LCD (Liquid Crystal Display) Liquid Crystal Display. The structure of LCD is to place a Liquid Crystal cell between two parallel Glass substrates. The lower substrate Glass is equipped with TFT (thin film Transistor), and the upper substrate Glass is equipped with a color filter. The signal and voltage on the TFT are changed to control the liquid crystal molecules. Rotate the direction, so as to achieve the display purpose by controlling whether the polarized light of each pixel point is emitted or not. LCD has replaced CRT as the mainstream, and the price has dropped a lot, and it has been fully popularized.
A few days ago, according to data from CINNO Research, 2020 is expected to be the first year of mass production and launch of fingerprint recognition mobile phones under the LCD screen.
LCD Type
According to the different Backlight sources, LCDs can be divided into two types: CCFL displays and LED displays.
Misunderstanding:
Many users believe that liquid crystal displays can be divided into LEDs and LCDs. To some extent, this understanding is misguided by advertisements.
The LED display on the market is not a true LED display. To be precise, it is an LED-backlit liquid crystal display. The liquid crystal panel is still a traditional LCD display. In a sense, this is somewhat fraudulent. nature! South Korea’s Samsung was once convicted by the British Advertising Association as violating the country’s advertising laws because its “LED TV” LCD TVs were suspected of misleading consumers. For liquid crystal displays, the most important key is the type of liquid crystal panel and Backlight, while the liquid crystal panels of displays on the market generally use TFT panels, which are the same. The difference between LED and LCD is only their Backlight type: LED The backlight and CCFL backlight (that is, fluorescent lamps) are Diodes and cold cathode lamps, respectively.
LCD is the acronym for Liquid Crystal Display, which means “liquid crystal display”, that is, liquid crystal display. The LED display refers to a type of liquid crystal display (LCD), that is, a liquid crystal display (LCD) with LED (light emitting Diode) as the backlight source. It can be seen that LCD includes LEDs. Corresponding to the LED display is actually a CCFL display.
(1) CCFL
Refers to a liquid crystal display (LCD) with CCFL (Cold Cathode Fluorescent Lamp) as the backlight source.
The advantage of CCFL display is good color performance, but the disadvantage is higher power consumption.
(2) LED
Refers to a liquid crystal display (LCD) that uses LEDs (light emitting Diodes) as a backlight source, and generally refers to WLEDs (white light LEDs).
The advantages of LED displays are small size and low power consumption. Therefore, using LEDs as a backlight source can achieve high brightness while taking into account lightness and thinness. The main disadvantage is that the color performance is worse than that of CCFL monitors, so most professional graphics LCDs still use traditional CCFL as the backlight source.
Technical parameters
(1) Low cost
Generally speaking, reducing costs has become an important rule for companies to survive. Throughout the development history of TFT-LCD, it is not difficult to find that increasing the size of glass substrates, reducing the number of masks, increasing base station productivity and product yield, and purchasing raw materials nearby are the continuous efforts of many TFT-LCD manufacturers. .
Glass substrate is an important raw material for the production of TFT-LCD, and its cost accounts for about 15% to 18% of the total cost of TFT-LCD. It has developed from the first generation line (300mm × 400mm) to the current tenth generation line (2,850mm ×3,050). mm), it has only gone through a short period of twenty years. However, due to the extremely high requirements for the chemical composition, performance and production process conditions of TFT-LCD glass substrates, the global TFT-LCD glass substrate production technology and market have long been used by Corning in the United States, Asahi Glass and Electric Glass, etc. Monopolized by a few companies. Under the strong promotion of market development, the mainland of my country also began to actively participate in the R&D and production of TFT-LCD glass substrates in 2007. A number of TFT-LCD glass substrate production lines of the fifth generation and above have been built in China and planned In the second half of 2011, two 8.5-generation high-generation liquid crystal glass substrate production line projects were launched. This provides an important guarantee for the localization of upstream raw materials for TFT-LCD manufacturers in mainland my country and a significant reduction in manufacturing costs.
The most core part of TFT production technology is the photolithography process, which is not only an important part of determining product quality, but also a key part that affects product cost. In the photolithography process, the most attention is paid to the mask. Its quality determines the quality of TFT-LCD to a large extent, and the reduction of its use can effectively reduce equipment investment and shorten the production cycle. With the change of TFT structure and the improvement of production process, the number of masks used in the manufacturing process is correspondingly reduced. It can be seen that the TFT production process has evolved from the early 8-mask or 7-mask lithography process to the commonly used 5-mask or 4-mask lithography process, which greatly reduces the TFT-LCD production cycle and production costs.
4 Mask lithography process has become the mainstream in the industry. In order to continuously reduce production costs, people have been trying to explore how to further reduce the number of masks used in the photolithography process. In recent years, some Korean companies have made breakthroughs in the development of the 3-mask lithography process, and have announced mass production. However, due to the difficulty of the 3-mask process and the low yield rate, further development is still being made. And improving. From a long-term development perspective, if Inkjet (inkjet) printing technology makes a breakthrough, the realization of maskless manufacturing is the ultimate goal that people pursue.
(2) High resolution
In order to realize a large-area high-resolution liquid crystal display, it is usually necessary to use low-impedance metal materials, high-performance switching elements, and high-precision processing techniques. Aluminum is the most researched and used material for making TFT buses with low-impedance metals. By solving the problems of easy formation of hillocks, chemical corrosion and oxidation of aluminum, alloy methods (such as Al-Cu, Al-Si, Al-Nd and Al-Ti, etc.) and interlayer methods (such as Mo/Al/Mo) have been reported successively. , Cr/Al/Cr and Ti/Al/Ti, etc.), the alloy method is relatively simple in process, but the material has a higher resistivity. In May 1998, IBM developed a 16.3-inch ultra-high resolution (200ppi) a-Si TFT display using Al-Nd alloy as the gate electrode, and mass production has been achieved. In April 1999, Toshiba introduced the 20.8-inch 16-SVGA (3, 200 × 2, 400) a-Si TFT-LCD, which can be said to represent the highest level of a-Si TFT-LCD in terms of high resolution and high capacity. .
According to Display Search in the third quarter of 2011, the global flat panel display research report “QuarterlyWorldw ide Flat Pane l Dis play Fore cas t Re port” pointed out that the average pixels per inch (ppi) in large-size LCD panels (>9.1 inches) ) Will grow from 88ppi in 2010 to 98ppi in 2015. The ppi of small and medium-sized LCD panels (<9.0 inches) will grow from 180ppi to 300ppi in the same period. With the rise of smart phones, mobile phones will be the most obvious application product for ppi’s growth.
Another important way to realize high-resolution liquid crystal display is to develop LT p-Si TFT technology. The resolution of published p-SiTFT-LCD products is generally around 200ppi. Compared with a-Si TFT-LCD, LT p-Si TFT-LCD has a smaller volume of thin film transistors and storage capacitors. Therefore, it has a larger penetration area per inch, resulting in a brighter display. , And save more power. When the market demands higher ppi, low-temperature polysilicon (LTPS) technology becomes the best choice for manufacturing high-resolution thin-film Transistor liquid crystal displays.
(3) Contrast
The control ICs, filters and oriented films used in LCD manufacturing are related to the Contrast of the panel. For general users, a Contrast ratio of 350:1 is sufficient, but such Contrast in the professional field does not satisfy users. Demand. Compared with CRT monitors easily reaching 500:1 or even higher contrast, only high-end LCD monitors can achieve this level. The first-tier LCD monitors on the market, such as Samsung, Asus, LG, etc., can reach a contrast ratio of 1000:1. However, because the contrast is difficult to accurately measure with instruments, you have to go and see it yourself when you pick it.
Tip: Contrast is very important. It can be said that the selection of LCD is a more important indicator than bright spots. When you understand that your customers buy LCDs for entertainment and watching DVDs, you can emphasize that contrast is more important than no dead pixels. We When watching streaming media, the brightness of the source is generally not large, but to see the contrast of light and dark in the scene of the character, the texture change from gray to black hair must be shown by the level of contrast. The 256-level grayscale in the test software In the test, more small gray grids can be seen clearly when looking up, which means that the contrast is better!
(4) Brightness
LCD is a substance between solid and liquid. It can’t emit light by itself, so it needs additional light source. Therefore, the number of lamps is related to the brightness of the liquid crystal display. The earliest liquid crystal displays had only two upper and lower tubes, the lowest of the popular type was four lamps, and the high-end one was six lamps. The four-lamp design is divided into three types of placement: one is that there is a lamp on each of the four sides, but the disadvantage is that there will be dark shadows in the middle. The solution is to arrange the four lamps from top to bottom. The last one is the “U”-shaped placement form, which is actually two lamp tubes produced by two lamps in disguise. The six-lamp design actually uses three lamps. The manufacturer bends all three lamps into a “U” shape, and then places them in parallel to achieve the effect of six lamps.
(5) Signal
The response time refers to the response speed of the liquid crystal display to the input signal, that is, the response time of the liquid crystal from dark to bright or from bright to dark (the time for the brightness to change from 10%–>90% or 90%–>10%) , Usually in milliseconds (ms). To make this clear, we have to start with the human eye’s perception of dynamic images. There is a phenomenon of “visual residue” in the human eye, and a high-speed motion picture will form a short-term impression in the human brain. Animations, movies, etc. to the latest games are the application of the principle of visual residue, allowing a series of gradual images to be displayed in rapid succession in front of people’s eyes, forming a dynamic image. The acceptable display speed of the picture is generally 24 frames per second, which is the origin of the movie playback speed of 24 frames per second. If the display speed is lower than this standard, people will obviously feel the picture pause and discomfort. Calculated according to this index, the display time of each picture needs to be less than 40ms. In this way, for the liquid crystal display, the response time of 40ms becomes a hurdle, and the display above 40ms will have obvious picture flicker, which makes people feel dizzy. If you want the image screen to reach the level of non-flicker, it is best to achieve a speed of 60 frames per second.
a very simple formula to calculate the number of frames per second under the corresponding response time as follows:
Response time 30ms=1/0.030=approximately 33 frames per second
Response time 25ms=1/0.025=approximately 40 frames per second
Response time 16ms=1/0.016=approximately 63 frames per second
Response time 12ms=1/0.012=approximately 83 frames per second
Response time 8ms=1/0.008=approximately 125 frames per second
Response time 4ms=1/0.004=approximately 250 frames per second
Response time 3ms=1/0.003=approximately 333 frames per second
Response time 2ms=1/0.002=approximately 500 frames per second
Response time 1ms=1/0.001=approximately 1000 frames per second
(6) Viewing angle
The viewing angle of the LCD is a headache. When the backlight passes through the polarizer, liquid crystal and alignment layer, the output light becomes directional. In other words, most of the light is emitted vertically from the screen, so when viewing the LCD from a larger angle, the original color cannot be seen, and even the entire white or all black can only be seen. In order to solve this problem, manufacturers have also started to develop wide-angle technology. There are three popular technologies: TN+FILM, IPS (IN-PLANE-SWITCHING) and MVA (MULTI-DOMAIN VERTICAL alignMENT).
The TN+FILM technology is to add a layer of wide viewing angle compensation film on the original basis. This layer of compensation film can increase the viewing angle to about 150 degrees, which is a simple and easy method and is widely used in liquid crystal displays. However, this technology cannot improve performance such as contrast and response time. Perhaps for manufacturers, TN+FILM is not the best solution, but it is indeed the cheapest solution, so most Taiwanese manufacturers use this Method to build a 15-inch LCD display.
IPS (IN-PLANE-SWITCHING) technology claims to allow up, down, left, and right viewing angles to reach a greater 170 degrees. Although the IPS technology increases the viewing angle, the use of two electrodes to drive the liquid crystal molecules requires more power consumption, which will increase the power consumption of the liquid crystal display. In addition, the fatal thing is that the response time of driving the liquid crystal molecules in this way will be relatively slow.
The principle of MVA (MULTI-DOMAIN VERTICAL alignMENT) technology is to add protrusions to form multiple visible areas. The liquid crystal molecules are not completely arranged vertically when they are static. After voltage is applied, the liquid crystal molecules are arranged horizontally so that light can pass through the layers. MVA technology increases the viewing angle to more than 160 degrees and provides a shorter response time than IPS and TN+FILM. This technology was developed by Fujitsu, and Taiwan’s Chi Mei (Chi Mei is a subsidiary of Chi Mei in the mainland) and Taiwan AUO are authorized to use this technology. ViewSonic’s VX2025WM is the representative of this type of panel. The horizontal and vertical viewing angles are both 175 degrees, basically no blind spots, and promises no bright spots; viewing angles are divided into parallel and vertical viewing angles, and the horizontal angle is based on LCD The vertical axis is the center, moving to the left and right, you can clearly see the angle range of the image. The vertical angle is centered on the parallel central axis of the display screen, moving up and down, the angular range of the image can be clearly seen. The viewing angle is in “degrees”. The more commonly used labeling format is to directly mark the total horizontal and vertical ranges, such as 150/120 degrees, and the lowest viewing angle is 120/100 degrees (horizontal/vertical), which is lower than This value is unacceptable, and it is best to reach 150/120 degrees or more.
Working Principle
We have known for a long time that matter has three types: solid, liquid, and gas. Although the arrangement of the centroids of liquid molecules does not have any regularity, if these molecules are elongated (or flat), their molecular orientation may be regular. So we can subdivide the liquid into many forms. Liquids with no regular molecular direction are called liquids directly, while liquids with molecular directional properties are called “liquid crystals” or “liquid crystals” for short. Liquid crystal products are not unfamiliar to us. The mobile phones and calculators we commonly see are all liquid crystal products. Liquid crystal was discovered by the Austrian botanist Reinitzer in 1888. It is an organic compound with regular molecular arrangement between solid and liquid. Generally, the most commonly used liquid crystal type is nematic liquid crystal. The molecular shape is a slender rod with a length and width of about 1nm～10nm. Under the action of different electric currents and electric fields, the liquid crystal molecules will be regularly rotated by 90 degrees to produce light transmittance. The difference, so that the difference between light and dark occurs when the power is ON/OFF, and each pixel is controlled according to this principle to form the desired image.
The principle of liquid crystal display is that liquid crystals will exhibit different light characteristics under the action of different voltages. Liquid crystals are physically divided into two categories, one is passive (also called passive), this type of liquid crystal itself does not emit light and requires external Provide a light source, according to the position of the light source, it can be divided into two types: reflective and transmissive. The cost of Passive liquid crystal display is lower, but the brightness and contrast are not large, and the effective viewing angle is small, and the color saturation of the color passive liquid crystal display is relatively high. Small, so the color is not bright enough. The other type is powered, mainly TFT (Thin Film Transistor). Each liquid crystal is actually a Transistor that can emit light, so strictly speaking, it is not a liquid crystal. A liquid crystal display is composed of many liquid crystals. Arranged in an array, in a monochrome liquid crystal display, a liquid crystal is a pixel, and in a color liquid crystal display, each pixel is composed of three liquid crystals, red, green and blue. At the same time, each liquid crystal can be considered There is an 8-bit register on the back. The value of the register determines the respective brightness of the three liquid crystal cells. However, the value of the register does not directly drive the brightness of the three liquid crystal cells, but is accessed through a “palette”. It is unrealistic to equip each pixel with a physical register. In fact, only one row of registers is equipped. These registers are connected to each row of pixels in turn and load the content of the row. Drive all pixel rows once and display one The complete picture (Frame).
Liquid crystal looks like a liquid in terms of shape and appearance, but its crystal molecular structure shows a solid state. Like a metal in a magnetic field, when it is affected by an external electric field, its molecules will produce a precise and orderly arrangement; if the arrangement of the molecules is properly controlled, the liquid crystal molecules will allow light to penetrate; the path of light penetrating the liquid crystal can be It is determined by the arrangement of the molecules that make up it, which is again a characteristic of solids. Liquid crystal is an organic compound composed of long rod-shaped molecules. In the natural state, the long axes of these rod-shaped molecules are roughly parallel. The first feature of the liquid crystal display (Liquid Crystal Display, hereinafter referred to as LCD) is that the liquid crystal must be filled between two planes with fine grooves to work normally. The grooves on these two planes are perpendicular to each other (intersect at 90 degrees). That is to say, if the molecules on one plane are arranged in the north-south direction, the molecules on the other plane are arranged in the east-west direction, and the molecules located between the two planes are Forced into a state of 90-degree twist. Since the light travels along the direction of the arrangement of the molecules, the light is also twisted 90 degrees when passing through the liquid crystal. But when a voltage is applied to the liquid crystal, the molecules will be re-arranged vertically, so that the light can be directed out without any twisting. The second characteristic of LCD is that it relies on polarization filters and the light itself. Natural light diverges randomly in all directions. Polarization filters are actually a series of thinner and thinner parallel lines. These lines form a net to block all the light that is not parallel to these lines. The line of the polarization filter is exactly perpendicular to the first one, so it can completely block the light that has been polarized. Only when the lines of the two filters are completely parallel, or the light itself has been twisted to match the second polarized filter, the light can penetrate. LCD is composed of two polarized filters that are perpendicular to each other, so under normal circumstances, all light that attempts to penetrate should be blocked. However, because the two filters are filled with twisted liquid crystals, after the light passes through the first filter, it will be twisted 90 degrees by the liquid crystal molecules, and finally pass through the second filter. On the other hand, if a voltage is applied to the liquid crystal, the molecules will rearrange and be completely parallel, so that the light will no longer be twisted, so it will be blocked by the second filter. Taking Synaptics TDDI technology as an example, the touch controller and display driver are integrated into a single chip, which reduces the number of components and simplifies the design. ClearPad 4291 supports a hybrid multi-point in-cell design, which uses existing layers in a liquid crystal display (LCD), eliminating the need for a discrete touch sensor. ClearPad 4191 has taken another step forward, using the electrodes already in the LCD, thus achieving a more concise system architecture. Both solutions make the touch screen thinner and the display brighter, helping to improve the overall aesthetics of smartphone and tablet designs. For the reflective TN (twisted nematic) liquid crystal display, its structure is composed of the following layers: polarization filter, glass, mutually insulated and transparent two sets of electrodes, liquid crystal, electrode, glass, polarization Filters, reflectors.
(1) Passive matrix
The display principles of TN-LCD, STN-LCD and DSTN-LCD are basically the same. The difference is that the twist angle of the liquid crystal molecules is somewhat different. Let’s take a typical TN-LCD as an example to introduce its structure and working principle.
In a TN-LCD liquid crystal display panel with a thickness of less than 1 cm, it is usually a plywood made of two large glass substrates with a color filter, an alignment film, etc., and two polarizing plates on the outside. They can determine the maximum luminous flux and color production. The color filter is a filter composed of three colors of red, green, and blue, which are regularly fabricated on a large glass substrate. Each pixel is composed of three color units (or called sub-pixels). If a panel has a resolution of 1280×1024, it actually has 3840×1024 transistors and sub-pixels. The upper left corner (gray rectangle) of each sub-pixel is an opaque thin film transistor, and the color filter can produce the three primary colors of RGB. Each interlayer contains electrodes and grooves formed on the alignment film, and the upper and lower interlayers are filled with multiple layers of liquid crystal molecules (the liquid crystal space is less than 5×10-6m). In the same layer, although the position of the liquid crystal molecules is irregular, the long axis orientation is parallel to the polarizer. On the other hand, between different layers, the long axis of the liquid crystal molecules is continuously twisted 90 degrees along the plane parallel to the polarizer. Among them, the orientation of the long axis of the two layers of liquid crystal molecules adjacent to the polarizing plate is consistent with the polarization direction of the adjacent polarizing plate. The liquid crystal molecules near the upper interlayer are arranged in the direction of the upper groove, and the liquid crystal molecules in the lower interlayer are arranged in the direction of the lower groove. Finally, it is packaged into a liquid crystal box and connected with the driver IC, the control IC and the printed circuit board.
Under normal circumstances, when light is irradiated from top to bottom, usually only one angle of light can penetrate, through the upper polarizing plate into the groove of the upper interlayer, and then passing through the lower polarizing plate through the passage of the twisted arrangement of liquid crystal molecules. Form a complete path of light penetration. The interlayer of the liquid crystal display is attached with two polarizing plates, and the arrangement and light transmission angle of the two polarizing plates are the same as the groove arrangement of the upper and lower interlayers. When a certain voltage is applied to the liquid crystal layer, due to the influence of the external voltage, the liquid crystal will change its initial state, and will no longer be arranged in a normal way, but will become an upright state. Therefore, the light passing through the liquid crystal will be absorbed by the second layer of polarizing plate and the entire structure will appear opaque, resulting in a black color on the display screen. When no voltage is applied to the liquid crystal layer, the liquid crystal is in its initial state and will twist the direction of the incident light by 90 degrees, so that the incident light from the backlight can pass through the entire structure, resulting in white on the display. In order to achieve the color you want for each individual pixel on the panel, multiple cold cathode lamps must be used as the backlight of the display.
(2) Active matrix
The structure of the TFT-LCD liquid crystal display is basically the same as that of the TN-LCD liquid crystal display, except that the electrodes on the upper interlayer of the TN-LCD are changed to FET transistors, and the lower interlayer is changed to a common electrode.
There are many differences between the working principle of TFT-LCD and TN-LCD. The imaging principle of the TFT-LCD liquid crystal display is to use the “back-through” illumination method. When the light source is irradiated, it first penetrates upward through the lower polarizing plate, and transmits light with the help of liquid crystal molecules. Since the upper and lower interlayer electrodes are changed to FET electrodes and common electrodes, when the FET electrodes are turned on, the arrangement of the liquid crystal molecules will also change, and the purpose of display is achieved by shielding and transmitting light. But the difference is that because the FET transistor has a capacitance effect and can maintain a potential state, the previously transparent liquid crystal molecules will remain in this state until the FET electrode is energized next time to change its arrangement.