A High Sensitivity PDCLC-based Electro-optic Modulator for TFT Array Inspector Jeoung-Yeon Hwang 1 , Junge Kyu Park 2 , Jong Sun Choi 3 , Hyungtak Kim 3 , Jae-Hoon Kim 1,2 , and Chang-Jae Yu 1,2* 1 Dept. of Information Display Engineering, Hanyang University, Seoul, Korea 2 Dept. of Electronic Engineering, Hanyang University, Seoul, Korea 3 School of Electronic and Electrical Engineering, Hongik University, Seoul, Korea Abstract We proposed a high sensitive electro-optic modulator based on a polymer-dispersed cholesteric liquid crystal (PDCLC) for the TFT array inspector. Since the PDCLC is operated in a reflective mode, the PDCLC-based inspection system is very simple since a dielectric mirror, which is generally used in a conventional TFT array inspector, is not required. 1. Introduction Thin-film-transistor (TFT) liquid crystal displays (LCDs) have the advantages of picture quality such as contrast, response, and brightness. To reduce costs for the widespread use of TFT-LCDs, the yield and throughput of the fabrication process of TFT arrays must be improved. In order to increase yield and reduce costs, it is necessary to test TFT arrays and detect defects during the manufacturing process. A manufacturing the TFT-LCD with no pixel defects is one of the important issues in the TFT-LCD manufacturing industry. Therefore, the importance of the efficiency enhancement on LCD production process has been increasing and the ALDI (Automated LCD Defect Inspection) system is demanded for this efficiency enhancement. There are several established methods for various inspections [1-7]. In general, the inspector shows a common electrode configuration in a single substrate and the TFT array is acted as the other pixel electrode for inspection. Most of them are noncontact methods that use a two-dimensional electro-optic (EO) modulator and detect the surface potential of the TFT array induced by phase modulation [5, 6]. The positional resolution of the modulator is required to level of several tens of micrometers. However, the fringe electric field from the edges on the pixel electrode has influence on liquid crystal configurations even at the neighboring pixel regions. Thus, the EO variation penetrates into the neighboring pixels. The variation is increased with increasing the applied voltage and thus it gives rise to degrading the detection sensitivity of the inspector. With increasing resolution TFT-LCD, it is hard to detect a fine pitch pixel of TFT-LCDs by using the modulator based on polymer-dispersed liquid crystal (PDLC) [6]. In this research, we propose a novel high sensitive EO modulator based on the polymer-dispersed cholesteric liquid crystal (PDCLC) for TFT array inspector. The non-contact inspecting system for the novel light modulator based on PDLC is shown in Fig. 1. A conventional PDLC-based modulator has to require dielectric mirror. Conversely, the proposed modulator based on PDCLC is operated in the reflected mode. Therefore, the proposed modulator has the advantages of simple structure without the dielectric mirror instead of conventional modulators. 2. Experimental The polyimide (PI) alignment layer (RN1199, Nissan Chemical industries Co.) for LC alignment was spin-coated on the indium- tin-oxide (ITO) substrate. The layer was soft- baked to evaporate solvent under 100 C for 10 min and then was hard-baked under 220 C for 1 hour 30 min. After rubbing the alignment layer, two rubbed substrates were assembled in anti-parallel direction. The mixtures for PDCLC device presented in this work are consisted of CLCs with chiral dopant (R-811, Merck Co.) mixed in the nematic LC MLC6608 (Merck Co.) and UV curable optical adhesive prepolymer NOA65 (Norland Products Inc.). The CLCs and prepolymer NOA65 are mixed in a ratio 50:50. The CLC mixtures with the uniform cell gap of 15 μm were injected into the assembled cells with antiparallel rubbed homogeneous alignment layer by capillary action at a clearing temperature. And then, the UV light was exposed for 90 min which uses different UV intensities with 0.05 mW/cm 2 , 8 mW/cm 2 , 32 mW/cm 2 . The samples were measured by a polarizing optical microscope (POM) equipped with a hot stage and computer program-assisted controller. The reflection spectra were measured using Ocean Optics spectrometer equipped with the spectroscopy operating software to calculate a variety of color-space values from the reflection spectra. Figure 1. Schematic non-contact testing system for novel light modulator based on (a) cholesteric liquid crystal droplet and (b) pillar structure. 3. Results and Discussion Figure 2 shows the microscopic textures and image profiling of a convention modulator based on PDLC and proposed modulator based on PDCLC which uses different UV intensities with 0.05, 8, and 32 mW/cm 2 . The size of droplet on PDCLC according to UV intensity is about 7.34 m (32 mW/cm 2 ), 14.4 m (8 mW/cm 2 ), and 32.3 m (0.05 mW/cm 2 ), respectively. As UV intensity decreases, the size of droplet on PDCLC is increased. Especially, the pillar structure on the PDCLC with the lowest UV intensity (0.05 mW/cm 2 ) is obtained as shown in Fig. 2(d). When the 50 V voltage on PDCLC based modulator is applied, it have changed to focal conic state from planar state, and then we can admit no pixel defect in TFT array substrate. The light variation induced by the fringe effect electric field of the edge on the ITO electrode is obtained by image processor as shown in Fig. 2. As shown in Fig. 2(a), the variation value of the modulator based on PDLC is about 20, and 33 m at 20, and 50 V respectively. The variation is increased as increasing the size of droplets in the PDCLC. ISSN 0097-966X/13/4403-1341-$1.00 © 2013 SID P-94 / J.-Y. Hwang SID 2013 DIGEST • 1341