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polymethacrylic acid composite abrasive and its CMP performance on glass substrate
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Microelectronic Engineering 85 (2008) 714–720 www.elsevier.com/locate/mee
Preparation of a-alumina/polymethacrylic acid composite abrasive and its CMP performance on glass substrate Zefang Zhang a,b, Hong Lei a,* a
Research Center of Nanoscience and Nanotechnology, Shanghai University, Shanghai 200444, China b School of Sciences, Shanghai University, Shanghai 200444, China Received 26 June 2007; accepted 3 January 2008 Available online 16 January 2008
Abstract Chemical mechanic polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of key elements during CMP process. a-Alumina particles, as a kind of widely used abrasive in CMP slurries, often cause surface defects due to its high hardness and agglomeration. In order to enhance the dispersion stability and prevent agglomeration of pure alumina abrasives in CMP, a-alumina grafted with polymethacrylic acid (a-Al2O3-g-PMAA) composite abrasives was prepared and characterized by means of Fourier transform infrared (FTIR), Mastersizer 2000 instrument and scanning electron microscope (SEM). The results indicated that the prepared a-Al2O3-g-PMAA composite abrasives have better dispersion stability than pure a-Al2O3 abrasives. The CMP performances of the a-Al2O3-g-PMAA composite abrasives on glass substrate were investigated by SPEEDFAM-16B-4M CMP equipment with different process parameters. By using the optimum process parameters, the prepared abrasives exhibit better glass substrate CMP performance than pure a-Al2O3 abrasives. Further, the CMP mechanism of glass substrate was deduced preliminarily. Ó 2008 Elsevier B.V. All rights reserved. Keywords: CMP; a-Al2O3-g-PMAA composite abrasive; Glass substrate
1. Introduction Chemical mechanical polishing (CMP) has become a more essential technique in manufacturing of semiconductor and digital compact disc (CD) glass substrate [1–5]. There are up to 16 variables that needed to be controlled to achieve a stable process, and one of the major variables is related to slurry compositions [6]. As the basic element of slurry, i.e., the hardness, sizes, and chemical properties of abrasives are crucial for a desired CMP performance. In CMP slurry, abrasives can be classified as two types: the traditional inorganic particles and the composite particles [7]. Traditional inorganic abrasives, such as silica, alumina, ceria, have been widely studied [3–6] and used in the commercial slurries, but just one kind of inorganic abra*
Corresponding author. E-mail address: [email protected] (H. Lei).
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.001
sives used in slurries often leads to undesired CMP performance. Recently, with the increasing demand of improving the polishing performance while minimizing roughness and defects of the polished surface, composite particles as abrasives in slurry have been paid much attentions [7–11]. It is thought that the coating on the surface of particles with harder substance can improve the removal rate while keeping the density of defects constant, however the hard particles coated with a softer material can reduce CMP defects [8]. Yano et al. [9] developed a kind of slurry with inorganic/resin abrasive for the Al/low k damascene wiring CMP. The slurry resulted in less scratching and better planarity. In SiO2 CMP, it was found that composite abrasive slurry exhibited the reduction of the dishing and erosion depth, which was reduced to less than 80 nm after first step CMP and less than 70 nm after second step CMP [10]. Lei and Zhang [7] developed a novel kind of slurry containing
Z. Zhang, H. Lei / Microelectronic Engineering 85 (2008) 714–720
a-alumina/silica core-shell abrasives for hard disk substrate. The slurry resulted in lower surface roughness and waviness as well as lower topographical variations and less scratch than that containing pure alumina abrasive under the same testing conditions. At present, a-alumina particle as a kindly of widely used abrasives has been used in CMP [12,13], but its high hardness and agglomeration often leads to more surface defects. With advanced electro-manufacture developing so fast, glass substrate, as a kind of widely used material in the home electronic products such as VCD, DVD, CVD, HDVD, etc., is forced to ultra-smooth. Although there have been some studies [7,8–10] on composite abrasives, in which almost no study is involved in glass substrate CMP. In the present study, a novel a-Al2O3-g-PMAA composite abrasive was prepared and its CMP performances on glass substrate have been studied. 2. Experimental 2.1. Preparation of a-Al2O3-g-PMAA composite abrasives and their its based slurry Preparation of a-Al2O3-g-PMAA composite abrasives was carried out via a two-step process: surface activation of a-Al2O3 with c-methacryloxypropyl trimethoxy silane coupling agent (KH570) in toluene followed by graft polymerization of methacrylic acid (MAA) in water with ammonium persulfate (APS) initiator. 2.1.1. Silylation Prior to silylation, the a-alumina particles were dried in an oven at 110 °C under vacuum for 10 h in order to get rid of the physically absorbed and weakly chemically absorbed species. Subsequently, 5 g dried alumina particles and 2 g KH570 in 50 ml were charged into a 150 ml flask equipped with a reflux condenser. The mixture was refluxed at the boiling temperature for 5 h under vacuum. The silylated alumina particles were centrifuged, and the precipitate was extracted with toluene solvent for 8 h to remove adsorbed silane molecules, followed by drying at 80 °C in a vacuum oven for 12 h to remove the remaining solvent. 2.1.2. Graft polymerization Free-radical graft polymerization of MAA onto KH570modified alumina particles was carried out in a flask equipped with a condenser at 80 °C under vacuum. Firstly, 5 g silylated alumina and 100 g deionized (DI) water were put in the temperature-controlled reactor with stirring. When the reactant reached the desired temperature, 0.04 g APS was charged, and then 4 g MAA was added after 20 min. After the graft polymerization, the resultant suspension was centrifuged, and the precipitate was extracted with DI water for 8 h to remove the absorbed polymers. Then the grafted alumina particles were dried under vacuum at 80 °C. The purified a-Al2O3-g-PMAA
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composite particles were characterized by FTIR, SEM and Mastersizer 2000 instrument. 2.1.3. Preparation of the slurry 2.5 wt.% composite particles and 1 wt.% functional additives were added to DI water in a container under continuously stirring. Then the mixture was milled for 2 h in a vibrator containing ZrO2 balls as abrasives. Finally, the mixture was filtrated with a 20 lm pore strainer. 2.2. Polishing tests Polishing tests were conducted with a SPEEDFAM16B-4 M CMP equipment (SPEEDFAM Co. LTD, Japan). The down force was varied between 106 kg and 166 kg; the rotating speed was varied between 9 rpm/min and 18 rpm/ min and the polishing time varied from 10 min to 180 min. The slurry supplying rate was 1 L/min. The samples were U 160 mm sodium–calcium glass substrates. The surface average roughness (Ra) of disk substrates before polishing is about 500 nm. The polishing pad was a Rodel porous polyurethane pad. After polishing, the substrates were washed with ultrasonic in a cleaning solution containing 0.5 wt.% surfactant in DI water. Finally, they were dried by a multi-functional drying system. 2.3. Examination of the polished surfaces The polished surface features, the surface average roughness (Ra), was measured to evaluate the polishing effects in different slurries. Ra was measured by using a Zygo surface profiler (Zygo Corp., France) with the resolu˚ . The measuring area was 0.357 mm tion of 0.1 A 0.268 mm. The mass of glass substrate was measured by using an analytical balance. The difference of mass before and after polishing divided by the polishing time gives material removal rate. All the data were the mean values of six times. 3. Results and discussion 3.1. Structure and dispersibility of a-Al2O3-g-PMAA composite particles Fig. 1 shows the FTIR spectra of pure alumina particles, silylated alumina particles and PMAA-grafted alumina particles. By comparison with pure alumina, a series of absorption peaks of silylated alumina particles at 1659 cm1, 1732 cm1, 2844 cm1 and 2917 cm1 which belong to C@C, C@O, –CH3, respectively show that pure alumina particles were silylated successfully. For PMAAgrafted alumina, there is a strong absorption peak of C@O in PMAA at 1707 cm1, which suggests that the PMAA were successfully grafted onto the surface of alumina particles through covalent bonding.
Z. Zhang, H. Lei / Microelectronic Engineering 85 (2008) 714–720
Transmittance/%
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4000
3500
3000
2500
2000
1500
1000
Wavenumber/cm-1 Fig. 1. FTIR spectra of pure alumina particles (a), silylated alumina particles (b) and PMAA-grafted alumina particles (c).
In order to analyze the dispersibility of alumina particles before and after surface modification, the morphology of the alumina particles before and after surface modification was observed by SEM as shown in Fig. 2. It is found that the surface modified alumina particles have better dispersibility, while obvious agglomeration was observed for pure alumina particles. And it is also noticed that, by comparison with the sharp appearance of pure alumina particles, the surface modified alumina particles seem to become round. In addition, the size difference of alumina particles before and after surface modification as shown in Fig. 3, median diameter (d50) of pure alumina is 3.107 lm and aAl2O3/PMAA is 0.780 lm, which have further suggested that surface modified alumina particles have a better dispersibility. It may be explained by the grafted PMAA have prevented the agglomeration of alumina particles, which resulted that the size of surface modified alumina particles become smaller. 3.2. CMP performance of slurry containing a-Al2O3-g-PMAA composite particles A better understanding on the effect of polishing process parameters on the CMP is very important. Although there have been a lot of studies [14–20] about the effects of process parameters on CMP performance, in which almost no study in involved in CMP of glass substrate. So, the effects of polishing parameters such as polishing time, down force and rotation speed on material removal rate (MRR) and surface average roughness (Ra) was studied in glass substrate CMP with these prepared Al2O3-g-PMAA composite abrasives. 3.2.1. Effect of different process parameters on MRR and Ra 3.2.1.1. Rotating speed. Fig. 4. shows MRR and Ra of glass substrate with the increase of the rotating speed. The removal rate of glass substrate by change of the rotating
Fig. 2. SEM images of alumina particles before (a) and after (b) surface modification.
speed showed the liner tendency of steep increase, which was caused by the increasing chance of friction between glass substrate and polishing pad as well as abrasives. The highest removal rate of 1.68 mg/min was obtained in 18 rpm/min but the lowest Ra was obtained in 15 rpm/ min. Therefore, the optimum rotating speed of glass substrate CMP was 15 r/nin considering both the material removal rate and the surface average roughness. 3.2.1.2. Down force. Fig. 5 shows the relationship of MRR and Ra of glass substrate as a function of down force. As the down force increases, the material removal rate increases linearly up to a critical point at 146 kg. Conversely, the surface average roughness decreases linearly up to a critical point at 146 kg. We conclude that as the down force increases, the removal rate increases and the surface average roughness is improved. With the down
Z. Zhang, H. Lei / Microelectronic Engineering 85 (2008) 714–720
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Fig. 3. Size distribution of alumina particles before (a) and after (b) surface modification.
force of 146 kg, the relatively high removal rate of 1.42 mg/ min and the relatively low surface average roughness of 59 nm were obtained. 3.2.1.3. Polishing time. Fig. 6 shows MRR and Ra of glass substrate according to polishing time without any change of other process parameters. The material removal rate by change of the polishing time showed the linear tendency of steep decrease. This estimates that the surface of substrate has many rough peaks in initial polishing process which were firstly removed in the polishing process. With the increasing of polishing time, the number of rough peaks decreased, which induce the surface roughness to decrease as shown in the Fig. 6(b). But the surface average roughness almost shows no difference from 60 min to 180 min. In other words, polishing time of 60 min is enough for the prepared abrasives. Prolonging the polishing time cannot improve surface quality any more. Table 1 summarizes the optimum process parameters obtained by the present study. 3.2.2. Different CMP performance between pure alumina and a-Al2O3-g-PMAA composite abrasives The MRR and Ra of glass substrate polished by pure alumina abrasives and Al2O3-g-PMAA core/shell abrasives with the optimum process parameters were given in Table 2. It is found that the introduction of PMAA on the surface
of alumina particles decrease the MRR and Ra. The lower Ra value is, the higher surface planarization has been achieved. In other words, the prepared Al2O3-g-PMAA core/shell abrasives have higher surface planarization than pure alumina abrasives. CMP is a complex erosive wear process. As the supplier of mechanical effect, abrasive wear accounts for main material removal process. Wear volume (V) of abrasive wear may be described as following equation [21]: V ¼ kW =H s ; where k is wear coefficient, W is applied load and Hs is hardness of polished surface. k is determined by abrasive properties, such as hardness, shape and size of abrasive. The harder, sharper and larger abrasive is, the higher wear loss will be [21–24]. The reduction in MRR of the a-Al2O3-g-PMAA composite abrasives can be attributed to the reduction of the hardness of the alumina particles (since PMAA coating is relatively softer thana-Al2O3 core) as well as their smoother shape and smaller particles. 3.3. CMP mechanism of glass substrate Glass polishing in the prepared a-Al2O3/PMAA core/ shell abrasives based slurry is a chemical mechanical polishing process. CMP process is regarded as a combination
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a
a
1.8
1.4
Removal Rate/mg.min-1
Removal Rate/mg.min-1
1.6 1.4 1.2 1.0 0.8
1.2
1.0
0.8
0.6 8
10
12
14
16
18
100
Rotating Speed/r.min-1
b
120
140
160
Down Force/kg
b 110
200
100
90
Ra/nm
Ra/nm
160
120
80
70
80
60 8
10
12
14
16
18 100
Rotating Speed/r.min-1
140
160
Down Force/kg
Fig. 4. Effect of rotating speed on (a) MRR and (b) Ra (down force = 146 kg; polishing time = 60 min).
of chemical effect and mechanical effect [25–29]. Although there is still lack of a model that is able to describe the entire available CMP process, erosion and abrasive wear may be agreed to be two basic effects during the material removal process. The prepared CMP slurry consists of abrasive, functional additives and water. Abrasives and water containing chemical additives may provide mechanical grinding and chemical erosion effects, respectively. The two effects in CMP process can be described as follows. At first, a glass surface can be hydrolyzed in the water [30– 33]. The glass substrate polished in the experiment is sodium–calcium glass. The structure of sodium–calcium glass is classified into two kinds, one is the continuous network, „Si–O–Si„, and the other is the collapsing structure, „Si–O–Na (or terminal structure). The terminal structure of the glass surface reacts with water as shown in reaction (1): B Si A O A Na + H2 O !B SiOH + Naþ + OH
120
ð1Þ
The sodium or calcium bond near the interface breaks because of transference of a sodium ion. Therefore, the oxygen combines with a hydrogen from H2O in order to
Fig. 5. Effect of down force on (a) MRR and (b) Ra (rotating speed = 15 rpm/min; polishing time = 60 min).
meet its energy field, and forms „SiOH. The free OH ion interacts with the silicon–oxygen bond further, which causes the glass to dissolve as shown in reaction (2): B Si A O A Si B + OH !B SiOH + B SiO
ð2Þ
This reaction proceeds after the first reaction (1) happens, so the strong bond, „Si–O–Si„ breaks. One disconnecting end combines with hydroxyl while the other end, „SiO, combines with water as shown in reaction (3): B SiO + H2 O !B SiOH + OH
ð3Þ
The corrosion of the glass surface can reach several hun˚ in depth [34]. The functional additives added here dred A can promote the hydrolysis of glass. The hydrolysis of glass may form intenerated film of silicic acid gel in the surface. Then the hydrolyzed film is easy to be worn away by a-Al2O3/PMAA core/shell abrasives particles since CMP is a machining based on direct contact under applied pressure. The protruding region is removed faster than the recessed region [35], resulting in global planarization for the glass substrate surface.
Z. Zhang, H. Lei / Microelectronic Engineering 85 (2008) 714–720
Removal Rate/mg.min-1
a
The real CMP mechanism may be very complicated. More works need to be done on these in the future.
1.6 1.5
4. Conclusions
1.4 1.3 1.2 1.1 1.0
30
60
90
120
150
180
Polishing Time/min
b 200 180 160
Ra/nm
719
Novel a-Al2O3/PMAA core-shell abrasives were prepared via a two-step process: surface activation followed by graft polymerization and characterized by means of laser Mastersizer 2000, FTIR and SEM. The results indicate that modified a-Al2O3 has better dispersion stability than pure a-Al2O3. After the effect of different process parameters on the material removal rate and the surface average roughness in chemical mechanical polishing (CMP) of the glass substrate with the novel abrasives was examined. The optimum process parameters have been obtained considering the material removal rate and the surface average roughness as follows: 15 rpm/min, 146 kg, 60 min. The prepared abrasives have better glass substrate CMP performance than pure a-Al2O3 abrasives under the same polishing conditions. Finally, the CMP mechanism of glass substrate was deduced.
140 120
Acknowledgements
100
The work was supported by National Natural Science Foundation of China (Grant No. 50575131), Natural Science Foundation of Shanghai (Grant No. 07ZR14039) and Key subject of Shanghai Municipal Education Commission (Grant No. J50102).
80 60 20
40
60
80
100
120
140
160
180
Polishing Time/min Fig. 6. Effect of polishing time on (a) MRR and (b) Ra (rotating speed = 15 rpm/min; down force = 146 kg).
Table 1 Optimum process parameters obtained by the present study Process parameters
Optimum value
Rotating speed Down force Polishing time
15 rpm/min 146 kg 60 min
Table 2 The MRR and Ra of glass substrate polished by pure alumina and Al2O3g-PMAA composite abrasives with the optimum process parameters Type of abrasives
Removal rate (mg/min)
Surface average roughness (nm)
Pure alumina abrasives Al2O3-g-PMAA core/shell abrasives
2.87 1.42
70 59
Meanwhile, the two effects may be simultaneous and interactional. An optimal combination of the two effects in CMP may dominate the final surface quality [36]. Nevertheless, an online examination of the physical and chemical changes during the polishing process is lacked.
References [1] W.J. Patrick, W.L. Guthrie, C.L. Standley, J. Electrochem. Soc. 138 (6) (1991) 1778–1784. [2] J.M. Steiferwald, S.P. Muraaka, R.J. Getmann, Chemical Mechanical Planarization of Microelectronic Materials, Wiley, New York, 1997. [3] Lei Hong, Lu Haishen, Sub-nanometer precesion polishing of glass substrate, in: 7th International Conference on Frontiers of Design and Manufacturing, V1, pp. 445–448. [4] Lei Hong, Zhang Pengzhen, Lu Haishen, Lubr. Eng. 1 (2006) 31–34. [5] Liangyong Wang, Kailiang Zhang, Zhitang Song, Songlin Feng, Appl. Surf. Sci. 253 (2007) 4951–4954. [6] A. Sorooshian, R. Ashwani, H.K. Choi, Effect of particle interaction on agglomeration of silica-based CMP slurries, in: Materials Research Society Symposium Proceedings 816, 2004, pp. 125–131. [7] H. Lei, P.Z. Zhang, Appl. Surf. Sci. 253 (2007) 8754–8761. [8] R.K. Singh, R. Bajaj, MRS Bull. 27 (10) (2002) 743. [9] H. Yano, Y. Matsui, G. Minamihaba, et al., Mater. Res. Soc. Symp. Proc. 671 (2001) M5.1. [10] H. Shiho, Y. Manabe, N. Kawahashi, J. Mater. Chem. 10 (2000) 333. [11] Kawahashi Nobuo, Hattori Masayuki, Mater. Res. Soc. Symp. Proc. 671 (2001) M2.2.1–M2.2.8. [12] Lei Hong, Zhu Yuliang, et al., J. Funct. Mater. 36 (9) (2005) 1425– 1428, in Chinese. [13] H. Lei, J.B. Luo, X.C. Lu, Chin. J. Mech. Eng. (English Edition) 19 (4) (2006) 496–499. [14] Nam-Hoon Kim, Min-Ho Choi, Sang-Yong Kim, Eui-Goo Chang, Microelectron. Eng. 83 (2006) 506–512. [15] Sung-Woo Park, Chul-Bok Kim, Sang-Yong Kim, Yong-Jin Seo, Microelectron. Eng. 66 (2003) 488–495.
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[16] Gwon-Woo Choi, Kang-Yeon Lee, Nam-Hoon Kim, Jin-Seong Park, Yong-Jin Seo, Woo-Sun Lee, Microelectron. Eng. 83 (2006) 2213–2217. [17] Shih-Chieh Lin, Meng-Long Wu, Int. J. Mach. Tool Manuf. 42 (2002) 99–103. [18] Woong Cho, Yoomin Ahn, Chang-Wook Baek, Yong-Kweon Kim, Microelectron. Eng. 65 (2003) 13–23. [19] Z. Stavreva, D. Zeidler, M. P16tner, K. Drescher, Microelectron. Eng. 37/38 (1997) 143–149. [20] Y.-J. Seo et al., Microelectron. Eng. (2007), doi:10.1016/ j.mee.2007.03.005. [21] Markus Forsberg, Microelectron. Eng. 77 (2005) 319–326. [22] Wen Shizhu, Tribology, Tsinghua University Press, Beijing, 1990 (pp. 408 (in Chinese)). [23] A.A. Torrance, Wear 258 (2005) 281–293. [24] D.V. De Pellegrin, G.W. Stachowiak, Wear 258 (2005) 208–216.
[25] N.B. Kirk, J.V. Wood, Br. Ceram. Trans. 93 (1) (1994) 25– 30. [26] Robert Sabia, Harrie J. Stevens, James R. Varner, J. Non-cryst. Solids 249 (1999) 123–130. [27] Hong Lei, Jianbin Luo, Wear 257 (2004) 461–470. [28] J.F. Luo, D.A. Dornfeld, IEEE Trans. Semicond. Manuf. 2 (2001) 112–133. [29] J. Larsen-Basse, H. Liang, Wear 233–235 (1999) 647–654. [30] H. Liang, F. Kaufman, R. Sevilla, Wear 211 (1997) 271–279. [31] A. Kaller, On the Polishing of Glass, particularly the Precision. Polishing of Optical Surfaces, Glasstech. Ber., 1991, 9, 241. [32] T. Izumitani, S. Harada, Glass Technol. 12 (1971) 131. [33] R.J. Charles, J. Appl. Phys. 29 (11) (1958) 1549–1553. [34] L.M. Cook, J. Non-cryst. Solids 120 (1990) 152–171. [35] D.E. Clark, Surf. Sci. 100 (1980) 53–70. [36] H. liang, G.H. Xu, Scripta Mater. 46 (2002) 343–347.
Microelectronic Engineering 85 (2008) 714–720 www.elsevier.com/locate/mee
Preparation of a-alumina/polymethacrylic acid composite abrasive and its CMP performance on glass substrate Zefang Zhang a,b, Hong Lei a,* a
Research Center of Nanoscience and Nanotechnology, Shanghai University, Shanghai 200444, China b School of Sciences, Shanghai University, Shanghai 200444, China Received 26 June 2007; accepted 3 January 2008 Available online 16 January 2008
Abstract Chemical mechanic polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of key elements during CMP process. a-Alumina particles, as a kind of widely used abrasive in CMP slurries, often cause surface defects due to its high hardness and agglomeration. In order to enhance the dispersion stability and prevent agglomeration of pure alumina abrasives in CMP, a-alumina grafted with polymethacrylic acid (a-Al2O3-g-PMAA) composite abrasives was prepared and characterized by means of Fourier transform infrared (FTIR), Mastersizer 2000 instrument and scanning electron microscope (SEM). The results indicated that the prepared a-Al2O3-g-PMAA composite abrasives have better dispersion stability than pure a-Al2O3 abrasives. The CMP performances of the a-Al2O3-g-PMAA composite abrasives on glass substrate were investigated by SPEEDFAM-16B-4M CMP equipment with different process parameters. By using the optimum process parameters, the prepared abrasives exhibit better glass substrate CMP performance than pure a-Al2O3 abrasives. Further, the CMP mechanism of glass substrate was deduced preliminarily. Ó 2008 Elsevier B.V. All rights reserved. Keywords: CMP; a-Al2O3-g-PMAA composite abrasive; Glass substrate
1. Introduction Chemical mechanical polishing (CMP) has become a more essential technique in manufacturing of semiconductor and digital compact disc (CD) glass substrate [1–5]. There are up to 16 variables that needed to be controlled to achieve a stable process, and one of the major variables is related to slurry compositions [6]. As the basic element of slurry, i.e., the hardness, sizes, and chemical properties of abrasives are crucial for a desired CMP performance. In CMP slurry, abrasives can be classified as two types: the traditional inorganic particles and the composite particles [7]. Traditional inorganic abrasives, such as silica, alumina, ceria, have been widely studied [3–6] and used in the commercial slurries, but just one kind of inorganic abra*
Corresponding author. E-mail address: [email protected] (H. Lei).
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.001
sives used in slurries often leads to undesired CMP performance. Recently, with the increasing demand of improving the polishing performance while minimizing roughness and defects of the polished surface, composite particles as abrasives in slurry have been paid much attentions [7–11]. It is thought that the coating on the surface of particles with harder substance can improve the removal rate while keeping the density of defects constant, however the hard particles coated with a softer material can reduce CMP defects [8]. Yano et al. [9] developed a kind of slurry with inorganic/resin abrasive for the Al/low k damascene wiring CMP. The slurry resulted in less scratching and better planarity. In SiO2 CMP, it was found that composite abrasive slurry exhibited the reduction of the dishing and erosion depth, which was reduced to less than 80 nm after first step CMP and less than 70 nm after second step CMP [10]. Lei and Zhang [7] developed a novel kind of slurry containing
Z. Zhang, H. Lei / Microelectronic Engineering 85 (2008) 714–720
a-alumina/silica core-shell abrasives for hard disk substrate. The slurry resulted in lower surface roughness and waviness as well as lower topographical variations and less scratch than that containing pure alumina abrasive under the same testing conditions. At present, a-alumina particle as a kindly of widely used abrasives has been used in CMP [12,13], but its high hardness and agglomeration often leads to more surface defects. With advanced electro-manufacture developing so fast, glass substrate, as a kind of widely used material in the home electronic products such as VCD, DVD, CVD, HDVD, etc., is forced to ultra-smooth. Although there have been some studies [7,8–10] on composite abrasives, in which almost no study is involved in glass substrate CMP. In the present study, a novel a-Al2O3-g-PMAA composite abrasive was prepared and its CMP performances on glass substrate have been studied. 2. Experimental 2.1. Preparation of a-Al2O3-g-PMAA composite abrasives and their its based slurry Preparation of a-Al2O3-g-PMAA composite abrasives was carried out via a two-step process: surface activation of a-Al2O3 with c-methacryloxypropyl trimethoxy silane coupling agent (KH570) in toluene followed by graft polymerization of methacrylic acid (MAA) in water with ammonium persulfate (APS) initiator. 2.1.1. Silylation Prior to silylation, the a-alumina particles were dried in an oven at 110 °C under vacuum for 10 h in order to get rid of the physically absorbed and weakly chemically absorbed species. Subsequently, 5 g dried alumina particles and 2 g KH570 in 50 ml were charged into a 150 ml flask equipped with a reflux condenser. The mixture was refluxed at the boiling temperature for 5 h under vacuum. The silylated alumina particles were centrifuged, and the precipitate was extracted with toluene solvent for 8 h to remove adsorbed silane molecules, followed by drying at 80 °C in a vacuum oven for 12 h to remove the remaining solvent. 2.1.2. Graft polymerization Free-radical graft polymerization of MAA onto KH570modified alumina particles was carried out in a flask equipped with a condenser at 80 °C under vacuum. Firstly, 5 g silylated alumina and 100 g deionized (DI) water were put in the temperature-controlled reactor with stirring. When the reactant reached the desired temperature, 0.04 g APS was charged, and then 4 g MAA was added after 20 min. After the graft polymerization, the resultant suspension was centrifuged, and the precipitate was extracted with DI water for 8 h to remove the absorbed polymers. Then the grafted alumina particles were dried under vacuum at 80 °C. The purified a-Al2O3-g-PMAA
715
composite particles were characterized by FTIR, SEM and Mastersizer 2000 instrument. 2.1.3. Preparation of the slurry 2.5 wt.% composite particles and 1 wt.% functional additives were added to DI water in a container under continuously stirring. Then the mixture was milled for 2 h in a vibrator containing ZrO2 balls as abrasives. Finally, the mixture was filtrated with a 20 lm pore strainer. 2.2. Polishing tests Polishing tests were conducted with a SPEEDFAM16B-4 M CMP equipment (SPEEDFAM Co. LTD, Japan). The down force was varied between 106 kg and 166 kg; the rotating speed was varied between 9 rpm/min and 18 rpm/ min and the polishing time varied from 10 min to 180 min. The slurry supplying rate was 1 L/min. The samples were U 160 mm sodium–calcium glass substrates. The surface average roughness (Ra) of disk substrates before polishing is about 500 nm. The polishing pad was a Rodel porous polyurethane pad. After polishing, the substrates were washed with ultrasonic in a cleaning solution containing 0.5 wt.% surfactant in DI water. Finally, they were dried by a multi-functional drying system. 2.3. Examination of the polished surfaces The polished surface features, the surface average roughness (Ra), was measured to evaluate the polishing effects in different slurries. Ra was measured by using a Zygo surface profiler (Zygo Corp., France) with the resolu˚ . The measuring area was 0.357 mm tion of 0.1 A 0.268 mm. The mass of glass substrate was measured by using an analytical balance. The difference of mass before and after polishing divided by the polishing time gives material removal rate. All the data were the mean values of six times. 3. Results and discussion 3.1. Structure and dispersibility of a-Al2O3-g-PMAA composite particles Fig. 1 shows the FTIR spectra of pure alumina particles, silylated alumina particles and PMAA-grafted alumina particles. By comparison with pure alumina, a series of absorption peaks of silylated alumina particles at 1659 cm1, 1732 cm1, 2844 cm1 and 2917 cm1 which belong to C@C, C@O, –CH3, respectively show that pure alumina particles were silylated successfully. For PMAAgrafted alumina, there is a strong absorption peak of C@O in PMAA at 1707 cm1, which suggests that the PMAA were successfully grafted onto the surface of alumina particles through covalent bonding.
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Transmittance/%
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4000
3500
3000
2500
2000
1500
1000
Wavenumber/cm-1 Fig. 1. FTIR spectra of pure alumina particles (a), silylated alumina particles (b) and PMAA-grafted alumina particles (c).
In order to analyze the dispersibility of alumina particles before and after surface modification, the morphology of the alumina particles before and after surface modification was observed by SEM as shown in Fig. 2. It is found that the surface modified alumina particles have better dispersibility, while obvious agglomeration was observed for pure alumina particles. And it is also noticed that, by comparison with the sharp appearance of pure alumina particles, the surface modified alumina particles seem to become round. In addition, the size difference of alumina particles before and after surface modification as shown in Fig. 3, median diameter (d50) of pure alumina is 3.107 lm and aAl2O3/PMAA is 0.780 lm, which have further suggested that surface modified alumina particles have a better dispersibility. It may be explained by the grafted PMAA have prevented the agglomeration of alumina particles, which resulted that the size of surface modified alumina particles become smaller. 3.2. CMP performance of slurry containing a-Al2O3-g-PMAA composite particles A better understanding on the effect of polishing process parameters on the CMP is very important. Although there have been a lot of studies [14–20] about the effects of process parameters on CMP performance, in which almost no study in involved in CMP of glass substrate. So, the effects of polishing parameters such as polishing time, down force and rotation speed on material removal rate (MRR) and surface average roughness (Ra) was studied in glass substrate CMP with these prepared Al2O3-g-PMAA composite abrasives. 3.2.1. Effect of different process parameters on MRR and Ra 3.2.1.1. Rotating speed. Fig. 4. shows MRR and Ra of glass substrate with the increase of the rotating speed. The removal rate of glass substrate by change of the rotating
Fig. 2. SEM images of alumina particles before (a) and after (b) surface modification.
speed showed the liner tendency of steep increase, which was caused by the increasing chance of friction between glass substrate and polishing pad as well as abrasives. The highest removal rate of 1.68 mg/min was obtained in 18 rpm/min but the lowest Ra was obtained in 15 rpm/ min. Therefore, the optimum rotating speed of glass substrate CMP was 15 r/nin considering both the material removal rate and the surface average roughness. 3.2.1.2. Down force. Fig. 5 shows the relationship of MRR and Ra of glass substrate as a function of down force. As the down force increases, the material removal rate increases linearly up to a critical point at 146 kg. Conversely, the surface average roughness decreases linearly up to a critical point at 146 kg. We conclude that as the down force increases, the removal rate increases and the surface average roughness is improved. With the down
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Fig. 3. Size distribution of alumina particles before (a) and after (b) surface modification.
force of 146 kg, the relatively high removal rate of 1.42 mg/ min and the relatively low surface average roughness of 59 nm were obtained. 3.2.1.3. Polishing time. Fig. 6 shows MRR and Ra of glass substrate according to polishing time without any change of other process parameters. The material removal rate by change of the polishing time showed the linear tendency of steep decrease. This estimates that the surface of substrate has many rough peaks in initial polishing process which were firstly removed in the polishing process. With the increasing of polishing time, the number of rough peaks decreased, which induce the surface roughness to decrease as shown in the Fig. 6(b). But the surface average roughness almost shows no difference from 60 min to 180 min. In other words, polishing time of 60 min is enough for the prepared abrasives. Prolonging the polishing time cannot improve surface quality any more. Table 1 summarizes the optimum process parameters obtained by the present study. 3.2.2. Different CMP performance between pure alumina and a-Al2O3-g-PMAA composite abrasives The MRR and Ra of glass substrate polished by pure alumina abrasives and Al2O3-g-PMAA core/shell abrasives with the optimum process parameters were given in Table 2. It is found that the introduction of PMAA on the surface
of alumina particles decrease the MRR and Ra. The lower Ra value is, the higher surface planarization has been achieved. In other words, the prepared Al2O3-g-PMAA core/shell abrasives have higher surface planarization than pure alumina abrasives. CMP is a complex erosive wear process. As the supplier of mechanical effect, abrasive wear accounts for main material removal process. Wear volume (V) of abrasive wear may be described as following equation [21]: V ¼ kW =H s ; where k is wear coefficient, W is applied load and Hs is hardness of polished surface. k is determined by abrasive properties, such as hardness, shape and size of abrasive. The harder, sharper and larger abrasive is, the higher wear loss will be [21–24]. The reduction in MRR of the a-Al2O3-g-PMAA composite abrasives can be attributed to the reduction of the hardness of the alumina particles (since PMAA coating is relatively softer thana-Al2O3 core) as well as their smoother shape and smaller particles. 3.3. CMP mechanism of glass substrate Glass polishing in the prepared a-Al2O3/PMAA core/ shell abrasives based slurry is a chemical mechanical polishing process. CMP process is regarded as a combination
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a
a
1.8
1.4
Removal Rate/mg.min-1
Removal Rate/mg.min-1
1.6 1.4 1.2 1.0 0.8
1.2
1.0
0.8
0.6 8
10
12
14
16
18
100
Rotating Speed/r.min-1
b
120
140
160
Down Force/kg
b 110
200
100
90
Ra/nm
Ra/nm
160
120
80
70
80
60 8
10
12
14
16
18 100
Rotating Speed/r.min-1
140
160
Down Force/kg
Fig. 4. Effect of rotating speed on (a) MRR and (b) Ra (down force = 146 kg; polishing time = 60 min).
of chemical effect and mechanical effect [25–29]. Although there is still lack of a model that is able to describe the entire available CMP process, erosion and abrasive wear may be agreed to be two basic effects during the material removal process. The prepared CMP slurry consists of abrasive, functional additives and water. Abrasives and water containing chemical additives may provide mechanical grinding and chemical erosion effects, respectively. The two effects in CMP process can be described as follows. At first, a glass surface can be hydrolyzed in the water [30– 33]. The glass substrate polished in the experiment is sodium–calcium glass. The structure of sodium–calcium glass is classified into two kinds, one is the continuous network, „Si–O–Si„, and the other is the collapsing structure, „Si–O–Na (or terminal structure). The terminal structure of the glass surface reacts with water as shown in reaction (1): B Si A O A Na + H2 O !B SiOH + Naþ + OH
120
ð1Þ
The sodium or calcium bond near the interface breaks because of transference of a sodium ion. Therefore, the oxygen combines with a hydrogen from H2O in order to
Fig. 5. Effect of down force on (a) MRR and (b) Ra (rotating speed = 15 rpm/min; polishing time = 60 min).
meet its energy field, and forms „SiOH. The free OH ion interacts with the silicon–oxygen bond further, which causes the glass to dissolve as shown in reaction (2): B Si A O A Si B + OH !B SiOH + B SiO
ð2Þ
This reaction proceeds after the first reaction (1) happens, so the strong bond, „Si–O–Si„ breaks. One disconnecting end combines with hydroxyl while the other end, „SiO, combines with water as shown in reaction (3): B SiO + H2 O !B SiOH + OH
ð3Þ
The corrosion of the glass surface can reach several hun˚ in depth [34]. The functional additives added here dred A can promote the hydrolysis of glass. The hydrolysis of glass may form intenerated film of silicic acid gel in the surface. Then the hydrolyzed film is easy to be worn away by a-Al2O3/PMAA core/shell abrasives particles since CMP is a machining based on direct contact under applied pressure. The protruding region is removed faster than the recessed region [35], resulting in global planarization for the glass substrate surface.
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Removal Rate/mg.min-1
a
The real CMP mechanism may be very complicated. More works need to be done on these in the future.
1.6 1.5
4. Conclusions
1.4 1.3 1.2 1.1 1.0
30
60
90
120
150
180
Polishing Time/min
b 200 180 160
Ra/nm
719
Novel a-Al2O3/PMAA core-shell abrasives were prepared via a two-step process: surface activation followed by graft polymerization and characterized by means of laser Mastersizer 2000, FTIR and SEM. The results indicate that modified a-Al2O3 has better dispersion stability than pure a-Al2O3. After the effect of different process parameters on the material removal rate and the surface average roughness in chemical mechanical polishing (CMP) of the glass substrate with the novel abrasives was examined. The optimum process parameters have been obtained considering the material removal rate and the surface average roughness as follows: 15 rpm/min, 146 kg, 60 min. The prepared abrasives have better glass substrate CMP performance than pure a-Al2O3 abrasives under the same polishing conditions. Finally, the CMP mechanism of glass substrate was deduced.
140 120
Acknowledgements
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The work was supported by National Natural Science Foundation of China (Grant No. 50575131), Natural Science Foundation of Shanghai (Grant No. 07ZR14039) and Key subject of Shanghai Municipal Education Commission (Grant No. J50102).
80 60 20
40
60
80
100
120
140
160
180
Polishing Time/min Fig. 6. Effect of polishing time on (a) MRR and (b) Ra (rotating speed = 15 rpm/min; down force = 146 kg).
Table 1 Optimum process parameters obtained by the present study Process parameters
Optimum value
Rotating speed Down force Polishing time
15 rpm/min 146 kg 60 min
Table 2 The MRR and Ra of glass substrate polished by pure alumina and Al2O3g-PMAA composite abrasives with the optimum process parameters Type of abrasives
Removal rate (mg/min)
Surface average roughness (nm)
Pure alumina abrasives Al2O3-g-PMAA core/shell abrasives
2.87 1.42
70 59
Meanwhile, the two effects may be simultaneous and interactional. An optimal combination of the two effects in CMP may dominate the final surface quality [36]. Nevertheless, an online examination of the physical and chemical changes during the polishing process is lacked.
References [1] W.J. Patrick, W.L. Guthrie, C.L. Standley, J. Electrochem. Soc. 138 (6) (1991) 1778–1784. [2] J.M. Steiferwald, S.P. Muraaka, R.J. Getmann, Chemical Mechanical Planarization of Microelectronic Materials, Wiley, New York, 1997. [3] Lei Hong, Lu Haishen, Sub-nanometer precesion polishing of glass substrate, in: 7th International Conference on Frontiers of Design and Manufacturing, V1, pp. 445–448. [4] Lei Hong, Zhang Pengzhen, Lu Haishen, Lubr. Eng. 1 (2006) 31–34. [5] Liangyong Wang, Kailiang Zhang, Zhitang Song, Songlin Feng, Appl. Surf. Sci. 253 (2007) 4951–4954. [6] A. Sorooshian, R. Ashwani, H.K. Choi, Effect of particle interaction on agglomeration of silica-based CMP slurries, in: Materials Research Society Symposium Proceedings 816, 2004, pp. 125–131. [7] H. Lei, P.Z. Zhang, Appl. Surf. Sci. 253 (2007) 8754–8761. [8] R.K. Singh, R. Bajaj, MRS Bull. 27 (10) (2002) 743. [9] H. Yano, Y. Matsui, G. Minamihaba, et al., Mater. Res. Soc. Symp. Proc. 671 (2001) M5.1. [10] H. Shiho, Y. Manabe, N. Kawahashi, J. Mater. Chem. 10 (2000) 333. [11] Kawahashi Nobuo, Hattori Masayuki, Mater. Res. Soc. Symp. Proc. 671 (2001) M2.2.1–M2.2.8. [12] Lei Hong, Zhu Yuliang, et al., J. Funct. Mater. 36 (9) (2005) 1425– 1428, in Chinese. [13] H. Lei, J.B. Luo, X.C. Lu, Chin. J. Mech. Eng. (English Edition) 19 (4) (2006) 496–499. [14] Nam-Hoon Kim, Min-Ho Choi, Sang-Yong Kim, Eui-Goo Chang, Microelectron. Eng. 83 (2006) 506–512. [15] Sung-Woo Park, Chul-Bok Kim, Sang-Yong Kim, Yong-Jin Seo, Microelectron. Eng. 66 (2003) 488–495.
720
Z. Zhang, H. Lei / Microelectronic Engineering 85 (2008) 714–720
[16] Gwon-Woo Choi, Kang-Yeon Lee, Nam-Hoon Kim, Jin-Seong Park, Yong-Jin Seo, Woo-Sun Lee, Microelectron. Eng. 83 (2006) 2213–2217. [17] Shih-Chieh Lin, Meng-Long Wu, Int. J. Mach. Tool Manuf. 42 (2002) 99–103. [18] Woong Cho, Yoomin Ahn, Chang-Wook Baek, Yong-Kweon Kim, Microelectron. Eng. 65 (2003) 13–23. [19] Z. Stavreva, D. Zeidler, M. P16tner, K. Drescher, Microelectron. Eng. 37/38 (1997) 143–149. [20] Y.-J. Seo et al., Microelectron. Eng. (2007), doi:10.1016/ j.mee.2007.03.005. [21] Markus Forsberg, Microelectron. Eng. 77 (2005) 319–326. [22] Wen Shizhu, Tribology, Tsinghua University Press, Beijing, 1990 (pp. 408 (in Chinese)). [23] A.A. Torrance, Wear 258 (2005) 281–293. [24] D.V. De Pellegrin, G.W. Stachowiak, Wear 258 (2005) 208–216.
[25] N.B. Kirk, J.V. Wood, Br. Ceram. Trans. 93 (1) (1994) 25– 30. [26] Robert Sabia, Harrie J. Stevens, James R. Varner, J. Non-cryst. Solids 249 (1999) 123–130. [27] Hong Lei, Jianbin Luo, Wear 257 (2004) 461–470. [28] J.F. Luo, D.A. Dornfeld, IEEE Trans. Semicond. Manuf. 2 (2001) 112–133. [29] J. Larsen-Basse, H. Liang, Wear 233–235 (1999) 647–654. [30] H. Liang, F. Kaufman, R. Sevilla, Wear 211 (1997) 271–279. [31] A. Kaller, On the Polishing of Glass, particularly the Precision. Polishing of Optical Surfaces, Glasstech. Ber., 1991, 9, 241. [32] T. Izumitani, S. Harada, Glass Technol. 12 (1971) 131. [33] R.J. Charles, J. Appl. Phys. 29 (11) (1958) 1549–1553. [34] L.M. Cook, J. Non-cryst. Solids 120 (1990) 152–171. [35] D.E. Clark, Surf. Sci. 100 (1980) 53–70. [36] H. liang, G.H. Xu, Scripta Mater. 46 (2002) 343–347.