Transistor LCD Technology

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

TFT-LCD display screen includes array glass substrate, color filter film and liquid crystal material. The array glass substrate preparation process is: using three photolithography masks, first continuously deposit ITO film (thickness 20~50nm) and Cr film (thickness 50~100nm) on the glass substrate, and photolithography patterns, and then continuously Deposit insulating gate film SiN: (thickness about 400nm), and then intrinsic a-Si (thickness 50~100nm) m) and n+a-Si layer, and photolithography pattern (dry method) is used to deposit the Al film, photolithography of the drain-source electrode, and finally, using the drain-source electrode as a mask, self-aligned etching of the Cr film and TFT on the pixel electrode n+a-Si film between source and drain. This is the simple manufacturing process of TFT inverse staggered structure. The next step is to apply a polyimide alignment layer on the glass substrate and rub it with a flannel in a certain direction to form fine channels in the same direction on the surface of the alignment layer to control the alignment of liquid crystal molecules. Under the condition that the groove directions of the upper and lower orientation grooves of the two glass substrates are orthogonal, the two glass substrates are sealed into a box. The gap between the boxes is generally only a few microns (such as 10 μm), and then the liquid crystal material is evacuated and filled.

Color filter (Color Filter) is referred to as CF. The color display of TFT-LCD actually uses light passing through the array substrate to illuminate the color film, and the display screen can display colors. The color filter film (like colored cellophane) can be made on top of the transparent electrode (between the transparent electrode and the liquid crystal layer), or under the transparent electrode (between the transparent electrode and the glass), with the upper and lower glass substrates and CF The film alignment accuracy is very high, which requires the black and white matrix of the CF film to be exactly aligned 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 polarizers. The principle of color display can be briefly described as follows: Divide a pixel of TFT-LCD into three primary colors of red, green, and blue (R, G, B), and correspond to the RGB of the CF film. The LCD that acts as a light valve controls the transmission of light. The three colors of light of the CF film are 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 light-shielding black matrix (Black Matrix), referred to as BM, must be installed at the gap. For stability and smoothness, acrylic resin and epoxy resin are used to make a protective layer (oe cota) with a thickness of 0.5 to 2 μm, referred to as OC. Then a shared electrode, a transparent electrode film, is formed on this protective layer. The BM layer is usually made of metallic chromium (Cr). In order to reduce surface reflection, chromium oxide (CrOx) or resin is also used. The thickness of metallic chromium is about 1000 to 1500 angstroms, and it is colored with resin, dye or pigment as a colored layer. The coloring pattern of each pixel differs depending on the purpose of the TFT-LCD. For example, it can be arranged in strips, mosaics, triangles, etc. The characteristics of CF film are expressed in 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 material. According to incomplete statistics, there are more than 10,000 kinds of polymer compounds that can be used as liquid crystal materials. It is usually difficult to use a kind of liquid crystal material to meet the main technical indicators such as 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 representative liquid crystal materials can be divided into three major 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, and the molecules can not only rotate and slide, but also move up and down; the second is cholesteric liquid crystal. In this kind of liquid crystal material, the molecules are oriented on different planes. On the same plane, the long axis of the molecules is parallel to the director of each plane, and twists layer by layer to show a spiral change; the third is near-phase liquid crystal.

This kind of liquid crystal material has molecules arranged in a layered shape. 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 layers. The main characteristics of liquid crystal materials are: they have a slender molecular structure, and their layer electrical conductivity, dielectric constant, and refractive index are different in two directions perpendicular and parallel to the molecular director, and vary with external conditions such as temperature and driving frequency. change. In addition, the refractive index anisotropy is large, and the liquid crystal cell can be made thinner while producing the same optical effect. The electric field strength at the same voltage can speed up the response speed of the liquid crystal cell.

TFT-LCD backlight. The liquid crystal itself does not emit light, and external illumination must be applied. This external illumination is called a backlight. The backlight of an LCD display can be roughly divided into three types: edge type, direct type and self-illuminating type according to the relative position of the liquid crystal display surface and the light source. Incandescent lamps and white halogen lamps are point light sources, fluorescent lamps (hot cathodes, cold cathodes) are line light sources, and electroluminescence (EL) and matrix light-emitting diodes are surface light sources. Edge backlights are fluorescent lamps that assemble line light sources on the sides of the display area. In order to ensure the uniformity of brightness in the display area, edge-type backlights adopt light collection and light guide measures. The light collection is to effectively make the incident light emit from one side, and the light guide is to reflect the light emitted from the collection to make it a plane light source; the direct backlight is directly below the display area, equipped with 1 or Several side-by-side cold cathode lamps are equipped with diffuse scattering plates on top of the cold cathode lamps to eliminate spots caused by the cold cathode lamps; the self-illuminating backlight is equipped with an electroluminescent plate below the display area. Electroluminescence is surface luminescence, which can emit light uniformly over the entire surface without spots. The luminescence colors are green, blue, and white, and the brightness is 30 to 100 nits. The development trend of TFT-LCD backlight is: large screen, high brightness, wide viewing angle, thinness, lightweight, low power consumption and low price. .

TFT-LCD drive circuit. In order to display arbitrary graphics, TFT-LCD uses a progressive scan matrix display with m×n dots arranged. When designing the drive circuit, we must first consider that liquid crystal electrolysis will cause the liquid crystal material to deteriorate. To ensure longevity, AC drive is generally used. The driving modes that have been formed include: voltage selection mode, ramp mode, DAC mode and analog mode, etc. Since TFT-LCD is mainly used in notebook computers, the drive circuit is roughly divided into: signal control circuit, power circuit, gray scale voltage circuit, common electrode drive circuit, data line drive circuit and addressing line drive circuit (gate drive IC).

The main functions of the above-mentioned drive 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 drive IC; the power circuit supplies the required power voltage to the digital IC and gate Driving IC; the gray voltage circuit supplies the 10 gray voltages generated by the digital driving circuit to the data driver respectively; common The electrode driving circuit supplies the common voltage to the shared electrode relative to the pixel electrode; the data line driving circuit latches each 6-bit display data and clock signal of the RGB signal sent from the signal control circuit in a timing sequence and continues them internally, and then This display data is converted into an analog signal by a 6-bit DA converter, and then converted into an impedance by the output circuit and supplied to the data line of the LCD screen; the gate drive circuit switches the clock signal from the signal control circuit to the ON/OFF voltage of the output circuit through the shift register conversion action, and then applies it to the LCD screen in sequence. Finally, the drive circuit is assembled on the TAB (automatic welding flexible circuit board) and connected to the LCD display using ACF (anisotropic conductive adhesive film) and TCP (drive circuit flexible tape).

TFT-LCD working principle. First, the display principle is introduced. The principle of liquid crystal display is based on the characteristic that the light transmittance of liquid crystal changes with the magnitude of the voltage applied to it. When the light passes through the upper polarizer, it becomes linearly polarized light. The polarization direction is consistent with the vibration direction of the polarizer, and is consistent with 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. And because the two beams of light propagate at different speeds (with the same phase), when the two beams of light are combined, the vibration direction of the vibrating light will inevitably change. Light passing through the liquid crystal layer is gradually distorted. When the light reaches the lower polarizer, its optical axis vibration direction is twisted by 90 degrees and remains 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 applying voltage, the liquid crystal is oriented under the action of the electric field and the distortion disappears. At this time, the linearly polarized light that passes 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 under the modulation of an external light source. During 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: 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 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.

Key technologies of TFT-LCD. There are many key technologies for TFT-LCD, mainly including the following major aspects:

The first is to improve the opening rate technology. The aperture ratio refers to the ratio of the light-transmitting part and the opaque part of the TFT-LCD display. The greater the aperture ratio, the higher the brightness. The main factors that affect the aperture ratio are gate and source bus width, TFT size, upper and lower substrate box alignment accuracy, storage capacitor size and black matrix size, etc. In order to improve the aperture ratio, the method adopted is to make both the black and white matrix and the color filter on the TFT substrate. This method avoids the decrease in the aperture ratio caused by the box alignment accuracy, but the yield is not very high and the cost will increase accordingly. The other is the gate-source bus, which uses integrated circuit micro-machining technology. In the 1990s, the TFT matrix was micro-machined to about 10 μm, with an aperture rate of 35%. When the micro-machining reached 5 μm, the aperture rate was 80%. The third is to use self-aligned lithography technology.

The main purpose is to eliminate the parasitic capacitance formed by the overlap of gate and source and drain. Using self-aligned photolithography technology, the gate electrode is used as a mask to photoetch n+a-Si and source and drain electrodes to reduce the overlap between the gate and source electrodes. The last step is to improve gate and source materials. In order to increase the aperture ratio, the bus width should be kept as small as possible, 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 clad with Al, so that a low resistance bus can be obtained.

The second is perspective-expanding technology. The anisotropy of liquid crystal molecules determines the different spatial distribution of liquid crystal molecules and the different light transmittances at different solid angles. This is an important reason for uneven display contrast. Therefore, expanding the viewing angle is one of the key issues in liquid crystal display technology. The technical measures generally adopted include: compensation membrane technology. On the LCD screen, a light diffusion film and a light intensity compensation film are attached to diffuse the light passing through the LCD screen evenly and compensate for the light intensity at certain angles. In addition, multi-domain technology is used to divide more than two different liquid crystal molecule arrangement areas within the pixel to form multi-domain liquid crystal molecule orientation, thereby achieving the purpose of expanding the viewing angle. Expanding viewing angle technologies also include methods and measures such as IPS and ASM.

The third is to simplify the TFT array process. Generally, the number of etching times for the TFT array process is 7 to 9 times. The process flow is too long, which affects the product qualification rate and production capacity. Foreign literature reports that there have been four overlay processes, which is half the number of conventional TFT array processes.

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 discussed here.