Frictional Characteristics of a Nanoporous SiO 2 Film with a Surface-Treated by Chemical Mechanical Polishing

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1 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009, pp Frictional Characteristics of a Nanoporous SiO 2 Film with a Surface-Treated by Chemical Mechanical Polishing Hojoong Kim SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon Daekwang Woo, Ji Hun Kim, Young Ze Lee, Atul Kulkarni and Taesung Kim Department of Mechanical Engineering, Sungkyunkwan University, Suwon Min Suk Kang, Jeong Kuk Shon and Ji Man Kim Department of Chemistry, Sungkyunkwan University, Suwon (Received 16 June 2008) The use of nanoporous thin lms has proven to be benecial when surface interactions occur. The porous structure inside the lms has the capacity to store lubricating materials, such as oil or carbon, which can provide good wear resistance and lubrication performance. If such performance is to be obtained, a uniform pore size and structure is very important. In this study, nanoporous SiO 2 lms were deposited on silicon wafers by using a sol-gel method. The porous structure was developed using a surfactant of pluronic (F127) and cetyltrimethylammonium bromide (CTAB). The surfaces of these lms were treated using a chemical mechanical polishing (CMP) process. Before and after the surface treatment, the lms were evaluated using a ball-on-disk-type friction and wear tester to obtain the frictional characteristics. The lms with a porous structure showed a lower coecient of friction (COF) than those without pores. The COF decreased even more after the lms were surface treated by CMP. The surface roughness appeared to aect the frictional characteristics more strongly after CMP. In addition, the wear resistance diered with the type of porous structure. PACS numbers: Pq, Ps, Rm Keywords: Nanoporous, Mesoporous, Thin lm, Chemical mechanical polishing, Tribology I. INTRODUCTION The discovery of highly ordered mesoporous silica materials was reported by scientists at the Mobil Corporation in 1992 [1]. Subsequently, nanoporous lms were fabricated with pores in a uniform two-dimensional hexagonal shape and with tunable diameters between 2 and 30 nm [2]. Nanoporous silica materials have a variety of applications on account of their large surface area (700 { 1500 m 2 /g), high chemical and thermal stability and easy functionalization. The application of the rst generation of mesoporous silica was reported in 1996 [3] and nanoporous silica lms have been implemented successfully as membranes, sensors, catalysts and low-dielectric (low-k) materials [4{7]. Typically, porous materials are used on the surfaces of bearings and act as a lubricant container to impart a self-lubrication behavior [8]. Thin lms consisting of a nanoporous structure may also have similar self- tkim@skku.edu; Fax: lubrication behavior due to their containing lubricants, such as oil and carbon, inside the pores. Tribological investigations of thin lms on nonporous structures, such as diamond-like carbon (DLC) lms and oxide metals, have been carried out by many researchers [9{15]. There are several processes available for lm formation but the sol-gel process has been the most widely used lm formation technique since the middle of the 1900s [16]. The sol-gel process is a wet-chemical technique that is used mainly for the fabrication of metal oxides and glass like inorganic silica. The advantages of the sol-gel process are high-purity products, its being a low-temperature process, its usually mild ph conditions, high homogeneity, a controlled level of porosity, etc. [17]. The nanoporous structure inside the lm is constructed using the concept of self-assembly supramolecular chemistry [18{20]. The sol-gel process allows the mixing of inorganic and organic components on a nanometer scale, in what is known as a hybrid organic-inorganic nanocomposite [21,22]. Hybridization is carried out by using organic template growth in stable inorganic materials, usually inside the pores. The hybrid organic-inorganic con

2 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 cept reinforces the nanoporous structure by lling the pores with functional groups [23]. Silica is usually used in this nanoporous lm structure on account of its ability for functionalization. This study focused on the tribological behavior of nanoporous silica lms. Two types of nanoporous SiO 2 lms were prepared by using a sol-gel process with surfactants such as cetyltrimethylammonium bromide (CTAB) and pluronic F127 (henceforth referred to as F127 only in the text). The lms were characterized by using atomic force microscopy (AFM), X-ray diraction (XRD), transmission electron microscopy (TEM) and ellipsometry. The surface treatments were carried out using a chemical mechanical polishing (CMP) process. The coecient of friction (COF) of the nanoporous lm surface was examined using a ball-on-disk type friction and wear tester. II. EXPERIMENT During the sol-gel process, hydrochloric acid (HCl), ethanol (EtOH), water (H 2 O) and tetraethylorthosilicate (TEOS) were mixed. TEOS (98 % purity) was used as the silica precursor, HCl was used as a condensation catalyst and the remaining two species, EtOH and H 2 O, were used as volatile species. The fraction of volatile species determines the kinetics of the hydrolysiscondensation reactions. During the experiment, separate solutions were prepared by adding two types of surfactants, CTAB and F127, to evaluate the formation of the self-assembled micelle structures. The surfactant CTAB was chosen due to its well-known properties. The surfactant F127 was chosen to observe the eect of dierent porous structure. For the CTAB solution, g of surfactant, 6 g of TEOS, 2 g of HCl and 6.54 g of EtOH were mixed whereas 0.6 g, 2 g, 1.5 g and 15 g of the respective compounds were mixed for the F127 solution. The prepared solutions were spin-coated on a silicon wafer for 1 min at rpm. In self-assembly of the micelle structure, a surfactant concentration that exceeds the critical micelle concentration played a vital role. During the \evaporation induced self assembly (EISA)" process, solution evaporation at the air/lm interface was the driving force for self-assembly. This process was used to form highly-ordered nanoporous structures [24]. The lms were aged for 24 hs at 40 C in an oven to form a stable structure. The lms were further heat treated at 450 C for 3 hs to remove the surfactant. The nanoporous lms had dimensions of 25 mm 25 mm 1 mm. A surface-treatment method, chemical mechanical polishing (CMP) (Logitech, PM-5) process, was carried out to obtain a surface planarization eect. In addition, exposure of the underlying pores to the lm surface was also one of anticipated eects of CMP. CMP was carried out for 30 s at a pressure of 4 psi, xing the spindle and the plate velocity to 50 rpm. The slurry ow was kept constant at 0.14 lpm. Commercialized slurry was used for polishing the silica substrate during the process (Cheil industries, starplanar-4000). The frictional characteristics were determined using a ball-on-disk type sliding tester (Daekyung tech, DTA- 408) with 10 mm, AISI steel bearing balls (hardness: 62HRC, Ra: 0.02 m,). The COF of the lms was measured under dry friction conditions. The sliding velocity between the ball and the lm and the contact force were xed at 15 rpm (11.78 mm/s) and 2.6 N (265 g), respectively. The wear track diameter was 30 mm and the friction force was measured using a load cell attached to the equipment. The frictional characteristics under boundary lubrication conditions were evaluated under conditions similar to those used for the dry friction case by using mineral oil. The two frictional conditions, dry and boundary, were compared. XRD (Rigaku diractometer) and TEM (JEOL, HETEM) were used to observe the porous structure of the lm. The XRD results for the lms formed by using dierent surfactants were compared to determine the relative size of the pores [25]. The lm's surface and thickness were determined by using AFM (Thermomicroscopes), SEM (Hitachi, FESEM) and ellipsometry (Ellipsotech, EHi-SE-F). III. RESULT AND DISCUSSION Figure 1 shows the XRD patterns of the nanostructured lms formed using the two surfactants, CTAB and F127. The peak signals for both SiO 2 lms were observed at approximately for CTAB and 1 2 for F127. The sharp peaks in the XRD pattern indicate that the pore structure is well ordered. The angle of the peak provides information on the d-spacing of the lm and the surfactant CTAB forms a smaller micelle structure than the polymer surfactant F127. Due to the resulting d-spacing and size of the surfactants, the F127 lm is expected to have a pore size larger than the CTAB lms. To give the pore size in numbers, we calculated the size by using the methods reported by Zhao et al. [26]. The resulting pore sizes of the CTAB and the F127 lms were approximately 3 nm and 8 nm, respectively. Similar results were obtained by using TEM, as shown in Figure 2. In both images, the scale bar is 50 nm. Both TEM images show well-ordered porous structures with the desired pore sizes. The surfactant F127 has a larger pore size than CTAB, conrming the XRD results. The detailed nanostructure was not examined in this study. However, the TEM image showed that the F127 lm had mostly a 3D cubic structure whereas the CTAB lm had only a 2D hexagonal structure. The conditions used to obtain this result were reviewed by Nicole et al. [27]. The thicknesses of the F127 and the CTAB lms were measured by using AFM, SEM and ellipsometry. The average thicknesses of the F127 lms using these tech-

3 Frictional Characteristics of a Nanoporous SiO 2 Film { Hojoong Kim et al Fig. 1. XRD pattern of the nanostructured lms formed using a surfactant: (a) CTAB and (b) F127. Fig. 3. AFM images of the surface morphology before and after the CMP process: (a) CTAB before, (b) F127 before, (c) CTAB after and (d) F127 after. (e) Surface prole of CTAB along the indicated line and (f) Surface prole of F127. Fig. 2. TEM image of nanoporous lms formed using a surfactant: (a) CTAB and (b) F127. For both the images, the scale bar is 50 nm niques were 307 nm, 202 nm and 232 nm, respectively. The result shows that the thickness measured by using AFM is comparatively higher than that measured using the other two methods. This is due to the steep boundary of the lm. Therefore, the SEM measurements were considered as a reference for further analysis. Similarly, the mean thickness of the CTAB lm was approximately 900 nm, which is larger than that of the F127 lm. This dierence was induced by the dierent viscosities of the two solutions. The solution with the CTAB surfactant showed a higher viscosity than the solution with F127. This dierence caused a dierent layer thickness during spin coating. Figure 3 shows AFM images of the surface morphologies of the lms formed before and after the CMP process. The results show a good planarization eect due to the CMP process for both the F127 and the CTAB cases. Before the CMP process, the average surface roughness (R a ) was approximately 19 A for the F127 lm whereas it was 29 A for the CTAB lm. After 30 s CMP, the R a was changed to 1.9 A for the F127 lm and to 4.4 A for the CTAB lm. However, surface-exposed pores were not observed by using AFM. As shown in Figures 3(e) and (f), the cross sectional proles of the lms were observed after the CMP process, but no exposed pores were found. In addition, a line patterned surface prole was observed for both lms after CMP, which were thought to be caused by abrasive wear by the slurry particles during CMP. This abrasive wear might be a reason for the lack of exposed pores because it induced a collapse of the surface porous structure.

4 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 Fig. 4. COF of the nanoporous lms (a) before surface treatment, (b) after a CMP process with dry friction and (c) after CMP with boundary lubrication. Figure 4(a) shows the value of the COF for the F127 and the CTAB lms before the surface treatment under dry friction conditions. The COFs of both lms were lower than that of the wafer substrate itself and that of a SiO 2 lm with no pore structure. The CTAB lm showed a lower frictional coecient than the F127 lm. These results demonstrate that the lms with a porous structure have a lower frictional coecient. The lms with surfaces treated using the CMP process showed signicantly improved frictional performance, as shown in Figure 4(b). The resulting COF of the F127 lm was lower than that of the CTAB lm. However, with increasing number of wear cycles, the COF increased due to the low wear resistance of the lms. All the friction tests suggest that the surface roughness and the porous structure have a strong inuence on the COF. The surface roughness of the CTAB lm was higher than that of the F127 lm before the CMP process. However, the resulting COFs did not follow that trend. As shown in Figure 4(a), the COF in the F127 case behaves similar to that of a silica lm without pores, whereas the COF of the CTAB case shows the lowest value. This is due to the dierent wear resistances in the two cases. During the friction test, the silica lm in the CTAB case collapsed more than that in the F127 case and the residue of the collapsed silica act as a lubricant during test. On the other hand, the lm in the F127 case has a harder structure and behaves similar to a silica lm without pores. During the CMP process, the lms under test were pressed, causing collapse of the porous structure. Due to this, the COF test results after the CMP process depended on the surface roughness. Hence, we observe that the COF of CTAB is higher than that of F127 and the result is shown in Figure 4(b). Figure 4(c) shows the results of the boundary lubrication experiment with mineral oil, which was carried out to observe the function of the porous structure as a lubricant container. The results show that there is no signicant change in the COF during the experiment. This suggests a possibility that a porous structure may be successfully used as a self-lubrication material. Further functionalization can be performed to contain the lubricants in the porous structure. The observations of

5 Frictional Characteristics of a Nanoporous SiO 2 Film { Hojoong Kim et al ACKNOWLEDGMENTS Nanoporous SiO 2 lms were prepared using a sol-gel method with two dierent surfactants CTAB and F127. These nanoporous lms were evaluated systematically for their frictional characteristics. The lms with the F127 surfactant had a larger pore size than those with the CTAB. The nanoporous lms with the F127 and the CTAB surfactants had thicknesses on the order of 200 nm and 900 nm, respectively, as measured by using SEM and ellipsometry. The planarization eect of the CMP process enhanced the frictional performance of the lms. The surface roughness aect the frictional behavior more after CMP than before. Before CMP, the existence of a porous structure had a more signicant eect. The lms showed potential for being used as a lubricant reservoir. However, further functionalization will be needed to ll the pores with lubricants. In addition, the nanoporous lms showed dierent wear resistances for dierent surfactants. This study was supported by grant No. (R ) from the Basic Research Program of the Korea Science & Engineering Foundation. REFERENCES Fig. 5. Wear volume of the F127 and the CTAB lms. Figures 4(b) and (c) are similar to those in the work of Piwonski [17], which showed that the COF depends on the surface roughness. The porous structures of the F127 and the CTAB lms also inuenced the wear volume. Figure 5 shows the wear volume of the track after the ction test, which shows that the CTAB case has a weaker structure than the F127 case. This is mainly due to the surfactant behaviors being dierent during the formation of lm. A cationic surfactant, such as CTAB, forms rod-shaped micelles and each micelle repels each other, resulting in a structure with a thinner framework wall. However, the surfactant F127 consists of a non-ionic polymer and it has no repulsive or attractive forces, resulting in a thicker framework wall. This leads to more structural damage as a result of friction for the CTAB case. IV. CONCLUSION [1] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 359, 710 (1992). [2] D. H. Everett, Pure Appl. Chem. 31, 578 (1972). [3] H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara and G. A. Ozin, Nature 379, 703 (1996). [4] R. D. Miller, Science 286, 421 (1999). [5] C. Y. Tsai, S. Y. Tam, Y. Lu and C. J. Brinker, J. of Membrane Sci. 169, 255 (2000). [6] P. Yang, G. Wirnsberger, H. C. Huang, S. R. Cordero, M. D. McGehee, B. Scott, T. Deng, G. M. Whitesides, B. F. Chmelka, S. K. Buratto and G. D. Stucky, Science 287, 465 (2000). [7] G. Wirnsberger, B. J. Scott and G. D. Stucky, Chem. Commun. 119 (2001). [8] W. T. Rouleau, Trans. ASME, Ser. D: J. Basic Eng. 84, 205 (1962). [9] A. Grill, Wear 168, 143 (1993). [10] S. J. Bull, Diamond Relat. Mater. 4, 827 (1995). [11] W. Zhang, W. Liu and Q. Xue, Mater. Res. Bull. 36, 1903 (2001). [12] W. Zhang, W. Liu and C. Wang, Wear 253, 377 (2002). [13] C. W. Cho and Y. Z. Lee, Wear 254, 383 (2003). [14] E. S. Y and R. A. Singh, J. Korean Phys. Soc. 52, 656 (2008). [15] Y. S. Park, H. J. Cho and B. Hong, J. Korean Phys. Soc. 51, 1119 (2007). [16] L. L. Hench and J. K. West, Chem. Rev. 90, 33 (1990). [17] I. Piwonski, Thin Solid Films 515, 3499 (2007). [18] D. Kundu, H. S. Zhou and I. Honma, J. Mater. Sci. Lett. 17, 2089 (1998). [19] D. Grosso, A. R. Balkenende, P. A. Albouy, A. Ayral, H. Amenitsch and F. Babonneau, Chem. Mater. 13, 1848 (2001). [20] J. C. K, S. Y. Kwon, B. I. Choi, H. S. Nham and Y.-G. Kim, J. Korean Phys. Soc. 47, 991 (2005). [21] D. A. Loy and K. J. Shea, Chem. Rev. 95, 1431 (1995). [22] U. Schubert, N. Huesing and A. Lorenz, Chem. Mater. 7, 2010 (1995). [23] S. Inagaki, S. Guan, T. Ohsuna and O. Terasaki, Nature 416, 304 (2002). [24] C. J. Brinker, Y. Lu, A. Sellinger and H. Fan, Adv. Mater. 11, 579 (1999). [25] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc. 114, (1992). [26] P. Y. D. Zhao, N. Melosh, J. Feng, B. E. Chmelka and C. D. Stucky, Adv. Mater. 10, 1380 (1998). [27] L. Nicole, C. Boissiere, D. Grosso, A. Quach and C. Sanchez, J. Mater. Chem. 15, 3598 (2005).