Optical and Mechanical Properties of Toluene-TEOS Hybrid Plasma-Polymer Thin Films Deposited by Using PECVD

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1 Journal of the Korean Physical Society, Vol. 55, No. 5, November 2009, pp Optical and Mechanical Properties of Toluene-TEOS Hybrid Plasma-Polymer Thin Films Deposited by Using PECVD S.-J. Cho, I.-S. Bae and J.-H. Boo Department of Chemistry, Sungkyunkwan University, Suwon S. Lee and D. Jung Department of Physics, Sungkyunkwan University, Suwon (Received 2 September 2008, in final form 11 March 2009) Toluene-tetraethoxysilane (TEOS) hybrid plasma - polymer thin films were deposited on silicon(100) substrates by using plasma enhanced chemical vapor deposition (PECVD) with a homemade stainless-steel vacuum chamber. Toluene and TEOS were utilized as the organic and the inorganic precursors with hydrogen gas for the toluene bubbler and argon gas for both the TEOS bubbler and the carrier gas. To analyze the thermal, optical, and mechanical properties of the plasma polymerized thin films, we grew hybrid plasma-polymer thin films at various radio-frequency (RF, MHz) powers, bubbling ratios of TEOS to toluene precursors, and annealing temperatures. The thin films were analyzed by using ellipsometry and Fourier-transform infrared spectroscopy (FT- IR). The refractive indices varied with the RF power, the bubbling ratio of TEOS to toluene, and the annealing temperature. Also, the IR spectra showed that the hybrid plasma-polymer thin films had totally different chemical functionalities from the liquid toluene and the TEOS precursors and that the chemical functionalities of the thin films changed with the RF power, the bubbling gas ratio of TEOS to toluene, and the annealing temperature. The SiO peak intensity increased with increasing TEOS bubbling ratio, and the -OH and the CO peak intensities decreased with increasing annealing temperature. From the field emission scanning electron microscopy (FE-SEM) results, the thickness of the thin films was determined before and after the annealing, with the thickness shrinkage (%) being measured by using SEM cross-sectional images. The MTS nanoindenter xpr was used to measure the hardness and Young s modulus and showed that both these values increased as the deposition RF power increased; these values also changed with the bubbling ratio of TEOS to toluene and with the annealing temperature. PACS numbers: x, d, g Keywords: PECVD, Dielectrics, Low-, Optical and mechanical properties DOI: /jkps I. INTRODUCTION Advances in the performance of the ultra-large-scale integrated (ULSI) circuit have been hindered in recent years by difficulties in introducing materials in the interconnect part of the Si chip technology while a large effort was invested for many years to replace the SiO 2 dielectric with materials having a significantly lower dielectric constant [1]. The problems of propagation delay, cross-talk noise, and power dissipation, due to resistancecapacitance (R-C) coupling, given the increased wiring capacitance, especially the interline capacitance between the metal lines on the same metal level, have become significant, and although the resistance is mainly affected jhboo@skku.edu; Tel: ; Fax: by the conducting materials, the capacitance is determined by the dielectric materials [2, 3]. The use of advanced inter-metal dielectrics to reduce the capacitance seems to be more important than the decrease in resistivity provided by the substitution of copper for aluminum [4]. Therefore, thin films with relatively low dielectric constants (κ < 3.0) are under intense study for their application as interlayer dielectrics. Polymer thin films give their low-κ values are considered as possible candidates for low-κ materials [5 7]. Additionally, the mechanical and thermal processing and the packaging associated with making the final chip exert significant stress on the multilevel interconnect structure. Therefore, there are many chemical and physical property requirements for ILD (interlayer dielectric) materials to fit into current industrial processes. The thermal properties of the dielectric are of particular importance and must be consistent with subsequent pro

2 Optical and Mechanical Properties of Toluene-TEOS S.-J. Cho et al cessing temperatures and thermal conductivity requirements for heat transfer within a chip. The material should be able to withstand repeated temperature cycling to high temperatures, preferably up to 450 C, without appreciable weight loss and shrinkage [8]. Mechanically, the layers must be strong enough to withstand the chemical mechanical polishing (CMP) process associated with planarization and thermal cycling. The Young s modulus, hardness, and fracture toughness must also be sufficiently high to prevent cracking and delamination [9]. II. EXPERIMENT The experiment was carried out in a homemade stainless-steel plasma enhanced chemical vapor deposition (PECVD) system. Silicon (100) wafers were wetcleaned by sonication with acetone, methyl alcohol, distilled water, and isopropyl alcohol. In addition, substrates were dry-cleaned by in-situ Ar-plasma bombardment at 100 W for 15 min. The hybrid-polymer thin films were deposited by using a PECVD method. Toluene and tetraethoxysilane (TEOS) were utilized as the organic and the inorganic precursors with co-deposition precursor ratios of TEOS to toluene of 1:10, 3:10, and 5:10. The basic properties of the precursors are shown in Table 1. Each precursor was preheated at 60 and 80 C, and bubbled with 50 sccm of hydrogen and 5, 15, and 25 sccm of argon gas. An additional 50 sccm of argon was used as the carrier gas. The deposition time was between 30 and 60 min to achieve the same thickness, 300 nm, and depended on the RF power. The general deposition pressure and temperature were Torr and 150 C, respectively. The typical condition of the PECVD process applied in this study for film deposition was a RF power of 20, 40, or 60 W. The films were then annealed at temperatures ranging from 300 to 500 C under an Ar atmosphere for 5 min by using rapid thermal process (RTP). The κ values were determined from measurements of the capacitance in the Al/hybrid polymer thin film/si/al metal-insulator-silicon-metal (MISM) structure. The capacitance was determined at 1 MHz with the MISM structure. The chemical bonding type of plasma hybridpolymer thin films was investigated by Fourier-transform infrared spectroscopy (FT-IR, Bruker Optik IFS66v/S). The ex-situ ellipsometry data of all investigated films were produced by using a Gaertner auto gain ellipsometer L116B (single wavelength, 632 nm). The thickness was measured by using field-emission scanning electron microscopy (FE-SEM, JEOL JSM700F). A nanoindenter (MTS NanoindenterR XP) was used to measure the hardness and Young s modulus of thin films. Measurements were made applied by applying a constant strain rate to the diamond indenter tip, followed by indenter unloading, while monitoring the load versus total Fig. 1. Relationship between the dielectric constant and the refractive index. indenter displacement. The hardness and modulus were continuously determined from load-displacement curves by using the continuous stiffness method. The nanoindentation depth was 300 nm. In addition, the nanoindentation results only up to a penetration depth of h tf/10 were considered in all analyses, where t f was the thickness of the hybrid plasma-polymer film on the Si substrate [10,11]. III. RESULTS AND DISCUSSION The dielectric constant is a frequency-dependent, intrinsic material property that can be divided into 3 components resulting from the electronic, ionic, and dipolar polarizations. The dielectric constants measured at 1 MHz consist of electronic ( ε e ), ionic ( ε i ), and dipolar ( ε d ) contributions, as expressed in κ(at1mhz) = 1 + ε e + ε i + ε d (1) The dielectric constant of a material can also be calculated from the refractive index, as expressed in κ(λ) = n 2 (λ) k 2 (λ) + 2ink(λ) (2) where κ is a relative dielectric constant, n is the real part of the refractive index, k is the extinction coefficient, i 2 = -1, and λ is the wavelength of the light source. The pure electronic contribution to the dielectric constant ( ε e ) was calculated from the refractive index obtained in the UV/Vis region. Since the extinction coefficient(k) of SiO 2 -based materials is normally negligible in this region, the relative dielectric constant(κ) in Eq. (2) can be simply expressed as [12 14], κ = n 2 (3) Figure 1 shows the dielectric constants measured at 1 MHz and the refractive indices measured at 632 nm. From Eq. (3), the measurement of the refractive index leads to the dielectric constant in the optical frequency

3 Journal of the Korean Physical Society, Vol. 55, No. 5, November 2009 Table 1. The basic properties of each precursor. Name Formula M.W. B.P. M.P. d Toluene C 6H 5-CH C -95 C 0.86 Tetraethoxysilane Si(OC 2H 5) C C 0.94 Fig. 2. Refractive index (n) of hybrid thin films at (a) various ratios of TEOS to toluene deposited at an RF power of 20 W and (b) RF powers of 40 and 60 W deposited at a 3:10 ratio of TEOS to toluene. region, which is contributed by electronic states. Thus, in Eq. (1), its contribution is related to the dielectric constant (at 1 MHz). We know that the refractive index increases with increasing of the dielectric constant. Therefore, the dielectric constant can be predicted by measuring of the refractive index. Thus, Figure 1 shows evidence of Eq. (3) and the relationship between the dielectric constant and the refractive index. As the dielectric constant is frequency-dependent, the dielectric constant at one region cannot be compared with that at a different frequency region. However, in this case, the refractive index n is related to the density of the material (when the extinction coefficient is negligible), so it can be related to the static dielectric constant, κ. Figure 2 shows the change in the refractive index at different annealing temperatures. Figure 2(a) shows the refractive indices of a hybrid-polymer thin film deposited at different ratios of TEOS to toluene at an RF power of 20 W. The refractive index of the as-deposited thin film increases with increasing ratio of TEOS to toluene. The refractive index of each sample increased with increasing annealing temperature. Each sample had only a small difference in refractive indices until 400 C, but the differences increased suddenly at 500 C. In particular, each sample had different rates of increased for the refractive indices for different values of the TEOS-to-toluene ratio. A high TEOS ratio made little differences in the refractive indices compared to low TEOS ratios during the rapid thermal process at temperatures from 300 to 500 C. This indicated that increasing the ratio of TEOS to toluene contributed to decreasing the difference in refractive indices between as-deposited and post-annealed thin films. Figure 2(b) shows the refractive indices of the hybrid-polymer thin films deposited at various RF powers at a fixed 3:10 ratio of TEOS to toluene. The refractive index of the as-deposited thin film increased with increasing deposition RF power, the same trend as in Figure 1, denoting that the refractive indices of these samples correlate with increasing RF power. The deposition RF power contributed to a slightly decreased difference of refractive index during the annealing. Therefore, Figure 2 shows that the thermal stability of the optical property (refractive index) was improved by increasing the TEOS ratio. Figure 3 shows the bonding state of the hybrid plasma thin films and the change in the functional group intensity by using FT-IR spectroscopy over the range of cm 1. The IR spectra exhibit absorption peaks at cm 1, corresponding to the Si-O stretching vibration band. On the other extreme, bands of , , 1611, 1724, , and cm 1 correspond to Si-CH 3, CH x bending vibration, C=C, C=O, CH x stretching vibration, and -OH bands, respectively [15]. The intensity of the Si-O peak didn t decrease with annealing; thus, the SiO species was not degassed by heat. SiO has chemical and thermal stability in the range of C. Figures 3(a), 3(b), and 3(c) show the effects of annealing temperature for different ratios of TEOS to toluene. The peak intensities of C=O, -OH, C=C, and CH x decreased with increasing annealing temperature, which shows a decrease in the organic chemical species in the hybrid-polymer thin film with increasing annealing temperature. Especially, only the -OH peak intensity decreased without decreasing the intensity of the C=O, C=C, and CH x, in spite of the 300 C annealing, in the case of a 5:10 TEOS-to-toluene rate at 20 W. For TEOS:toluene = 5:10, decrease in the amount of organic chemical species (C=O, C=C, and CH x ) was relatively little in comparison with the other

4 Optical and Mechanical Properties of Toluene-TEOS S.-J. Cho et al Fig. 4. The (a) hardness and (b) Young s modulus of hybrid thin films at various ratios of TEOS to toluene deposited an RF power of 20 W. The (c) hardness and (d) Young s modulus at RF powers of (d) 40 and (e) 60 W deposited at a 3:10 ratio of TEOS to toluene. Fig. 3. IR spectra of hybrid thin films at various ratio of TEOS to toluene, (a) 1:10, (b) 3:10, and (c) 5:10, deposited at an RF power of 20 W and at RF powers of (d) 40 and (e) 60 W deposited at a 3:10 ratio of TEOS to toluene. samples because the increase in the TEOS ratio led to thermal stability. The influence of annealing temperature was investigated for various deposition RF powers as shown in Figures 3(b), 3(d), and 3(e). Each sample, which was deposited at a different deposition RF power, had the same tendency of decreasing organic chemical species, regardless of the RF power, signifying that thermal stability was chemically improved by increasing the TEOS ratio rather than the RF power. Figure 4 shows the hardness and Young s modulus before and after the annealing process. Each sample had a minimum value at 400 C, except the sample with a TEOS-to-toluene ratio of 3:10 at 60 W. The hardness and Young s modulus decreased with increasing annealing temperature because the organic chemical species were degassed during the annealing process. However, the hardness and Young s modulus increased at 500 C by increasing thin film density because of micro-pores and vacancies of the annealed thin film collapsed down and was filled by components of the thin films during an overly-heated annealing process. In the case of a TEOSto-toluene ratio of 3:10 at 60 W, the primary status of the physical strength was much higher than it was for other samples. Thus, the organic chemical species were not degassed enough to collapse the thin films during the annealing process. The hardness and Young s modulus increased with increasing RF power and increasing Fig. 5. Thickness (%) of hybrid thin films at (a) various ratios of TEOS to toluene, deposited an RF power of 20 W and at (b) RF powers of 40 and 60 W deposited at a 3:10 ratio of TEOS to toluene. TEOS bubbling ratio. A more effective parameter for the hardness and Young s modulus was the RF power because the high ion density of the plasma generated a high-density thin film [16].

5 Journal of the Korean Physical Society, Vol. 55, No. 5, November 2009 Figure 5 shows that the change in film thickness (%) caused by post annealing. In Figure 5(a), each sample deposited at 20 W at different ratios of TEOS to toluene had the same tendency of decreasing film thickness. The rate of decrease of the thickness by increasing annealing temperature reduced slightly by increasing the TEOS ratio because the SiO species played the role of a supporter in the hybrid polymer thin film. However, the effect of the deposition RF power is larger than the effect of the SiO content ratio as shown in Figure 5(b) [16]. Therefore, a rate of decrease of the thickness was greatly decreased by the thermal degassing of the organic chemical species because of the high-density thin film deposited at a high RF power (60 W). Support of this research by the BK 21 project of the Ministry of Education, Korea, is gratefully acknowledged. This work was also supported by the Korea Research Foundation Grant funded by the Korean Government (MEST, KRF J0072) and by the Korea Science and Engineering Foundation by the Korean Government (MEST, ). REFERENCES IV. CONCLUSION Organic-inorganic hybrid-polymer films were deposited on Si(100) by using the PECVD method with co-deposition of TEOS and toluene. Through IR and refractive index analyses, the thermal stability of the optical and the chemical properties were found to be increased by increasing the TEOS-to-toluene bubbling ratio. FT-IR results also showed that the as-grown hybrid films had -CH x, C=O, C=C, -OH, and Si-O x functional groups, and overall different shapes and structures from each liquid precursor. Furthermore the annealing process changed each chemical species, especially the peak intensities of C=O, C=C, CH x, and -OH, which were decreased by annealing. The hardness and Young s modulus increased with increasing RF power and TEOS bubbling ratio. A more effective parameter for the hardness and Young s modulus was the RF power, given the high ion density in the plasma that generated a high-density thin film by using the PECVD process. Increasing the RF power increased thermal stability in thermal shrinkage. Thus, the initial density affects the thermal shrinkage of the as-deposited film. From these results, the thermal stability of the chemical species in the hybrid-polymer thin film and the dielectric constant (a refractive index) are substantially influenced by the TEOS bubbling ratio while the thermal stability of the thickness (thermal shrinkage), the hardness, and Young s modulus are influenced by the RF power. ACKNOWLEDGMENTS [1] A. Grill and V. Patel, Appl. Phys. Lett. 79, 803 (2001). [2] I-S. Bae, S-H. Cho, Z. T. Park, J-G. Kim and J-H. Boo, J. Vac. Sci. Technil. A, Vac. Surf. Films 23, 875 (2005). [3] S. H. Kim, R. Navamathavan, A. S. Jung, Y. J. Jang, K- M. Lee and C. K. Choi, J. Korean Phys. Soc. 50, 1814 (2007). [4] I-S. Bae, C-K. Jung, S-H. Jeong, S-J. Cho, Y. J. Yu, J. G. Kim and J-H. Boo, Thin Solid Films 515, 407 (2006). [5] Y. C. Quan, J. Joo and D. Jung, Jpn. J. Appl. Phys. 38, 1356 (1999). [6] R. Navamathavan and C. K. Choi, J. Korean Phys. Soc. 48, 1675 (2006). [7] S-J. Cho, I-S. Bae, Y. S. Park, B. Hong and J-H. Boo J. Korean Phys. Soc. 53, 1634 (2008). [8] W. N. Gill, S. Rogojevic and T. Lu, in Low Dielectric Constant Materials for IC Applications, edited by P. S. Ho, J. Leu and W. W. Lee (Springer, Berlin, Germany, 2003). [9] B. D. Hatton, K. Landskron, W. J. Hunks, M. R. Bennett, D. Shukaris, D. D. Perovic and G. A. Ozin, Mater. Today. 9, 22 (2006). [10] N. Chérault, G. Carlotti, N. Casanova, P. Geraud, C. Goldberg, O. Tomas and M. Verdier, Microelectronic Engin. 82, 368 (2005). [11] A. Gouldstone, H-J. Koh, K-J. Zeng, A. E. Giannakopoulos and S. Suresh, Acta Materialia 48, 2277 (2000). [12] W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics (Wiley, New York, 1976). [13] S. M. Han and E. S. Aydil, J. Appl. Phys. 83, 2172 (1998). [14] J. Y. Kim, M. S. Hwang, Y-H. Kim, H. J. Kim and Y. Lee, J. Appl. Phys. 90, 2469 (2001). [15] Robert M. Silverstein and Francis X. Webster, Spectrometric Identification of Organic Compounds (John Wiley & Sons, Inc., New York, USA, 1998). [16] G. Y. Liang, G. Zhao, J. L. Zeng and J. R. Shi, J. Quan. Spect. Rad. Trans. 102, 473 (2006).