Thin film transistor liquid crystal display technology
TFT-LCD structure. Thin-film transistor LCD is composed of three core components: display screen, backlight and drive circuit.
TFT-LCD display, including array glass substrate, color filter film and liquid crystal material. The preparation process of the array glass substrate is: using three photolithographic mask plates, firstly continuously depositing ITO film (thickness 20-50 nm) and Cr film (thickness 50-50 nm) on the glass substrate, and lithographically patterning, and then continuous Deposition of insulating gate film SiN: (thickness about 400nm), then intrinsic a-Si (thickness 50 ~ 100nm) and n + a-Si layer, and lithographic pattern (dry method) deposit Al film, lithography leak source The electrode, and finally using the drain-source electrode as a mask, self-aligned etching n + a-Si film between the Cr film on the pixel electrode and the source and drain of the TFT. This is a simple manufacturing process of the TFT reverse interleaving structure. The next step is: coating the polyimide alignment layer on the glass substrate, rubbing with a velvet cloth in a certain direction, to form a fine channel in the same direction on the surface of the alignment layer, and control the alignment of the liquid crystal molecules. Under the condition that the groove directions of the two glass substrates are aligned orthogonally, the two glass substrates are sealed up and down into a cell, the cell gap is generally only a few microns (such as 10 μm), and then the liquid crystal material is encapsulated by vacuum.
Color Filter (CF) is abbreviated as CF. The color display of TFT-LCD is actually the light passing through the array substrate, irradiated on the color film, and the display screen can display the color. The color filter film (like colored cellophane) can be made on the transparent electrode (between the transparent electrode and the liquid crystal layer) or under the transparent electrode (between the transparent electrode and the glass), the upper and lower glass substrates and the CF The film alignment accuracy is very high, requiring the CF film black and white matrix to be aligned exactly with the edge of the ITO pixel electrode, the CF film is attached to the surface of the liquid crystal cell, and then the liquid crystal cell is sandwiched between two colorless polarizing plates. The principle of color display can be briefly described as: a pixel point of the TFT-LCD is divided into three primary colors of red, green and blue (R, G, B), and corresponds to the RGB of the CF film, and the LCD that functions as a light valve transmits The three-color light amount of the CF film is balanced and adjusted to obtain the desired color. If the incident light passing through the CF film leaks, it will affect the contrast of the TFT-LCD, so a black matrix (Black Matrix) for light shielding shall be set at the gap for short. For stability and smoothness, an acrylic-based resin and an epoxy resin are used to make a protective layer (oe cota) with a thickness of 0.5-2 μm, referred to as OC. Then, a shared electrode, that is, a transparent electrode film is formed on this protective layer. The BM layer is usually made of metal chromium (Cr). In order to reduce surface reflection, chromium oxide (CrOx) or resin is also used. The thickness of metallic chromium is about 1000-1500 angstroms, and it is colored with resin, dye or pigment as the coloring layer. The coloring pattern of each pixel is different depending on the application of TFT-LCD. For example, it can be arranged in the shape of bar, Marseille and triangle. The characteristics of CF films are expressed by transmittance, color purity, contrast, and low reflectance, so the requirements for CF films are: high transmittance and color purity; high contrast and flatness, and extremely low diffuse reflection.
Liquid crystal material. According to incomplete statistics, there are more than 10,000 polymer compounds that can be used as liquid crystal materials. It is usually difficult to use a liquid crystal material to meet the main technical indicators of the temperature range, elastic coefficient, dielectric constant, refractive index anisotropy, and viscosity required by the device. In the project, the mixed liquid crystal must be used to modulate the physical properties. Commonly used representative liquid crystal materials can be divided into three categories according to different molecular arrangement directions: one is nematic liquid crystal. In this liquid crystal material, the long axes of the molecules are parallel. In addition to rotating and sliding, the molecules can also move up and down; the second is cholesteric liquid crystal. In this liquid crystal material, the molecules are oriented on different planes. On the same plane, the long axis of the molecule is parallel to the directors of each plane, and it twists layer by layer in a spiral change; third, it is a near-phase crystal liquid crystal. In this liquid crystal material, the molecules are arranged in layers, and the long axes of the molecules in each layer are parallel and can move parallel to each other, but the molecules cannot slide freely between the layers. The main characteristics of the liquid crystal material are: it has an elongated molecular structure, which is perpendicular and parallel to the molecular director, its layer electrical conductivity, dielectric constant, and refractive index are different, and it varies with the external conditions such as temperature and driving frequency. Variety. In addition, the refractive index anisotropy is large, and when the same optical effect occurs, the liquid crystal cell can be thinned. The electric field strength under the same voltage can accelerate the response speed of the liquid crystal cell.
TFT-LCD backlight. The liquid crystal itself does not emit light, and external irradiation light must be applied. This external irradiation light is called a backlight. According to the relative position of the liquid crystal display surface and the light source, the backlight of the liquid crystal display can be roughly divided into three types: edge type, direct type and self-luminous type. Incandescent lamps and white halogen lamps are point light sources, fluorescent lamps (hot cathodes, cold cathodes) are linear light sources, and electroluminescence (EL) and matrix light-emitting diodes are surface light sources. The edge-type backlight is a fluorescent lamp equipped with a linear light source on the side of the display area. In order to ensure the uniformity of the brightness of the display area, the edge-type backlight source adopts light collection and light guide measures. The light collection is to effectively make the incident light can be emitted from one side, the light guide is to reflect the light emitted from the light collection, so that it becomes a planar light source; the direct type backlight is directly below the display area, equipped with 1 or Several cold cathode lamps arranged side by side are equipped with diffuse diffuser plates on top of the cold cathode lamps at the same time to eliminate the spots caused by the cold cathode lamps; the self-luminous backlight is equipped with electroluminescent plates below the display area. Electroluminescence is surface-emitting, which can emit light uniformly and without spots on the whole surface. The light-emitting color is green, blue, and white, and the brightness is 30-100 nits. The development trend of TFT-LCD backlight source is: large screen, high brightness, wide viewing angle, thinness, light weight, low power consumption and low price.
TFT-LCD drive circuit. In order to display arbitrary graphics, the TFT-LCD is displayed with a progressive scan matrix arranged in m × n dots. When designing the drive circuit, first consider that the electrolysis of the liquid crystal will deteriorate the liquid crystal material. In order to ensure the service life, the AC drive method is generally used. The driving methods that have been formed are: voltage selection method, ramp method, DAC method and analog method. Since the TFT-LCD is mainly used for notebook computers, the driving circuit is roughly divided into: a signal control circuit, a power supply circuit, a gray-scale voltage circuit, a common electrode driving circuit, a data line driving circuit and an addressing line driving circuit (gate driving IC). The main functions of the above drive circuit are: the signal control circuit supplies digital signals, control signals and clock signals to the digital IC, and the control signals and clock signals to the gate drive IC; the power supply circuit supplies the required power supply voltage to the digital IC and the gate Driver IC; the gray voltage circuit supplies the 10 gray voltages generated by the digital drive circuit to the data drive; the common electrode drive circuit supplies the common voltage to the shared electrode relative to the pixel electrode; the data line drive circuit sends the signal control circuit Each 6-bit display data of the RGB signal and clock signal are latched in sequence and continued into the interior, and then this display data is converted into an analog signal by a 6-bit DA converter, and then converted into impedance by the output circuit, which is supplied to the LCD screen. Data line; The gate drive circuit converts the clock signal sent by the signal control circuit to the ON / OFF voltage through the shift register conversion action, and sequentially adds it to the LCD screen. Finally, the drive circuit is assembled on the TAB (automatically soldered flexible circuit board), and connected with the LCD screen using ACF (anisotropic conductive adhesive film) and TCP (flexible lead tape for the drive circuit).
TFT-LCD working principle. First introduce the display principle. The principle of liquid crystal display is based on the characteristic that the light transmittance of liquid crystal changes with the magnitude of the applied voltage. When the light passes through the upper polarizer, it becomes linearly polarized light, and the polarization direction is consistent with the vibration direction of the polarizer and the arrangement order of the liquid crystal molecules on the upper and lower glass substrates. When light passes through the liquid crystal layer, linearly polarized light is split into two beams due to the refraction of the liquid crystal. Since the two beams have different propagation speeds (the same phase), when the two beams are combined, the vibration direction of the vibration light will inevitably change. The light passing through the liquid crystal layer is gradually distorted. When the light reaches the lower polarizer, the vibration direction of its optical axis is twisted by 90 degrees and is consistent with the vibration direction of the lower polarizer. In this way, the light passes through the lower polarizer to form a bright field. After the voltage is applied, the liquid crystal aligns under the action of the electric field, and the distortion disappears. At this time, the linearly polarized light passing through the upper polarizer no longer rotates in the liquid crystal layer, and cannot pass through the lower polarizer to form a dark field. It can be seen that the liquid crystal itself does not emit light, and can be displayed only under the modulation of the external light source. During the entire display process, the liquid crystal functions as a voltage-controlled light valve. The working principle of TFT-LCD can be briefly described as: when the forward voltage of the gate is greater than the applied voltage, the drain-source electrode is turned on, and when the forward voltage of the gate is equal to 0 or negative voltage, the drain-source electrode is turned off. The drain electrode is connected to the ITO pixel electrode, the source electrode is connected to the source line (column electrode), and the gate electrode is connected to the gate line (row electrode). This is the simple working principle of TFT-LCD.
The key technology of TFT-LCD. There are many key technologies of TFT-LCD, mainly including the following major aspects:
One is to improve the aperture ratio technology. The aperture ratio refers to the ratio of the light-transmitting part and the non-transmitting part of the TFT-LCD display screen. The larger the aperture ratio, the higher the brightness. The main factors that affect the aperture ratio are the width of the gate and source bus lines, the size of the TFT, the accuracy of the upper and lower substrates to the box, the size of the storage capacitor, and the size of the black matrix. In order to improve the aperture ratio, the method adopted is to make both the black and white matrix and the color film on the TFT substrate. This method avoids the decline in aperture ratio caused by the accuracy of the box, but the yield is not very high, and the cost will increase accordingly. The other is the gate-source bus, which uses integrated circuit micromachining technology. In the 1990s, the microfabrication of the TFT matrix was about 10 μm, and the aperture ratio was 35%. When the micromachining reached 5 μm, the aperture ratio was 80%. The third is to use self-aligned lithography. Mainly to eliminate the parasitic capacitance formed by the overlapping of the gate and the source and drain. Using self-aligned lithography, the gate electrode is used as a mask, and the n + a-Si and source-drain electrodes are etched to reduce the overlap between the gate and source electrodes. The last is to improve the gate source material. In order to increase the aperture ratio, the bus width should be as small as possible, but the problem of reducing the contrast due to excessive bus resistance, input signal delay, and insufficient driving should be considered. Usually adopt the method of Cr or MoTa clad with Al, so that low resistance bus can be obtained.
The second is to expand the perspective technology. The anisotropy of liquid crystal molecules determines the difference in the spatial distribution of liquid crystal molecules. Different solid angles have different light transmittances, which is an important cause of uneven display contrast. Therefore, widening the viewing angle is one of the key topics of liquid crystal display technology. The technical measures generally adopted are: compensation membrane technology. On the liquid crystal display screen, a light diffusion film and a light intensity compensation film are attached, so that the light passing through the liquid crystal screen is evenly diffused, and the light intensity at certain angles is compensated. In addition, multi-domain technology is used to divide more than two different liquid crystal molecule arrangement regions within the pixel to form a multi-domain liquid crystal molecule orientation, so as to achieve the purpose of expanding the viewing angle. There are also methods and measures such as IPS and ASM to expand the viewing angle technology.
The third is to simplify the TFT array process. Generally, the etching times of the TFT array process are 7 to 9 times, and the process flow is too long, which affects the product qualification rate and production capacity. Foreign literature reports that there have been four sets of engraving process, which is half of the conventional TFT array process.
Of course, the key technologies of liquid crystal displays are not just the above three aspects, but they are the most critical technologies that affect the quality of TFT-LCD. Other key technologies will not be repeated here.
Led Garden Lights,Best Solar Garden Lights,Solar Garden Lanterns,Solar Outdoor Christmas Light
Jiangmen Biaosheng Solar Energy Technology Co., Ltd. , https://www.bsprosolar.com