CeO2 hybrid particles for enhanced chemical mechanical polishing performance

Tribology International 82 (2015) 211–217

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Tribology International journal homepage: www.elsevier.com/locate/triboint

Polymethylmethacrylate (PMMA)/CeO2 hybrid particles for enhanced chemical mechanical polishing performance Yang Chen a,n, Zhina Li a, Naiming Miao b a b

School of Materials Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, PR China School of Mechanical Engineering, Changzhou University, Changzhou 213016, Jiangsu, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 21 July 2014 Received in revised form 16 October 2014 Accepted 21 October 2014 Available online 30 October 2014

The organic/inorganic hybrid particles with polymethylmethacrylate (PMMA) spheres (350–360 nm) as inner cores, and ceria nanoparticles (5–10 nm) as outer shells were synthesized via electrostatic interaction. Chemical mechanical polishing (CMP) tests for silica films showed that the hybrid particles exhibited lower surface roughness, fewer scratches as well as lower topographical variations than those of solid ceria particles and (PMMA þCeO2) mixed particles. The improved CMP behavior might be attributed to the synergetic effect of the PMMA cores and the ceria shells. Furthermore, we discussed the material removal mechanism of hybrids in CMP process based on the proposal that the core/shell composite abrasives might be considered as micro-fixed abrasive pads. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Hybrid particle Ceria Fixed abrasive pad Chemical mechanical polishing

1. Introduction The chemical mechanical polishing (CMP) technique has been widely applied in the fabrication of advanced integrated circuit and semiconductor industry [1,2]. Ceria (CeO2) particles are one of the key abrasive materials in CMP process [3,4]. Ceria-based slurries have been widely used in the process of polishing silicon oxide films due to their high material removal rate (MRR). Several models were proposed to explain this high MRR for oxide-CMP. It is commonly considered that the cerous has less effect to polish and is easy to adhere, whereas ceria polishes silica quickly and is easy to be removed [5,6]. However, ceria particles synthesized by common methods are irregular in shape and possess sharp edges, corners, and apexes [7], which possibly result in scratch defects and mechanical damages and limit their farther application. Organic/inorganic core/shell nanostructures have attracted much attention due to their unique structure and interesting properties, thus having wide potential applications in biotechnological, optical, electronic, magnetic, catalytic and sensing devices [8–10]. Generally, core/shell structured hybrid particles exhibit special mechanical properties (lower elastic modulus and hardness) as compared to traditional inorganic particles [11]. In CMP applications, it is likely to combine several advantageous properties in the core/shell structure. The inorganic shell may exhibit an enhanced mechanical and/or chemical action. Meanwhile, the

n

Corresponding author. Tel./fax: þ 86 519 86330066. E-mail address: [email protected] (Y. Chen).

http://dx.doi.org/10.1016/j.triboint.2014.10.013 0301-679X/& 2014 Elsevier Ltd. All rights reserved.

elastic organic cores can increase the contact area between wafer and abrasive and decrease the contact stress. In addition, the mechanical properties of polymer cores can be highly controllable by adjustment of polymerization parameters. The synergistic effect between organic core and inorganic shell may be beneficial for reducing mechanical damages and increasing MRR in the CMP process. Many approaches have been explored to synthesize the core/ shell hybrid particles with silica or ceria shells. To reduce the defectivity after polishing, Armini and coworkers [12,13] investigated the effects of particle (fumed and colloidal silica) shape and size on oxide CMP performances. The experimental results indicated that fewer and shallower scratches were detected for the composites with a colloidal silica shell as compared with colloidal silica, as well as the composites with a fumed silica shell. Besides, the highest MRR was observed for the largest diameter cores combined with the smallest silica particles. In our previous work [14–16], the hybrid particles with polystyrene (PS) as a core and CeO2 nanoparticles as a shell were prepared by in-situ decomposition reaction of Ce(NO3)3 on the surfaces of the negatively charged PS spheres. Oxide-CMP behaviors and elastic moduli of the PS/CeO2 hybrid particles were evaluated by atomic force microscopy (AFM). Armini and coworkers [17] synthesized the polymethylmethacrylate (PMMA)/CeO2 composites comprising polymer cores (ca. 300 nm) coated by ceria nanoparticles (ca. 14 nm). The hybrid particles were achieved by either using silane coupling agents (composite A) or tuning the pH in order to form electrostatic attractive interactions between the core and the shell (composite B). However, the SEM images of composite A and B

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indicated that ceria nanoparticles were unevenly coated on the surfaces of PMMA cores and the bare PMMA and separated ceria nanoparticles could be clearly observed. Herein, we described a simple and efficient route to prepare well-defined core/shell structured PMMA/CeO2 hybrid particles via electrostatic interaction. The whole process required neither surface treatment for PMMA cores nor addition of surfactant. It is crucial that CeO2 nanoparticles are uniformly located on the surfaces of PMMA cores, and there were few secondary CeO2 particles as residues. In addition, the difference of oxide-CMP behavior among traditional solid ceria particles, (PMMAþCeO2) mixed particles and hybrid particles were explored by AFM. Furthermore, we proposed for the first time that the core/shell composites could be considered as micro-fixed abrasive pads (MFAPs). In this sense, the organic cores could also be regarded as micro-polishing pads, while the inorganic nanoparticles were fixed on the surfaces of organic cores. The efficient and damage-free CMP mechanism of the non-rigid core/shell composite abrasives was discussed.

2. Experimental section 2.1. Synthesis and characterization of PMMA/CeO2 hybrid particles PMMA core particles were prepared by emulsifier-free emulsion polymerization [18]. Briefly, 10 g methyl methacrylate (MMA, purified with an inhibitor removal column prior to use) and 150 ml deionized (DI) water were added into a 250 ml round bottom flask with a reflux condenser and magnetic stirrer. Then the mixture was heated to 70 1C gradually in an oil bath under constant stirring at 250 rpm. After 5 min, 0.2 g potassium persulfate (KPS, dissolved in 30 g DI water) was added to initiate polymerization. Finally, PMMA colloids were derived after reacting for another 7 h. To synthesize PMMA/CeO2 hybrid particles, 4 ml PMMA colloids were charged into 200 ml DI water under ultrasonic vibration for 5 min. Subsequently, 1.5 g cerium nitrate hexahydrate (Ce(NO3)3  6H2O) and 2.4 g hexamethylenetetramine (HMT) were added. The mixture was initially dispersed by means of an ultrasonic bath for 10 min to form homogeneous suspension. Then, the mixture was reacted at 75 1C for 2 h. The resulting precipitates were centrifuged, washed with DI water and absolute ethanol, and dried overnight at 80 1C. The phase of the samples was examined by a Rigaku D/Max 2500 PC diffractometer with Cu Kα radiation. The morphology and microstructure of the samples were observed by a scanning electron microscope (SEM, JSM-6360LA), a field emission scanning electron microscope (FESEM, ZEISS, SUPRA 55), and a transmission electron microscope (TEM, JEOL-2100). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Avatar 370 spectrometer using KBr pellet technique. Thermogravimetric analysis (TGA) was performed in air at a rate of 10 1C/min on a Netzsch TG 209 F3 thermal analyzer. 2.2. CMP test Thermal oxide silicon wafers were used as substrates for CMP, and the thickness of oxide films was about 1200 nm. All of the polishing tests were conducted with a TegraForce-1/TrgraPol-15 polisher (Struers, Denmark). The model of polishing pad was MDChem (porous polyurethane pad) from Struers Company. A diamond pad conditioner was utilized to abrade the pad prior to each CMP test. The pH value of the slurry containing the PMMA/CeO2 hybrid particles was adjusted to 10 with ammonia (25–28 wt%), and 0.1 wt% sodium dodecyl benzene sulfonate (SDBS) was added as dispersant. The CMP process parameters were summarized as follows. The speed of carrier head and platen was 120 and 90 rpm

respectively. The slurry flow rate was 100 ml/min, and the solid content (for abrasive particles) of slurries was 1 wt%. The head pressure was 4 psi, and the polishing time was 1 min. For comparison, traditional solid ceria spherical particles (360– 380 nm) and sphere-like CeO2 nanoparticles (10–20 nm) were prepared according to the procedure proposed by Chen et al. [19,20]. To obtain the (PMMAþ CeO2) mixed particles, the PMMA spheres were simply mixed with CeO2 nanoparticles. The mass ratio of PMMA: CeO2 in the mixed system was 61%: 39%, which was determined by the TGA analysis of the PMMA/CeO2 hybrid particles. The slurries containing solid ceria particles or (PMMAþCeO2) mixed particles were prepared with the same procedures, and the polishing tests were performed under the same conditions. After ultrasonic post-cleaning with DI water and surfactant solution, the polished wafers were placed in ethanol and dried naturally in a super clean room. All wafers polished by different abrasives were cleaned by the same methods. The topography, roughness and profile curve of the wafer surface before and after CMP was characterized by AFM (Dimention V, Bruker) at room temperature. Silicon tips used in tapping mode were purchased from BudgetSensors (Tap300Al-G) with tip radius smaller than 10 nm. The resonance frequency of the tip was 300 kHz, and the scanning rate was 1 Hz. The sample/line was 512. The spring constant of the tip was about 40 N/m. The MRR data was calculated according to the method described elsewhere [14]. The MRR, average roughness (Ra) and root mean square roughness (RMS) data are the average of three individual tests.

3. Results and discussion 3.1. Material characterization From the low magnification image of PMMA colloids (Fig. 1a), it could be observed that the PMMA particles were all in spherical shape with narrow size distributions and an average size of ca. 350–360 nm. It can be seen from the high-magnification SEM image (Fig. 2b) that the surfaces of the obtained PMMA spheres were smooth. Fig. 2 shows the FT-IR spectrum for the as-prepared hybrid particles. The characteristic peaks for PMMA were found at ca. 1731 cm  1 (the C ¼O stretching vibration), 1389 and 2951 cm  1 (the aliphatic C–H stretching) [21,22]. The typical absorption peaks of CeO2 at ca. 400 and 480 cm  1 [23], were also presented in Fig. 1 for hybrid particles. These results indicated that the final products were composed of PMMA and CeO2. The crystalline phases of the hybrid particles were further characterized by XRD (Fig. 3). The characteristic peaks at 2θ ¼28.51, 33.11, 47.51, 56.31, 59.11 and 69.31 could be indexed as a face centered cubic-phase CeO2 (JCPDS 340394) [24]. In addition, we found no amorphous diffraction peak at ca. 201 for PMMA. It might be attributed to the formation of CeO2 shell on the PMMA surfaces. Similar phenomenon was also observed by Kim and coworkers [25]. These results suggested that the hybrid particles exhibited a core (PMMA)–shell (CeO2) structure. The morphologies of the obtained PMMA/CeO2 hybrid particles were investigated by SEM. The panoramic FESEM image (Fig. 4a) of the composites exhibited large-scale uniform spheres and similar to the PMMA colloids. It was clearly observed that the surface morphology of the hybrid particles was rather rougher than PMMA spheres, which can be confirmed from the high-magnification image (Fig. 4b). The size of hybrid particles was determined to be 390–410 nm from the particle size analyzer while the size of PMMA cores was ca. 350–360 nm. It indicated that the CeO2 coating thickness was about 15–30 nm. More detailed structural characterization of the PMMA/CeO2 hybrid particles, solid ceria particles and

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Fig. 4. FESEM images for PMMA/CeO2 hybrid particle samples.

Fig. 2. FT-IR spectrum for PMMA/CeO2 hybrid particle samples.

(PMMAþ CeO2) mixed particles was performed by TEM analysis. As shown in Fig. 5a, well-defined core–shell structure could be obviously detected for the as-synthesized hybrid particles. The rugged surface morphology clearly indicated that the PMMA core was covered by a dense layer of CeO2 nanoparticles and there were few free CeO2 particles as residues. The average CeO2 shell thickness estimate by TEM image was ca. 20 nm, which agreed well with the FESEM results (Fig. 4). From the higher-resolution image (inset in Fig. 5a), it could be estimated that the mean size of the CeO2 nanoparticles in the shells was about 5–8 nm. As shown in Fig. 5b, most of CeO2 nanoparticles (10–20 nm) in (PMMAþ CeO2) mixed

particles system was separate from each other, while a small amount of CeO2 nanoparticles absorbed onto the surfaces of PMMA spheres. The obtained solid ceria particles (Fig. 5c) revealed monodispersed submicron spheres with a relatively uniform diameter of 360–380 nm. A possible mechanism for the formation of PMMA/CeO2 hybrid particles is described as follows. Because KPS was used as anionic initiator, the obtained PMMA was negative-charged [18]. During the synthesis process, Ce3 þ cations could be absorbed onto the surfaces of the negatively charged PMMA cores via electrostatic forces. Subsequently, Ce3 þ combined with opposite-charged OH 

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Mass residue/%

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Fig. 5. TEM images of (a) PMMA/CeO2 hybrid particles, (b) (PMMAþ CeO2) mixed particles and (c) solid submicron ceria particle samples.

slowly hydrolyzed from HMT driven by electrostatic attraction. Then, Ce3 þ hydrolyzes to form gelatinous hydrous cerium hydroxide on the surfaces of PMMA cores. Followed by dryness process in air, Ce3 þ was oxidized to Ce4 þ due to its intrinsic crystallization tendency [26], and the CeO2 shell was formed. In order to determine the amounts of ceria components in (PMMA þCeO2) mixed system, TGA for the PMMA/CeO2 hybrid particles was performed (Fig. 6). The first weight loss stage was associated with the release of absorbed water between 50 and 150 1C, and the second weight loss stage, at 200–450 1C, was associated with the decomposition of PMMA [27]. The weight loss of hybrid particles was about 61 wt% during the calcination due to decomposition of PMMA cores. Therefore, the residual weight of ca. 39 wt% should be the weight fraction of the CeO2 nanoparticles in the PMMA/CeO2 hybrid particles. 3.2. Polishing performance Fig. 7 exhibits the typical two-dimensional (2D) AFM morphologies of the wafer surface before and after polishing. The original

wafer surface (Fig. 7a) appeared very rough, and the scratches and concaves might be resulted from grinding damage. The RMS and Ra roughness values were 0.965 and 0.664 nm, respectively. After polishing with the slurry containing the (PMMA þCeO2) mixed particles, plenty of residual particles adhered to the polished wafer surfaces (Fig. 7b), and some scratches could be clearly observed. The 2D AFM images of the wafer surfaces after polishing with the slurry containing solid submicron ceria (Fig. 7c) and nanometer ceria (Fig. 7d) particles revealed some scratches, indicating mechanical damage occurred possibly during the material removal process. By comparison with (PMMA þCeO2) mixed particles or solid ceria particles, the PMMA/CeO2 hybrid particles achieved a global smooth wafer surface (Fig. 7e). Defects such as scratches and residual particles could be hardly observed on the polished surface, which indicated that the as-synthesized PMMA/CeO2 hybrid particles could eliminate effectively surface damages. The surface roughness values and MRR data obtained with different abrasive types were summarized in Table 1. The PMMA/ CeO2 hybrid particles were useful to maintain a relatively high MRR (266.2 nm/min) with low surface roughness (RMS¼ 0.201 nm, Ra ¼0.154 nm), revealed a significant reduction in surface roughness. The profile curves of the wafer surfaces after CMP were also characterized by AFM (Fig. 8). For contrast, the diagonals in 2D AFM images were selected in profilogram measurements. As shown in Fig. 8, the wafer surfaces polished by hybrid particles exhibited lower topographical variations than those obtained by mixed particles or traditional solid ceria particles. To explore the material removal mechanism of the PMMA/CeO2 hybrid particles, the present polishing behaviors of different abrasives were compared for further analyses. The hybrid abrasives revealed much higher MRR and lower surface roughness values than those of (PMMAþ CeO2) mixed particles, which might be attributed to the special core–shell structure, and the enhanced PMMA surface hardness resulting from the ceria shell. By comparison with solid ceria particles, the hybrid abrasives exhibited lower roughness values and fewer scratches. The elastic polymer cores in the hybrids might increase the contact areas between wafers and particles and decrease the indentation depth of particles onto the polished surfaces during CMP, which was in favor of reducing roughness and mechanical damage. According to the existing literatures [16,28], the compressive Young’s moduli of a variety of the polymer colloids (PS or PMMA) and the core–shell organic/inorganic hybrid particles (PMMA/SiO2, PS/SiO2 and PS/CeO2) were determined using AFM nanoindentation technique. The reported results confirmed that the moduli of the hybrid particles were much lower than those of inorganic shell materials and much close to those of their polymer cores. Compared

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CMP with solid nanometer CeO2 particles

CMP with PMMA/CeO2 hybrid particles Fig. 7. Typical AFM images of the wafer surface (a) before and after CMP with (b) (PMMA þCeO2) mixed particles, (c) solid submicron ceria particles, (d) solid nanometer ceria particles and (e) PMMA/CeO2 hybrid particles.

with traditional inorganic particles, these hybrid particles exhibited the especial non-rigid mechanical properties. Moreover, the elastic properties of the composites appeared to be determined exclusively by the properties of the polymer cores. Meanwhile, the inorganic shells stiffened the organic cores. During the CMP process of core– shell organic/inorganic hybrid abrasives, the polished wafer surface directly contact with the inorganic nanoparticles in the shell. In the real contact area between hybrid particle and wafer, the separate

inorganic nanoparticles in the shell may not individually deform when the entire composites deform. Consequently, the hybrid particle exhibits non-rigid mechanical property as a whole, while the composite is rigid in the real contact area between particle and wafer. The elastic deformation of organic cores, which act as the supports for inorganic nanoparticles in shells, may increase the real contact area between abrasive and wafer. In other words, the number of inorganic nanoparticles contacted directly with wafer

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increase accordingly. It may be contribute to decrease the contact stress during CMP and the indentation depth of abrasive onto the surface of wafer, which is help to reducing mechanical damage. Moreover, the springlike structure coming from elastic polymer cores may allow the hybrid particles to easily adapt to the polishing pad asperities, which is also in favor of improving surface planarization [29]. In order to achieve an insight into the organic/inorganic composite abrasives, we proposed for the first time that the composites could be considered as MFAPs. In this sense, the organic cores could also be regarded as micro-polishing pads, while the inorganic nanoparticles were fixed on the surfaces of organic cores. During the CMP process with the slurries containing composite particles, the interface between actual polishing pad and wafer exist plenty of MFAPs. Many advantages are expected from the MFAPs as compared with traditional solid inorganic abrasives. It is in general difficult to avoid nanoparticles aggregation, which easily scratches the wafer surface and decreases the amount of active abrasives. The MFAPs might maintain the physical contact between nanoparticle and wafer always occurred in the real contact interface, which is beneficial to decrease the mechanical damages resulted from hard agglomerates. Besides, the MFAPs might also increase the probability of the direct contact between abrasive and wafer. Moreover, it might increase the Table 1 Surface roughness values and material removal rates data obtained with different abrasives. Abrasives

Roughness (5 μm  5 μm)

Before CMP CMP with (PMMAþ CeO2) mixed particles CMP with solid submicron ceria particles CMP with solid nanometer ceria particles CMP with hybrid particles

RMS (nm)

Ra (nm)

0.965 0.412 0.424 0.286 0.201

0.664 0.236 0.304 0.214 0.154

MRR (nm/min)

/ 137.7 312.6 175.5 266.2

number of active abrasives and MRR correspondingly. Wang and coworkers [30] explored the CMP behavior of the mixed abrasives slurries containing B4C (ca. 2 μm) and CeO2 (ca. 78 nm) particles for sapphire wafer. The CeO2 particles in MAS were seated on the surfaces of B4C particles in terms of electrostatic force, and the improvement in MRR and surface roughness may be attributed to the direct contact between ceria and sapphire hydration layer. Based on the above discussion we could conclude that the nominal dimensions of composite abrasives might be inappropriate for directly calculating the indentation depth of composites onto the wafer surfaces. Furthermore, the size and morphology of the inorganic nanoparticles in the shell should be taken into consideration. In summary, the definite roles for the core/shell organic/inorganic composite abrasives in the material removal during the CMP process still remain unclear and ambiguous. In order to offer insights into the novel abrasives with special structure and property, more detailed analyses and physical– mathematical models are required.

4. Conclusions The core/shell PMMA/CeO2 hybrid particles were prepared through in-situ chemical precipitation. The hybrid particles revealed a spherical shape with an average size of 350–360 nm in diameter. The CeO2 nanoparticles immobilized on the surface of the negatively charged PMMA colloids, and the ceria shell thickness was about 20 nm. The silica films after CMP with hybrid particles exhibited lower surface roughness, lower topographical variations as well as less scratches and residual particles than those of solid ceria particles or (PMMAþ CeO2) mixed particles. The enhanced polishing performance might be attributed to the synergetic effect of the PMMA cores and the ceria shells. The composites exhibit non-rigid mechanical property as a whole, while the composite is rigid in the real contact area between particle and wafer. Furthermore, we discussed the material removal mechanism of hybrids in CMP process based on the

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proposition that the core/shell composite abrasives might be considered as micro-fixed abrasive pads. Acknowledgements The work was supported financially by the National Natural Science Foundation of China (51205032) and the Natural Science Foundation of Jiangsu Province of China (BK2012158, BK2011238). References [1] Thomas ELH, Nelson GW, Mandal S, Foord JS, Williams OA. Chemical mechanical polishing of thin film diamond. Carbon 2014;68:473–9. [2] Zhou Y, Pan G, Shi X, Gong H, Luo G, Gu Z. Chemical mechanical planarization (CMP) of on-axis Si-face SiC wafer using catalyst nanoparticles in slurry. Surf Coat Technol 2014;251:48–55. [3] Yang JC, Kim H, Kim T. Study of polishing characteristics of monodisperse ceria abrasive in chemical mechanical planarization. J Electrochem Soc 2010;157: H235–H240. [4] Dandu PRV, Peethala BC, Babu SV. Role of different additives on silicon dioxide film removal rate during chemical mechanical polishing using ceria-based dispersions. J Electrochem Soc 2010;157:H869–74. [5] Cook LM. Chemical processes in glass. J Non-Cryst Solids 1990;120:152–71. [6] Hoshino T, Kurata Y, Terasaki Y, Susa K. Mechanism of polishing of SiO2 films by CeO2 particles. J Non-Cryst Solids 2001;283:129–36. [7] Feng X, Sayle DC, Wang ZL, Paras MS, Santora B, Sutorik AC, Sayle TXT, Yang Y, Ding Y, Wang X, Her Y. Converting ceria polyhedral nanoparticles into singlecrystal nanospheres. Science 2006;312:1504–8. [8] Shi F, Li Y, Wang H, Zhang Q. Formation of core/shell structured polystyrene/ anatase TiO2 photocatalyst via vapor phase hydrolysis. Appl Catal, B: Environ 2012;123–124:127–33. [9] Fang FF, Kim JH, Choi HJ. Synthesis of core–shell structured PS/Fe3O4 microbeads and their magnetorheology. Polymer 2009;50:2290–3. [10] Xu D, Ji X, Liu H, Wang M, Ge X, Lam MH. Synthesis of triangle hybrid particles by radiation-induced seeded emulsion polymerization based on polystyrene/ SiO2 core–shell particles. Mater Lett 2012;79:61–4. [11] Guo D, Xie G, Luo J. Mechanical properties of nanoparticles: basics and applications. J Phys D: Appl Phys 2014;47:013001. [12] Armini S, Whelan CM, Moinpour M, Maex K. Composite polymer core–silica shell abrasives: the effect of the shape of the silica particles on oxide CMP. J Electrochem Soc 2008;155:H401–6. [13] Armini S, Whelan CM, Maex K. Engineering polymer core–silica shell size in the composite abrasives for CMP applications. Electrochem Solid-State Lett 2008;11:H280–4.

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