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Novel Alignment Mechanism of Liquid Crystal on a Hydrogenated Amorphous Silicon Oxide Kyung Chan Kim, Han Jin Ahn, Jong Bok Kim, Byoung Har Hwang, and Hong Koo Baik* Department of Metallurgical Engineering, Yonsei University, Shinchon 134, Seodeamoon, Seoul 120-749, Korea Received March 30, 2005. In Final Form: September 27, 2005 The mechanism of liquid crystal (LC) alignment has been investigated during the last few decades for inorganic materials as well as for organic materials; however, it has not been clearly confirmed for some alignment materials. Inorganic alignment materials such as amorphous silicon oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H) are deposited on indium tin oxide (ITO) films on glass by reactive sputtering deposition. After deposition, the inorganic alignment materials are irradiated using an Ar+ ion beam (IB) for LC alignment. On the basis of the experimental results, a-SiOx films deposited by the sputtering do not align the LC, but a-SiOx:H films treated with varying IB energies, IB incident angles, IB doses, and IB irradiation times have excellent alignment properties and electrooptical properties, identical to those of polyimide (PI). These results imply that inorganic alignment layers irradiated by IB can be adopted as an LC alignment layer instead of rubbed PI. Additionally, hydrogen plays an important role in LC alignment because of the difference in alignment properties between a-SiOx films and a-SiOx:H films. We investigate the mechanism of IB-treated inorganic alignment layers and suggest that LCs are aligned by chemical effects, such as van der Waals interaction, more than by physical effects, such as morphology effects, in the inorganic alignment layer irradiated by IB.
1. Introduction Liquid crystal displays (LCDs) are widely used in various information display devices, for example, mobile phone displays, notebook computer monitors, laptop monitors, personal digital assistant (PDA) displays, calculators, and large area TV applications. The unidirectional uniform alignment of liquid crystal (LC) molecules on treated substrate surfaces is very important in LC science and technology. In the many techniques proposed for the uniform alignment of LC molecules, mechanical rubbing is the most widely used because it is a very simple method of obtaining uniform LC alignment, and it ensures high productivity. Although the rubbing method has many advantages, this method also has many problems because it is a physical contact, among them, the generation of dust and static electricity. Polyimide (PI) also requires a high heat treatment (up to 200 °C), limiting the choice of substrates and materials. To overcome the demerits of the mechanical rubbing process, several alignment methods have been proposed as potential replacements, such as oblique evaporation,1-4 the PI Langmuir-Blodgett (LB) film method,5-7 and photoalignment.8-9 Since the LC photoalignment method * Corresponding author. Tel: +82-2-2123-2838; fax: +82-2-3125375; e-mail address:
[email protected]. (1) Janning, J. L. Appl. Phys. Lett. 1972, 21, 173. (2) Crossland, W. A.; Morrissy, J. H.; Needham, B. J. Phys. D: Appl. Phys. 1976, 9, 2001. (3) Guyon, E.; Pieranski, P.; Boix, M. Lett. Appl. Eng. Sci. 1973, 1, 19. (4) Armitage, D. J. Appl. Phys. 1980, 51, 2552. (5) Murata, M.; Awaji, H.; Isurugi, M.; Uekita, M.; Tawada, Y. Jpn. J. Appl. Phys. 1992, 31, L189. (6) Murata, M.; Uekita, M.; Nakajima, Y.; Saitoh, K. Jpn. J. Appl. Phys. 1993, 32, L679. (7) Ikeno, H.; Oh-saki, A.; Nitta, M.; Ozaki, N.; Yokoyama, Y.; Nakaya, K.; Kobayashi, S. Jpn. J. Appl. Phys. 1988, 27, 475. (8) Wu, Y.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 349. (9) Usami, K.; Sakamoto, K.; Ushioda, S. J. Appl. Phys. 2003, 93, 9523.
is free from the drawbacks of the traditional method of mechanical rubbing, it is preferable for next-generation LCDs. However, there are several technical issues associated with photoalignment that need to be addressed, such as weak anchoring strength, pretilt angle control, image sticking, and low thermal stability. Recently, as one of the noncontact alignment methods, an alignment method by ion beam (IB) irradiation was reported by Chaudhari and Doyle.10-11 This alignment method does not have the drawbacks seen with rubbing because it is a noncontact method. In addition, they used inorganic material instead of PI to avoid the heat treatments. Few reports have appeared about this technique as of yet, and suitable inorganic materials have to be developed to increase the processing window; also, a complete understanding of the LC alignment mechanism by IB irradiation is still lacking. Therefore we need an accurate study to understand the IB alignment mechanism. In recent years, our group has investigated the IB method and various inorganic materials.12-15 In this paper, we use inorganic materials, such as amorphous silicon (10) Chaudhari, P.; Lacey, J.; Doyle, J. P.; Galligan, E. A.; Lien, S. C.; Callegari, A. C.; Hougham, G.; Lang, N. D.; Andry, P. S.; John, R.; Yang, K. H.; Cai, C.; Speidell, J. L.; Purushothaman, S.; Ritsko, J.; Samant, M. G.; Stohr, J.; Nakagawa, Y.; Katoh, Y.; Saitoh, Y.; Sakai, K.; Satoh, H.; Satoh, H.; Odahara, S.; Nakano, H.; Nakagaki, J.; Shiota, Y. Nature 2001, 56, 56. (11) Doyle, J. P.; Chaudhari, P.; Lacey, L.; Galign, E. A.; Lien, S. C.; Callegari, A. C.; Lang, N. D.; Lu, M.; Nakagawa, Y.; Nakano, H.; Okazaki, N.; Odahara, S.; Katoh, Y.; Saitoh, Y.; Sakai, K.; Satoh, H.; Shiota, Y. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 206, 467. (12) Rho, S. J.; Lee, D. K.; Baik, H. K.; Hwang, J. Y.; Jo, Y. M.; Seo, D. S. Thin Solid Films 2002, 420-421, 259. (13) Lee, D. K.; Rho, S. J.; Baik, H. K.; Hwang, J. Y.; Jo, Y. M.; Seo, D. S.; Lee, S. J.; Song, K. M. Jpn. J. Appl. Phys. 2002, 41, L1399. (14) Song, K. M.; Rho, S. J.; Ahn, H. J.; Kim, K. C.; Baik, H. K.; Hwang, J. Y.; Jo, Y. M.; Seo, D. S.; Lee, S. J. Jpn. J. Appl. Phys. 2004, 43, 1577. (15) Ahn, H. J.; Kim, K. C.; Kim, J. B.; Hwang, B. H.; Baik, H. K. Jpn. J. Appl. Phys. 2005, 44, 4092.
10.1021/la050839y CCC: $30.25 © 2005 American Chemical Society Published on Web 10/28/2005
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Figure 2. Schematic of a cross section of the LCD with an IB-treated alignment layer.
Figure 1. IB irradiation system.
oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H), not only because they are widely used in the industry, but also because they are inexpensive and easily controlled. Also, the IB treatment is a noncontact process and can be applied to large area displays. To investigate the LC alignment mechanism, we measure thin-film properties and the degradation of electrooptical (E/O) properties using various analysis equipment. 2. Experiment The a-SiOx and a-SiOx:H thin films were deposited onto either indium tin oxide (ITO) glass or Si wafers using radio frequency (RF) magnetron sputtering. Prior to the deposition of the films, all substrates were cleaned ultrasonically; cleaning was carried out with trichloroethylene (TCE), acetone, and alcohol for 10 min each. For the Si wafer, after standard cleaning, native oxide was etched in a buffered oxide etchant (BOE). In this experiment, a SiO2 (99.9999%) target was used as the sputtering source; the base pressure in the chamber was 3 × 10-6 Torr, and the working pressure in the deposition chamber was 5 × 10-3 Torr. The substrate holder was rotated at 6.5 rpm to deposit a uniform alignment layer. The deposition power was fixed at 100 W, and the gas flow ratio (Rg) of the Ar and H2 gases entering the chamber for sputter deposition was changed from 0 to 0.8 (Rg ) H2/[Ar + H2]). The a-SiOx and a-SiOx:H thin films had anisotropic properties after Ar+ IB irradiation, and the LC was aligned by IB-treated a-SiOx:H films. The Ar+ IB system is shown in Figure 1. The IB irradiation time, IB incident angle from the surface plane, IB energy, and current density were 0-2 min, 30-60°, 100-300 eV, and 40.52 µA/cm2, respectively. The schematic diagram of the alignment layer and IB irradiation is shown in Figure 2. To understand the LC alignment mechanism, we measured the surface morphology and the chemical bonding state using atomic force microscopy (AFM), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) equipment. In addition, the a-SiOx and a-SiOx:H films had excellent properties for use as an alignment layer, not only including good transmittance, alignment property, pretilt angle, voltage-transmittance (V-T), response time (RT), and the voltage holding ratio (VHR), but also stability. Therefore, the a-SiOx:H films were characterized using polarized optical microscopy (POM), ultraviolet visible spectroscopy (UV/vis), and various E/O property measurement systems.
3. Results and Discussion One requirement for an alignment layer to be used in an LCD is high transmittance. Therefore, we determined the relation between wavelengths from 200 to 800 nm and the transmittance for each sample using UV/vis spectroscopy. Figure 3 shows the transmittance of
Figure 3. Transmittance of a-SiOx:H thin films as a function of H2 and IB.
a-SiOx:H thin films as a function of H2 and IB. To confirm the change in transmittance, we choose the transmittance of bare ITO glass as the standard transmittance (100%) at a specific wavelength (633 nm). Figure 3 shows that a-SiOx:H film deposited at a gas flow ratio of 0.25 has the highest transmittance, and, as the content of hydrogen in the thin film is increased, the transmittance of the a-SiOx:H film decreases. The transmittance of a-SiOx:H film is changed as a function of film thickness and hydrogen content. However, we deposit the same thickness (∼10 nm) to remove the thickness effect and only consider the effect of hydrogen content. When H2 is incorporated in the alignment layer during deposition, the larger the amount of H2, the greater the increases in the number of Si-H and Si-Si bonds because the Si-O-Si network structure is broken by H2.16 We evaluated the alignment properties of a-SiOx:H films using LCs with a positive dielectric constant (Merck, MJ001929). Figure 4 shows the alignment properties of a-SiOx and a-SiOx:H films before and after IB irradiation. From the results, a-SiOx films deposited by the sputtering are not aligned regardless of the IB irradiation conditions. However, when IB irradiation is parallel to the polarizer direction, good homogeneous alignment properties only occurred in the IB-irradiated a-SiOx:H film because of a normal black state without an applied electric field. It is implied that hydrogen, along with IB irradiation, plays an important part in LC alignment in view of the results achieved thus far. We think that hydrogen incorporated in the thin film deforms the Si-O-Si network structure and makes new bonding structures such as Si-H, which (16) Janssen, R.; Janotta, A.; Dimova-Malinovska, D.; Stutzmann, M. Phys. Rev. B 1999, 60, 13561.
Liquid Crystal Alignment on a-SiOx:H
Figure 4. POM results of a-SiOx and a-SiOx:H thin films using positive LC before and after IB irradiation. (a) a-SiOx film before IB irradiation. (b) a-SiOx film after IB irradiation (1 min/45°/ 200 eV). (c) a-SiOx:H film before IB irradiation. (d) a-SiOx:H film after IB irradiation (1 min/45°/200 eV).
are polar groups. When the IB is irradiated, it selectively destroys polar groups on the surface. Then LCs are aligned parallel to the direction of IB irradiation because of the interactions between the LCs and the alignment layer. Nevertheless, this is uncertain; therefore, we have to perform more investigations on LC alignment mechanisms. However, it is difficult to verify this hypothesis. On the basis of the experimental results (Figure 4), for IB-irradiated a-SiOx:H film, LCs are only aligned. In the previously published literature, the alignment mechanism of LC molecules on rubbed PI was the anisotropic intermolecular interactions between the LC molecules and the surface of the alignment layer and elastic energy anisotropy due to surface morphology that is defined by surface columns and microgrooves.17 Moreover, it is reported that silicon oxide deposited by oblique evaporation causes LCs to be aligned by the morphological effects of the silicon oxide surface.1-3 Therefore we measure the surface morphology effect in preference to any other. Figure 5 shows the surface morphology of a-SiOx and a-SiOx:H films. From the experimental results, the surface roughness (rms) of a-SiOx, a-SiOx:H film, and IBirradiated a-SiOx:H film is 2.4, 4.5, and 2.6 nm, respectively. In this case, the IB irradiation angle is 45° and surface anisotropy such as unidirectional orientation in rubbed PI does not occur according to AFM data. When silicon oxide is unidirectionally deposited by the evaporation, an adatom deposited on the substrate has a kinetic energy between 0.03 and 0.2 eV, both of which are too low to enable it to move on the substrate. The obliquely evaporated silicon oxide has a rough surface, which has a high enough rms value for the morphology effect to occur. In our experiments, however, the deposited adatom on (17) Shioda, T.; Okada, Y.; Chung, D. H.; Takanishi, Y.; Ishikawa, K.; B. C. Park, Takezoe, H. Jpn. J. Appl. Phys. 2002, 41, L266.
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Figure 5. AFM data of a-SiOx and a-SiOx:H film before and after IB irradiation. (a) a-SiOx film before IB irradiation. (height: ∼500 Å, width: 5 µm). (b) a-SiOx:H film before IB irradiation. (height: ∼500 Å, width: 2 µm). (c) a-SiOx:H film after IB irradiation. (height: ∼500 Å, width: 2 µm).
the substrate, which is rotated at 6.5 rpm has a kinetic energy of ∼5 eV, which is sufficiently high to enable it to move on the substrate. The a-SiOx:H has a relatively smooth surface (rms ) 2.6 nm).18 The IB-irradiated a-SiOx:H film has a smooth surface, and the IB irradiation causes a decrease in the surface roughness due to oblique irradiation.19 Therefore, we consider that LCs are aligned by a chemical effect such as chemical bonding as a major factor then by a morphological effect as a minor factor. However, when we measure the surface anisotropy using high-resolution X-ray reflectivity (XRR), the surface anisotropy of the alignment layer might occur, although the surface anisotropy is not found by AFM results; further investigation is needed for the morphology effect. LCs are aligned in a-SiOx:H film after IB irradiation. Therefore, we use FT-IR (Figure 6) and XPS (Figure 7) measurements to confirm the change in the chemical bonding state after IB irradiation in the a-SiOx:H film. IB irradiation was performed at 45° from the surface plane, and the IB energy and IB irradiation time was 200 eV and 1 min, respectively. In the FT-IR data (Figure 6), we assume that a-SiOx:H film before IB irradiation has various Si-O-Si, Si-Si, Si-H, and OH bonds.20-23 Although we cannot find the changes in Si-H bonding in the FT-IR spectra, we can consider the change in Si-H (18) Smith, D. L. Thin-Film Deposition: principles and practice; McGraw-Hill: Singapore, 1997. (19) Bardley, R. M.; Harper, J. M. E. J. Vac. Sci. Technol., A 1988, 6, 2390. (20) Theil, J. A.; Tsu, D. V.; Watkins, M. W.; Kim, S. S.; Lucovsky, G. J. Vac. Sci. Technol., A 1990, 8, 1374. (21) Chang, W. J.; Houng, M. P.; Wang, Y. H. Jpn. J. Appl. Phys. 1999, 38, 4642. (22) Delfino, M.; Tsai, W.; Reynolds, G.; Day, M. E. Appl. Phys. Lett. 1993, 63, 3426. (23) Yoshimaru, M.; Koizumi, S.; Shimokawa, K. J. Vac. Sci. Technol., A 1997, 15, 2915.
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Figure 6. FT-IR data of a-SiOx:H film before and after IB irradiation.
Figure 8. Pretilt angle vs (a) IB exposure time, (b) incident angle, or (c) IB energy. Dotted lines are visual aids.
Figure 7. XPS narrow scan of a-SiOx:H film before (a) and after (b) IB irradiation.
bonding by IB irradiation because Si-H bonding is weaker than Si-Si bonding, and Si-Si bonding is eliminated by IB irradiation in Figure 7. As a result, many of the OH and Si-H bonds are removed, and new bonding structures are formed in the a-SiOx:H thin film after IB irradiation. The selective deconstruction of the OH and Si-H bonds, which have surface polarity, makes the anisotropic property of a-SiOx:H film after IB irradiation, and such anisotropic surface polarity formed by IB irradiation aligns LC toward the IB direction. The removal of Si-Si bonds in the a-SiOx:H film after IB irradiation is confirmed by XPS measurement (Figure 7). The Si-O and Si-Si peaks exist in the a-SiOx:H film before IB irradiation, but the Si-Si peak disappeared along with an increase in the Si-O peak area after IB irradiation. Although the Si-H peak is not detected because of XPS limitations, it seems that Si-H and Si-Si bonds are eliminated by IB irradiation because the dissociation energy of the Si-H bond is lower than that of the Si-Si bond (Si-H: 299.2 D°298/KJ mol-1 < Si-Si: 326.8 D°298/KJ mol-1). Although there is
an increase in the Si-O peak area, we cannot see the change in the Si-O peak in FT-IR spectra, and we find that a new Si-O-Si bond is formed where the bond was broken. To prove the effect of IB irradiation, we control various factors such as IB exposure time, IB incident angle, and IB energy. Figure 8 shows the pretilt angle and IB conditions, and it indicates that the pretilt angle, which has an error range within 0.2°, is controlled by IB irradiation. From Figure 8, we get the variation of pretilt angle in the range of 0.11°∼0.96° and surmise that the alignment layer has anisotropic property after IB irradiation. When the IB energy exceeds 200 eV, the pretilt angle is decreased. It is believed that the anisotropic properties of a-SiOx:H film is decreased by excessively high IB energy irradiation.11 A good alignment layer also has excellent E/O properties such as V-T with a low threshold voltage, a fast RT, and a high VHR to apply to LCDs. Therefore, we measured the E/O properties of IB-irradiated a-SiOx:H films as a homogeneous alignment layer and compared them with those of rubbed PI because this is the industry standard (Figure 9 and Table 1). As shown in the experimental results, the rubbed PI and a-SiOx:H films have a low threshold voltage (∼1.2 V). The RT of an a-SiOx:H film is shorter than that of rubbed PI (18.355 ms). We estimate that the anchoring energy of IB-irradiated alignment is lower than that of rubbed PI, as indicated in the published results of many researchers. In addition, Figure 9 also shows VHR characteristics in IB-irradiated a-SiOx:H films. The a-SiOx:H film has a high VHR value (about
Liquid Crystal Alignment on a-SiOx:H
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Figure 9. V-T (a), RT (b), and VHR (c) curves of rubbed PI and IB-induced a-SiOx:H films. Table 1. E/O Properties of Rubbed PI and IB-Irradiated a-SiOx:H Thin Film V-T (V)
RT (ms)
sample
V90
V10
rising
falling
RT
rubbed PI a-SiOx:H
1.263 1.231
2.037 2.174
7.678 3.848
10.677 9.781
18.355 13.629
98%), which is the same as the VHR of rubbed PI. Therefore, we expect that the a-SiOx:H film irradiated by IB will have excellent alignment properties and can be used in place of rubbed PI for LCD technology. The surface change in the alignment layer after IB irradiation and the aging test is schematically shown in Figure 10. If the alignment layer without immediate LC cell fabrication after IB irradiation is maintained in air, the impurity is adsorbed onto the surface of the alignment
layer. Therefore, we infer that many dangling bonds produced by IB irradiation are reacted with a number of impurities in the LC and the sealant, and the impurities lead to degradation of the alignment properties and the E/O properties (not shown). We immediately make the LC cell after IB irradiation and investigate the stability of an IB-treated inorganic alignment layer after a time lapse. Figure 11 and Table 2 show the V-T and RT value measured after 100 days and the VHR value measured after 7 days. As shown in the experimental results, the E/O properties were rarely degraded and maintained similar values. In addition, the VHR measured after 7 days has almost the same value in the measurement system error range, which is within 1%. This indicates that IB-irradiated a-SiOx:H films have neither many dangling bonds, such as active reaction sites, nor sufficient
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Figure 11. V-T (a) and VHR (b) measurement in IB-irradiated a-SiOx:H film after aging process. Table 2. V-T and RT Values in IB-Irradiated a-SiOx:H Film after Aging Process V-T (V)
Figure 10. Schematic diagram of alignment layer after IB treatment and after aging test. (a) After IB irradiation, immediately fabricated LC cell. (b) After IB irradiation, fabricated LC cell using alignment layer degraded by air exposure over the course of 5 days.
impurities occurring during degradation. Therefore, we concluded that IB-irradiated a-SiOx:H films have good stability during the aging, on the basis of the experimental results; however, we do not know what factor is dominant. 4. Conclusions We investigated inorganic alignment layers and IB irradiation for LC alignment. Sputter-deposited a-SiOx films were found not to align in homogeneous alignment. An a-SiOx:H film irradiated with an IB has excellent transmittance, alignment properties, E/O properties (such as V-T, RT, and VHR), and stability, comparable to that of rubbed PI. Generally, many researchers insist that LC is aligned by surface physical effects, such as morphological effects and orientating the PI molecules, and surface chemical effects, such as π-π electron coupling effect, dipole-dipole interaction, and van der Waals interaction.
RT (ms)
sample
V90
V10
rising time
decay time
RT
0h 18 h 42 h 66 h 100 day
1.173 1.163 1.226 1.204 1.223
2.109 2.045 2.188 2.148 2.315
3.333 3.547 3.475 3.303 3.560
6.408 7.029 6.815 6.015 7.831
9.741 10.576 10.290 9.318 11.391
In our experimental results, we cannot find surface morphology effects from AFM results. Therefore, we provisionally conclude that the LC is aligned by a chemical effect such as van der Waals interaction as a major factor then by a morphological effect as a minor factor because a-SiOx:H film does not have π-π electron coupling effects and dipole-dipole interaction. Although we cannot exactly confirm the effect of hydrogen in a-SiOx:H film, we surmise that hydrogen plays an important part in changing the alignment layer structure and in the selective deconstruction of OH and Si-H bonds, which have surface polarities that create the anisotropic properties of a-SiOx:H film after IB irradiation, and such anisotropic surface polarity formed by IB irradiation works to align LCs toward the IB direction. We also believe that the IB process overcomes the limitations of the rubbing process and may be applied to large-area displays. In conclusion, we suggest that inorganic alignment layers irradiated using IB can be adopted as an LC alignment layer instead of rubbed PI. Acknowledgment. The authors are grateful to Professor Y. B. Kim of Konkuk University for allowing the use of his devices. LA050839Y
