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Abstract
The phenomenon of a liquid crystal (LC) display panel includes deterioration with time, the quality management being monitored, gap-induced faults in harsh conditions such as high temperature, depressurization with low temperature, and hitting on the panels to remove the possibility of any problems at anywhere in the world. Therefore, obtaining the LC amount margin is very important at up and down enough margins by any changes in the environment. In particular, we derive the main factors affecting for well-management of the LC amount margins due to the increasing complexity of the panel architecture and the development of materials when there are frequently a lot of changes. A method was found to approximate the three-dimensional structure in the panel to reflect the measured values as representative values; based on such values, a simulation system that automatically generates the structure based on the design file was built. To evaluate the consistency, we compared the actual LC amount for 14 products and the simulation calculated value and secured an accuracy of about 98.9% the result of the comparison between the real and simulation, including the margin section, secured a linear correlation of 92.6%, and its consistency was verified. It has been verified that the amount of LC drop can be automatically adjusted.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Although liquid crystal (LC) materials were discovered more than a century ago, their useful electro-optic effects and stability were developed only in the late 1960s and 1970s. In the early stage, passive matrix LC Displays (LCDs) were found useful in electronic calculators and wrist watches [1]. With the advance of thin-film-transistors (TFTs) [2], color filter (CF) [3], and low-voltage LC effects [4], active-matrix LCDs have gradually penetrated into the market of notebook computers, desktop monitors and televisions (TVs). Today, LCDs had found widespread uses in everyday life, including mobile applications such as mobile phones, personal digital assistants, navigation systems, notebook personal computers, office applications such as desktop computers and video projectors and home applications such as large-screen TVs [5]. In the display market, new display modes, which are organic light-emitting diode [6–10] and quantum dot displays [11–13], to replace LCD has been proposed, but recently, LCD has been continuously developed and commercialized for augmented reality and virtual reality, which requires the use of a display with high resolution and long lifetime [14,15].
As mentioned above, the developed LCD product, which is applied to many aspects, have been used anywhere in the world. The most important performance of a display device is optical characteristics, and the thickness of the LC layer having optical anisotropy is the most important factor for LCD’s optical characteristics. This thickness is called the cell gap. Uniform cell gap management with appropriate margins is essential in order to make products with uniform functions anywhere regardless of the circumstance conditions such as pressure and temperature. The influence of the pressure and the temperature is depended by alpine and hot or cold weather, respectively. In this case, the phenomenon of the LCD panel is the change with time depending on the differences of pressure and temperature due to nature of the LC molecules and its effect is closely affected. For instance, the most recent end user’s claim about the faulty of LC unfilled panel area had occurred from Bolivia as a world class alpine area. The poor gap problem was caused by the time-dependent change of LCs in a harsh environment, and this cell gap related quality problem often occurs as a customer issue. In general, this optical property problem is expressed as a mura defect due to the non-uniform thickness of the LCs. [16,17] In order to predict the deterioration with time before shipment of products, the quality management is to monitor the phenomenon faults in harsh conditions such as high temperature, depressurization with low temperature and hitting on the panels for removing the possibility of any problems at anywhere in the world, Therefore, obtaining the margin of LCs is very important at up and down enough margins in the center position by any changes in the environment. In particular, the margin management of the thickness of the LC layer is required due to manufacturing process variation because of the complex product architecture (PA) of many functional layers in recent LCD products.
In this paper, to identify the main influence factors of the thickness fluctuation of the LC layer inside the panel of vertical aligned (VA) LCD, [18–23] which is the main method of current LCD business and especially those having a designed CF on TFT array (COA) structure, three-dimension (3D) structure generation and simulation evaluation of the computational simulation program was conducted considering the physical properties of various processes and materials. And the LC amount margin result in the actual panel based on the actual mass production measurement was compared with the simulation result. Ultimately, based on this, a method for automatically correcting the optimal LC amount in the one drop filling process for injecting LC material was proposed by constructing a prediction system.
2. Evaluation of process margin
2.1 Margin of the LC amount
LCDs are a display device and transmittance are the most basic of its display quality and the voltage-transmittance curve is as follows.
Here, φ and Γ is azimuthal angle and optical retardation of LC layer and optical retardation factor ${\Gamma\ =\ 2\pi}\varDelta {nd/\lambda }$ and Δn, d, and λ note that difference of phase retardation, cell gap, and wavelength of the LCs, respectively.
The display quality performance of LCD is most affected by the thickness of the LC layer, which determines the degree of light transmission between crossed polarizers. After prepare top and bottom substrates, which have a TFT array and a CF, overcoat (OC), black matrix (BM) and columnar spacer (CS) [24], both top and bottom substrates are assembled having process of a one drop filling (ODF) of LCs and drawing of seal line. At this point, the volume of the inside of the panel assembled of both TFT and CF substrates determines the amount of injected LCs. In this case, the current passed qualification standards is less than 2% of the upper and lower margin about the center value of proper amount of LCs. However, if less than or exceed the volume margin of LCs, the faulty is shown below as shown in Fig. 1.
Fig. 1. Schematic diagrams and images of an excessive and an insufficient amount of LCs. (a) Excessive amount of LCs and (b) insufficient amount of LCs called active unfilled area (AUA). Images of (C) expanded (EXPD) and (d) AUA in the actual panel, respectively.
Figure 1 (a) and (c) show a schematic diagram of an excessive amount of LCs called expanded (EXPD) of the LC panel and its image of the EXPD, respectively. The image of the EXPD appears yellowish spot and/or more bright in the edge of the panel. When the cell gap increases according to the excessive amount of LC layer in the panel. As a result, the blue color rate in the sum of the color amounts decreases, the output of voltage-transmittance of the panel shift occurs and thus the panel shows a relatively yellowish. On the other hand, its schematic diagram and image of an insufficient amount of LCs show that of an active unfilled area (AUA) as shown in Fig. 1 (b) and (d), respectively. The image of the AUA is appeared black spot or region without injected LCs. [25]
Figure 2 shows cross-sectional of PA in the VA LCDs having a COA structure. For the performance of display quality, the most important is the optical characteristic based on the voltage-transmittance curve. Its optical properties have optical anisotropy between crossed polarizers and are determined by the behavior of LC directors that determine light transmission and blocking. As can be seen from the previously mentioned transmittance formula, the thickness of the LC layer has the greatest influence on determining the change in transmittance other than the fixed parameters. LCs with fluidity must be properly injected into the volume of the space between the upper and lower glass substrates as shown in Fig. 2, so that faults such as EXPD and AUA do not occur as well as achieving the goal of optical properties. In particular, the thickness of CF of COA is more than 0.2 $\mu \rm{m}$ thicker than that of LC layer in VA LCD, which is optimally designed with a cell gap of 3 $\mu \rm{m}$. The contact (CNT) hole for connecting the TFT array on the bottom of the COA and the pixel electrode on the top of the COA and the hole for removing the fume generated after processing of organic materials such as photo resist, CF, OC, and etc. There are many factors that cause the LCs to deviate and the curvature of the surface in contact with the LC layer due to the complex PA also acts as a cause to change the volume of the entire LC layer. In order to understand the influence of these factors and to find a process control plan, the evaluation was carried out by simulating the internal volume structure of the actual panel. In the simulated structure, the respective number indicate a main CS height (①), BM (②), LC molecules (③), pattern height (PH, ④), OC (⑤), CNT hole (⑥), pattern width (PW, ⑦), CF overlap height (⑧), CS x and y axis overlay (⑨), differences of height between main and sub CS (⑩) and CS aspect ratio between diameter and height ($\textcircled{\raisebox{0.9pt}{\tiny\hbox{$11$}}}$), respectively. Each parameter is a factor of an individual measurement step that is measured to check whether the upper and lower glass substrates are properly manufactured compared to the design before the LCs are injected in the assembled two substrates.
Fig. 2. Cross-sectional of panel architectures in the liquid crystal display having a designed with color filter on TFT array.
2.2 Concept of the computational simulation
Figure 3 shows the process diagram of a computational simulation using a layout-based LC filling design simulator (TVolumeX, Sanayi System, Korea) [26]. In this simulation, the full graphical design system (GDS) files of the actual panel, the panel information data to model the shape of the pixel is used. The GDS has an active area that is repeated in the same structure and same size for display, and also has a gate integrated circuits (ICs) and a data ICs connection for panel drive to be used as a final product for display panel on the outside of active area, and seal line is used to bond the upper and lower plates together. Therefore, we classified the panel area into several areas, which have periodic characteristics in spatial dimension. So, the whole panel including active area, area of the seal line, a gate and data IC is proceeded to calculate the total volume. After add the information of each layer within a pixel in the panel, it is implemented as a 3D structure. And then mechanical analysis by considering main and sub CSs prepared to support the top and bottom substrates, injected LCs inside of the panel and the material properties of the layer against CS is calculated. Based on the results of this progress within the panel and the output section of the simulation indicate the proper amount of the LCs, cell gap and the margins of LCs. In addition, the function of the simulator is the calculation of the displacement of the specific positions and has the ability of view the 3D shape and a cross-sectional view of the panel in a certain part.
Fig. 3. The process diagram of a TVolumeX computational simulation. The image represents the step process of (a) input step, (b) processing step, and (c) output step, respectively.
The 3D structure was confirmed by atomic force microscope and focus ion-beam scanning electron microscope in order to shape the structure of the multi-layer manufactured by the upper and lower plate processes as closely as possible to the real one. In actual mass production, the measurement methods of the critical dimension meter, which measure the length, the ellipsometer and alpha-step, which measure the thickness and thickness step, and the white light scanning interferometer, which measure the 3D shape, were respectively regularized. Figure 4 shows how to model the internal structure of the panel. In particular, the essential leveling index (LI) for modeling the upper layer when manufacturing multi-layers in a continuous process was calculated for each material. The formula for calculating LI (= T'/T) was applied and the taper angle, TA = arctan (T/TW), was additionally measured. Based on these parameter, six types of curved structures were shaped to realize the internal structure of 3D panel structure.
Fig. 4. Shaping of internal structure of panel (a) multi-layer structure formed by continuous process (b) various curved structure.
Together with the index, which is an inherent constant for each material, which has been confirmed in advance and applied as a basic parameter, the internal structure of the three-dimensionally shaped panel based on the measured values of each step of the sequential multi-process is shown in Fig. 5. Figure 5 (a) is the structure of the active area of the pixel realizing the actual color and Fig. 5 (b) is the structure of the area where the TFT circuit part of the lower plate and CS of the upper plate contact each other at the boundary of the sub-pixel divided into high and low in the polymer stabilized VA LCDs [22,23].
Fig. 5. (a) The structure of the active area of the pixel realizing the actual color display and its cross-sectional planar structure (b) the structure of the area where the TFT circuit and COA array part of the lower plate and CS of the upper plate contact each other at the boundary of the sub-pixel divided into high and low in the PSVA LCDs and its cross-sectional planar structure, respectively.
2.3 Evaluation of the LC margin
In the already confirmed PA of VA LCD, the individual LC amount distribution according to the process deviation for each item is checked, and by reflecting this, the expected LC amount fluctuation that can occur during the entire PA process shows a ± 7.91% range. This is about 4 times higher than the condition within ±2%, which is the level at which products can be shipped to organically respond to environmental factors.
A computational simulation of the appropriate LC amount margin for a VA LCD panel with a COA structure was performed using the TVolumeX simulator. In order to reflect the environmental impact of various customers mentioned in the introduction, the simulation was evaluated by applying 103.5, 100, and 96.5 g/cm3 as the density of LC molecules according to the three temperatures of low temperature, room temperature, and high temperature, respectively. And young’s modulus of respective inner layer such as BM, OC, indium-tin-oxide, CS, capping on CNT hole, CFs, passivation and glass are 240000, 6320, 10020, 99800, 1720, 998000, 5180, 126320 and 24000 Kg f/cm2 and other layers are only 1 Kg f/cm2, respectively.
The system was built based on the creation of a 3D structure, which is the already confirmed PA of VA LCD as shown in Fig. 2, that improved consistency by matching the results of the actual panel. And the process distribution of each factor measured in the actual mass production line was identified for one month. Then, an expected change value of the LC amount reflecting the deviation was calculated. The results shown in Table 1 above are the results, and based on the results, the expected LC amount change according to the process distribution of each factor for the representative LCD products was calculated.
Table 1. Process dispersion in actual mass production by major factors of the internal structure presented in Fig. 2, and the expected change in the amount of LC.
3. Results and discussion
After shaping the 3D structure inside the panel, other factors were fixed, and the results of simulation of the LC amount margin in the range from the maximum to the minimum of the process margin for one factor are as follows for each item. And the actual panel LC margin evaluation results and simulation results for various inch and resolution products were compared.
3.1 Impact simulation results for each item
Figure 6 shows the simulation evaluation results of the effect of the contact hole radius and the step height of BM and OC, respectively. For well-management of the LC margins with margin faulty free, the split evaluation of the skew of the contact hole radius are from -4 µm to 4 µm in the standard of the medium of the radius of contact hole skew. As shown in Fig. 6 (a), the simulation results of the proper amount of LCs is continuously reduced of the negative linearity. Base on the reference 22 µm as the median of contact hole skew of mass production, the different of the boundary of margin is ±2 µm at the up and down areas. For next item, the split evaluation of the OC leveling and BM thickness are from -0.2 µm to 0.2 µm in the standard of the flatness at all regions, respectively. As shown in Fig. 6 (b), the simulation results of the proper amount of LCs is continuously increased of the positive linearity. Based on the constant 0 of mass production, the boundary difference between the upper and lower margins of OC leveling is ±0.08 µm for smooth management of the LC margin. In the case of BM, there was no concern that the LC amount margin would fail in the process margin section that may occur.
Fig. 6. The simulation results of amount of LC margins about (a) contact hole skew radius from -4 µm to 4 µm (b) OC leveling on the BM layer from -0.2 µm to 0.2 µm (c) BM thickness from -0.2 µm to 0.2 µm, respectively. The circles symbols note the proper amount of LCs and blue and red solid lines note maximum and minimum LC margins, respectively.
Figure 7 shows the result of evaluation of the LC amount margin effect of the overlapped width and height of the CF layer with an overlap structure for current capping of display driving operation, and the portion that rises compared to the adjacent part in the COA structure where CF is formed on the upper part of the TFT array.
Fig. 7. The simulation results of amount of LC margins about (a) CS against (PH) step height from -0.06 µm to 0.06 µm (b) color filter overlap (PW) width from -15 µm to 15 µm (c) PW height from -0.5 µm to 0.5 µm, respectively. The circles symbols note the proper amount of LCs and blue and red solid lines note maximum and minimum LC margins, respectively.
In order to well-management of the LC margins without margin failure, the split evaluation of the PH, which is called thickness of point of CS against on the bottom substrate and defined the difference of thickness between top point coated RGB on the TFT channel and flatness of the active area, are from -0.06 µm to 0.06 µm in the standard of condition of flatness all regions. As shown in Fig. 7 (a), the simulation results of the proper amount of LCs is continuously reduced of the negative linearity. The different of the boundary of margin is ± 0.03 µm at the up and down areas. For next items, the split evaluation is divided two categories such as PW width and PW height. At first, the split evaluation of the PW width, which is called RGB overlap width and defined the point between slope of two colors overlap and flatness of the active area, are from -15 µm to 15 µm in the standard of zero point of the PW width as like RGB overlap free. As shown in Fig. 7 (b), the simulation results of the proper amount of LCs is continuously reduced of the negative linearity. Base on the reference zero point of the PW width, the different of the boundary of margin is from -12 µm to 10 µm at the up and down areas. At the second split evaluation of the PW height, which is called RGB overlap height and defined the length between top of two colors overlap and flatness of the active area, are from -0.5 µm to 0.5 µm in the standard of zero point of the PW height as like RGB overlap free. As shown in Fig. 7 (c), the simulation results of the proper amount of LCs is continuously reduced of the negative linearity. Base on the reference zero point of the PW height, the different of the boundary of margin is from -0.3 µm to 0.25 µm at the up and down areas.
Figure 8 shows the simulation results of amount of LC margins about CS overlay through x-axis or y-axis from -6 µm to 6 µm. Blue and red solid lines and circle symbol note that maximum and minimum of LC amount margin and the proper amount of LCs, respectively. According to the impact of CS overlay of x-axis and y-axis, the volume changes do not affect the proper amount of LCs. Due to the direction of pixel in a PA of evaluated product is arranged in a transverse in the panel, x-axis overlay isn’t effect on the volume change and y-axis overlay have an effect on the volume change affecting the margin of the minimum and maximum amount of LCs as shown in Fig. 8 (b). Otherwise, x-axis overlay is effective factor about the volume change in the other size devices has the longitudinal direction of the pixel but y-axis overlay has no effect on inner volume in the panel.
Fig. 8. The simulation results of amount of LC margins about (a) X-axis overlay from -6 µm to 6 µm (b) Y-axis overlay from -6 µm to 6 µm, respectively. The circles symbols note the proper amount of LCs and blue and red solid lines note maximum and minimum LC margins, respectively.
3.2 Evaluation of the actual panel's LC amount
The LC amount margin for TV products of various inches and resolutions was evaluated by comparing the actual and simulation. TV's inches ranged from 65 inches to 32 inches, which are typical products, and resolutions were applied to ultra-high definition (UHD), full high definition (FHD), and high definition (HD) products. This was to disprove whether the parameter gathering for the proposed structure for various structures inside the panel is sufficient. It was predicted through split of three density values for temperature. In the case of the actual periodic layout inspection, which is the last evaluation method before shipment to the customer, the amount of LC drop at the ODF process was split in 20 panels and compared with the results of the actual panel evaluated in a chamber that simulated extreme environments of high and low temperatures. As a result, as a result of the evaluation of a total of 14 LCD products, the consistency was verified by securing an average of 98.9% and a linear correlation of 92.6% between the amount of liquid crystal at the center of the actual panel and the amount of LC at the center of the simulation result as shown in Fig. 9. Based on this, in the actual mass production process, an automatically ODF control system was established that automatically adjusts the amount of ±1% LC, which corresponds to 50% of the ±2% used as spec, according to the expected internal volume calculated in advance before LC injection process.
Fig. 9. Comparison of evaluation results of LC amount margin between actual TV panel and computational simulation for various inches and resolutions.
4. Conclusion
We have developed a system that calculates the optimal amount of LC with an appropriate margin. LCD products made with LCs that are highly dependent on temperature and pressure and have fluidity must maintain normal display’s quality while maintaining uniform optical characteristics in various environments such as high temperature, low temperature, and reduced pressure. So, based on the 3D LC quantity simulator called TVolumeX, based on the design file of the LCD panel, the necessary measurement steps, items, and digitization methods were found to simulate the structure of the actual panel as closely as possible to the real thing. In order to secure the consistency of the automatically formed 3D structure based on the measured values, the improvement of the consistency and the impact evaluation were conducted for each individual factor. Based on the finally secured technology, the results derived through evaluation and simulation of the central LC amount and margin section of the actual panel for 14 various product groups were compared. The accuracy of the central LC amount was excellent with an average of 98.9%, and the result of comparison between the real and simulation, including the margin section, secured a linear correlation of 92.6%, and its consistency was verified. Accordingly, the appropriate LC amount was calculated in advance based on the simulation result in which a 3D structure was formed by reflecting the numerical values measured in the mass production line, and based on this, an automatic LC drop control system within ±1% was built in the LCD mass production process.
Disclosures
The authors declare no conflicts of interest. Disclosures.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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