TFT-LCD technology

TFT-LCD technology

TFT-LCD structure. Thin-film transistor liquid crystal display consists of three core components: display screen, backlight source and drive circuit.

TFT-LCD display screen includes an array glass substrate, a color filter film and a liquid crystal material. The preparation process of the array glass substrate is: using three photolithography masks, firstly, continuously depositing ITO film (20-50nm thick) and Cr film (50-100nm thick) on the glass substrate, and photolithography pattern, then continuously depositing insulating gate film SiN: (about 400nm thick), then intrinsic a-Si (50-100nm thick) and n+a-Si layer, and photolithography pattern (dry method) depositing Al film, photolithography drain source electrode, and finally using the drain source electrode as a mask, self-aligning etching the Cr film on the pixel electrode and the n+a-Si film between the TFT source and drain. This is a simple manufacturing process of the TFT anti-staggered structure. The next step is: coating a polyimide orientation layer on the glass substrate, rubbing it with a flannel in a certain direction, so that the surface of the orientation layer forms a fine channel with a consistent direction, and controls the directional arrangement of the liquid crystal molecules. Under the condition that the directions of the upper and lower orientation grooves of the two glass substrates are orthogonal, the two glass substrates are sealed up and down into a box. The gap between the two glass substrates is generally only a few microns (such as 10μm), and then the liquid crystal material is vacuum-sealed.

Color filter is abbreviated as CF. The color display of TFT-LCD is actually the light through the array substrate, shining on the color filter, so that the display screen can display the color. The color filter (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 alignment accuracy of the upper and lower glass substrates and the CF film is very high, requiring the black and white matrix of the CF film to be aligned with the edge of the ITO pixel electrode. The CF film is attached to the surface of the liquid crystal box, and then the liquid crystal box is clamped with two colorless polarizers. The principle of color display can be briefly described as follows: a pixel of 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. The LCD, which acts as a light valve, balances and adjusts the amount of light of the three colors passing through the CF film 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) abbreviated as BM should be set at the gap to block the light. For stability and smoothness, a protective layer (oe cota) of 0.5 to 2 μm thick is made of acrylic resin and epoxy resin, referred to as OC. Then a common electrode, i.e. 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 metal chromium is about 1000 to 1500 angstroms, and resin, dye or pigment is used as a coloring layer for coloring. The coloring pattern of each pixel is different depending on the purpose of TFT-LCD. For example, it can be arranged in stripes, mosaics, triangles, etc. The characteristics of CF film are expressed by transmittance, color purity, contrast and low reflection, so the requirements for CF film are: high transmittance and color purity; high contrast and flatness and extremely low diffuse reflection.

Liquid crystal materials. 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 single liquid crystal material to meet the main technical indicators such as the temperature range, elastic coefficient, dielectric constant, refractive index anisotropy and viscosity required by the device. In engineering, mixed liquid crystals must be used to modulate physical properties. Commonly used and representative liquid crystal materials can be divided into three categories according to the different molecular arrangement directions: one is nematic liquid crystal. In this liquid crystal material, the long axes of the molecules are parallel, and the molecules can move up and down in addition to rotation and sliding; the second is cholesteric liquid crystal. In this liquid crystal material, the molecules are oriented on different planes. On the same plane, the long axes of the molecules are parallel to the directors of each plane, and they twist layer by layer to form a spiral change; the third is 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. They can move parallel to each other, but the molecules cannot slide freely between layers. The main characteristics of liquid crystal materials are: they have a slender molecular structure, and their layer dielectric constant, dielectric constant, and refractive index are different in the two directions perpendicular and parallel to the molecular director, and they change with external conditions such as temperature and driving frequency. In addition, the large anisotropy of the refractive index can make the liquid crystal cell thinner while producing the same optical effect. The electric field strength under the same voltage can speed up the response speed of the liquid crystal cell.

TFT-LCD backlight. Liquid crystal itself does not emit light, and external illumination must be applied. This external illumination is called backlight. The backlight of the LCD can be roughly divided into three types according to the relative position of the liquid crystal display surface and the light source: edge type, direct type and self-luminous type. Incandescent lamps and white halogen lamps are point light sources, fluorescent lamps (hot cathode, cold cathode) are line light sources, electroluminescence (EL) and matrix light-emitting diodes are surface light sources. The edge backlight is a fluorescent lamp with a line light source installed on the side of the display area. In order to ensure the uniformity of the brightness of the display area, the edge backlight adopts light collection and light guiding measures. Light collection is to effectively make the incident light emitted from one side, and light guiding is to reflect the light emitted by the light collection to make it a plane light source; the direct backlight is installed directly below the display area, with one or several cold cathode lamps arranged side by side, and a diffuse scattering plate is installed on the cold cathode lamp to eliminate the spots caused by the cold cathode lamp; the self-luminous backlight is installed below the display area with an electroluminescent panel. Electroluminescence is a surface luminescence, which can emit light evenly across the entire surface without spots. The luminous colors are green, blue, and white, and the brightness is 30 to 100 nits. The development trend of TFT-LCD backlight sources is: large screen, high brightness, wide viewing angle, thinness, light weight, low power consumption, and low price.

TFT-LCD driving circuit. In order to display any graphics, TFT-LCD uses a progressive scanning matrix with m×n dots. When designing the driving circuit, the first thing to consider is that liquid crystal electrolysis will deteriorate the liquid crystal material. In order to ensure the life span, AC driving is generally used. The driving modes that have been formed include: voltage selection mode, ramp mode, DAC mode and analog mode. Since TFT-LCD is mainly used in notebook computers, the driving circuit is roughly divided into: signal control circuit, power supply circuit, grayscale voltage circuit, common electrode driving circuit, data line driving circuit and addressing line driving circuit (gate driving IC). The main functions of the above driving circuit are: the signal control circuit supplies digital signals, control signals and clock signals to the digital IC, and supplies the control signals and clock signals to the gate driving IC; the power supply circuit supplies the required power supply voltage to the digital IC and the gate driving IC; the grayscale voltage circuit supplies the 10 grayscale voltages generated by the digital driving circuit to the data driver; the common electrode driving circuit supplies the common voltage to the shared electrode relative to the pixel electrode; the data line driving circuit latches and continues to the internal 6-bit display data and clock signal of the RGB signal sent by the signal control circuit in a timely sequence, and then the display data is converted into an analog signal by a 6-bit DA converter, and then converted into impedance by the output circuit and supplied to the data line of the LCD screen; the gate driving circuit switches the output circuit to ON/OFF voltage through the shift register conversion action of the clock signal sent by the signal control circuit, and sequentially applies it to the LCD screen. Finally, the driving circuit is assembled on TAB (automatic welding flexible circuit board) and connected to the LCD screen with ACF (anisotropic conductive film) and TCP (driving circuit flexible lead).

Working principle of TFT-LCD. First, let's introduce the display principle. The principle of liquid crystal display is based on the characteristic that the transmittance of liquid crystal changes with the voltage applied to it. When 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 liquid crystal molecules on the upper and lower glass substrates. When light passes through the liquid crystal layer, the linearly polarized light is decomposed into two beams of light due to refraction by the liquid crystal. Since the two beams of light have different propagation speeds (the same phase), when the two beams of light are synthesized, the vibration direction of the 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 distorted 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 is oriented 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 only be displayed under the modulation of an external light source. In the entire display process, the liquid crystal acts as a voltage-controlled light valve. The working principle of TFT-LCD can be briefly described as follows: when the gate forward voltage is greater than the applied voltage, the drain-source electrode is turned on, and when the gate forward voltage is equal to 0 or a negative voltage, the drain-source electrode is disconnected. The drain electrode is connected to the ITO pixel electrode, the source electrode is connected to the source line (column electrode), and the gate is connected to the gate line (row electrode). This is the simple working principle of TFT-LCD.

Key technologies of TFT-LCD. There are many key technologies of TFT-LCD, mainly in the following aspects:
First, the technology of improving the aperture ratio. The aperture ratio refers to the ratio of the light-transmitting part to the non-transmitting part of the TFT-LCD display screen. The larger the aperture ratio, the higher the brightness. The main factors affecting the aperture ratio are the width of the gate and source bus, the size of the TFT, the accuracy of the upper and lower substrates, the size of the storage capacitor and the size of the black matrix. In order to improve the aperture ratio, the method adopted is: both the black and white matrix and the color film are made on the TFT substrate. This method avoids the decrease in the aperture ratio caused by the accuracy of the box, but the yield is not very high, and the cost will increase accordingly. Another is the gate-source bus, which uses integrated circuit micromachining technology. In the 1990s, the micromachining of the TFT matrix was about 10μm, and the aperture ratio was 35%. When the micromachining reached 5μm, the aperture ratio was 80%. The third is to use self-aligned lithography technology. It is mainly to eliminate the parasitic capacitance formed by the overlap of the gate and the source and drain. Using self-aligned lithography technology, the gate electrode is used as a mask plate, and the n+a-Si and source and drain electrodes are lithographically etched to reduce the overlap between the gate and source electrodes. Finally, the gate source material is improved. In order to increase the aperture ratio, the bus width should be minimized, but the problem of reduced contrast due to excessive bus resistance, input signal delay, and insufficient drive should be considered. Usually, Cr or MoTa metal is used to encapsulate Al, so that a low resistance bus can be obtained.

The second is the technology of expanding the viewing angle. The anisotropy of liquid crystal molecules determines the different spatial distribution of liquid crystal molecules. The light transmittance at different solid angles is different, which is an important reason for the uneven display contrast. Therefore, expanding the viewing angle is one of the key topics of liquid crystal display technology. The technical measures generally taken are: compensation film technology. On the liquid crystal display screen, a light diffusion film and a light intensity compensation film are attached to make the light passing through the liquid crystal screen diffuse evenly and compensate for the light intensity at certain angles. Another is to use multi-domain technology to divide more than two different liquid crystal molecule arrangement areas within the pixel to form a multi-domain liquid crystal molecule orientation, thereby achieving 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. The general TFT array process requires 7 to 9 etching times, which is too long and affects the product qualification rate and production capacity. Foreign literature reports that there are already 4 overlay processes, which is half the number of conventional TFT array processes.

Of course, the key technologies of LCDs are not limited to the above three aspects, but they are the most critical technologies that affect the quality of TFT-LCD. Other key technologies will not be discussed here one by one.