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Improvement of electrical properties of surfactant-templated mesoporous silica thin films by plasma treatment
Thin Solid Films 506 – 507 (2006) 360 – 363 www.elsevier.com/locate/tsf
Improvement of electrical properties of surfactant-templated mesoporous silica thin films by plasma treatment Sang-Bae Jung, Hyung-Ho Park * Department of Ceramic Engineering, Yonsei University, 134 Sinchon-dong, Seodaemoon-ku, Seoul, 120-749, South Korea Available online 8 September 2005
Abstract Surfactant-templated mesoporous silica film draws a great attention due to its superior properties as low-k dielectrics. In this study, electrical properties of the film using Brij-76 surfactant were evaluated after plasma treatment. The selected gases were H2, O2, and Ar. X-ray diffraction pattern revealed that mesoporous silica film was highly textured but pore ordering was destroyed when applied rf power and atomic mass of gas increased. Strained Si – O bond near gel pore was reduced and incompletely oxidized Si bond could be controlled after plasma treatment. Optimizing treatment condition can control the structural defects in the wall and as a result, electrical property of the film could be improved. Mesoporous film after H2 plasma treatment at rf power of 25 W and 100 mTorr showed current density of 3.6 10 6 A/cm2 at 1.6 MV/cm. As a conclusion, electrical properties of mesoporous silica film can be improved by plasma treatment through controlling reactivity of gas and ion bombardment effect with low power. D 2005 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Low-k; Brij-76; Plasma treatment
1. Introduction Due to the rapid decrease in the dimensions of UltraLarge-Scaled-Integration (ULSI) devices, interconnect RC time delay is a serious problem which cannot be neglected [1]. This time delay will be overcome both by using low resistivity Cu or low dielectric material [2,3]. Low-k dielectrics should have low dielectric constant, low leakage current, high breakdown strength, and mechanical stability. Recently, highly ordered mesoporous silica film with pore size of 2 –10 nm draws a great attention due to its superior properties as low-k dielectric [4]. From the homogeneous solution of silica and surfactant dissolved in alcohol below critical micelle concentration, preferential evaporation of solvent during dip or spin coating drives cooperative self-assembly of silica and surfactant [5]. This process, called evaporation-induced self-assembly (EISA),
* Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 365 5882. E-mail address: [email protected] (H.-H. Park). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.082
enables the formation of highly ordered mesoporous silica film after calcination. Ordered mesoporous silica film can give robust mechanical strength. However, structural defects such as incompletely oxidized Si and Si –OH groups remain after calcination due to incomplete polycondensation reaction [6]. Because electrical behavior of porous film is not only dependent on the porosity but also chemical species, the control of structural defects determines the electrical performance of film. It was reported that surface chemical species of nanoporous silica film could be controlled by plasma treatment using H2, O2, and Ar gases [7,8]. In this work, ordered mesoporous silica film was fabricated using Brij-76 surfactant (C 18 H 37 (OCH 2 CH2)10OH). Brij-76 surfactant can form a 3-dimensional structure with thick wall and suitable pore size for low-k application [3]. Ordered mesoporous film was exposed to plasma and electrical properties of the film after plasma treatment were evaluated. Selected gases were H2, O2, and Ar from the consideration of atomic mass and reactivity.
S.-B. Jung, H.-H. Park / Thin Solid Films 506 – 507 (2006) 360 – 363
361
Precursor solution was prepared with two-step process. First, solution A was prepared by dissolving Brij-76 block copolymer in ethyl alcohol (EtOH) –H2O –HCl solution with the molar ratio of 0.05 Brij-76:15 EtOH:H2O:0.0028 HCl. And then, solution B was prepared by mixing tetraethoxyorthosilicate (TEOS) and EtOH followed by the addition of acid catalyst in H2O. The molar ratio of catalyst was chosen for the minimization of condensation reaction [9]. After stirring solution B for a while, solution B was added to solution A. Final composition of TEOS/EtOH/ H2O/HCl/Brij-76 was 1:20:5:0.01:0.05. Silica sol was spindeposited at 2000 rpm for 30 s at optimized aging time. Ordered mesoporous silica film with thickness of 240 nm could be fabricated by removing the surfactant at 400 -C with a heating rate of 1 -C/min. For plasma treatment, parallel-plate reactor was used. Electrodes were interconnected with 13.56 MHz rf generator. Rf power was applied to upper electrode and samples were mounted on lower electrode. The H2, O2 and Ar gases were admitted into the reactor through a showerhead in the top electrode. Plasma exposure time was fixed at 5 min and upper electrode rf power was varied from 25 to 100 W at 100 mTorr. For the investigation of pore structure, X-ray powder diffraction (XRD) patterns were collected using Fe Ka ˚ . Fourier transform radiation with wavelength of 1.9373 A infrared (FT-IR; Jasco 300Z) spectroscopic analysis was performed to reveal chemical species of silica wall. X-ray photoelectron spectroscopy (XPS, VG Scientific, ESCALAB 220i-XL) measurement was carried out with excitation source of monochromatic Al Ka radiation. The dielectric constant in metal-insulator-semiconductor (MIS) structure was obtained using HP 4284A, impedance/gain-phase analyzer at 1 MHz. Current – voltage (I – V) characteristic was measured using HP 4145B, semiconductor parameter analyzer. For the electrical measurements, Al was deposited on films as upper electrode.
3. Results and discussion XRD patterns of as-prepared and calcined mesoporous silica films were given in Fig. 1 as (a) and (b). Asprepared film exhibited sharp diffraction peak with plane ˚ . Any other diffraction peak was not spacing of 46.8 A observed except second harmonic in high 2h region. So, as-prepared film exhibited highly textured pore structure. It was reported that mesoporous silica using Brij-76 surfactant forms the pore structure of polytypic intergrowth of hexagonal close-packed (hcp) and cubic close-packed (ccp) structure [10]. So, diffraction peak in Fig. 1a can be corresponded as (002) of hcp or (111) of ccp structure. After the calcination, the intensity of diffraction peak was increased due to enhanced crystallinity, i.e., increased
Intensity (arb. unit)
2. Experimental procedures
(e) (d) (c) (b) (a) 2.0
2.4
2.8
3.2
3.6
4.0
2θ Fig. 1. XRD patterns of (a) as-prepared and (b) calcined, and plasma treated mesoporous films using (c) H2, (d) O2, and (e) Ar gases at 25 W.
uniformity of pore-wall and pore-size and regularity of pore-distribution, and the texture characteristic was maintained except anisotropy shrinkage along normal direction to film surface as shown in Fig. 1b. The porosity of calcined mesoporous film is nearly 40%. Fig. 1c, d, and e corresponds to XRD patterns of plasma-treated mesoporous films using H2, O2, and Ar gases, respectively. Plasma treatment condition was 25 W and 100 mTorr. The position of diffraction peak was slightly shifted to higher angle even at present treatment condition, which means that ordered pore structure was shrunken. However, H2 plasma treatment shows the smallest peak shift due to the lightest atomic mass of H2. In order to investigate the effect of ion bombardment on the pore structure after plasma treatment, applied rf power was varied and diffraction peak intensity from XRD data as a function of rf power was shown in Fig. 2. Because rf power was applied to upper electrode, the charged ion energy is controlled by self-bias across the sheath region, which increases with applied rf power. As the applied rf power increases, diffraction intensity was decreased. However, intensity of diffraction peak was more steadily decreased with H2 plasma treatment when compared with treatments using O2 or Ar. Therefore, it was concluded that pore structure of the mesoporous film was severely destroyed after plasma treatment in case of high rf power and heavy atom. Fig. 3 shows the leakage current characteristic of ordered mesoporous film after plasma treatment with rf power of 25 W. Ordered mesoporous film shows the current density of 1.4 10 3 A/cm2 at applied electric field of 1.6 MV/cm. After H2 plasma treatment, leakage behavior was greatly improved as 3.6 10 6 A/cm2 and in case of O2 plasma treatment, 1.17 10 4 A/cm2 was obtained. However, Ar plasma treatment did not show an improved behavior. Fig. 4 represents the leakage current density for mesoporous film treated using Ar plasma with the variation of rf power from
362
S.-B. Jung, H.-H. Park / Thin Solid Films 506 – 507 (2006) 360 – 363
H2 O2 Ar
0.9
Current density (A/cm2)
Relative intensity ratio
1.0
0.8 0.7 0.6 0.5 0
20
40
60
80
100
10 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10
Rf power (W) Fig. 2. XRD peak intensity ratio of calcined and plasma treated mesoporous films with various gas as a function of applied rf power.
25 W to 100 W. As the applied rf power increased, leakage behavior of mesoporous film was severely degraded. Especially with 100 W of rf power, a hard breakdown of the film was observed around 1 MV/cm of applied electric field. The result showed a close relation between microstructure of mesoporous film and its electrical property. Partial collapse of thin mesoporous film with regular distributed and 3-dimensionally connected meso-pores could create an electric current path in the mesoporous film and this induces high leakage current density and furthermore electrical breakdown. From the results given in Figs. 3 and 4, it can be said that leakage current is greatly improved only with light gas plasma of low rf power, for example, H2 plasma treatment with 25 W in this experiment. Fig. 5 shows FT-IR spectra of mesoporous silica film before and after various plasma treatments with rf power of 25 W. As shown in Fig. 5a, FT-IR spectrum of mesoporous film shows characteristic absorption behavior near 1080 cm 1 by Si– O – Si of TO3, 1130 cm 1 by cyclic Si –O – Si and 960 cm 1 by Si –OH vibration. The Si –O network structure with TO3 is known to be more stable than cyclic
(c) (b) (a)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Electric field (MV/cm) Fig. 4. Leakage current density of Ar plasma treated mesoporous films; (a) 25 W, (b) 50 W, and (c) 100 W.
[11,12]. In surfactant-templated mesoporous silica film at acidic condition, cyclic species exists near the surface of gel pores with strain energy [12]. Such a strained bond increased the leakage current density [13,14]. After the plasma treatments, more stable TO3 vibration was relatively increased irrespective of admitted gas. This structural transition can give an improvement on the leakage current characteristic. However, Si –OH group did not change after the treatment. In order to investigate the framework bonding nature in detail, XPS analysis was performed. Fig. 6 shows the Si 2p photoelectron spectra before and after the H2 plasma treatment at 25 W. In Fig. 6a, photoelectron spectrum is made up of framework Si –O bond appeared at 103 eV and additional bond at low binding energy side when compared with Fig. 6b. This can be related to incompletely oxidized Si bond in the silica wall of mesoporous silica film. After the H2 plasma treatment, it was found that incompletely oxidized Si bond reduced as shown in Fig. 6b and this can be related with the transform of Si– O network from cyclic to TO3.
0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9
calcined mesoporous film H2 O2 Ar
Intensity (arb. unit)
Current density (A/cm2)
0.1
(d) (c) (b) (a)
1E-10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Electric field (MV/cm) Fig. 3. Leakage current density of calcined and plasma treated mesoporous film with various gases at 25 W.
900
1000
1100
1200
1300
Wavenumber (cm-1) Fig. 5. FT-IR spectra of (a) calcined and plasma treated mesoporous films using (b) H2, (c) O2, and (d) Ar gases at 25 W.
S.-B. Jung, H.-H. Park / Thin Solid Films 506 – 507 (2006) 360 – 363
(b)
(a)
110
108
106
104
102
100
98
Binding energy (eV) Fig. 6. Si 2p photoelectron spectra of (a) calcined and (b) H2 plasma treated mesoporous films at 25 W.
Conduction through thick insulator is governed by bulklimited conduction [15]. Ionized H2 gas can reduce the concentration of structural defect in the film. As shown in Figs. 5b and 6b, structural defects such as incompletely oxidized Si and cyclic species were reduced after the treatment. So leakage current reduction in Fig. 3 by H2 plasma treatment is attributed to the control of structural defect with suppressed ion bombardment effect [6]. As mentioned in Figs. 1 and 2, pore structure of the film was heavily destroyed due to the physical ion bombardment effect of O2 and Ar plasma. In case of reactive O2 plasma, reduction of defects can be anticipated, but it is considered that massive ion generates an impact-damage in the film even at rf power of 25 W. On the other hand, improvement of leakage current characteristic by reduction of cyclic species is in competition with degradation by ion bombardment effect for massive Ar plasma. As the applied rf power increased, leakage current of mesoporous film exhibited the breakdown behavior. It is considered to the effect of the ion bombardment, but the effect can be exaggerated for thin mesoporous insulator (240 nm) because ion energy can be mainly applied to the pore channel due to textured pore structure.
4. Conclusions In this study, ordered mesoporous silica film was exposed to H2, O2, and Ar plasmas and their electrical
363
properties were evaluated. XRD pattern revealed that mesoporous silica film was highly textured and pore ordering was destroyed as the applied rf power and atomic mass increase. FT-IR observation revealed that strained Si– O bond, which degraded the leakage current behavior, was reduced after various plasma treatments. Incompletely oxidized Si bond was controlled after H2 plasma treatment, which was confirmed by XPS analysis. Leakage current density of the film after H2 plasma treatment with rf power of 25 W was greatly improved but after O2 plasma treatment, it was improved slightly due to the ion bombardment effect. However, Ar plasma treatment did not give the improved leakage current density value in spite of reduced strained Si –O bond due to non-reactive massive Ar ion. It was observed that leakage current behavior was degraded as the applied rf power increases due to increased ion energy and ordered pore structure. As a conclusion, electrical properties of mesoporous silica film can be improved by plasma treatment through controlling reactivity of gas and ion bombardment effect with low power.
Acknowledgement The authors are indebted to the financial support from KISTEP (Program No. M1-0214-00-0228).
References [1] S.K. Chiang, C.L. Lassen, Solid State Technol. 10 (1999) 42. [2] S.B. Jung, H.H. Park, Thin Solid Films 420/421 (2002) 503. [3] F.K.D. Theije, A.R. Balkenende, M.A. Verheijen, M.R. Baklanov, K.P. Mogilnikov, Y. Furukawa, J. Phys. Chem., B 107 (2003) 4280. [4] J.Y. Chen, F.M. Pan, A.T. Cho, K.J. Chao, T.G. Tsai, B.W. Wu, C.M. Yang, L. Chang, J. Electrochem. Soc. 150 (2003) F123. [5] C.J. Brinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 11 (1999) 579. [6] A.T. Cho, T.G. Tsai, C.M. Yang, K.J. Chao, F.M. Pan, Electrochem. Solid-State Lett. 4 (2001) G35. [7] J.J. Kim, H.H. Park, S.H. Hyun, Thin Solid Films 377/378 (2000) 525. [8] H.R. Kim, H.H. Park, S.H. Hyun, G.Y. Yeom, Thin Solid Films 332 (1998) 444. [9] C.J. Brinker, G.W. Scherer, Sol-Gel Science, 2nd edR, Academic Press, San Diego, 1990, p. 213. [10] Y. Sakamoto, I. Dı´az, O. terasaki, D. Zhao, J.P. Pariente, J.M. Kim, G.D. Stucky, J. Phys. Chem., B 106 (2002) 3118. [11] T.M. Parrill, J. Mater. Res. 7 (1992) 2230. [12] P. Innocenzi, P. Falcaro, D. Grosso, F. Babonneau, J. Phys. Chem., B 107 (2003) 4711. [13] C.F. Yeh, T.J. Chen, C.L. Fan, J.S. Kao, J. Appl. Phys. 83 (1998) 1107. [14] P. Solomon, J. Appl. Phys. 48 (1977) 3843. [15] M.H. Jo, H.H. Park, Appl. Phys. Lett. 72 (1998) 1391.
Improvement of electrical properties of surfactant-templated mesoporous silica thin films by plasma treatment Sang-Bae Jung, Hyung-Ho Park * Department of Ceramic Engineering, Yonsei University, 134 Sinchon-dong, Seodaemoon-ku, Seoul, 120-749, South Korea Available online 8 September 2005
Abstract Surfactant-templated mesoporous silica film draws a great attention due to its superior properties as low-k dielectrics. In this study, electrical properties of the film using Brij-76 surfactant were evaluated after plasma treatment. The selected gases were H2, O2, and Ar. X-ray diffraction pattern revealed that mesoporous silica film was highly textured but pore ordering was destroyed when applied rf power and atomic mass of gas increased. Strained Si – O bond near gel pore was reduced and incompletely oxidized Si bond could be controlled after plasma treatment. Optimizing treatment condition can control the structural defects in the wall and as a result, electrical property of the film could be improved. Mesoporous film after H2 plasma treatment at rf power of 25 W and 100 mTorr showed current density of 3.6 10 6 A/cm2 at 1.6 MV/cm. As a conclusion, electrical properties of mesoporous silica film can be improved by plasma treatment through controlling reactivity of gas and ion bombardment effect with low power. D 2005 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Low-k; Brij-76; Plasma treatment
1. Introduction Due to the rapid decrease in the dimensions of UltraLarge-Scaled-Integration (ULSI) devices, interconnect RC time delay is a serious problem which cannot be neglected [1]. This time delay will be overcome both by using low resistivity Cu or low dielectric material [2,3]. Low-k dielectrics should have low dielectric constant, low leakage current, high breakdown strength, and mechanical stability. Recently, highly ordered mesoporous silica film with pore size of 2 –10 nm draws a great attention due to its superior properties as low-k dielectric [4]. From the homogeneous solution of silica and surfactant dissolved in alcohol below critical micelle concentration, preferential evaporation of solvent during dip or spin coating drives cooperative self-assembly of silica and surfactant [5]. This process, called evaporation-induced self-assembly (EISA),
* Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 365 5882. E-mail address: [email protected] (H.-H. Park). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.082
enables the formation of highly ordered mesoporous silica film after calcination. Ordered mesoporous silica film can give robust mechanical strength. However, structural defects such as incompletely oxidized Si and Si –OH groups remain after calcination due to incomplete polycondensation reaction [6]. Because electrical behavior of porous film is not only dependent on the porosity but also chemical species, the control of structural defects determines the electrical performance of film. It was reported that surface chemical species of nanoporous silica film could be controlled by plasma treatment using H2, O2, and Ar gases [7,8]. In this work, ordered mesoporous silica film was fabricated using Brij-76 surfactant (C 18 H 37 (OCH 2 CH2)10OH). Brij-76 surfactant can form a 3-dimensional structure with thick wall and suitable pore size for low-k application [3]. Ordered mesoporous film was exposed to plasma and electrical properties of the film after plasma treatment were evaluated. Selected gases were H2, O2, and Ar from the consideration of atomic mass and reactivity.
S.-B. Jung, H.-H. Park / Thin Solid Films 506 – 507 (2006) 360 – 363
361
Precursor solution was prepared with two-step process. First, solution A was prepared by dissolving Brij-76 block copolymer in ethyl alcohol (EtOH) –H2O –HCl solution with the molar ratio of 0.05 Brij-76:15 EtOH:H2O:0.0028 HCl. And then, solution B was prepared by mixing tetraethoxyorthosilicate (TEOS) and EtOH followed by the addition of acid catalyst in H2O. The molar ratio of catalyst was chosen for the minimization of condensation reaction [9]. After stirring solution B for a while, solution B was added to solution A. Final composition of TEOS/EtOH/ H2O/HCl/Brij-76 was 1:20:5:0.01:0.05. Silica sol was spindeposited at 2000 rpm for 30 s at optimized aging time. Ordered mesoporous silica film with thickness of 240 nm could be fabricated by removing the surfactant at 400 -C with a heating rate of 1 -C/min. For plasma treatment, parallel-plate reactor was used. Electrodes were interconnected with 13.56 MHz rf generator. Rf power was applied to upper electrode and samples were mounted on lower electrode. The H2, O2 and Ar gases were admitted into the reactor through a showerhead in the top electrode. Plasma exposure time was fixed at 5 min and upper electrode rf power was varied from 25 to 100 W at 100 mTorr. For the investigation of pore structure, X-ray powder diffraction (XRD) patterns were collected using Fe Ka ˚ . Fourier transform radiation with wavelength of 1.9373 A infrared (FT-IR; Jasco 300Z) spectroscopic analysis was performed to reveal chemical species of silica wall. X-ray photoelectron spectroscopy (XPS, VG Scientific, ESCALAB 220i-XL) measurement was carried out with excitation source of monochromatic Al Ka radiation. The dielectric constant in metal-insulator-semiconductor (MIS) structure was obtained using HP 4284A, impedance/gain-phase analyzer at 1 MHz. Current – voltage (I – V) characteristic was measured using HP 4145B, semiconductor parameter analyzer. For the electrical measurements, Al was deposited on films as upper electrode.
3. Results and discussion XRD patterns of as-prepared and calcined mesoporous silica films were given in Fig. 1 as (a) and (b). Asprepared film exhibited sharp diffraction peak with plane ˚ . Any other diffraction peak was not spacing of 46.8 A observed except second harmonic in high 2h region. So, as-prepared film exhibited highly textured pore structure. It was reported that mesoporous silica using Brij-76 surfactant forms the pore structure of polytypic intergrowth of hexagonal close-packed (hcp) and cubic close-packed (ccp) structure [10]. So, diffraction peak in Fig. 1a can be corresponded as (002) of hcp or (111) of ccp structure. After the calcination, the intensity of diffraction peak was increased due to enhanced crystallinity, i.e., increased
Intensity (arb. unit)
2. Experimental procedures
(e) (d) (c) (b) (a) 2.0
2.4
2.8
3.2
3.6
4.0
2θ Fig. 1. XRD patterns of (a) as-prepared and (b) calcined, and plasma treated mesoporous films using (c) H2, (d) O2, and (e) Ar gases at 25 W.
uniformity of pore-wall and pore-size and regularity of pore-distribution, and the texture characteristic was maintained except anisotropy shrinkage along normal direction to film surface as shown in Fig. 1b. The porosity of calcined mesoporous film is nearly 40%. Fig. 1c, d, and e corresponds to XRD patterns of plasma-treated mesoporous films using H2, O2, and Ar gases, respectively. Plasma treatment condition was 25 W and 100 mTorr. The position of diffraction peak was slightly shifted to higher angle even at present treatment condition, which means that ordered pore structure was shrunken. However, H2 plasma treatment shows the smallest peak shift due to the lightest atomic mass of H2. In order to investigate the effect of ion bombardment on the pore structure after plasma treatment, applied rf power was varied and diffraction peak intensity from XRD data as a function of rf power was shown in Fig. 2. Because rf power was applied to upper electrode, the charged ion energy is controlled by self-bias across the sheath region, which increases with applied rf power. As the applied rf power increases, diffraction intensity was decreased. However, intensity of diffraction peak was more steadily decreased with H2 plasma treatment when compared with treatments using O2 or Ar. Therefore, it was concluded that pore structure of the mesoporous film was severely destroyed after plasma treatment in case of high rf power and heavy atom. Fig. 3 shows the leakage current characteristic of ordered mesoporous film after plasma treatment with rf power of 25 W. Ordered mesoporous film shows the current density of 1.4 10 3 A/cm2 at applied electric field of 1.6 MV/cm. After H2 plasma treatment, leakage behavior was greatly improved as 3.6 10 6 A/cm2 and in case of O2 plasma treatment, 1.17 10 4 A/cm2 was obtained. However, Ar plasma treatment did not show an improved behavior. Fig. 4 represents the leakage current density for mesoporous film treated using Ar plasma with the variation of rf power from
362
S.-B. Jung, H.-H. Park / Thin Solid Films 506 – 507 (2006) 360 – 363
H2 O2 Ar
0.9
Current density (A/cm2)
Relative intensity ratio
1.0
0.8 0.7 0.6 0.5 0
20
40
60
80
100
10 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10
Rf power (W) Fig. 2. XRD peak intensity ratio of calcined and plasma treated mesoporous films with various gas as a function of applied rf power.
25 W to 100 W. As the applied rf power increased, leakage behavior of mesoporous film was severely degraded. Especially with 100 W of rf power, a hard breakdown of the film was observed around 1 MV/cm of applied electric field. The result showed a close relation between microstructure of mesoporous film and its electrical property. Partial collapse of thin mesoporous film with regular distributed and 3-dimensionally connected meso-pores could create an electric current path in the mesoporous film and this induces high leakage current density and furthermore electrical breakdown. From the results given in Figs. 3 and 4, it can be said that leakage current is greatly improved only with light gas plasma of low rf power, for example, H2 plasma treatment with 25 W in this experiment. Fig. 5 shows FT-IR spectra of mesoporous silica film before and after various plasma treatments with rf power of 25 W. As shown in Fig. 5a, FT-IR spectrum of mesoporous film shows characteristic absorption behavior near 1080 cm 1 by Si– O – Si of TO3, 1130 cm 1 by cyclic Si –O – Si and 960 cm 1 by Si –OH vibration. The Si –O network structure with TO3 is known to be more stable than cyclic
(c) (b) (a)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Electric field (MV/cm) Fig. 4. Leakage current density of Ar plasma treated mesoporous films; (a) 25 W, (b) 50 W, and (c) 100 W.
[11,12]. In surfactant-templated mesoporous silica film at acidic condition, cyclic species exists near the surface of gel pores with strain energy [12]. Such a strained bond increased the leakage current density [13,14]. After the plasma treatments, more stable TO3 vibration was relatively increased irrespective of admitted gas. This structural transition can give an improvement on the leakage current characteristic. However, Si –OH group did not change after the treatment. In order to investigate the framework bonding nature in detail, XPS analysis was performed. Fig. 6 shows the Si 2p photoelectron spectra before and after the H2 plasma treatment at 25 W. In Fig. 6a, photoelectron spectrum is made up of framework Si –O bond appeared at 103 eV and additional bond at low binding energy side when compared with Fig. 6b. This can be related to incompletely oxidized Si bond in the silica wall of mesoporous silica film. After the H2 plasma treatment, it was found that incompletely oxidized Si bond reduced as shown in Fig. 6b and this can be related with the transform of Si– O network from cyclic to TO3.
0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9
calcined mesoporous film H2 O2 Ar
Intensity (arb. unit)
Current density (A/cm2)
0.1
(d) (c) (b) (a)
1E-10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Electric field (MV/cm) Fig. 3. Leakage current density of calcined and plasma treated mesoporous film with various gases at 25 W.
900
1000
1100
1200
1300
Wavenumber (cm-1) Fig. 5. FT-IR spectra of (a) calcined and plasma treated mesoporous films using (b) H2, (c) O2, and (d) Ar gases at 25 W.
S.-B. Jung, H.-H. Park / Thin Solid Films 506 – 507 (2006) 360 – 363
(b)
(a)
110
108
106
104
102
100
98
Binding energy (eV) Fig. 6. Si 2p photoelectron spectra of (a) calcined and (b) H2 plasma treated mesoporous films at 25 W.
Conduction through thick insulator is governed by bulklimited conduction [15]. Ionized H2 gas can reduce the concentration of structural defect in the film. As shown in Figs. 5b and 6b, structural defects such as incompletely oxidized Si and cyclic species were reduced after the treatment. So leakage current reduction in Fig. 3 by H2 plasma treatment is attributed to the control of structural defect with suppressed ion bombardment effect [6]. As mentioned in Figs. 1 and 2, pore structure of the film was heavily destroyed due to the physical ion bombardment effect of O2 and Ar plasma. In case of reactive O2 plasma, reduction of defects can be anticipated, but it is considered that massive ion generates an impact-damage in the film even at rf power of 25 W. On the other hand, improvement of leakage current characteristic by reduction of cyclic species is in competition with degradation by ion bombardment effect for massive Ar plasma. As the applied rf power increased, leakage current of mesoporous film exhibited the breakdown behavior. It is considered to the effect of the ion bombardment, but the effect can be exaggerated for thin mesoporous insulator (240 nm) because ion energy can be mainly applied to the pore channel due to textured pore structure.
4. Conclusions In this study, ordered mesoporous silica film was exposed to H2, O2, and Ar plasmas and their electrical
363
properties were evaluated. XRD pattern revealed that mesoporous silica film was highly textured and pore ordering was destroyed as the applied rf power and atomic mass increase. FT-IR observation revealed that strained Si– O bond, which degraded the leakage current behavior, was reduced after various plasma treatments. Incompletely oxidized Si bond was controlled after H2 plasma treatment, which was confirmed by XPS analysis. Leakage current density of the film after H2 plasma treatment with rf power of 25 W was greatly improved but after O2 plasma treatment, it was improved slightly due to the ion bombardment effect. However, Ar plasma treatment did not give the improved leakage current density value in spite of reduced strained Si –O bond due to non-reactive massive Ar ion. It was observed that leakage current behavior was degraded as the applied rf power increases due to increased ion energy and ordered pore structure. As a conclusion, electrical properties of mesoporous silica film can be improved by plasma treatment through controlling reactivity of gas and ion bombardment effect with low power.
Acknowledgement The authors are indebted to the financial support from KISTEP (Program No. M1-0214-00-0228).
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