Oval Mura of Curved LCM Analysis and Improvement

Transcription

1 Oval Mura of Curved LCM Analysis and Improvement Wu-Chang Tsai, Chi-Chen Huang and Ren-Mei Tzeng AU Optronics Corporation, Hsinchu, Taiwan, ROC Abstract A method of improving oval Mura of curved Liquid Crystal Display (LCD) is studied. The easy using predicted model to predict oval Mura are proposed in this study. The predicted model is base on structure analysis software (finite element calculation method) and gathered statistics of result of curved LCD from 32 televisions to 65 televisions. Meanwhile, experimental results are well correspond to the predicted model results. Finally, we use the predicted model to modify the LCD panel design in curve display and reduced lots of research and manufacture cost. 1. Introduction Curved TVs were introduced to customer since about 2013 and becomes popular quickly. With curved screen, viewer can have a wider field of view for a curved screen than the same physical screen size flat screen. (Fig.1) Figure 2: AUO curved LCD module However, LCD is originally designed for flat display. Without changing original design and manufacturing method, we fund the picture quality of curved LCD is not as good as flat ones. It result in some kinds of Mura (unevenness image) and the Mura is very clear as the L0 pattern (level 0, the darkest display of LCD,) and this kind of Mura is now, because of its shape, called Oval Mura. (Fig.3) Figure 3: LCD Oval Mura Figure 1: The actual size and felt size compare between flat and curved display The curved LCD is not only provided the wider field of view but also enhanced users immersive experience with its curved form. Therefore, we makes lot of research and announced the next generation UHD 4K curved LCD TV displays have been designed with the world s leading 4000R golden curvature (Based on the available market research information as of August 26, 2014.) We focused on the center of oval Mura and zooming in to the pixel scale and fund out the structure of LCD were no longer as our design. The structure of LCD is normal as figure 4. It means there is red, blue, and green pixel on color filter (CF) and each pixel are well corresponding to pixels of TFT array glass. Figure 4: LCD Oval Mura The well corresponding relationship is no longer good in the curved LCD and is compared as figure 5.

2 Figure 6: The cross-section of a flat LCD. Figure 5: Well corresponding pixel and shifted pixel. Although the pixel shift result in oval Mura were known, there hasn t been a good method to predict or solve it so far. So this article aims at finding out the method to deal with this problem. It makes lots of cost and time if we using the original pixel design of flat LCD and measure the shift value and than modify the design. The good method should be quickly and correctly predict before manufacturing LCD and should also be easy to use. Therefore, in order to improve the oval Mura, the numerical analysis method is firstly considered. The simulation software of finite element method which is using numerical analysis method were used and results from simulation software were well correspond to experimental results. After that results are gather statistics as the inequalities that consist of view size of LCD, curvature of LCD, and thickness of color filter and TFT glass. Finally, the inequalities are combined with the contour to be a predicted model, and the model is used to improve the oval Mura. 2. Simulation method introduction In this section, the pixel shift of LCD is analyzed firstly, and the corresponding simulation method is introduced next. Figure 6 is the cross-section of a flat LCD. The seal combines color filter and TFT-Array substrate, and all the pixels on CF are corresponding to the pixels on TFT-Array substrate. Figure 7 shows as flat LCD bending to curved LCD, some pixels of CF and TFT are no longer well corresponding to each other. That is because the combination of CF and TFT are around the edges of them, and extension of CF and TFT are not the same at the other area after bending. Which is meaning that the well corresponding pixel only at the central and the four edges. Figure 7: Pixel shift of bending flat LCD to curved LCD Since it is a force deformation of continuous media issue, we can discuss it with Hooke s Law, show as equation (1). = E (1) Where is the stress tensor, E is the Yong s modulus and is the strain tensor. In a Cartesian coordinate system, the stress and strain tensors can be presented by 3*3 matrices (2) & (3) = (2) = (3) The analogous of Hooke's spring law for continuous media is then (4).

3 3 3 i j c i j k l k= 1 l= 1 = (4) k l Where cijkl are the properties of the material, and often depend on physical state variables such as temperature, pressure, and microstructure. It can be show as Figure 8. The results of simulation and experiments are compared and are well correspond. For example, the figure 11 shows correlations of simulation and experiments of the 55 LCD with 2500mm curvature. Figure 8: The stress and strain in Cartesian coordinate system In order to quickly find out the stain and displacement of each pixel, we use the finite element method and chose the commercial CAE (Computer Aided Engineering) software, LS-Dyna and Abaqus to simulate the curved display module. 3. Verification of simulation model In this section, the experimental result of oval Mura and the result of CAE model of finite element method to simulate oval Mura is compared. Several curved LCD had been set on different curvature jig to study the oval Mura. There was a 55 LCD curved as 2500mm radius and compared with the one curved as 6600mm radius and shown as Figure 9. Figure 11: The correlations of simulation and experiments of the 55 LCD with 2500mm curvature These results of simulation shows the curved LCD are all with the similar contour of the pixel displacement. We compare several curved LCD with experiment and also get the similar contour of the pixel displacement. With the well corresponded simulation method and several simulation results, the pixel shift of curved LCD can now be predicted before manufacture. 4. Oval Mura improvement In this section, simulation results of oval Mura collected from many sizes, many thickness, and many curvatures firstly. Next, the simple predicted model of pixel displacement of curved LCD is built. Finally, we offer some solution to improve oval Mura. 4.1 Results collection According to the experience from section 3, the contours of pixel shift are similar. Thus, we collect many simulation result of size from 32 to 65, thickness (t) of CF and TFT from 0.5mm to 0.7mm, and curvature (R) from 2500mm to 9000mm. Figure 12 shows the 3 examples such as 65, t=0.7mm,r=3000mm, 65,t=0.5mm,R=5000mm, and 42,t=0.7mm,R=6600mm. Figure 9: R2500mm Vs. R6600mm of 55 curved LCD. Here we chose the 55 LCD which was curved 2500mm radius to verify the simulation model. The simulation model is shown as figure 10. Figure 10: Simulation model of 1/4 symmetric LCD

4 Figure 14: Typical LCD cross-section of a pixel Figure 12: 3 samples of collected simulation results 4.2 Predicted model set up In order to better use these results, we normalize the data in figure 10 and gather all the simulation data statistics and result in figure 13. The D will be a cubic equation of R, t, and S. As the previous sections discussed, the pixel of CF will be no longer correspond to TFT pixels after LCD curved. Therefore, to move the pixels of CF or TFT, or to make BM wider to cover the displacement will be good to improve the issue. Pixel designer could use the predicted model which is announced in section 4.2 and modify the pixels at every position. It can be shown as figure 15 and the shift d is the maximum pixel shift weighted with the level of the contour of predicted model. Figure 13: Simple predicted model 4.3 Solutions of improving oval Mura With the predicted model, we can easy predict the displacement of pixel shift of many kinds of active area sizes, substrate thickness, and LCD curvatures. Since the pixel shift is known, we can try to reduce or remove the displacement between CF and TFT. One easy way to remove the displacement of pixels is to make CF pixel and TFT pixel on the same subtract. However it will increase the cost and decrease the performance of current manufacture processes. Therefore, we claimed some simple solutions to improve the oval Mura. For the purpose of improving oval Mura, to modify the black matrixes (BM) of LCD is one of the least cost solution. The typical LCD cross-section of a pixel is shown as figure 14. Figure 15: Oval Mura improving diagram by shifting pixel Furthermore, the oval Mura can also be improved by just modifying the BM. All the BM can be added the width which is equal to the maximum pixel shift of predicted model or each BM at different position can be added the width which is equal to the maximum pixel displacement and weighted with the level of the contour of predicted model. It can be shown as figure 16. Figure 16: Oval Mura improving diagram by adding BM width

5 Another solution is making the color filter on array shown as figure 17. The BM could be removed from top glass side. 3. To improve the oval Mura, modified pixel design which is shifted according to predicted model is one kind of solution. 4. Another solution which is easy to modify and cost less is according to predicted model to make the BM wider. 5. Using new pixel design to remove vertical BM of top glass. Figure 17: Removing BM from top glass side 5. Conclusion We can summarize the key points of this study as follows: 1. According to the experiment and simulation result, the proposed simulation method is effective and comprehensive to evaluate oval Mura. 2. The easy using predicted model is announced by lots of simulation results. References 1. Liquid Crystals: Application and Uses; Birendra Bahadur, Singapore, Cheng-Liang Ye, et al. The Application of BOA on Curved Panel SID 2016 DIGEST, pp Yongmin Park, et al. Quantification model of proper curvature for large-sized curved TVs JSID 2015, pp K. Hemanth Vepakomma, et al. Stress Induced Substrate Mura in Curved LCD SID 2015 DIGEST, pp