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Preparation of La-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates
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Precision Engineering xxx (2015) xxx–xxx
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Preparation of La-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates Hong Lei a,b,∗ , Kaiyu Tong a a b
School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China Research Center of Nano-science and Nano-technology, Shanghai University, Shanghai 200444, China
a r t i c l e
i n f o
Article history: Received 26 June 2015 Received in revised form 21 October 2015 Accepted 31 October 2015 Available online xxx Keywords: CMP SiO2 composite abrasives Sapphire Solid-state reaction
a b s t r a c t Chemical mechanical polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of the key elements in CMP process. In order to enhance removal rate and improve surface quality of sapphire substrate, a series of novel La-doped colloidal SiO2 composite abrasives were prepared by seed-induced growth method. The CMP performance of the La-doped colloidal SiO2 composite abrasives on sapphire substrate were investigated using UNIPOL-1502 polishing equipment. The analyses on the surface of polished sapphire substrate indicate that slurries containing the La-doped colloidal SiO2 composite abrasives achieve lower surface roughness, higher material removal rate than that of pure colloidal SiO2 abrasive under the same testing conditions. Furthermore, the acting mechanism of the La-doped colloidal silica in sapphire CMP was investigated. X-ray photoelectron spectroscopy analysis shows that solid-state chemical reactions between La-doped colloidal SiO2 abrasive and sapphire surface occur during CMP process, which can promote the chemical effect in CMP and lead to the improvement of material removal rate. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Sapphire, single crystal form of alpha-alumina, is widely used in a variety of modern high-technology applications for its excellent optical, chemical and mechanical properties such as high hardness, great thermal stability, chemical inertness, especially as a substrate material in the photoelectronic, microelectronic and semiconductor industries, ranging from optical windows, read/write laser sources to GaN-based light emitting diodes (LEDs), etc [1–6]. In all of these applications, the surface flatness of sapphire substrate is a key factor that influences its performances. Undoubtedly, its planarization machining is very important [7]. However, the intrinsic nature of sapphire (high hardness and chemical inertness) poses great challenges to such machining. In order to achieve high surface quality, several polishing technologies are put forward by researchers [8]. At present, chemical mechanical polishing (CMP) is the only widely used global planarization technology in manufacturing of semiconductor and sapphire substrates [9–12]. In CMP, abrasives are one of the key influencing factors on the polished surface quality. The sizes and distribution,
∗ Corresponding author at: Shanghai Institute of Technology, School of Materials Science and Engineering, Shanghai 201418, China. Tel.: +86 13331906218; fax: +86 21 66137104. E-mail address: hong [email protected] (H. Lei).
dispersibility, hardness and species of abrasive are crucial for a desired CMP performance [13]. In order to improve surface planarization and material removing rate (MRR) of sapphire, several polishing abrasives have been studied. Zhu et al. [14] have studied the effect of abrasive hardness on the CMP of (0 0 0 1) plane sapphire. They found that, hard abrasives (such as monocrystalline and polycrystalline diamond, ␣-Al2 O3 and ␥-Al2 O3 ) can improve MRR of sapphire, but arithmetic average surface roughness (Ra ) is higher. Nevertheless, using silica sol as polishing abrasives for sapphire can get lower Ra , but MRR of sapphire is lower. Hu et al. [15] have researched the CMP of sapphire wafers with boron carbide and colloidal silica abrasives. They found that, B4 C abrasive can gain a higher removal rate in sapphire wafer, but the colloidal silica can achieve a nanoscale flatness of sapphire surface. Xiong et al. [16] adopted silicon carbide, alumina and silica sol as polishing abrasives for CMP of sapphire. The results show that, under the same condition, Ra of sapphire polished with silica sol abrasive is lower than that with silicon carbide and alumina abrasives. In general, silica sol is widely regarded as ideal polishing abrasive for sapphire CMP since it can get better surface quality [17,18]. However, MRR of sapphire with colloidal silica as polishing abrasive is still lower and needs to be improved. Currently, silica-based composite particles as CMP abrasives capture increasing attention, and chemical modifying of silica abrasive has been proved to be an effective way to improving its polishing performances in CMP. Zhao [19] synthesized SiO2 /CeO2
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Please cite this article in press as: Lei H, Tong K. Preparation of La-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.10.009
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Fig. 1. SEM micrographs of La-doped colloidal SiO2 composite abrasives with different La(OH)3 contents (a) 0 wt.% of La(OH)3 , (b) 0.5 wt.% of La(OH)3 , (c) 1.0 wt.% of La(OH)3 , (d) 1.5 wt.% of La(OH)3 .
abrasive for CMP of oxide wafer and Lei [20] developed porous Fe2 O3 /SiO2 abrasive for CMP of hard disk substrate. Zhang et al. [21] prepared silica composite abrasive by coating with a layer of polystyrene. Xu et al. [22] reported the slurry of colloidal silicon dioxide (SiO2 ) containing Fe–N-x/C, which exhibits higher MRR than that containing pure SiO2 abrasive in the CMP of sapphire substrates under the same conditions. In a word, the silica-based composite abrasives can result in both high planarization efficiency and good planarization quality. Especially, rare earth compounds including cerium are often used as polishing abrasive and exhibit good polishing performances [19]. As rare earth elements (REE), lanthanum and cerium have many similar natures and lanthanum has high catalytic activity for many chemical reactions [23–25]. Therefore, lanthanum-modified silica sol may be expected to have better polishing performances. In this paper, La-doped colloidal SiO2 composite abrasives were prepared and their CMP performances on sapphire substrates were investigated.
colloidal SiO2 composite abrasive solution was obtained after the end of reaction. In above synthesis process of composite abrasives, the lanthanum was doped into colloidal SiO2 abrasives in the form of lanthanum hydroxide since the reaction liquid is alkali, where the reaction La3+ + 3OH− → La(OH)3 happened at pH of 9 to 10. By adding different amounts of 0.23 wt.% La(NO3 )3 solution versus crystal silica seed solution, La-doped colloidal SiO2 composite abrasives containing 0.5 wt.%, 1.0 wt.% and 1.5 wt.% La(OH)3 (mass ratios La(OH)3 of vs SiO2 ) were obtained. The pure colloidal SiO2 abrasive was prepared by the same method mentioned above except adding La(NO3 )3 solution. Preparation of polishing slurries: Before polishing, 10 wt.% prepared colloidal solutions were diluted to solid concentration of 6 wt.%, then 3.0 wt.% sodium hexametaphosphate as dispersant were added into the solution. Finally, the above-mentioned mixtures were filtrated with a 10 m pore filter to obtain polishing slurries.
2. Experimental details 2.1. Preparation of La-doped colloidal SiO2 composite abrasives and polishing slurries Synthesis of La-doped colloidal SiO2 abrasives with different La(OH)3 contents: 10 wt.% crystal silica seed solution were heated to 100 ◦ C in a four-neck flask. A certain amount of 2.5 wt.% active silicic acid solution, 0.23 wt.% La(NO3 )3 solution and 0.5 wt.% NaOH solution were simultaneously added in drops into the crystal silica seed solution in the four-neck flask under continuously stirring. The liquid volume in the four-neck flask was kept constant by controlling evaporating speed of water and feeding speed of the solutions. The reaction liquid was keeping at pH of 9.0–11.0 by controlling the dropping velocity of NaOH. The seed-induced growth was carried out at 100 ◦ C for 7 h at under continuously stirring, and then cooled to room temperature. The solid concentration of 10 wt.%
2.2. Characterization of La-doped colloidal SiO2 composite abrasives The composite abrasives were characterized by means of scanning electron microscopy (SEM), time of flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). Elementary analysis of the sample which is processed solid SiO2 composite abrasives was measured by Model 2100 Trift II timeof-flight secondary ion mass spectroscopy. The size, shape and distribution of composite abrasives were characterized by SEM, and SEM analyses were accomplished using a JEOL JSM-6700F field emission scanning electron microscope with voltage of 10 kV. The element type of composite abrasives after polishing were analyzed by XPS, and XPS spectrum was obtained by ESCALAB 250Xi electron spectrometer using a focused monochromatized Al K␣ radiation
Please cite this article in press as: Lei H, Tong K. Preparation of La-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.10.009
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Fig. 2. TOF-SIMS spectra of pure colloidal SiO2 abrasive (a) and La-doped colloidal SiO2 abrasive with 1.5 wt.% La(OH)3 (b). Table 1 The experiment data error analysis of three polishing tests. The mass of La(OH)3 (wt.%)
0
0.5
1
1.5
1 2 3 Mean Standard deviation
0.446 0.439 0.408 0.431 0.0202
0.536 0.578 0.584 0.566 0.0262
0.674 0.665 0.651 0.663 0.0112
0.683 0.692 0.668 0.681 0.0121
(h = 1486.6 eV). The binding energy of C 1s (285.44 eV) was used as reference.
2.3. Polishing test
Fig. 3. Effect of the mass content of La(OH)3 on material removal rate. (Red line is error bar that is ±1.96 standard deviation and represent a description of 95% confident of which the mean represents the true MRR value.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Polishing tests were conducted by using a UNIPOL-1502 polishing equipment (Shenyang Kejing instrument, Co. LTD, China). The polishing pad was a DPC5150 porous polyurethane pad (Rodel, U.S.A). The slurries supplying rate is 180 ml/min. The parameters of polishing were down force of 6 kg, plate rotating speed of 60 rpm and polishing time of 2 h. Work pieces were ϕ50.8 mm × 0.45 mm ␣-Al2 O3 sapphire substrates with c (0 0 0 1) orientation. The Ra and of the sapphire substrates are about 2.5 nm. After polishing, the sapphire substrates were washed with ultrasonic in a cleaning solution
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containing 0.5 wt.% surfactant in deionized (DI) water. Finally, they were dried by a multifunctional drying system. The Ra was measured by an Ambios XI-100 Plus surface profiler (Ambios Technology Corp., U.S.A) with the resolution of 0.1 nm, and the measuring area of 500 m × 500 m. The depth of focus is 3.0 m, the working distance is 7.4 mm, and the numerical aperture is 0.30. The mass of the sapphire substrate was measured by a AL104 analytical balance with accuracy of 0.1 mg (METTLER TOLEDO, Switzerland). The material removal rate was calculated by the following formula: MRR =
m × 106 R2
(2-1)
where MRR—material removal rate (m/h), m-mass of material removed (g), R—radius of sapphire substrate (mm), —polishing time (h) (in the test, = 2 h), —density (g/cm3 ) (density of sapphire substrate, = 3.98 g/cm3 ). 3. Result and discussion 3.1. Characterization of La-doped colloidal SiO2 composite abrasives La-doped colloidal SiO2 composite abrasives were diluted 100 times, filmed on silicon wafers and dried, and then analyzed by SEM. Fig. 1 is SEM micrographs of pure colloidal SiO2 abrasive and La-doped colloidal SiO2 composite abrasives. The results show that, by comparison with the pure colloidal SiO2 abrasive (0 wt.% of La(OH)3 ), the particle sizes of La-doped colloidal SiO2 composite abrasives have almost no change. And the prepared La-doped
Fig. 4. Ra and RMS as functions of mean mass ratio of La(OH)3 . (Red line is error bar which is ±1.96 standard deviation of measurement value of 3 times and represent a description of 95% confident of which mean represents the true Ra value. And measurement error is about ± 0.1 nm.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
colloidal SiO2 abrasives have consistent spherical shape and excellent dispersibility. TOF-SIMS is an effective method for element analysis because of its high sensitivity and mass and space resolution. In order to analyze the composition of La-doped colloidal SiO2 composite abrasives, elementary analysis of the samples of the pure colloidal SiO2 abrasive and the composite abrasive were conducted by TOF-SIMS.
Fig. 5. Surface profiles of sapphire substrates (a) before polishing, Ra = 2.5 nm; (b) polished by pure colloidal SiO2 , Ra = 2.2 nm; (c) polished by La-doped colloidal SiO2 containing 0.5 wt.% La(OH)3 , Ra = 1.7 nm; (d) polished by La-doped colloidal SiO2 containing 1.5 wt.% La(OH)3 , Ra = 1.2 nm.
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Fig. 6. TOF-SIMS spectra of the slurry containing La-doped colloidal SiO2 with 1.5 wt.% La(OH)3 after polishing (a) total graph, (b) aluminum partial enlarged drawing and (c) lanthanum partial enlarged drawing.
The results are shown in Fig. 2. Horizontal axis is ion mass (amu), and vertical axis is number of ions counted. It is seen that silicon and lanthanum are present in the composite abrasive in Fig. 2b while the silicon is only present for pure colloidal SiO2 in Fig. 2a, which suggests that lanthanum was successfully introduced into colloidal SiO2 composite abrasives. 3.2. CMP performances of the La-doped colloidal SiO2 composite abrasives In order to investigate the difference in polishing performances between the prepared La-doped colloidal SiO2 composite abrasives and pure colloidal SiO2 abrasive, the MRR and Ra of polished sapphire substrate surfaces were analyzed. Fig. 3 and Table 1 reveal the effects of different La(OH)3 contents on the MRR of the sapphire. It can be seen that, with increasing of the mass content of La(OH)3 , the MRR of sapphire substrates increases gradually. The MRR of sapphire substrates polished in pure colloidal SiO2 slurry is 0.431 m/h, while the MRRs of La(OH)3 -doped composite abrasives with 0.5% La(OH)3 , 1.0% La(OH)3 and 1.5% La(OH)3 give 0.575 m/h, 0.663 m/h and 0.681 m/h, respectively. In other words, the prepared La-doped colloidal SiO2 composite abrasives possess higher MRR than pure colloidal SiO2 abrasive.
In addition, to get more details about the differences in polishing performances between the composite abrasives and pure colloidal SiO2 abrasive, the Ra and RMS values and topographical micrographs of polished sapphire surfaces were obtained by an AMBIOS TECHNOLOGY XI-100 Optical Surface Profiler. As displayed in Fig. 4, with increasing of the mass content of La(OH)3 , the Ra and RMS of sapphire substrates decrease gradually. Before polishing, the roughness is as high as 2.5 nm. After polishing by the pure colloidal SiO2 , the surface of the sapphire substrate reaches Ra of 2.2 nm. And it is found that the prepared composite abrasive with 0.5%, 1.0% and 1.5% La(OH)3 doping content give Ra of 1.7 nm, 1.5 nm and 1.2 nm, respectively. The lower Ra and RMS value mean the higher surface planarization. Fig. 5 shows topographical micrographs of sapphire surfaces before and after polishing. As displayed in Fig. 5, the surface before polishing (Fig. 5a) is very rough and the roughness is as high as 2.5 nm. After polished by pure SiO2 abrasive (Fig. 5b), the surface topography fluctuations are reduced. However, after polished by the Ladoped colloidal SiO2 abrasive (Fig. 5c and d), the surfaces become smoother and the topography fluctuations can hardly be observed. To sum up, the prepared La-doped colloidal SiO2 composite abrasives possess higher surface planarization than pure colloidal SiO2 abrasive.
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Table 2 Binding energies of abrasives containing La-doped colloidal SiO2 with 1.5 wt.% La(OH)3 after polishing. Elements in sample
Si
Al
La
Binding energy (eV)
101.88 102.85
74.88 75.2273.8874.92
834.48 838.08
slurry containing La-doped colloidal SiO2 composite abrasive after polishing was analyzed by TOF-SIMS and XPS. The slurry containing La-doped colloidal SiO2 composite abrasive with 1.5 wt.% La(OH)3 after polishing was dried and characterized by TOF-SIMS. The slurry containing La-doped colloidal SiO2 composite abrasive with 1.5 wt.% La(OH)3 after polishing was centrifuged. The residual of the solid precipitation was washed with deionized water for many times and dried at normal temperature, and then analyzed by XPS. Fig. 6 shows the results of TOF-SIMS. Aluminum is present in the slurry containing La-doped colloidal SiO2 composite abrasive after polishing. It means that aluminum had been introduced into slurry. In order to further reveal the mechanism of La-doped colloidal SiO2 abrasives on sapphire during CMP, XPS analysis of the residual of the solid precipitation of polishing slurry was conducted. Fig. 7 shows the XPS narrow scan spectra result of element Si (a), element Al (b) and element La (c) of La-doped colloidal SiO2 abrasive with 1.5 wt.% La(OH)3 after polishing, and the binding energy of different material is presented in Table 2. In Fig. 7, the black line shows the real total intensity measured by XPS test, and the red line shows the total intensity after curve fitting by using the Thermo Advantage software. The closer between the experiment results (black line) and the fitting results (red line) will lead to more accurate analysis results in Fig. 7. The peak at the binding energy (BE) of 102.85 eV (Fig. 7a) corresponds to Si2p in SiO2 (gel) state and the peak at the binding energy (BE) of 74.88 eV (Al2p) corresponds to Al2 O3 sapphire (Fig. 7b). Fig. 7a and b reveals that the peak at the binding energy (BE) of 101.88 eV corresponds to Si2p in Al2 SiO5 state as well as the peak at the binding energy (BE) of 74.29 eV corresponds to Al2p in Al2 SiO5 state [26], and the peak at the binding energy (BE) of 73.88 eV corresponds to Al2p in AlOOH state. The result illustrates solid-state chemical transformation react between SiO2 and AlOOH. This change can be expressed in the following equations: Al2 O3 + H2 O → 2AlOOH
(1)
2AlOOH + SiO2 → Al2 SiO5 + H2 O
(2)
In addition, the peak at the binding energy (BE) of 838.08 eV (Fig. 7c) corresponds to La3d5/2 as AlLaO3 as well as the peak at the binding energy (BE) of 834.48 eV (Fig. 7c) corresponds to La3d5/2 as LaOOH. This change can be expressed in the following equations:
Fig. 7. The XPS narrow scan spectra results of element Si (a), element Al, (b) and element La and (c) of La-doped colloidal SiO2 composite abrasive containing 1.5 wt.% La(OH)3 after polishing sapphire (black lines are experiment results and red lines are fitting results). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Mechanism discussion CMP is a chemical action and mechanical action process of mutual coordination. In order to explore the acting mechanism of La-doped colloidal SiO2 composite abrasives to sapphire CMP, the
La(OH)3 → LaOOH + H2 O
(3)
Al2 O3 + LaOOH → AlLaO3 + AlOOH
(4)
To sum up, the result shows that solid-state chemical reaction occurs during the polishing process. In alkaline conditions, the doping of lanthanum makes the chemical bond of Al O Al rupture easier, and the formation of softening film made up of AlOOH more easily on the surface [24,25,27]. Simultaneously, the reaction between lanthanum and substrate surface generates AlLaO3 . The softening film, AlLaO3 and Al2 SiO5 which is the reaction product of AlOOH and SiO2 can be easily removed by mechanical action [27,28]. The improvements of sapphire CMP may be attributed to the solid-chemical reaction between sapphire surface and La-doped colloidal SiO2 composite abrasives. Simultaneously, the softer La(OH)3 introduced into colloidal SiO2 abrasive leads to little
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scratches on the sapphire surface comparing to pure colloidal SiO2 abrasive. 4. Conclusions Novel La-doped colloidal SiO2 composite abrasives were prepared. The results show that the La-doped colloidal SiO2 composite abrasives have good dispersibility and consistent spherical shape. The slurries containing La-doped colloidal SiO2 composite abrasives achieve lower Ra and higher MRR than that containing pure SiO2 abrasive in the CMP of sapphire substrates under the same conditions. The improvements of sapphire CMP may be attributed to its solid-chemical reaction between sapphire surface and La-doped colloidal SiO2 composite abrasives. Meanwhile, softer La(OH)3 doping and good dispersibility of the composite abrasives endue their higher surface planarization in sapphire CMP. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 51475279, no. 51375291). Research Fund for Doctoral Program of Higher Education of China (grant no. 20123108110016). References [1] Gentilman RL, Maguire EA, Starrett HS, Hartnett TM, Kirchner HP. Strength and transmittance of sapphire and strengthened sapphire. J Am Ceram Soc 1981;64:116. [2] Schmid F, Khattak CP, Felt DM. Am Ceram Soc Bull 1994;73:39. [3] Nakamura S. Annu Rev Mater Sci 1998;28:125. [4] Li JL, Nutt SR, Kirby KW. J Am Ceram Soc 1999;82:3260. [5] Nakamura S, et al. InGaN-based laser diodes. Annu Rev Mater Sci 1998;28:125. [6] Niu X, Lu G, Zha W, Niu X, Lu G, Zhang W, et al. Dislocation of Cz-sapphire substrate for GaN growth by chemical etching method. Trans Nonferrous Met Soc China 2006;16:187. [7] Zhu HL, Tessaroto LA, Sabia R, Greenhut VA, Smith M, Niesz DE. Appl Surf Sci 2004;236:120.
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[8] Zhuo ZG, Zhou H, Xu XM, et al. The progress on polishing technology of sapphire substrate used in LED. Mach Des Manuf 2013;4:249. [9] Niu XH, Liu YL, Tan BM, Han LY, Zhang JX, Zhang JX. Trans Nonferrous Met Soc China 2006;16:732. [10] Wang YZ, Liu SL, Peng GL, Zhou SM, Xu J. J Cryst Growth 2005;274:241. [11] Wang YZ, Zhou SM, Xu J. Research of chemical mechanical polishing technique of sapphire substrate. J Synth Cryst 2004;33:441. [12] Aida H, Doi T, Takeda H, et al. Ultraprecision CMP for sapphire. GaN, and SiC for advanced optoelectronic materials. Curr Appl Phys 2012;12:41. [13] Zhou CH, Shan L, Hight R. Tribol Trans 2002;45:232. [14] Zhu HL, Niesz DE, Greenhut VA. The effect of abrasive hardness on the chemicalassisted polishing of (0 0 0 1) plane sapphire. J Mater Res 2005;20:504. [15] Hu XK, Song ZT, Pan ZC, et al. Planarization machining of sapphire wafers with boron carbide and colloidal silica as abrasives. Appl Surf Sci 2009;255:8230. [16] Xiong W, Chu XF, Dong YP, et al. Effects of different abrasives on sapphire chemical-mechanical polishing. J Synth Cryst 2013;42:1064. [17] Zhao YC, Tan BM, Niu XH, et al. Effects of different abrasives on the sapphire CMP. Micronanoelectron Technol 2014;51:120. [18] Niu XH, Wu T, Zhao XH, et al. Study on CMP mechanism and technology of sapphire substrate for photo-conducting device. ECS Trans 2009;18:435. [19] Zhao Xiaobing, Long Renwei, Chen Yang, Chen Zhigang. Synthesis, characterization of CeO2 @SiO2 nanoparticles and their oxide CMP behavior. Microelectron Eng 2010;87:1716. [20] Lei Hong, Chu Fengling, Xiao Baoqi. Preparation of silica/ceria nano composite abrasive and its CMP behavior on hard disk substrate. Microelectron Eng 2010;87:1474. [21] Zhang L, Wang HB, Zhang ZF, et al. Preparation of monodisperse polystyrene/silica core-shell nano-composite abrasive with controllable size and its chemical mechanical polishing performance on copper. Appl Surf Sci 2011;258:1217. [22] Xu L, et al. Fe–Nx/C assisted chemical–mechanical polishing for improving the removal rate of sapphire. Appl Surf Sci 2015;343:115S–20S. [23] Garbarino G, et al. A study of Ni/Al2 O3 and Ni–La/Al2 O3 catalysts for the steam reforming of ethanol and phenol. Appl Catal B: Environ 2015;174–175:21–34. [24] Zhang ZY, Lu XC, Han BL, Luo JB. Rare earth effect on the microstructure and wear resistance of Ni-based coatings. Mater Sci Eng A 2007;454–455:194–202. [25] Zhang ZY, Lu XC, Han BL, Luo JB. Rare earth effect on microstructure, mechanical and tribological properties of CoCrW coatings. Mater Sci Eng A 2007;444:92–8. [26] Vovk EA, et al. Mechanism of the Interaction between Al2 O3 and SiO2 during the chemical–mechanical polishing of sapphire with silicon dioxide. J Surf Invest: X-ray Synchrotron Neutron Tech 2012;6(1):115–21. [27] Jacob KT, et al. Use of composition-graded bi-electrolyte cells for thermodynamic studies on lanthanum aluminates. J Electrochem Soc 2014;161(6):H343–9. [28] Oganov AR, Price GD, Brodholt JP. Theoretical investigation of metastable Al2 SiO5 polymorphs. Acta Crystallogr Sect A: Found Crystallogr 2001;8:54–7.
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Precision Engineering journal homepage: www.elsevier.com/locate/precision
Preparation of La-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates Hong Lei a,b,∗ , Kaiyu Tong a a b
School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China Research Center of Nano-science and Nano-technology, Shanghai University, Shanghai 200444, China
a r t i c l e
i n f o
Article history: Received 26 June 2015 Received in revised form 21 October 2015 Accepted 31 October 2015 Available online xxx Keywords: CMP SiO2 composite abrasives Sapphire Solid-state reaction
a b s t r a c t Chemical mechanical polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of the key elements in CMP process. In order to enhance removal rate and improve surface quality of sapphire substrate, a series of novel La-doped colloidal SiO2 composite abrasives were prepared by seed-induced growth method. The CMP performance of the La-doped colloidal SiO2 composite abrasives on sapphire substrate were investigated using UNIPOL-1502 polishing equipment. The analyses on the surface of polished sapphire substrate indicate that slurries containing the La-doped colloidal SiO2 composite abrasives achieve lower surface roughness, higher material removal rate than that of pure colloidal SiO2 abrasive under the same testing conditions. Furthermore, the acting mechanism of the La-doped colloidal silica in sapphire CMP was investigated. X-ray photoelectron spectroscopy analysis shows that solid-state chemical reactions between La-doped colloidal SiO2 abrasive and sapphire surface occur during CMP process, which can promote the chemical effect in CMP and lead to the improvement of material removal rate. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Sapphire, single crystal form of alpha-alumina, is widely used in a variety of modern high-technology applications for its excellent optical, chemical and mechanical properties such as high hardness, great thermal stability, chemical inertness, especially as a substrate material in the photoelectronic, microelectronic and semiconductor industries, ranging from optical windows, read/write laser sources to GaN-based light emitting diodes (LEDs), etc [1–6]. In all of these applications, the surface flatness of sapphire substrate is a key factor that influences its performances. Undoubtedly, its planarization machining is very important [7]. However, the intrinsic nature of sapphire (high hardness and chemical inertness) poses great challenges to such machining. In order to achieve high surface quality, several polishing technologies are put forward by researchers [8]. At present, chemical mechanical polishing (CMP) is the only widely used global planarization technology in manufacturing of semiconductor and sapphire substrates [9–12]. In CMP, abrasives are one of the key influencing factors on the polished surface quality. The sizes and distribution,
∗ Corresponding author at: Shanghai Institute of Technology, School of Materials Science and Engineering, Shanghai 201418, China. Tel.: +86 13331906218; fax: +86 21 66137104. E-mail address: hong [email protected] (H. Lei).
dispersibility, hardness and species of abrasive are crucial for a desired CMP performance [13]. In order to improve surface planarization and material removing rate (MRR) of sapphire, several polishing abrasives have been studied. Zhu et al. [14] have studied the effect of abrasive hardness on the CMP of (0 0 0 1) plane sapphire. They found that, hard abrasives (such as monocrystalline and polycrystalline diamond, ␣-Al2 O3 and ␥-Al2 O3 ) can improve MRR of sapphire, but arithmetic average surface roughness (Ra ) is higher. Nevertheless, using silica sol as polishing abrasives for sapphire can get lower Ra , but MRR of sapphire is lower. Hu et al. [15] have researched the CMP of sapphire wafers with boron carbide and colloidal silica abrasives. They found that, B4 C abrasive can gain a higher removal rate in sapphire wafer, but the colloidal silica can achieve a nanoscale flatness of sapphire surface. Xiong et al. [16] adopted silicon carbide, alumina and silica sol as polishing abrasives for CMP of sapphire. The results show that, under the same condition, Ra of sapphire polished with silica sol abrasive is lower than that with silicon carbide and alumina abrasives. In general, silica sol is widely regarded as ideal polishing abrasive for sapphire CMP since it can get better surface quality [17,18]. However, MRR of sapphire with colloidal silica as polishing abrasive is still lower and needs to be improved. Currently, silica-based composite particles as CMP abrasives capture increasing attention, and chemical modifying of silica abrasive has been proved to be an effective way to improving its polishing performances in CMP. Zhao [19] synthesized SiO2 /CeO2
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Fig. 1. SEM micrographs of La-doped colloidal SiO2 composite abrasives with different La(OH)3 contents (a) 0 wt.% of La(OH)3 , (b) 0.5 wt.% of La(OH)3 , (c) 1.0 wt.% of La(OH)3 , (d) 1.5 wt.% of La(OH)3 .
abrasive for CMP of oxide wafer and Lei [20] developed porous Fe2 O3 /SiO2 abrasive for CMP of hard disk substrate. Zhang et al. [21] prepared silica composite abrasive by coating with a layer of polystyrene. Xu et al. [22] reported the slurry of colloidal silicon dioxide (SiO2 ) containing Fe–N-x/C, which exhibits higher MRR than that containing pure SiO2 abrasive in the CMP of sapphire substrates under the same conditions. In a word, the silica-based composite abrasives can result in both high planarization efficiency and good planarization quality. Especially, rare earth compounds including cerium are often used as polishing abrasive and exhibit good polishing performances [19]. As rare earth elements (REE), lanthanum and cerium have many similar natures and lanthanum has high catalytic activity for many chemical reactions [23–25]. Therefore, lanthanum-modified silica sol may be expected to have better polishing performances. In this paper, La-doped colloidal SiO2 composite abrasives were prepared and their CMP performances on sapphire substrates were investigated.
colloidal SiO2 composite abrasive solution was obtained after the end of reaction. In above synthesis process of composite abrasives, the lanthanum was doped into colloidal SiO2 abrasives in the form of lanthanum hydroxide since the reaction liquid is alkali, where the reaction La3+ + 3OH− → La(OH)3 happened at pH of 9 to 10. By adding different amounts of 0.23 wt.% La(NO3 )3 solution versus crystal silica seed solution, La-doped colloidal SiO2 composite abrasives containing 0.5 wt.%, 1.0 wt.% and 1.5 wt.% La(OH)3 (mass ratios La(OH)3 of vs SiO2 ) were obtained. The pure colloidal SiO2 abrasive was prepared by the same method mentioned above except adding La(NO3 )3 solution. Preparation of polishing slurries: Before polishing, 10 wt.% prepared colloidal solutions were diluted to solid concentration of 6 wt.%, then 3.0 wt.% sodium hexametaphosphate as dispersant were added into the solution. Finally, the above-mentioned mixtures were filtrated with a 10 m pore filter to obtain polishing slurries.
2. Experimental details 2.1. Preparation of La-doped colloidal SiO2 composite abrasives and polishing slurries Synthesis of La-doped colloidal SiO2 abrasives with different La(OH)3 contents: 10 wt.% crystal silica seed solution were heated to 100 ◦ C in a four-neck flask. A certain amount of 2.5 wt.% active silicic acid solution, 0.23 wt.% La(NO3 )3 solution and 0.5 wt.% NaOH solution were simultaneously added in drops into the crystal silica seed solution in the four-neck flask under continuously stirring. The liquid volume in the four-neck flask was kept constant by controlling evaporating speed of water and feeding speed of the solutions. The reaction liquid was keeping at pH of 9.0–11.0 by controlling the dropping velocity of NaOH. The seed-induced growth was carried out at 100 ◦ C for 7 h at under continuously stirring, and then cooled to room temperature. The solid concentration of 10 wt.%
2.2. Characterization of La-doped colloidal SiO2 composite abrasives The composite abrasives were characterized by means of scanning electron microscopy (SEM), time of flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). Elementary analysis of the sample which is processed solid SiO2 composite abrasives was measured by Model 2100 Trift II timeof-flight secondary ion mass spectroscopy. The size, shape and distribution of composite abrasives were characterized by SEM, and SEM analyses were accomplished using a JEOL JSM-6700F field emission scanning electron microscope with voltage of 10 kV. The element type of composite abrasives after polishing were analyzed by XPS, and XPS spectrum was obtained by ESCALAB 250Xi electron spectrometer using a focused monochromatized Al K␣ radiation
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Fig. 2. TOF-SIMS spectra of pure colloidal SiO2 abrasive (a) and La-doped colloidal SiO2 abrasive with 1.5 wt.% La(OH)3 (b). Table 1 The experiment data error analysis of three polishing tests. The mass of La(OH)3 (wt.%)
0
0.5
1
1.5
1 2 3 Mean Standard deviation
0.446 0.439 0.408 0.431 0.0202
0.536 0.578 0.584 0.566 0.0262
0.674 0.665 0.651 0.663 0.0112
0.683 0.692 0.668 0.681 0.0121
(h = 1486.6 eV). The binding energy of C 1s (285.44 eV) was used as reference.
2.3. Polishing test
Fig. 3. Effect of the mass content of La(OH)3 on material removal rate. (Red line is error bar that is ±1.96 standard deviation and represent a description of 95% confident of which the mean represents the true MRR value.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Polishing tests were conducted by using a UNIPOL-1502 polishing equipment (Shenyang Kejing instrument, Co. LTD, China). The polishing pad was a DPC5150 porous polyurethane pad (Rodel, U.S.A). The slurries supplying rate is 180 ml/min. The parameters of polishing were down force of 6 kg, plate rotating speed of 60 rpm and polishing time of 2 h. Work pieces were ϕ50.8 mm × 0.45 mm ␣-Al2 O3 sapphire substrates with c (0 0 0 1) orientation. The Ra and of the sapphire substrates are about 2.5 nm. After polishing, the sapphire substrates were washed with ultrasonic in a cleaning solution
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containing 0.5 wt.% surfactant in deionized (DI) water. Finally, they were dried by a multifunctional drying system. The Ra was measured by an Ambios XI-100 Plus surface profiler (Ambios Technology Corp., U.S.A) with the resolution of 0.1 nm, and the measuring area of 500 m × 500 m. The depth of focus is 3.0 m, the working distance is 7.4 mm, and the numerical aperture is 0.30. The mass of the sapphire substrate was measured by a AL104 analytical balance with accuracy of 0.1 mg (METTLER TOLEDO, Switzerland). The material removal rate was calculated by the following formula: MRR =
m × 106 R2
(2-1)
where MRR—material removal rate (m/h), m-mass of material removed (g), R—radius of sapphire substrate (mm), —polishing time (h) (in the test, = 2 h), —density (g/cm3 ) (density of sapphire substrate, = 3.98 g/cm3 ). 3. Result and discussion 3.1. Characterization of La-doped colloidal SiO2 composite abrasives La-doped colloidal SiO2 composite abrasives were diluted 100 times, filmed on silicon wafers and dried, and then analyzed by SEM. Fig. 1 is SEM micrographs of pure colloidal SiO2 abrasive and La-doped colloidal SiO2 composite abrasives. The results show that, by comparison with the pure colloidal SiO2 abrasive (0 wt.% of La(OH)3 ), the particle sizes of La-doped colloidal SiO2 composite abrasives have almost no change. And the prepared La-doped
Fig. 4. Ra and RMS as functions of mean mass ratio of La(OH)3 . (Red line is error bar which is ±1.96 standard deviation of measurement value of 3 times and represent a description of 95% confident of which mean represents the true Ra value. And measurement error is about ± 0.1 nm.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
colloidal SiO2 abrasives have consistent spherical shape and excellent dispersibility. TOF-SIMS is an effective method for element analysis because of its high sensitivity and mass and space resolution. In order to analyze the composition of La-doped colloidal SiO2 composite abrasives, elementary analysis of the samples of the pure colloidal SiO2 abrasive and the composite abrasive were conducted by TOF-SIMS.
Fig. 5. Surface profiles of sapphire substrates (a) before polishing, Ra = 2.5 nm; (b) polished by pure colloidal SiO2 , Ra = 2.2 nm; (c) polished by La-doped colloidal SiO2 containing 0.5 wt.% La(OH)3 , Ra = 1.7 nm; (d) polished by La-doped colloidal SiO2 containing 1.5 wt.% La(OH)3 , Ra = 1.2 nm.
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Fig. 6. TOF-SIMS spectra of the slurry containing La-doped colloidal SiO2 with 1.5 wt.% La(OH)3 after polishing (a) total graph, (b) aluminum partial enlarged drawing and (c) lanthanum partial enlarged drawing.
The results are shown in Fig. 2. Horizontal axis is ion mass (amu), and vertical axis is number of ions counted. It is seen that silicon and lanthanum are present in the composite abrasive in Fig. 2b while the silicon is only present for pure colloidal SiO2 in Fig. 2a, which suggests that lanthanum was successfully introduced into colloidal SiO2 composite abrasives. 3.2. CMP performances of the La-doped colloidal SiO2 composite abrasives In order to investigate the difference in polishing performances between the prepared La-doped colloidal SiO2 composite abrasives and pure colloidal SiO2 abrasive, the MRR and Ra of polished sapphire substrate surfaces were analyzed. Fig. 3 and Table 1 reveal the effects of different La(OH)3 contents on the MRR of the sapphire. It can be seen that, with increasing of the mass content of La(OH)3 , the MRR of sapphire substrates increases gradually. The MRR of sapphire substrates polished in pure colloidal SiO2 slurry is 0.431 m/h, while the MRRs of La(OH)3 -doped composite abrasives with 0.5% La(OH)3 , 1.0% La(OH)3 and 1.5% La(OH)3 give 0.575 m/h, 0.663 m/h and 0.681 m/h, respectively. In other words, the prepared La-doped colloidal SiO2 composite abrasives possess higher MRR than pure colloidal SiO2 abrasive.
In addition, to get more details about the differences in polishing performances between the composite abrasives and pure colloidal SiO2 abrasive, the Ra and RMS values and topographical micrographs of polished sapphire surfaces were obtained by an AMBIOS TECHNOLOGY XI-100 Optical Surface Profiler. As displayed in Fig. 4, with increasing of the mass content of La(OH)3 , the Ra and RMS of sapphire substrates decrease gradually. Before polishing, the roughness is as high as 2.5 nm. After polishing by the pure colloidal SiO2 , the surface of the sapphire substrate reaches Ra of 2.2 nm. And it is found that the prepared composite abrasive with 0.5%, 1.0% and 1.5% La(OH)3 doping content give Ra of 1.7 nm, 1.5 nm and 1.2 nm, respectively. The lower Ra and RMS value mean the higher surface planarization. Fig. 5 shows topographical micrographs of sapphire surfaces before and after polishing. As displayed in Fig. 5, the surface before polishing (Fig. 5a) is very rough and the roughness is as high as 2.5 nm. After polished by pure SiO2 abrasive (Fig. 5b), the surface topography fluctuations are reduced. However, after polished by the Ladoped colloidal SiO2 abrasive (Fig. 5c and d), the surfaces become smoother and the topography fluctuations can hardly be observed. To sum up, the prepared La-doped colloidal SiO2 composite abrasives possess higher surface planarization than pure colloidal SiO2 abrasive.
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Table 2 Binding energies of abrasives containing La-doped colloidal SiO2 with 1.5 wt.% La(OH)3 after polishing. Elements in sample
Si
Al
La
Binding energy (eV)
101.88 102.85
74.88 75.2273.8874.92
834.48 838.08
slurry containing La-doped colloidal SiO2 composite abrasive after polishing was analyzed by TOF-SIMS and XPS. The slurry containing La-doped colloidal SiO2 composite abrasive with 1.5 wt.% La(OH)3 after polishing was dried and characterized by TOF-SIMS. The slurry containing La-doped colloidal SiO2 composite abrasive with 1.5 wt.% La(OH)3 after polishing was centrifuged. The residual of the solid precipitation was washed with deionized water for many times and dried at normal temperature, and then analyzed by XPS. Fig. 6 shows the results of TOF-SIMS. Aluminum is present in the slurry containing La-doped colloidal SiO2 composite abrasive after polishing. It means that aluminum had been introduced into slurry. In order to further reveal the mechanism of La-doped colloidal SiO2 abrasives on sapphire during CMP, XPS analysis of the residual of the solid precipitation of polishing slurry was conducted. Fig. 7 shows the XPS narrow scan spectra result of element Si (a), element Al (b) and element La (c) of La-doped colloidal SiO2 abrasive with 1.5 wt.% La(OH)3 after polishing, and the binding energy of different material is presented in Table 2. In Fig. 7, the black line shows the real total intensity measured by XPS test, and the red line shows the total intensity after curve fitting by using the Thermo Advantage software. The closer between the experiment results (black line) and the fitting results (red line) will lead to more accurate analysis results in Fig. 7. The peak at the binding energy (BE) of 102.85 eV (Fig. 7a) corresponds to Si2p in SiO2 (gel) state and the peak at the binding energy (BE) of 74.88 eV (Al2p) corresponds to Al2 O3 sapphire (Fig. 7b). Fig. 7a and b reveals that the peak at the binding energy (BE) of 101.88 eV corresponds to Si2p in Al2 SiO5 state as well as the peak at the binding energy (BE) of 74.29 eV corresponds to Al2p in Al2 SiO5 state [26], and the peak at the binding energy (BE) of 73.88 eV corresponds to Al2p in AlOOH state. The result illustrates solid-state chemical transformation react between SiO2 and AlOOH. This change can be expressed in the following equations: Al2 O3 + H2 O → 2AlOOH
(1)
2AlOOH + SiO2 → Al2 SiO5 + H2 O
(2)
In addition, the peak at the binding energy (BE) of 838.08 eV (Fig. 7c) corresponds to La3d5/2 as AlLaO3 as well as the peak at the binding energy (BE) of 834.48 eV (Fig. 7c) corresponds to La3d5/2 as LaOOH. This change can be expressed in the following equations:
Fig. 7. The XPS narrow scan spectra results of element Si (a), element Al, (b) and element La and (c) of La-doped colloidal SiO2 composite abrasive containing 1.5 wt.% La(OH)3 after polishing sapphire (black lines are experiment results and red lines are fitting results). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Mechanism discussion CMP is a chemical action and mechanical action process of mutual coordination. In order to explore the acting mechanism of La-doped colloidal SiO2 composite abrasives to sapphire CMP, the
La(OH)3 → LaOOH + H2 O
(3)
Al2 O3 + LaOOH → AlLaO3 + AlOOH
(4)
To sum up, the result shows that solid-state chemical reaction occurs during the polishing process. In alkaline conditions, the doping of lanthanum makes the chemical bond of Al O Al rupture easier, and the formation of softening film made up of AlOOH more easily on the surface [24,25,27]. Simultaneously, the reaction between lanthanum and substrate surface generates AlLaO3 . The softening film, AlLaO3 and Al2 SiO5 which is the reaction product of AlOOH and SiO2 can be easily removed by mechanical action [27,28]. The improvements of sapphire CMP may be attributed to the solid-chemical reaction between sapphire surface and La-doped colloidal SiO2 composite abrasives. Simultaneously, the softer La(OH)3 introduced into colloidal SiO2 abrasive leads to little
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scratches on the sapphire surface comparing to pure colloidal SiO2 abrasive. 4. Conclusions Novel La-doped colloidal SiO2 composite abrasives were prepared. The results show that the La-doped colloidal SiO2 composite abrasives have good dispersibility and consistent spherical shape. The slurries containing La-doped colloidal SiO2 composite abrasives achieve lower Ra and higher MRR than that containing pure SiO2 abrasive in the CMP of sapphire substrates under the same conditions. The improvements of sapphire CMP may be attributed to its solid-chemical reaction between sapphire surface and La-doped colloidal SiO2 composite abrasives. Meanwhile, softer La(OH)3 doping and good dispersibility of the composite abrasives endue their higher surface planarization in sapphire CMP. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 51475279, no. 51375291). Research Fund for Doctoral Program of Higher Education of China (grant no. 20123108110016). References [1] Gentilman RL, Maguire EA, Starrett HS, Hartnett TM, Kirchner HP. Strength and transmittance of sapphire and strengthened sapphire. J Am Ceram Soc 1981;64:116. [2] Schmid F, Khattak CP, Felt DM. Am Ceram Soc Bull 1994;73:39. [3] Nakamura S. Annu Rev Mater Sci 1998;28:125. [4] Li JL, Nutt SR, Kirby KW. J Am Ceram Soc 1999;82:3260. [5] Nakamura S, et al. InGaN-based laser diodes. Annu Rev Mater Sci 1998;28:125. [6] Niu X, Lu G, Zha W, Niu X, Lu G, Zhang W, et al. Dislocation of Cz-sapphire substrate for GaN growth by chemical etching method. Trans Nonferrous Met Soc China 2006;16:187. [7] Zhu HL, Tessaroto LA, Sabia R, Greenhut VA, Smith M, Niesz DE. Appl Surf Sci 2004;236:120.
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