization that we used asores. It was observed that the PS microspheres were monodisperseith an average diameter of around 200 nm. Fig. 2(b–f) show the

ypical TEM images of the PS/CeO2 composite microspheres syn-hesized with different concentrations of Ce(NO3)3. Compared withhe smooth surfaces of the PS microspheres, the loaded PS micro-pheres exhibited very rough appearance, indicating that the CeO2

hells are composed of a large number of loosely packed nanopar-icles. It was clearly observed that the strong contrast betweenhe core and the shell. The grey part in the center represents therganic PS core, while the outside black part is considered to be

spheres (a) PS, (b) S1, (c) S2, (d) S3, (e) S4, (f) S5.

aggregation of inorganic CeO2 nanoparticles. Besides, we can con-clude that the coating layer composed of CeO2 nanoparticles onthe surface of PS spheres is uniform. The CeO2 shell thickness ofthe composite microspheres can be controlled by adjusting theamounts of Ce(NO3)3 in the reaction solution. As the concentra-tions of Ce(NO3)3·6H2O were 0.0023, 0.0069, 0.0115, 0.0184 and0.0230 M, the shell thickness were 10, 15, 20, 25 and 30 nm, and

the diameters of PS/CeO2 composite microspheres were about 220,230, 240, 250 and 260 nm, respectively.

Fig. 3 shows the particle size distribution curve obtained byDLS analysis of the PS/CeO2 composite microspheres S1. As shown,

8682 Y. Chen, R. Long / Applied Surface Science 257 (2011) 8679– 8685

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Table 1The RMS and MRR of oxide film before and after polished with pure CeO2 andPS/CeO2 composite abrasives.

Samples RMS (nm) MRR (mn/mim)Before polishing 0.847 –Polished with the pure CeO2 abrasives 0.606 96.8Polished with the composite abrasives S1 0.462 132.7

Fig. 3. DLS size distribution curve(c) of PS/CeO2 composite microspheres S1.

nly one narrow peak can be observed in particle size distributionurve, and the average size is about 261.8 nm. According to TEMnd DLS results, unreacted excess CeO2 nanoparticles were almostot observed.

Fig. 4 shows the TGA results of the as-synthesized PS/CeO2omposite microspheres under a nitrogen atmosphere. All of theamples followed similar decomposition curves. The compos-tes remarkably degraded starting at temperatures of 300 ◦C andttained final residual mass when the temperature was above50 ◦C. At temperatures above 500 ◦C, the PS core was completelyecomposed [14]. The residual weight that remained was thoughto be the weight of CeO2, and the CeO2 contents of the composite

icrospheres S1–S5 are about 13.8, 37.3, 39.2, 45.2 and 48.7 wt%espectively.

A possible mechanism of the formation of PS/CeO2 compos-te microspheres is described in Scheme 1. The first step involveshe preparation of negative-charged PS microspheres. For this rea-on, the reagent KPS was used as the initiator in order for thetyrene monomers to be polymerized [15]. Secondly, Ce3+ cationsould be absorbed onto the surfaces of the negatively chargedS microspheres. Under slow hydrolyzing of HMT, the opposite-harged OH− was released in the solution. Ce3+ combined withH− slowly hydrolyzed from HMT driven by electrostatic attrac-

ion. Then, Ce3+ hydrolyzed to form gelatinous hydrous ceriumydroxide (Ce(OH)3·xH2O) on the surfaces of the PS microspheres,

orming a core–shell structure [16]. Followed by dryness processn air, Ce3+ was oxidized to Ce4+ due to its intrinsic crystallization

endency [17], and the CeO2 shell was formed.

0 10 0 20 0 30 0 40 0 50 0 60 0

20

40

60

80

100

(c)(b)

(d)(e)

Weight(%)

Tempera ture(oC)

(a)

ig. 4. TGA curves of PS/CeO2 composite microspheres (a) S1, (b) S2, (c) S3, (d) S4,e) S5.

Polished with the composite abrasives S3 0.239 517.6Polished with the composite abrasives S5 0.264 366.3

2.2. Polishing performance

To investigate the surface features of silicon oxide layers, typi-cal topographical micrographs of the wafer surface before and afterpolishing with the pure CeO2 abrasives were analyzed by AFM, andthe results are shown in Fig. 5. In addition, diagonals in 2D AFMimages were selected for profilogram measurement. As shown inFig. 5a, the silicon oxide layer before CMP was rough. The typ-ical AFM images of the wafer surface after polishing with pureCeO2 abrasive revealed obvious scratches and residual particles (asshown in Fig. 5b).

Fig. 6(a–c) present the typical 3D AFM images of the SiO2 filmsurface after polishing with the as-prepared PS/CeO2 compositeabrasives S1, S3 and S5, respectively. By comparison with pure CeO2abrasive, as shown in Fig. 4b, the as-prepared PS/CeO2 core–shellcomposite abrasive led to lower topographical variations as well asfew scratches. The composite abrasives gave rise to ultra-smoothpolishing surfaces and low topographical variations, while thescratches can hardly be observed. The AFM results confirmed thatthe novel composite abrasives are beneficial due to elimination ofscratches and topographical variations.

In order to further investigate the difference in oxide CMPperformance between pure CeO2 and composite abrasives, theMRR and RMS of wafer polished by the pure CeO2 abrasives andthe as-prepared PS/CeO2 composite abrasives with different shellthickness were given in Table 1. The MRR of the composite abra-sives can reach 132.7, 517.6 and 366.3 nm/min, respectively. TheMRR of CeO2 abrasives was 95.8 nm/min, much lower than thatof the composite abrasives. The RMS within 5 �m × 5 �m area ofthe thermal oxide film polished by PS/CeO2 the composite abra-sives with different shell thickness (10, 20 and 30 nm) was 0.462,0.239, 0.264 nm, respectively, while pure the CeO2 abrasive givesRMS of 0.606 nm. The lower RMS value means the higher surfaceplanarization. The CMP results show that the composite abrasivesare useful to increase MRR and decrease RMS in the process of oxideCMP.

The improvement in CMP performances of the PS/CeO2 com-posite abrasives may be attributed to the core–shell structurecomprising polymer microspheres coated by CeO2 nanoparti-cles. Many advantages are expected from the elasticity of thepolymer core, such as reduced scratches, high polishing rate,and improved planarity [18]. The elastic deformation of softpolymer cores increases the contact areas between wafers andabrasives and decreases contact stress during CMP, which isin favor of reducing roughness and mechanical damage. Duringthe oxide CMP, ceria particles exhibit a chemical tooth prop-erty [19]. CeO2 shell improves the surface hardness of PS coreand possesses an enhanced chemical activity of composite abra-sives.

The oxide CMP is mainly affected by two related factors: thecontact area and the particle surface activity. According to con-tact area mechanism [20], the material removal rate increases

with increasing in contact area between the abrasives and the filmsurface. The higher MRR with respect to pure CeO2 abrasives isattributed to an increased filling factor, due to a higher numberof CeO2 nanoparticles that interact with polished surfaces [21].

Y. Chen, R. Long / Applied Surface Science 257 (2011) 8679– 8685 8683

of PS

Fsrcm(S

Scheme 1. Schematic synthesis

or a given PS core, the mechanical properties (Young’s moduli,urface hardness/activity, et al.) of composite microspheres areelated to the thickness of CeO2 shell coated on the surface of PSore. After polishing with the composite abrasives S1, CMP perfor-

ances including the topographical variations, surface roughness

RMS) and MRR was worse than those of the composite abrasives3 and S5. The reason may be attributed to the thin shell (the

Fig. 5. Typical 3D AFM images of the oxide wafer surface (a

/CeO2 composite microspheres.

thickness of CeO2 shell was about 10 nm) and low CeO2 content,which lead to lower surface hardness/activity of the compositemicrospheres. At a fixed solid content, the number of compositeabrasives in slurry decrease as the shell thickness increases, which

leads to a lower MRR for the composite abrasives S5 (the thicknessof CeO2 shell was about 30 nm). The CMP results showed that thecomposite abrasives with medium shell thickness (about 20 nm)

) before CMP, (b) after CMP with pure CeO2 abrasives.

8684 Y. Chen, R. Long / Applied Surface Science 257 (2011) 8679– 8685

e after

ga

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Fig. 6. Typical 3D AFM images of the oxide wafer surfac

ave lower surface roughness, lower topographical variations and

higher MRR.

The results presented above clearly indicate that there isn obvious effect of the composite abrasives shell thickness onhe oxide CMP performances. Therefore, the mechanical prop-

CMP with composite abrasives (a) S1, (b) S3 and (c) S5.

erties of the composite abrasives are of paramount importance

in CMP. The mechanical properties of the as-prepared PS/CeO2composite microspheres with different shell thickness are beinginvestigated by an AFM nanoindentation technique [21] in ourgroup.

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. Conclusions

Core–shell structured PS/CeO2 composite microspheres withontrolled shell thickness were synthesized by in situ chemicalrecipitation method. The diameter of PS/CeO2 composite micro-pheres was about 220–260 nm. The thickness of CeO2 shell ofhe composite microspheres was about 10, 15, 20, 25 and 30 nm,espectively. The SiO2 film after polished by the PS/CeO2 compositebrasives exhibited lower surface roughness, lower topographicalariations as well as fewer scratches than that of pure CeO2 abra-ives. The CMP results indicate that there is an obvious effect of thehell thickness of the as-synthesized PS/CeO2 composite abrasivesn oxide CMP performances.

cknowledgements

The work was supported financially by Youth Foundation ofhangzhou University (JQ201005).

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