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Preparation of porous alumina abrasives and their chemical mechanical polishing behavior
Thin Solid Films 520 (2012) 2868–2872
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Preparation of porous alumina abrasives and their chemical mechanical polishing behavior Hong Lei ⁎, Xin Wu, Ruling Chen 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 9 March 2011 Received in revised form 9 November 2011 Accepted 25 November 2011 Available online 2 December 2011 Keywords: Chemical mechanical polishing (CMP) Porous alumina abrasive Hard disk substrate Planarization
a b s t r a c t Porous alumina abrasives with different pore sizes were prepared using hydrothermal synthesis method by different hydrothermal temperatures. The pore structure, pore size and pore volume of the products were characterized by transmission electron microscopy and nitrogen adsorption desorption isotherm measurement. The chemical mechanical polishing (CMP) performances of porous alumina abrasives in hard disk substrate CMP were investigated. The results show that, the polished surface average roughness (Ra) decreases when the pore diameter of porous alumina abrasive increases. By comparison with solid alumina abrasive, the prepared porous alumina abrasives give lower Ra, and the porous alumina abrasive with 8.61 nm pore diameter has higher material removal rate under the same polishing conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Chemical mechanical polishing (CMP) has become a widely used global planarization technology in the manufacturing of semiconductor and computer hard disks driver [1–4]. Slurry is the main influencing factor on the polished surface quality during CMP process. Slurry consists of abrasive, lubricant, oxidizer, deionized water and so on. So abrasive is a key influencing factor on the polished surface quality. The size and distribution, dispersibility, hardness and species of abrasive are crucial for a desired CMP performances [5]. The traditional abrasives, such as silica, alumina, ceria, etc., have been widely studied [6–8] and used in the commercial slurries. But all these abrasives are compact solid particles and easy to cause polishing scratches. Reducing the hardness of abrasives is an effective way to improve the polished surface quality [9]. The porous abrasive because of their special pore channel structure is one of the effective ways to reduce the hardness of the abrasives. In our previous work, Liu and Lei [10] prepared porous silica abrasive, which showed better hard disk substrate CMP performances than pure solid silica abrasive. It showed that abrasive with narrow pores can improve the polished surface quality because of the abrasive's adsorption. The alumina is a widely used abrasive. But up to now, porous alumina as abrasive in CMP slurry has seldom been reported. So, porous alumina abrasives were prepared in the present paper and
⁎ Corresponding author. E-mail address: [email protected] (H. Lei). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.11.057
their CMP performances on hard disks have been investigated preliminarily. 2. Experimental methods 2.1. Preparation of porous alumina abrasives Porous alumina abrasives with different pore sizes were prepared using hydrothermal synthesis method by different hydrothermal temperatures [11]. Sodium dodecyl sulfate was used as a polymeric template. 150 g of Al(NO3)3·9H2O was dissolved in 200 mL deionized water and then mixed with 200 ml of 2 mol/L ammonia solution. The pH value of the aluminum nitrate solution was 2.8 after the addition of ammonia. Then 72 g of urea and 28.8 g of sodium dodecyl sulfate (SDS) were added into the above solution under stirring and the obtained homogeneous solution was put into a sealed Teflon-lined stainless autoclave vessel for static hydrothermal reaction at certain crystallization temperature for 48 h. After that, the resulting white precipitate was recovered by centrifugation, washed with deionized water for several times and dried at 110 °C for 12 h. The final product was obtained by calcination at 800 °C for 4 h in air. In this paper, porous alumina abrasives with different pore diameters were prepared by different crystallization temperatures such as 100 °C, 120 °C and 140 °C. 2.2. Characterization of porous alumina abrasives Nitrogen adsorption–desorption isotherms were measured at −196 °C on a Micromeritics ASAP 2020 M + C apparatus. The specific surface areas (SBET) of the samples were calculated with the BrunauerEmmett-Teller equation [12]. The total pore volumes (Vp) were
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determined at a relative pressure P/P0 (P is pressure of adsorbed solute, and P0 is saturated vapor pressure of sorbent) value of 0.995. The mean pore diameters (Dp) were calculated with the Barrett–Joyner–Halenda equation using the corresponding desorption branches of the isotherms [13]. The measurement uncertainties of the SBET, Vp and Dp were below 5%. Transmission electron microscopy (TEM) images were obtained on a JEOL 2000 CX electron microscope operating at an accelerating voltage of 200 kV. The samples were ultrasonically dispersed in ethanol and then dropped onto the carbon-coated copper grids prior to the observation. Particle size was determined by using a Malvern Zetasizer 3000HS laser particle size analyzer with measuring range from 2 nm to 3000 nm. 2.3. Preparation of porous alumina slurry and solid alumina slurry The preparation of porous alumina slurry and solid alumina slurry (a commercial alumina powder) have the same process. Firstly, 3 wt.% porous alumina (or solid alumina) powder and 4.5 wt.% functional additives were added into distilled water in a container under continuously stirring. Secondly, the mixture was milled for 4 h in a vibrator containing ZrO2 balls as the milling balls. After the milling, the average diameters of the three porous alumina were 203.1 nm, and the average diameter of the solid alumina was 180.2 nm (measured by Malvern Zetasizer 3000HS laser particle size analyzer). The particle diameters of the four abrasives were close. Thirdly, the mixture was filtrated with a 10 μm pore filter. Before polishing, 6.0 wt.% H2O2 as oxidant was added to the mixture to obtain the slurry. 2.4. Polishing tests Polishing tests were performed with a UNIPOL-1502 polishing equipment (Shenyang Kejing Instrument Co. LTD, China). Workpieces were 95 mm× 1.25 mm aluminum alloy hard disk substrates with NiP plated, which consisted of about 85 wt.% nickel and 15 wt.% phosphorus elements with an average roughness (Ra) of about 40 nm. The polishing pad was a Rodel porous polyurethane pad. The polishing conditions were as follows: the polishing pressure was varied between 2 psi and 10 psi; the rotating speed was varied between 20 rpm and 80 rpm and the polishing time was 30 min. The slurry supplying rate was 900 ml/min. After polishing, the hard disk substrates were washed with ultrasonic in a cleaning solution containing 0.5 wt.% surfactant in distilled water. Finally, they were dried by a multifunctional drying system. 2.5. Examination of the polished surfaces The surface average roughness (Ra) and material removal rate (MRR) were measured to evaluate the polishing effects in different polishing conditions. Ra was measured by an Ambios XI-100 surface profiler (Ambios Technology Corp., U.S.A) with the resolution of 0.1 Å. The measuring area was 93.5 μm × 93.5 μm. The mass of the hard disk substrate was measured by an analytical balance. All the data were the mean values of four tests. 3. Results and discussion
Fig. 1. TEM images of calcined porous alumina with different crystallization temperature(a= 100 °C , b = 120 °C, c = 140 °C).
3.1. Structure of porous alumina abrasives Fig. 1 shows the TEM of porous alumina prepared by SDS template. The examples show that the pore structures have wormhole-like appearance and no significant order in pore arrangement. The pore diameter and pore volume were measured by nitrogen adsorption desorption isotherms. Table 1 shows the pore diameter and pore volume of the porous alumina with different crystallization
Table 1 Pore parameters of porous alumina with different crystallization temperatures. Hydrothermal temperature
100 °C
120 °C
140 °C
Pore diameter (nm) Pore volume (cm3/g)
10.25 0.46
9.34 0.38
8.61 0.35
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temperatures. When the crystallization temperature increases, the pore diameter and pore volume of the porous alumina decrease. 3.2. CMP performances of slurry containing porous alumina abrasive Understanding the effect of abrasives with different pore diameter on the CMP is very important. In order to get more details of the difference in polishing performances, the effects of abrasives with different pore diameter on Ra and MRR were studied in hard disk substrate CMP with the same conditions. As shown in Fig. 2, with the increasing of the pore diameter, the MRR of the porous alumina firstly increases and then the MRR decreases. The MRR of the porous alumina with 8.61 nm pore diameter are higher than that of the solid alumina. This may be that porous alumina which has the pores can adsorb the more H2O2 than the solid alumina, and H2O2 can erode the material of the hard disk substrate. In addition, the slurry of porous alumina may contain more particles to enhance the MRR since the density of pore abrasive is lower than that of solid abrasive and particle sizes of the abrasives are similar [14]. When the pore diameter of the porous alumina increases further, the MRR decreases. This phenomenon may be due to the hardness decrease effect of the porous alumina. When the pore diameter of the porous alumina becomes larger, the porous alumina may become softer. And in this condition, the effect of hardness decrease on CMP is greater than that of the H2O2 adsorption increase on CMP, which leads to the MRR decreases. Fig. 3 shows the Ra of hard disk substrate polished by different pore diameter alumina. When the pore diameter becomes bigger, the porous alumina abrasive is softer. So the liner tendency of steep decrease in Ra value is caused by the decreasing hardness of porous alumina. There have been a lot of studies [15–20] about the effects of process parameters on CMP performances, in which almost no study in involved in CMP of hard disk substrate with porous alumina abrasive. So, the effects of polishing parameters such as polishing pressure and rotating speed on MRR and Ra were studied in hard disk substrate CMP with porous alumina abrasive crystallized at 140 °C (namely, 8.61 nm pore diameter) and solid alumina abrasive. Fig. 4 (a) shows the MRR of hard disk substrates polished by solid alumina abrasive and porous alumina abrasive with the different rotating speed. With the increasing of rotating speed, the MRR of the two abrasives both increase, which is attributed to the increasing chance of friction between hard disk substrate, polishing pad and abrasive. But the MRR of the two abrasives are close until the rotating speed is faster than 60 rpm. This may be due to channel structure of the porous abrasive. When the rotating speed is slower, the slurry in the polishing domain is enough and the slurry which absorbs in the channel cannot
Fig. 2. The MRR of hard disk substrate polished by solid alumina and porous alumina with different pore diameters (Polishing pressure= 8 psi; rotating speed = 60 rpm; polishing time = 30 min).
Fig. 3. The Ra of hard disk substrate polished by solid alumina and porous alumina with different pore diameters (Polishing pressure= 8 psi; rotating speed = 60 rpm; polishing time = 30 min).
have the significant effect. But when the rotating speed is faster, most of the slurry is throwing out from the pad, the slurry in the pad are not enough and the solid alumina abrasive cannot have the enough slurry to polish. But the porous alumina abrasive which can store slurry in the channels can provide the enough slurry to polish, so the MRR of the porous alumina abrasive is higher than solid alumina abrasive at high rotating speed. Fig. 4 (b) shows the Ra of hard disk substrate polished by solid alumina abrasive and porous alumina abrasive with the different
Fig. 4. The (a) MRR and (b) Ra of hard disk substrate polished by solid alumina and 8.61 nm pore diameter alumina with different rotating speed (Polishing pressure= 8 psi; polishing time = 30 min).
H. Lei et al. / Thin Solid Films 520 (2012) 2868–2872
rotating speed. It can be seen that, the two kinds of abrasives exhibit the similar changing trends. With the increasing of rotating speed, the Ra of the polished surfaces decreases at first and reaches the minimum at about 40 rpm, and then increases when the rotating speed is faster than 60 rpm. This may be attributed to the appearance of tremor for the polishing equipment at high rotating speed, which causes the unstability of polishing process and leads to the increasing of the Ra. It should be also noticed that, the Ra of the porous alumina abrasive is lower than that of the solid alumina abrasive. This may be due to the hardness difference of the two abrasives. The pore channel structure makes the porous alumina abrasive susceptible to small deformation. It is to say that the porous alumina abrasive is softer than the solid alumina abrasive, which can decrease excessive mechanical damage. So the porous alumina abrasive can reduce the Ra. Fig. 5 (a) shows the MRR of the hard disk substrates polished by solid alumina and porous alumina abrasives with the different polishing pressure. As the polishing pressure increases, the MRR of the two kinds of abrasives increases continuously. This may be due to a stronger mechanical grinding effect to impact and grind the substrate surfaces at high polishing pressure. And it is found that hard disk substrate polished by porous alumina abrasive gives higher MRR. This may be due to channel structure of the porous abrasive and the increased number of particles in slurry. The channel structure of the porous alumina abrasive may absorb H2O2 during CMP polishing, which can improve the chemical effect of slurry. And, the pore abrasive slurry will contain more particles than the solid abrasive slurry since the density of pore abrasive is lower than that of solid abrasive and particle sizes of the two abrasives are similar. More particles in slurry can enhance the
Fig. 5. The (a) MRR and (b) Ra of hard disk substrate polished by solid alumina and 8.61 nm pore diameter alumina with different polishing pressure (Rotating speed=60 rpm; polishing time=30 min).
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MRR [14]. So the MRR of the porous alumina is higher than that of the solid alumina. The relationship of Ra of the hard disk substrates polished by solid alumina and porous alumina abrasives as a function of the polishing
Fig. 6. Surface profiles of hard disk substrates before polishing (a), polished by solid alumina (b) and porous alumina abrasives (c). Polishing conditions: rotating speed=60 rpm; polishing pressure=8 psi; polishing time=30 min. (a) before polishing (Ra=37.28 nm). (b) Polished by solid alumina abrasive (Ra=18.08 nm). (c) Polished by 8.61 nm pore diameter alumina abrasive (Ra=14.84 nm).
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pressure is shown in Fig. 5 (b). With the increasing of the polishing pressure, the Ra of polished surfaces with the two different abrasives decreases gradually and reaches the minimum when the polishing pressure is 8 psi, and then increases when the polishing pressure is beyond 10 psi. The phenomenon is similar to the CMP of glass substrate in the literature [9]. This may be attributed to the appearance of asymmetric polishing and disk substrate distortion at high polishing pressure, which leads to the increasing of the Ra. And it is found that the Ra of the porous alumina abrasive is lower than that of the solid alumina abrasive under the testing polishing pressures. The lower hardness of the porous alumina abrasive may be the main reason. The porous alumina abrasive is softer than the solid alumina abrasive, so the porous abrasive may cause the lower mechanical damage and the Ra is lower. Further, in order to investigate the difference in polishing performances between solid alumina and porous alumina abrasive with 8.61 nm pore diameter, the surface topography of the polished hard disk substrate was measured by optical profilometry and the results are shown in Fig. 6. It is shown that the surface before polishing is very rough with many scratches and the Ra is 37.28 nm. After polishing in slurry containing solid alumina abrasive, the surface becomes smooth and Ra is 18.08 nm, but some small scratches still exist there. However, when polished with slurry containing porous alumina abrasive under the same polishing conditions, the surface becomes smoother, the scratches could hardly be observed and Ra is 14.84 nm. The lower Ra value means the higher surface planarization. In other words, the prepared porous alumina abrasive possesses higher surface planarization than solid alumina abrasive. The improvement in CMP performance of porous alumina abrasive comparing with solid alumina abrasive may be attributed to its pore channel structure. The pore channel structure makes the porous particles susceptible to small deformation, so the porous alumina abrasive is softer than the corresponding solid alumina abrasive. Thus the channel structure can decrease excessive mechanical damage. Therefore the scratching was prevented and surface roughness was reduced. However, the detailed CMP mechanism for porous abrasive needs to be studied further. 4. Conclusions Porous alumina abrasive was prepared by hydrothermal synthesis method. In hard disk substrate CMP, the Ra of substrate surface polished
by porous alumina abrasive decreases when the pore diameter of porous alumina abrasive increases. By comparison with solid alumina abrasive, the prepared porous alumina abrasives give lower surface average roughness, and the porous alumina abrasive with 8.61 nm pore diameter has higher material removal rate under the same polishing conditions. The result implies that the porous alumina which has lower hardness and good adsorption is capable of have good surface performances in CMP.
Acknowledgments The work was supported by National Natural Science Foundation of China (No. 90923016, 50905104 and 51175317), Leading Academic Discipline Project of Shanghai Municipal Education Commission (No. J50102)and Tribology Science Fund of State Key Laboratory of Tribology (No. SKLTKF09A04).
References [1] W.J. Patrick, W.L. Guthrie, C.L. Standley, J. Electrochem. Soc. 138 (6) (1991) 1778. [2] J.M. Steiferwald, S.P. Muraaka, R.J. Getmann, Chemical Mechanical Planarization of Microelectronics Material, Wiley, New York, 1997. [3] H. Lei, J.B. Luo, Wear 257 (2004) 461. [4] H. Lei, J.B. Luo, X.C. Lu, Chin. J. Mech. Eng. 19 (4) (2006) 496 (English edition). [5] C.H. Zhou, L. Shan, R. Hight, Tribol. Trans. 45 (2002) 230. [6] A. Sorooshian, R. Ashwani, H.K. Choi, Mater. Res. Soc. Symp. Proc. 816 (2004) 125. [7] H. Lei, Y.L. Zhu, X.F. Tu, J. Funct. Mater. 36 (9) (2005) 1425 (in Chinese). [8] P.Z. Zhang, H. Lei, J.P. Zhang, Opt. Technol. 32 (5) (2006) 682 (in Chinese). [9] Z.F. Zhang, H. Lei, Microelectron. Eng. 85 (2008) 714. [10] P. Liu, H. Lei, R.L. Chen, Int. J. Abras. Technol. 3 (3) (2010) 228. [11] Q. Liu, A.Q. Wang, X.D. Wang, T. Zhang, Microporous Mesoporous Mater. 100 (2007) 35. [12] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, second ed. Academic Press, London, 1982. [13] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [14] E. Paul, J. Horn, Y. Li, S.V. Babu, Electrochem. Solid-State Lett. 10 (2007) H131. [15] N.H. Kim, M.H. Choi, S.Y. Kim, E.G. Chang, Microelectron. Eng. 83 (2006) 506. [16] S.W. Park, C.B. Kim, S.Y. Kim, Y.J. Seo, Microelectron. Eng. 66 (2003) 488. [17] G.W. Choi, K.Y. Lee, N.H. Kim, J.S. Park, Y.J. Seo, W.S. Lee, Microelectron. Eng. 83 (2006) 2213. [18] S.C. Lin, M.L. Wu, Int. J. Mach. Tool Manuf. 42 (2002) 99. [19] W. Cho, Y. Ahn, C.W. Baek, Y.K. Kim, Microelectron. Eng. 65 (2003) 13. [20] Z. Stavreva, D. Zeidler, M. P16tner, K. Drescher, Microelectron. Eng. 37/38 (1997) 143.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Preparation of porous alumina abrasives and their chemical mechanical polishing behavior Hong Lei ⁎, Xin Wu, Ruling Chen 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 9 March 2011 Received in revised form 9 November 2011 Accepted 25 November 2011 Available online 2 December 2011 Keywords: Chemical mechanical polishing (CMP) Porous alumina abrasive Hard disk substrate Planarization
a b s t r a c t Porous alumina abrasives with different pore sizes were prepared using hydrothermal synthesis method by different hydrothermal temperatures. The pore structure, pore size and pore volume of the products were characterized by transmission electron microscopy and nitrogen adsorption desorption isotherm measurement. The chemical mechanical polishing (CMP) performances of porous alumina abrasives in hard disk substrate CMP were investigated. The results show that, the polished surface average roughness (Ra) decreases when the pore diameter of porous alumina abrasive increases. By comparison with solid alumina abrasive, the prepared porous alumina abrasives give lower Ra, and the porous alumina abrasive with 8.61 nm pore diameter has higher material removal rate under the same polishing conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Chemical mechanical polishing (CMP) has become a widely used global planarization technology in the manufacturing of semiconductor and computer hard disks driver [1–4]. Slurry is the main influencing factor on the polished surface quality during CMP process. Slurry consists of abrasive, lubricant, oxidizer, deionized water and so on. So abrasive is a key influencing factor on the polished surface quality. The size and distribution, dispersibility, hardness and species of abrasive are crucial for a desired CMP performances [5]. The traditional abrasives, such as silica, alumina, ceria, etc., have been widely studied [6–8] and used in the commercial slurries. But all these abrasives are compact solid particles and easy to cause polishing scratches. Reducing the hardness of abrasives is an effective way to improve the polished surface quality [9]. The porous abrasive because of their special pore channel structure is one of the effective ways to reduce the hardness of the abrasives. In our previous work, Liu and Lei [10] prepared porous silica abrasive, which showed better hard disk substrate CMP performances than pure solid silica abrasive. It showed that abrasive with narrow pores can improve the polished surface quality because of the abrasive's adsorption. The alumina is a widely used abrasive. But up to now, porous alumina as abrasive in CMP slurry has seldom been reported. So, porous alumina abrasives were prepared in the present paper and
⁎ Corresponding author. E-mail address: [email protected] (H. Lei). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.11.057
their CMP performances on hard disks have been investigated preliminarily. 2. Experimental methods 2.1. Preparation of porous alumina abrasives Porous alumina abrasives with different pore sizes were prepared using hydrothermal synthesis method by different hydrothermal temperatures [11]. Sodium dodecyl sulfate was used as a polymeric template. 150 g of Al(NO3)3·9H2O was dissolved in 200 mL deionized water and then mixed with 200 ml of 2 mol/L ammonia solution. The pH value of the aluminum nitrate solution was 2.8 after the addition of ammonia. Then 72 g of urea and 28.8 g of sodium dodecyl sulfate (SDS) were added into the above solution under stirring and the obtained homogeneous solution was put into a sealed Teflon-lined stainless autoclave vessel for static hydrothermal reaction at certain crystallization temperature for 48 h. After that, the resulting white precipitate was recovered by centrifugation, washed with deionized water for several times and dried at 110 °C for 12 h. The final product was obtained by calcination at 800 °C for 4 h in air. In this paper, porous alumina abrasives with different pore diameters were prepared by different crystallization temperatures such as 100 °C, 120 °C and 140 °C. 2.2. Characterization of porous alumina abrasives Nitrogen adsorption–desorption isotherms were measured at −196 °C on a Micromeritics ASAP 2020 M + C apparatus. The specific surface areas (SBET) of the samples were calculated with the BrunauerEmmett-Teller equation [12]. The total pore volumes (Vp) were
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determined at a relative pressure P/P0 (P is pressure of adsorbed solute, and P0 is saturated vapor pressure of sorbent) value of 0.995. The mean pore diameters (Dp) were calculated with the Barrett–Joyner–Halenda equation using the corresponding desorption branches of the isotherms [13]. The measurement uncertainties of the SBET, Vp and Dp were below 5%. Transmission electron microscopy (TEM) images were obtained on a JEOL 2000 CX electron microscope operating at an accelerating voltage of 200 kV. The samples were ultrasonically dispersed in ethanol and then dropped onto the carbon-coated copper grids prior to the observation. Particle size was determined by using a Malvern Zetasizer 3000HS laser particle size analyzer with measuring range from 2 nm to 3000 nm. 2.3. Preparation of porous alumina slurry and solid alumina slurry The preparation of porous alumina slurry and solid alumina slurry (a commercial alumina powder) have the same process. Firstly, 3 wt.% porous alumina (or solid alumina) powder and 4.5 wt.% functional additives were added into distilled water in a container under continuously stirring. Secondly, the mixture was milled for 4 h in a vibrator containing ZrO2 balls as the milling balls. After the milling, the average diameters of the three porous alumina were 203.1 nm, and the average diameter of the solid alumina was 180.2 nm (measured by Malvern Zetasizer 3000HS laser particle size analyzer). The particle diameters of the four abrasives were close. Thirdly, the mixture was filtrated with a 10 μm pore filter. Before polishing, 6.0 wt.% H2O2 as oxidant was added to the mixture to obtain the slurry. 2.4. Polishing tests Polishing tests were performed with a UNIPOL-1502 polishing equipment (Shenyang Kejing Instrument Co. LTD, China). Workpieces were 95 mm× 1.25 mm aluminum alloy hard disk substrates with NiP plated, which consisted of about 85 wt.% nickel and 15 wt.% phosphorus elements with an average roughness (Ra) of about 40 nm. The polishing pad was a Rodel porous polyurethane pad. The polishing conditions were as follows: the polishing pressure was varied between 2 psi and 10 psi; the rotating speed was varied between 20 rpm and 80 rpm and the polishing time was 30 min. The slurry supplying rate was 900 ml/min. After polishing, the hard disk substrates were washed with ultrasonic in a cleaning solution containing 0.5 wt.% surfactant in distilled water. Finally, they were dried by a multifunctional drying system. 2.5. Examination of the polished surfaces The surface average roughness (Ra) and material removal rate (MRR) were measured to evaluate the polishing effects in different polishing conditions. Ra was measured by an Ambios XI-100 surface profiler (Ambios Technology Corp., U.S.A) with the resolution of 0.1 Å. The measuring area was 93.5 μm × 93.5 μm. The mass of the hard disk substrate was measured by an analytical balance. All the data were the mean values of four tests. 3. Results and discussion
Fig. 1. TEM images of calcined porous alumina with different crystallization temperature(a= 100 °C , b = 120 °C, c = 140 °C).
3.1. Structure of porous alumina abrasives Fig. 1 shows the TEM of porous alumina prepared by SDS template. The examples show that the pore structures have wormhole-like appearance and no significant order in pore arrangement. The pore diameter and pore volume were measured by nitrogen adsorption desorption isotherms. Table 1 shows the pore diameter and pore volume of the porous alumina with different crystallization
Table 1 Pore parameters of porous alumina with different crystallization temperatures. Hydrothermal temperature
100 °C
120 °C
140 °C
Pore diameter (nm) Pore volume (cm3/g)
10.25 0.46
9.34 0.38
8.61 0.35
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temperatures. When the crystallization temperature increases, the pore diameter and pore volume of the porous alumina decrease. 3.2. CMP performances of slurry containing porous alumina abrasive Understanding the effect of abrasives with different pore diameter on the CMP is very important. In order to get more details of the difference in polishing performances, the effects of abrasives with different pore diameter on Ra and MRR were studied in hard disk substrate CMP with the same conditions. As shown in Fig. 2, with the increasing of the pore diameter, the MRR of the porous alumina firstly increases and then the MRR decreases. The MRR of the porous alumina with 8.61 nm pore diameter are higher than that of the solid alumina. This may be that porous alumina which has the pores can adsorb the more H2O2 than the solid alumina, and H2O2 can erode the material of the hard disk substrate. In addition, the slurry of porous alumina may contain more particles to enhance the MRR since the density of pore abrasive is lower than that of solid abrasive and particle sizes of the abrasives are similar [14]. When the pore diameter of the porous alumina increases further, the MRR decreases. This phenomenon may be due to the hardness decrease effect of the porous alumina. When the pore diameter of the porous alumina becomes larger, the porous alumina may become softer. And in this condition, the effect of hardness decrease on CMP is greater than that of the H2O2 adsorption increase on CMP, which leads to the MRR decreases. Fig. 3 shows the Ra of hard disk substrate polished by different pore diameter alumina. When the pore diameter becomes bigger, the porous alumina abrasive is softer. So the liner tendency of steep decrease in Ra value is caused by the decreasing hardness of porous alumina. There have been a lot of studies [15–20] about the effects of process parameters on CMP performances, in which almost no study in involved in CMP of hard disk substrate with porous alumina abrasive. So, the effects of polishing parameters such as polishing pressure and rotating speed on MRR and Ra were studied in hard disk substrate CMP with porous alumina abrasive crystallized at 140 °C (namely, 8.61 nm pore diameter) and solid alumina abrasive. Fig. 4 (a) shows the MRR of hard disk substrates polished by solid alumina abrasive and porous alumina abrasive with the different rotating speed. With the increasing of rotating speed, the MRR of the two abrasives both increase, which is attributed to the increasing chance of friction between hard disk substrate, polishing pad and abrasive. But the MRR of the two abrasives are close until the rotating speed is faster than 60 rpm. This may be due to channel structure of the porous abrasive. When the rotating speed is slower, the slurry in the polishing domain is enough and the slurry which absorbs in the channel cannot
Fig. 2. The MRR of hard disk substrate polished by solid alumina and porous alumina with different pore diameters (Polishing pressure= 8 psi; rotating speed = 60 rpm; polishing time = 30 min).
Fig. 3. The Ra of hard disk substrate polished by solid alumina and porous alumina with different pore diameters (Polishing pressure= 8 psi; rotating speed = 60 rpm; polishing time = 30 min).
have the significant effect. But when the rotating speed is faster, most of the slurry is throwing out from the pad, the slurry in the pad are not enough and the solid alumina abrasive cannot have the enough slurry to polish. But the porous alumina abrasive which can store slurry in the channels can provide the enough slurry to polish, so the MRR of the porous alumina abrasive is higher than solid alumina abrasive at high rotating speed. Fig. 4 (b) shows the Ra of hard disk substrate polished by solid alumina abrasive and porous alumina abrasive with the different
Fig. 4. The (a) MRR and (b) Ra of hard disk substrate polished by solid alumina and 8.61 nm pore diameter alumina with different rotating speed (Polishing pressure= 8 psi; polishing time = 30 min).
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rotating speed. It can be seen that, the two kinds of abrasives exhibit the similar changing trends. With the increasing of rotating speed, the Ra of the polished surfaces decreases at first and reaches the minimum at about 40 rpm, and then increases when the rotating speed is faster than 60 rpm. This may be attributed to the appearance of tremor for the polishing equipment at high rotating speed, which causes the unstability of polishing process and leads to the increasing of the Ra. It should be also noticed that, the Ra of the porous alumina abrasive is lower than that of the solid alumina abrasive. This may be due to the hardness difference of the two abrasives. The pore channel structure makes the porous alumina abrasive susceptible to small deformation. It is to say that the porous alumina abrasive is softer than the solid alumina abrasive, which can decrease excessive mechanical damage. So the porous alumina abrasive can reduce the Ra. Fig. 5 (a) shows the MRR of the hard disk substrates polished by solid alumina and porous alumina abrasives with the different polishing pressure. As the polishing pressure increases, the MRR of the two kinds of abrasives increases continuously. This may be due to a stronger mechanical grinding effect to impact and grind the substrate surfaces at high polishing pressure. And it is found that hard disk substrate polished by porous alumina abrasive gives higher MRR. This may be due to channel structure of the porous abrasive and the increased number of particles in slurry. The channel structure of the porous alumina abrasive may absorb H2O2 during CMP polishing, which can improve the chemical effect of slurry. And, the pore abrasive slurry will contain more particles than the solid abrasive slurry since the density of pore abrasive is lower than that of solid abrasive and particle sizes of the two abrasives are similar. More particles in slurry can enhance the
Fig. 5. The (a) MRR and (b) Ra of hard disk substrate polished by solid alumina and 8.61 nm pore diameter alumina with different polishing pressure (Rotating speed=60 rpm; polishing time=30 min).
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MRR [14]. So the MRR of the porous alumina is higher than that of the solid alumina. The relationship of Ra of the hard disk substrates polished by solid alumina and porous alumina abrasives as a function of the polishing
Fig. 6. Surface profiles of hard disk substrates before polishing (a), polished by solid alumina (b) and porous alumina abrasives (c). Polishing conditions: rotating speed=60 rpm; polishing pressure=8 psi; polishing time=30 min. (a) before polishing (Ra=37.28 nm). (b) Polished by solid alumina abrasive (Ra=18.08 nm). (c) Polished by 8.61 nm pore diameter alumina abrasive (Ra=14.84 nm).
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pressure is shown in Fig. 5 (b). With the increasing of the polishing pressure, the Ra of polished surfaces with the two different abrasives decreases gradually and reaches the minimum when the polishing pressure is 8 psi, and then increases when the polishing pressure is beyond 10 psi. The phenomenon is similar to the CMP of glass substrate in the literature [9]. This may be attributed to the appearance of asymmetric polishing and disk substrate distortion at high polishing pressure, which leads to the increasing of the Ra. And it is found that the Ra of the porous alumina abrasive is lower than that of the solid alumina abrasive under the testing polishing pressures. The lower hardness of the porous alumina abrasive may be the main reason. The porous alumina abrasive is softer than the solid alumina abrasive, so the porous abrasive may cause the lower mechanical damage and the Ra is lower. Further, in order to investigate the difference in polishing performances between solid alumina and porous alumina abrasive with 8.61 nm pore diameter, the surface topography of the polished hard disk substrate was measured by optical profilometry and the results are shown in Fig. 6. It is shown that the surface before polishing is very rough with many scratches and the Ra is 37.28 nm. After polishing in slurry containing solid alumina abrasive, the surface becomes smooth and Ra is 18.08 nm, but some small scratches still exist there. However, when polished with slurry containing porous alumina abrasive under the same polishing conditions, the surface becomes smoother, the scratches could hardly be observed and Ra is 14.84 nm. The lower Ra value means the higher surface planarization. In other words, the prepared porous alumina abrasive possesses higher surface planarization than solid alumina abrasive. The improvement in CMP performance of porous alumina abrasive comparing with solid alumina abrasive may be attributed to its pore channel structure. The pore channel structure makes the porous particles susceptible to small deformation, so the porous alumina abrasive is softer than the corresponding solid alumina abrasive. Thus the channel structure can decrease excessive mechanical damage. Therefore the scratching was prevented and surface roughness was reduced. However, the detailed CMP mechanism for porous abrasive needs to be studied further. 4. Conclusions Porous alumina abrasive was prepared by hydrothermal synthesis method. In hard disk substrate CMP, the Ra of substrate surface polished
by porous alumina abrasive decreases when the pore diameter of porous alumina abrasive increases. By comparison with solid alumina abrasive, the prepared porous alumina abrasives give lower surface average roughness, and the porous alumina abrasive with 8.61 nm pore diameter has higher material removal rate under the same polishing conditions. The result implies that the porous alumina which has lower hardness and good adsorption is capable of have good surface performances in CMP.
Acknowledgments The work was supported by National Natural Science Foundation of China (No. 90923016, 50905104 and 51175317), Leading Academic Discipline Project of Shanghai Municipal Education Commission (No. J50102)and Tribology Science Fund of State Key Laboratory of Tribology (No. SKLTKF09A04).
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