We first investigated the alignment characteristics of tin (IV) oxide (SnO2) thin films deposited by radio-frequency (RF) magnetron sputtering. This study demonstrates that liquid crystal (LC) molecules could be aligned homogeneously by controlling the Ion Beam (IB) irradiation energy densities. We also show that the pretilt angle of the LC molecules has a close relation with the surface energy. X-ray photoelectron spectroscopy (XPS) indicates that a non-stoichiometric SnO2-x surface converted by ion beam irradiation can horizontally align the LC molecules. The measured electro-optical (EO) characteristics showed high performance, comparable with those of rubbed and ion-beam irradiated polyimide (PI) layers.
©2010 Optical Society of America
Recently, uniform liquid crystal (LC) alignment on various layers using dissimilar surface treatment methods has been an important part in the manufacture of LC display (LCD) applications. The rubbing method  is widely used in industry because of its adaptability for mass production and because it is a low-cost treatment that can be applied over large areas. However, this process has serious drawbacks, such as debris creation, electrostatic discharge, and partial particles, which can introduce local defects, streaks, and resultant low-quality LCDs. For this reason, non-rubbing alternatives such as UV exposure , nano-imprint lithography , oblique deposition , and ion beam (IB) irradiation  have been actively investigated. In particular, the ion beam method, which results in induced alignment producing high-resolution displays in a controllable non-contact and non-stop process, has been investigated using inorganic materials. The use of many inorganic materials such as diamond-like carbon (DLC) , SiNx , and blended particles  has been reported. However, the fundamentals of alignment via IB-irradiation have differences according to the types and characteristics of the inorganics used. Therefore, an investigation of a new inorganic material with outstanding properties is needed in order to realize high quality LCDs.
We first investigated tin (IV) oxide (SnO2), which has excellent dielectric properties and good optical characteristics . In this paper, we introduce a previously unreported orientation mechanism of LC molecules on SnO2 thin films which were treated by IB-irradiation on the deposited SnO2 using RF sputtering.
The SnO2 thin films were deposited by an RF magnetron sputtering system with indium-tin-oxide (ITO)-coated glass (Samsung Corning 1737) substrates. In this investigation, a 2-inch-diameter and a pellet with a 99.99% SnO2 target was used. Before deposition, the ITO-coated glass was cleaned with a supersonic wave in a trichloroethyl-acetone-methanol-methanol-de-ionized water solution for 10 min and sequentially dried with N2 gas. In the deposition process, the total pressure was a uniform 5 mTorr, the growth temperature was 50 °C, and the deposition time was 30 min for an RF power of 150 W. The deposition was conducted in an Ar and O2 mixture with a ratio of Ar/O2 = 4/1 (Ar = 40 SCCM, O2 = 10 SCCM). The cross-section and RMS roughness of the SnO2 thin films were observed using field emission scanning electron microscopy (FESEM) (S-4200; Hitachi) and an atomic force microscope (AFM) in tapping mode. The optical transmittance of the SnO2 film was measured using an ultraviolet visible near-infrared scanning spectrophotometer (UV-3101PC; Shimadzu) in the spectral range of 250 – 800 nm. The thin films were irradiated with an Ar ion beam using a DuoPIGatron-type IB advanced system  for 2 min at an energy density of 0.6-3.0 keV and an incident angle of 45°. After the IB-irradiation, two types of test panels were fabricated: one with an anti-parallel configuration with cell gaps of 60 μm for the measurement of the pre-tilt angles, and one with a twisted-nematic (TN) panel with cell gaps of 5 μm for the electro-optical measurements. The pre-tilt angles were obtained using the crystal rotation method (TMA 107; Autronic) at room temperature. Each cell was filled with positive LC (Tc = 72 °C, Δε = 8.2; MJ001929, Merck) for fabrication. The transitions of the chemical bonding states were analyzed both before and after the IB irradiation using X-ray photoelectron spectroscopy (XPS) (ES-CALAB 220i-XL; VG Scientific). The EO characteristics for the voltage transmittance and response time were obtained using an LCD evaluation system (LCD-700; Otsuka Electronics).
3. Results and Discussion
Figure 1(a) shows the cross-sectional FESEM image of the SnO2 thin film deposited onto the ITO-coated substrates at a 50 °C growth temperature and a 30 min deposition time. The FESEM analysis indicates that the 200-nm-thick SnO2 thin film was deposited densely on the 200 nm ITO-coated substrate. The RMS roughness values which were obtained from the AFM data before and after the IB-irradiation are shown in Fig. 1(b) as a function of the IB-irradiation energy density levels applied for 2 min. The graph shows that the IB-irradiation does not strongly affect the roughness of the resulting thin films.
Transparency is the one of the important factor in the application of inorganic film in the LCDs. The optical transmittance at 250-800 nm was measured at room temperature of the as-deposited SnO2 on ITO film. The transmittance spectrum for the SnO2 on ITO-coated glass substrates are shown in Fig. 2 . For comparison, those of the PI film on ITO-coated glass substrates, plain ITO-coated glass substrates were also observed. The average optical transmittance of the ITO film, and PI-coated glass and SnO2/ITO films in the visible portion (420-780 nm) of the spectrum were 82.8%, 83.9%, and 91%, respectively. This result indicates that high transparency was observed on SnO2/ITO film compared with ITO film and PI-coated glass. This is because the sputtering-processed SnO2 was oxidation enhanced and, therefore, repaired the oxygen vacancies within the SnO2 layer , which could potentially improve the total LCD panel transmittance.
To confirm the alignment conditions and pre-tilt angles of the IB-irradiated SnO2 thin films to align the LC molecules, anti-parallel configurations were fabricated. Figure 3 shows the pre-tilt angles and pre-tilt errors of the LC molecules on the IB-irradiated SnO2 thin films as a function of the IB-irradiation energy density. In this figure, the LC molecules had low pre-tilt angles and low error values due to the uniform LC alignment at IB-energy densities higher than 1800 eV, while the high error values were observed when the IB-irradiation energy densities were lower than 1800 eV. These results indicate that a regular pre-tilt angle with a low pre-tilt error (less than 1%) in the LC molecules resulted in uniform LC alignment in the LCDs. The pre-tilt angles were observed in a low angle range of about 0.1° (upper 1800 eV), which can be fabricated in various LC modes with low pre-tilt angles, such as the twisted-nematic (TN) mode , in-plane switching (IPS) mode , and fringe-field switching (FFS) mode .
One of the important factors in controlling the pre-tilt angle of the LC molecular orientation is the surface energy . The contact angle measurement of a sessile liquid drop is easily performed using a contact angle analyzer (SEO Phoenix300) and the surface energy of the solid on which the liquid is placed. Figure 4(a) shows the variation in the contact angle on the SnO2 thin layers for various IB-irradiation energy densities. Generally, the contact angle is inversely proportional to the surface energy, as shown in Fig. 4(b). The hydrophilic properties decrease proportionally with the contact angle, so the pre-tilt angle decreases when the surface energy increases. When the surface energy is high, the intermolecular forces of the LC molecules are lower than the forces across the surface . This principle indicates that the LC molecules are horizontally aligned with the substrate (homogeneous alignment).
The XPS analysis was used to explain the effects of LC orientation on IB-irradiated SnO2 surfaces because the pre-tilt angle is generally affected by the anisotropic surface properties, which are a function of the chemical composition of the alignment surface.
Figure 5 shows the XPS spectra for the Sn3d5/2 peaks covering Sn2+, Sn4+, and Sn0 peaks from the SnO2 after IB-irradiation with various incident energy levels. The binding energies of the Sn3d5/2 without IB irradiation located at 486.3 ± 0.1 eV have a little change after IB irradiation. The growth in the Sn3d5/2 peak intensity was caused by outward atmospheric oxidation. In the IB-irradiation process, Sn atoms in the as-deposited sputtering SnO2 thin films can exist oxidizing steps . The increment ratio of the IB intensity leads to a decreased Sn2+ peak and an increased Sn4+ peak. The IB- irradiation below 1800 eV breaks the π-bond (Sn-O) of the SnO2 selectively, while IB-irradiation above 1800 eV can break a large amount of σ-bonds (Sn = O) between the Sn and O, which has a strong bonding energy. The intensity values of the Sn4+ were similar to each other (≈29k arb. units) when the IB-irradiation energy was over 1800 eV.
Figure 6 shows that the O 1s spectra displayed a changed intensity as function of IB-irradiation energy densities. The analysis of O 1s peak as-deposited sputtering SnO2 thin films confirmed that it is constructed as a mixture of some components. Two components exist in O 1s peak which corresponding to the O-Sn2+ and O-Sn4+ bonding have easily been differentiated. In respect of the O 1s peaks, the intensity of O-Sn4+ peak proportionally increased as increment of IB-irradiation energy densities, while, the intensity of O-Sn2+ relatively decreased. This result corresponds to alteration of Sn3d5/2 peak as shown in Fig. 5. It is evident that O atoms (ions) in combine with various Sn atoms (ions) at proper oxidizing steps. In process of oxidation, a large number of Sn4+ bonding with O atoms when the IB-irradiation energy was over 1800 eV. The XPS spectra for the Sn3d5/2 and O 1s of the SnO2 thin film clearly demonstrated that SnO2 surface sequentially modulated as function of IB energy density.
In other words, the surface of the SnO2 converted to the non-stoichiometric states of SnO2-x, which has high polarizability with delocalized electrons . The delocalized electrons on SnO2 surfaces cause aniso-tropic dipole polarizability  in the direction of IB-irradiation. This irradiation built to align homogeneous LC with stable pre-tilt angle. The van der Waals forces on the SnO2 surfaces change depending on IB-irradiation densities because the van der Waals forces are proportional to the polarizability . It means that IB-irradiated SnO2 with relatively higher van der Waals forces than LC molecular interactions. Also, the increased polarizability can lead to a large dispersion force, then, we can achieve the horizontally aligned LC molecules on the SnO2 surface  as shown in Fig. 7 .
The EO characteristics were confirmed to be appropriate for pragmatic LCD applications. Figure 8 shows the EO characteristics of TN-LCDs produced with cell gaps of 5 μm by the IB-irradiation of SnO2 thin layers. The IB-irradiation energy density, exposure time, and incident angle were 1800 eV, 2 min, and 45°, respectively. In addition, layers of rubbed and IB-irradiated polyimide (PI) under the same conditions were used for comparison.
Figure 8(a) is a plot of the response time (RT) curves: superior performance was obtained from the IB-irradiated SnO2 thin layer, with a rise time of 1.137 ms and a fall time of 4.594 ms, while the rubbed and IB-irradiated PI layers had rise times of 1.427 ms and 2.461 ms and fall times of 9.197 ms and 12.577 ms, respectively. The reduced response time of the LC has the potential for the high speed liquid crystal display applications which are needed in the industrial fields. The voltage-transmittance (V-T) curves are displayed in Fig. 8(b): the V-T characteristics were identical for each alignment when applying 5 V. At 10% transmittance, the threshold voltages of the TN-LCDs on the SnO2 thin layer and those of the rubbed and IB-irradiated PIs were 1.392, 1.407, and 1.427 V, respectively. Figure 8(c)-(d) shows the photomicrographs of TN-LCD cells with and without applying voltage (5 V). As can be seen, uniform LC alignment was formed on the SnO2 alignment layer. So, uniform switching behavior was observed without deviated alignment and local defects such as disclinations.
In conclusion, SnO2 irradiated using IB-irradiation was introduced as an LC alignment layer for the first time. We demonstrated the homogeneous alignment of the LC molecules on the SnO2 films. An IB irradiation energy density greater than 1800 eV resulted in uniform LC alignment on the SnO2 thin films. The low pre-tilt angle is closely related to the surface energy as a function of the IB-irradiation energy. XPS indicated that the IB-irradiation energy changed the chemical structure, and that oxidizing steps occurred on the SnO2 surface. In the process of oxidizing, the SnO2 structures were modified to SnO2-x. Delocalized electrons affected to anisotropic van der Waals forces and high polarizability. This principle confirmed that homogeneous LC molecules aligned on modulated SnO2 surfaces uniformly with low pre-tilt angle. Moreover, the superior EO characteristics were illustrated, showing tremendous potential for optical devices in LCDs.
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