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Anisotropy and Raman absorption of the polyimide surface irradiated by the ion beam for liquid crystal alignment
Thin Solid Films 517 (2009) 1803–1806
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Anisotropy and Raman absorption of the polyimide surface irradiated by the ion beam for liquid crystal alignment Phil Kook Son a, Jeung Hun Park b, Bong Kyun Jo a, Sung Pil Lee a, Joong Ha Lee a, Jae Chang Kim a, Tae-Hoon Yoon a,⁎, Taek Joon Lee c, Moonhor Ree c a b c
School of Electronics Engineering, Pusan National University, Busan 609-735, Republic of Korea Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90095, USA Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 June 2007 Received in revised form 9 September 2008 Accepted 18 September 2008 Available online 27 September 2008 Keywords: Raman scattering Polymers Ellipsometry Atomic force microscopy Liquid crystals Polyimide
a b s t r a c t In this paper, polyimide surfaces irradiated by an ion-beam for liquid crystal alignment are investigated by using atomic force microscopy, Raman spectroscopy, and spectroscopic ellipsometry. A liquid crystal cell aligned homogeneously through the ion-beam exposure exhibits electro-optic switching behavior similar to that of a rubbing-aligned liquid crystal cell. However, we found that the surface morphology and bonding molecules of ion-beam-treated polyimide surfaces show properties very different from mechanically-rubbed ones. Experimental results show that optical anisotropy of ion-beam-treated polyimide surfaces results in the formation of hydrogenated amorphous carbon-like structure with a short main-chain, while mechanical rubbing has little effect on structural and compositional variations of polyimide layers. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction Various techniques have been developed for liquid crystal (LC) alignment, such as obliquely evaporation of dielectrics [1], photoalignment of light-sensitive polymers [2,3], Langmuir–Blodgett films [4], atomic force microscopy (AFM) [5,6], lithography of polymers [7], and the rubbing process [8]. The ion-beam alignment method was introduced recently to overcome the drawbacks of rubbing, such as static charges, the creation of debris, and the degradation of the rubbing fabric [9]. However, mechanically rubbed polyimides are still widely used as alignment layers in the mass production of LC displays. The mechanism of aligning LCs has been an active research topic over the past several decades. However, many LC alignment issues remain unresolved regarding each alignment method: the rubbing process, photo-alignment, and atomic beam alignment. The orientational order of LC on a rubbed polyimide film is generated from the crystalline order of polymer chains [10]. The pretilt angle arises from tilted main- or side-chain segments at the rubbed polymer's surface that “guide” LC rods. The order on ion-beam-treated polyimides may be generated by selectively destroying π-bonds in imide rings by ion-beam exposure [11,12]. In the 1990s the mechanism of LC alignment was further studied at the molecular interaction level by using near edge X-ray ⁎ Corresponding author. E-mail address: [email protected] (Dr. Tae-Hoon Yoon).
absorption for fine structures [11], X-ray photoemission spectroscopy [12], second harmonic generation [13], photo-elastic modulation, AFM [5,8], etc. Stöhr et al. reported ion-beam effects on LC alignment layers to link the orientational bond order in rubbed polyimides, ion-beamtreated polyimides, and ion-beam-treated diamond-like-carbon films with the direction of LC alignment on these surfaces [11]. In this work we investigated the optical anisotropy of ion-beam-treated polyimide layers by using Raman spectroscopy and spectroscopic ellipsometry. 2. Experiments LC cells were prepared by two different methods: the mechanical rubbing method and the ion-beam-based process. Indium–tin-oxidecoated glass substrates were spin-coated with the polyimide RN-1702 (Nissan Chemical Co., Japan), pre-baked at 80 °C for 30 min, and cured at 230 °C for 1 h. Polyimide layers were bombarded by a low-energy argon ion beam. A cold hollow cathode (CHC) type of ion source was used to yield the ion beam. In order to collimate the ion beam, two perforated grids were used as electro-focusing lenses. The CHC type represents a separately cooled chamber supplied with a magnetic system and connected to a discharge chamber through an orifice. Argon gas was fed into the ion source through the CHC only. Discharge ignition in the cathode takes place at nominal discharge voltages and gas flow rates. A neutralizer filament outside the ion source serves as the source of the electrons necessary to compensated ion beam's spatial charges and reduces the repulsive force among ions.
0040-6090/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.084
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P.K. Son et al. / Thin Solid Films 517 (2009) 1803–1806
Raman spectra were measured by using a Raman microscope (Nanofinder 30, Tokyo Instruments Co.) with excitation at 488 nm in the backscattering configuration. We used a microscope with a 40× objective lens immersed in water in order to minimize the sample damage. Laser power incident on the sample was typically less than 200 mV to avoid substrate heating effect. The measurement was carried out over wave numbers of 400–3400 cm− 1. Variable angle spectroscopic ellipsometry (VASE) data for the ionbeam-treated and rubbed polyimide layer were acquired by using a J. A. Woollam VASE spectroscopic ellipsometer over the wavelengths of 400–800 nm in steps of 10 nm for the anisotropy measurement. VASE allows variation in the light's angle of incidence on the sample. In addition, the translator of the sample's stage can be rotated. This powerful option provides additional information on a sample's optical anisotropy. We take the x direction to be the film's optic axis. To find the sample's out-of-plane anisotropy, we selected the incident angles of 15°, 45°, and 75°. To get the sample's in-plane anisotropy, we rotated the translator from 0° to 180° in steps of 30°. Ellipsometry was measured 21 times for each sample. The optical axis in the film was induced by rubbing or ion-beam exposure was fitted in the same direction as the VASE' plane of incidence. We also characterized the film's surface morphology by using AFM with the tapping mode (Digital Instruments, Multimode AFM
Fig. 1. Optical micrographs of LC cells between two crossed polarizers; (a) no treatment, (b) rubbed, (c) ion-beam-treated.
Fig. 2. Measured pretilt angles of ion-beam-aligned homogeneous LC cells as a function of the ion-beam energy.
Nanoscope IIIa). An ultra-lever cantilever with a spring constant of 26 N/m and a resonance frequency of 268 kHz was used for scanning. 3. Results and discussion LC cells with a cell-gap of 3.4 μm were fabricated with a polyimide ‘RN-1702’ and a positive LC "Merck MLC-0223". Fig. 1 shows photographs of homogeneously aligned LC cells between crossed polarizers. The ion-beam energy, the incident angle, and the exposure time were 1000 eV, 30°, and 30 s, respectively. Without treating the polyimide layers, LC molecules are aligned randomly so that disclination lines are visible. LC molecules can be uniformly aligned by the ion-beam exposure or the rubbing process. Fig. 2 shows the measured pretilt angles of LC cells aligned homogeneously by ion-beam exposure on polyimide surfaces, as a function of the ion-beam energy. The ion beam's incident angle and exposure time were 30° and 30 s, respectively. Measured values of the pretilt angle were 1 ± 0.5° regardless of the ion-beam energy. We also found that an LC cell aligned homogeneously by ion-beam exposure exhibits the electro-optic switching behavior similar to that of a rubbed polyimide LC cell. We investigated characteristics of an ion-beam-exposed and a rubbing-treated polyimide layer by using AFM, Raman spectroscopy, and spectroscopic ellipsometry. The polyimide layer's surface morphology is shown in Fig. 3. The surface roughness increased from 0.57 nm to 1.12 nm by 30 s of exposure to an ion beam with energy of 500 eV and an incident angle of 30°. The rubbing process increased surface roughness to 1.51 nm, as shown in Fig. 3(b). It is apparent that the mechanical method modified surface morphology with very thin grooves aligned along the rubbing direction. In contrast, AFM observation revealed that ion-beam-treated polyimide surface exhibited no aligned grooves, but only spike-like patterns, due to the increased roughness. Therefore, we can propose that the binding structure of polyimide surfaces irradiated by an ion beam is very different from a rubbed surface. We examined the ion-beam energy's contribution to the polyimide layer's thickness change by using both scanning electron microscopy and spectroscopic ellipsometry. The polyimide layer's thickness decreased from 120 nm to 18 nm as the ion-beam energy increased from 0 to 1500 eV. On the other hand, the rubbing process did not affect the polyimide layer's thickness. To compensate for the loss of polyimide layers by the ion-beam treatment, we intentionally varied the polyimide layer's initial thickness. In our experiment, to maintain the polyimide layer's thickness at approximately 120 nm after the ion-beam exposure, regardless of the ion-beam energy, we linearly increased the polyimide layer's initial thickness from 120 nm to 222 nm by the spin coating method with an increase of the ion-beam energy from 0 to 1500 eV, respectively. During the ion-beam irradiation process, the incident angle was 30° and the exposure time was 30 s.
P.K. Son et al. / Thin Solid Films 517 (2009) 1803–1806
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Fig. 3. Surface morphologies of polyimide layers; (a) no treatment, (b) rubbed (rubbing direction: x axis), (c) ion-beam-treated (ion-beam direction: α = 30° on the y–z plane).
We employed Raman spectroscopy to examine the ion-beaminduced changes in optical properties due to the modifications in the polyimide layer's chemical structure. For the ion-beam conditions over the experiment, the ion-beam energy, the incident angle, and the exposure time were 1000 eV, 30°, and 30 s, respectively. Fig. 4 shows Raman spectra of a rubbed and an ion-beam-treated polyimide surfaces. Raman spectroscopy is considered to be an efficient way to obtain detailed information about carbon's bonding structure. The rubbing process did not change the polyimide surface's Raman absorption curve, which implies that the polyimide structure was maintained during the rubbing process. On the other hand, new absorption peaks appeared at 1570 cm− 1 and 1366 cm− 1 after ion-beam irradiation on polyimide film. This may be attributed to the hydrogenated amorphous carbon-like structure [14]. The peaks seem to be an intermediate stage before the heavily damaged amorphous structure formation, because they disappear with an increase in the ion-beam dose. For ion-beam energy during this process ranging from 300 to 1500 eV, the peak intensity at 1366 cm− 1 was almost unchanged, but that at 1570 cm− 1 decreased with the increase in energy. These results can be interpreted by the formation of a polycrystalline graphite phase under irradiation at high energy [15]. Therefore, we conclude that the polyimide surface irradiated by an ion beam was changed to a hydrogenated amorphous
carbon structure, which we attribute to the ion-beam-based LC alignment, very different from the rubbing process' results. Through spectroscopic ellipsometry, we measured refractive index spectra to observe the anisotropy of the ion-beam-treated and the rubbed polyimide layers. With no treatment, the polyimide layers
Fig. 4. Raman spectra of (a) a rubbed and (b) an ion-beam-treated polyimide layers.
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P.K. Son et al. / Thin Solid Films 517 (2009) 1803–1806
optical band-gap energy. We calculated optical band-gap energy of an ion-beam-treated and a rubbed polyimide layer. After the ion-beam exposure on polyimide layer surfaces, the optical band-gap energy of the polyimide layer was increased from roughly 5.76 eV to 5.97 eV with the increase of the ion-beam energy from 0 to 1000 eV. Interestingly the increase in the optical band-gap energy of the ionbeam-treated polyimide is comparable to the increase in the amount of the hydrogenated amorphous carbon-like structure. As for the rubbed polyimide layer, the optical band-gap energy of the polyimide was 5.81 eV. Less increase of the optical band-gap by the rubbing reveals that polyimide main chains are aligned with partly broken polyimide main chains. With the increase of the band-gap energy, the refractive index curve shifts toward lower wavelengths. Since the band-gap energy of the ion-beam-treated polyimide layer is higher than that of a rubbed polyimide layer, the refractive index of the former has lower wavelength dependence than that of the latter. 4. Conclusion In conclusion, our experimental results suggest that the ion-beam alignment process creates a carbon-like surface structure on the polyimide layer for LC alignment. Homogeneously aligned LC cell by ion-beam exposure exhibits electro-optic switching characteristics similar to those of rubbing-aligned LC cells. However, polyimide layer's surface morphology and bonding structures show alignment-methoddependent properties. The rubbing process maintains the polymer structure and generates very small anisotropy in the polyimide layer, but ion-beam exposure produces a hydrogenated amorphous carbonlike structure with a short main chain and high anisotropy. Acknowledgments
Fig. 5. Spectroscopic ellipsometry data of polyimide layers; (a) rubbed and (b) ionbeam-treated.
were isotropic because polyimide molecules were randomly distributed. However, after subjecting the polyimide surfaces to the rubbing process or ion-beam exposure, the refractive index nx along the direction of the ion-beam exposure or rubbing differ from the refractive index ny along the direction perpendicular to the ionbeam exposure or rubbing, because of the polyimide molecules' alignment. The ion-beam energy, the incident angle, and the exposure time were 1000 eV, 30°, and 30 s, respectively. The aligned grooves on polyimide surfaces irradiated by an ion beam were not visible in AFM images, but were observed by spectroscopic ellipsometry. The anisotropy generated by the ion-beam exposure was much larger than that generated by the rubbing process, as shown in Fig. 5. Anisotropy is very small in rubbed polyimide layers because the rubbing process influences polyimide surfaces only along the rubbing direction, resulting in a scarcely formed anisotropic layer. On the other hand, ion-beam exposure can modify the whole polyimide layer because of the large interaction volume among ions and polyimide layers. We also found that nx of an ion-beam-treated film shows less decrease with increasing wavelength than that of a rubbed film does. In general, spectroscopic ellipsometry data show that the refractive index curve shifts toward lower wavelengths with the increase of the
This work was supported in part by the Next-Generation Growth Engine Project of the Ministry of Commerce, Industry & Energy, the BK21 Program of the Ministry of Education & Human Resources Development, and the National Research Lab Program of the Korea Science & Engineering Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15]
J.L. Janning, Appl. Phys. Lett. 21 (1972) 173. S.C. Jain, H.-S. Kitzerow, Appl. Phys. Lett. 64 (1994) 2946. S.W. Lee, S.I. Kim, B. Lee, H.C. Kim, T. Chang, M. Ree, Langmuir 19 (2003) 10381. G. Barbero, A.G. Petrov, J. Phys.: Cond. Matter 6 (1994) 2291. J.L. West, L. Su, K. Artyushkova, J. Farrar, J.E. Fulghum, SID Intl. Symp. Digest Tech. Papers 33 (2002) 1102. A.J. Pidduck, S.D. Haslam, G.P.B. Broan, R. Bannister, I.D. Kitely, Appl. Phys. Lett. 71 (1997) 2907. B. Bahadur, Mol. Cryst. Liq. Cryst. 109 (1984) 1. C. Mauguin, Bull. Soc. Fr. Mineral. 34 (1911) 71. P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.-C.A. Lien, A. Callegari, G. Hougham, N.D. Lang, P.S. Andry, R. John, K.-H. Yang, M. Lu, C. Cai, J. Speidell, S. Purushothaman, J. Ritsko, M. Samant, J. Stöhr, Y. Nakagawa, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, S. Odahara, H. Nakano, J. Nakagaki, Y. Shiota, Nature 411 (2001) 56. M.F. Toney, T.P. Russell, J.A. Logan, H. Kikuchi, J.M. Sands, S.K. Kumar, Nature 374 (1995) 709. J. Stöhr, M.G. Samant, J. Luning, A.C. Callegari, P. Chaudhari, J.P. Doyle, J.A. Lacey, S.A. Lien, S. Purushothaman, J.L. Speidell, Science 292 (2001) 2299. J.S. Gwag, C.G. Jhun, J.C. Kim, T.-H. Yoon, G.-D. Lee, S.J. Cho, J. Appl. Phys. 96 (2004) 257. W. Chen, M.B. Feller, Y.R. Shen, Phys. Rev. Lett. 63 (1989) 2665. T. Ohnishi, I. Murase, T. Noguchi, M. Hirooka, Synth. Met. 18 (1987) 497. D. Xu, X.L. Xu, S.C. Zou, Mater. Lett. 12 (1991) 12.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Anisotropy and Raman absorption of the polyimide surface irradiated by the ion beam for liquid crystal alignment Phil Kook Son a, Jeung Hun Park b, Bong Kyun Jo a, Sung Pil Lee a, Joong Ha Lee a, Jae Chang Kim a, Tae-Hoon Yoon a,⁎, Taek Joon Lee c, Moonhor Ree c a b c
School of Electronics Engineering, Pusan National University, Busan 609-735, Republic of Korea Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90095, USA Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 June 2007 Received in revised form 9 September 2008 Accepted 18 September 2008 Available online 27 September 2008 Keywords: Raman scattering Polymers Ellipsometry Atomic force microscopy Liquid crystals Polyimide
a b s t r a c t In this paper, polyimide surfaces irradiated by an ion-beam for liquid crystal alignment are investigated by using atomic force microscopy, Raman spectroscopy, and spectroscopic ellipsometry. A liquid crystal cell aligned homogeneously through the ion-beam exposure exhibits electro-optic switching behavior similar to that of a rubbing-aligned liquid crystal cell. However, we found that the surface morphology and bonding molecules of ion-beam-treated polyimide surfaces show properties very different from mechanically-rubbed ones. Experimental results show that optical anisotropy of ion-beam-treated polyimide surfaces results in the formation of hydrogenated amorphous carbon-like structure with a short main-chain, while mechanical rubbing has little effect on structural and compositional variations of polyimide layers. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction Various techniques have been developed for liquid crystal (LC) alignment, such as obliquely evaporation of dielectrics [1], photoalignment of light-sensitive polymers [2,3], Langmuir–Blodgett films [4], atomic force microscopy (AFM) [5,6], lithography of polymers [7], and the rubbing process [8]. The ion-beam alignment method was introduced recently to overcome the drawbacks of rubbing, such as static charges, the creation of debris, and the degradation of the rubbing fabric [9]. However, mechanically rubbed polyimides are still widely used as alignment layers in the mass production of LC displays. The mechanism of aligning LCs has been an active research topic over the past several decades. However, many LC alignment issues remain unresolved regarding each alignment method: the rubbing process, photo-alignment, and atomic beam alignment. The orientational order of LC on a rubbed polyimide film is generated from the crystalline order of polymer chains [10]. The pretilt angle arises from tilted main- or side-chain segments at the rubbed polymer's surface that “guide” LC rods. The order on ion-beam-treated polyimides may be generated by selectively destroying π-bonds in imide rings by ion-beam exposure [11,12]. In the 1990s the mechanism of LC alignment was further studied at the molecular interaction level by using near edge X-ray ⁎ Corresponding author. E-mail address: [email protected] (Dr. Tae-Hoon Yoon).
absorption for fine structures [11], X-ray photoemission spectroscopy [12], second harmonic generation [13], photo-elastic modulation, AFM [5,8], etc. Stöhr et al. reported ion-beam effects on LC alignment layers to link the orientational bond order in rubbed polyimides, ion-beamtreated polyimides, and ion-beam-treated diamond-like-carbon films with the direction of LC alignment on these surfaces [11]. In this work we investigated the optical anisotropy of ion-beam-treated polyimide layers by using Raman spectroscopy and spectroscopic ellipsometry. 2. Experiments LC cells were prepared by two different methods: the mechanical rubbing method and the ion-beam-based process. Indium–tin-oxidecoated glass substrates were spin-coated with the polyimide RN-1702 (Nissan Chemical Co., Japan), pre-baked at 80 °C for 30 min, and cured at 230 °C for 1 h. Polyimide layers were bombarded by a low-energy argon ion beam. A cold hollow cathode (CHC) type of ion source was used to yield the ion beam. In order to collimate the ion beam, two perforated grids were used as electro-focusing lenses. The CHC type represents a separately cooled chamber supplied with a magnetic system and connected to a discharge chamber through an orifice. Argon gas was fed into the ion source through the CHC only. Discharge ignition in the cathode takes place at nominal discharge voltages and gas flow rates. A neutralizer filament outside the ion source serves as the source of the electrons necessary to compensated ion beam's spatial charges and reduces the repulsive force among ions.
0040-6090/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.084
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P.K. Son et al. / Thin Solid Films 517 (2009) 1803–1806
Raman spectra were measured by using a Raman microscope (Nanofinder 30, Tokyo Instruments Co.) with excitation at 488 nm in the backscattering configuration. We used a microscope with a 40× objective lens immersed in water in order to minimize the sample damage. Laser power incident on the sample was typically less than 200 mV to avoid substrate heating effect. The measurement was carried out over wave numbers of 400–3400 cm− 1. Variable angle spectroscopic ellipsometry (VASE) data for the ionbeam-treated and rubbed polyimide layer were acquired by using a J. A. Woollam VASE spectroscopic ellipsometer over the wavelengths of 400–800 nm in steps of 10 nm for the anisotropy measurement. VASE allows variation in the light's angle of incidence on the sample. In addition, the translator of the sample's stage can be rotated. This powerful option provides additional information on a sample's optical anisotropy. We take the x direction to be the film's optic axis. To find the sample's out-of-plane anisotropy, we selected the incident angles of 15°, 45°, and 75°. To get the sample's in-plane anisotropy, we rotated the translator from 0° to 180° in steps of 30°. Ellipsometry was measured 21 times for each sample. The optical axis in the film was induced by rubbing or ion-beam exposure was fitted in the same direction as the VASE' plane of incidence. We also characterized the film's surface morphology by using AFM with the tapping mode (Digital Instruments, Multimode AFM
Fig. 1. Optical micrographs of LC cells between two crossed polarizers; (a) no treatment, (b) rubbed, (c) ion-beam-treated.
Fig. 2. Measured pretilt angles of ion-beam-aligned homogeneous LC cells as a function of the ion-beam energy.
Nanoscope IIIa). An ultra-lever cantilever with a spring constant of 26 N/m and a resonance frequency of 268 kHz was used for scanning. 3. Results and discussion LC cells with a cell-gap of 3.4 μm were fabricated with a polyimide ‘RN-1702’ and a positive LC "Merck MLC-0223". Fig. 1 shows photographs of homogeneously aligned LC cells between crossed polarizers. The ion-beam energy, the incident angle, and the exposure time were 1000 eV, 30°, and 30 s, respectively. Without treating the polyimide layers, LC molecules are aligned randomly so that disclination lines are visible. LC molecules can be uniformly aligned by the ion-beam exposure or the rubbing process. Fig. 2 shows the measured pretilt angles of LC cells aligned homogeneously by ion-beam exposure on polyimide surfaces, as a function of the ion-beam energy. The ion beam's incident angle and exposure time were 30° and 30 s, respectively. Measured values of the pretilt angle were 1 ± 0.5° regardless of the ion-beam energy. We also found that an LC cell aligned homogeneously by ion-beam exposure exhibits the electro-optic switching behavior similar to that of a rubbed polyimide LC cell. We investigated characteristics of an ion-beam-exposed and a rubbing-treated polyimide layer by using AFM, Raman spectroscopy, and spectroscopic ellipsometry. The polyimide layer's surface morphology is shown in Fig. 3. The surface roughness increased from 0.57 nm to 1.12 nm by 30 s of exposure to an ion beam with energy of 500 eV and an incident angle of 30°. The rubbing process increased surface roughness to 1.51 nm, as shown in Fig. 3(b). It is apparent that the mechanical method modified surface morphology with very thin grooves aligned along the rubbing direction. In contrast, AFM observation revealed that ion-beam-treated polyimide surface exhibited no aligned grooves, but only spike-like patterns, due to the increased roughness. Therefore, we can propose that the binding structure of polyimide surfaces irradiated by an ion beam is very different from a rubbed surface. We examined the ion-beam energy's contribution to the polyimide layer's thickness change by using both scanning electron microscopy and spectroscopic ellipsometry. The polyimide layer's thickness decreased from 120 nm to 18 nm as the ion-beam energy increased from 0 to 1500 eV. On the other hand, the rubbing process did not affect the polyimide layer's thickness. To compensate for the loss of polyimide layers by the ion-beam treatment, we intentionally varied the polyimide layer's initial thickness. In our experiment, to maintain the polyimide layer's thickness at approximately 120 nm after the ion-beam exposure, regardless of the ion-beam energy, we linearly increased the polyimide layer's initial thickness from 120 nm to 222 nm by the spin coating method with an increase of the ion-beam energy from 0 to 1500 eV, respectively. During the ion-beam irradiation process, the incident angle was 30° and the exposure time was 30 s.
P.K. Son et al. / Thin Solid Films 517 (2009) 1803–1806
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Fig. 3. Surface morphologies of polyimide layers; (a) no treatment, (b) rubbed (rubbing direction: x axis), (c) ion-beam-treated (ion-beam direction: α = 30° on the y–z plane).
We employed Raman spectroscopy to examine the ion-beaminduced changes in optical properties due to the modifications in the polyimide layer's chemical structure. For the ion-beam conditions over the experiment, the ion-beam energy, the incident angle, and the exposure time were 1000 eV, 30°, and 30 s, respectively. Fig. 4 shows Raman spectra of a rubbed and an ion-beam-treated polyimide surfaces. Raman spectroscopy is considered to be an efficient way to obtain detailed information about carbon's bonding structure. The rubbing process did not change the polyimide surface's Raman absorption curve, which implies that the polyimide structure was maintained during the rubbing process. On the other hand, new absorption peaks appeared at 1570 cm− 1 and 1366 cm− 1 after ion-beam irradiation on polyimide film. This may be attributed to the hydrogenated amorphous carbon-like structure [14]. The peaks seem to be an intermediate stage before the heavily damaged amorphous structure formation, because they disappear with an increase in the ion-beam dose. For ion-beam energy during this process ranging from 300 to 1500 eV, the peak intensity at 1366 cm− 1 was almost unchanged, but that at 1570 cm− 1 decreased with the increase in energy. These results can be interpreted by the formation of a polycrystalline graphite phase under irradiation at high energy [15]. Therefore, we conclude that the polyimide surface irradiated by an ion beam was changed to a hydrogenated amorphous
carbon structure, which we attribute to the ion-beam-based LC alignment, very different from the rubbing process' results. Through spectroscopic ellipsometry, we measured refractive index spectra to observe the anisotropy of the ion-beam-treated and the rubbed polyimide layers. With no treatment, the polyimide layers
Fig. 4. Raman spectra of (a) a rubbed and (b) an ion-beam-treated polyimide layers.
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P.K. Son et al. / Thin Solid Films 517 (2009) 1803–1806
optical band-gap energy. We calculated optical band-gap energy of an ion-beam-treated and a rubbed polyimide layer. After the ion-beam exposure on polyimide layer surfaces, the optical band-gap energy of the polyimide layer was increased from roughly 5.76 eV to 5.97 eV with the increase of the ion-beam energy from 0 to 1000 eV. Interestingly the increase in the optical band-gap energy of the ionbeam-treated polyimide is comparable to the increase in the amount of the hydrogenated amorphous carbon-like structure. As for the rubbed polyimide layer, the optical band-gap energy of the polyimide was 5.81 eV. Less increase of the optical band-gap by the rubbing reveals that polyimide main chains are aligned with partly broken polyimide main chains. With the increase of the band-gap energy, the refractive index curve shifts toward lower wavelengths. Since the band-gap energy of the ion-beam-treated polyimide layer is higher than that of a rubbed polyimide layer, the refractive index of the former has lower wavelength dependence than that of the latter. 4. Conclusion In conclusion, our experimental results suggest that the ion-beam alignment process creates a carbon-like surface structure on the polyimide layer for LC alignment. Homogeneously aligned LC cell by ion-beam exposure exhibits electro-optic switching characteristics similar to those of rubbing-aligned LC cells. However, polyimide layer's surface morphology and bonding structures show alignment-methoddependent properties. The rubbing process maintains the polymer structure and generates very small anisotropy in the polyimide layer, but ion-beam exposure produces a hydrogenated amorphous carbonlike structure with a short main chain and high anisotropy. Acknowledgments
Fig. 5. Spectroscopic ellipsometry data of polyimide layers; (a) rubbed and (b) ionbeam-treated.
were isotropic because polyimide molecules were randomly distributed. However, after subjecting the polyimide surfaces to the rubbing process or ion-beam exposure, the refractive index nx along the direction of the ion-beam exposure or rubbing differ from the refractive index ny along the direction perpendicular to the ionbeam exposure or rubbing, because of the polyimide molecules' alignment. The ion-beam energy, the incident angle, and the exposure time were 1000 eV, 30°, and 30 s, respectively. The aligned grooves on polyimide surfaces irradiated by an ion beam were not visible in AFM images, but were observed by spectroscopic ellipsometry. The anisotropy generated by the ion-beam exposure was much larger than that generated by the rubbing process, as shown in Fig. 5. Anisotropy is very small in rubbed polyimide layers because the rubbing process influences polyimide surfaces only along the rubbing direction, resulting in a scarcely formed anisotropic layer. On the other hand, ion-beam exposure can modify the whole polyimide layer because of the large interaction volume among ions and polyimide layers. We also found that nx of an ion-beam-treated film shows less decrease with increasing wavelength than that of a rubbed film does. In general, spectroscopic ellipsometry data show that the refractive index curve shifts toward lower wavelengths with the increase of the
This work was supported in part by the Next-Generation Growth Engine Project of the Ministry of Commerce, Industry & Energy, the BK21 Program of the Ministry of Education & Human Resources Development, and the National Research Lab Program of the Korea Science & Engineering Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15]
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