TFT-LCD technology is based on semiconductor IC manufacturing processes, and is unique in that it uses glass substrates rather than traditional silicon wafers. For the TFT manufacturing process, thin film formation, such as CVD and PVD processes, is a very important part. The ODF process has been developed for the assembly of color filters and TFT substrates, and is used in large size LCDs.
In the following sections, we will introduce the process of TFT, Cell and Module one by one.
I. TFT panel process
First of all, the movement and arrangement of liquid crystal molecules need electrons to drive, so in the carrier of liquid crystal – TFT glass, there must be able to conduct the part to control the movement of liquid crystal, here will use ITO (Indium TIn Oxide, transparent conductive metal) to do this thing. ITO is transparent, also known as thin film conductive crystal so that it will not block the backlight.
The different arrangement of liquid crystal molecules and the rapid movement changes to ensure that each pixel accurately display the corresponding color, and the image changes precisely and quickly, which requires precision control of the liquid crystal molecules. ITO film requires special processing, as if printed circuitry on a PCB board, drawing conductive lines throughout the LCD board.
Conventional LCDs are back-channel etched (BCE) TFT display pixel structure. The specific structure is shown in the following figure.
For array panels with back-channel etched TFT structure.The main process can be divided into 5 steps (5 lightings) according to the sequence of the layers to be made and the interrelationship between the layers.
The first layer is Gate electrode.
The second layer is Gate insulation layer
The third layer is Source/Drain electrode
The fourth layer is Contact Hole
The fifth layer is ITO electrode.
The formation process is as follows.
Step 1: Gate and scan line formation
The process includes: Gate layer metal sputtering, Gate lithography, Gate wet lithography, and other processes. After these processes, scan lines and gate electrodes are formed on the glass substrate, i.e. gate electrodes. The graphical representation of the completed process is shown in the following figure.
Photolithography is the process of copying Mask graphic structures onto the glass substrate to be etched. There are three main processes: photoresist coating, exposure, and development
Wet etching: Wet etching uses an appropriate caustic solution to remove the film layer to be removed using a chemical reaction
Dry lithography: The chemical reaction between the process gas and the film, as well as the bombardment of the film by the plasma, is used to remove the film layer.
Step 2: Gate insulation layer and amorphous silicon island (Island) formation
The process includes: PECVD triple layer continuous film formation, island lithography, island dry lithography and other processes. After these processes, the final amorphous silicon island for TFT is formed on the glass substrate. The graphics obtained after the process is completed are shown in the following figure.
PVD (Physical Vapor DeposiTIon), i.e. physical vapor deposition, is an advanced surface treatment technology widely used in the international arena. Its working principle is to use gas discharge to partially dissociate the gas or evaporated material under vacuum conditions, and to deposit the evaporated material or its reactants on the substrate while the gas ions or the evaporated material ions bombard the action. At present, the most used PVD technology on the market is mainly divided into three categories: magnetron sputtering plating, multi-arc ion plating and evaporation plating.
Step 3: Source and drain electrode (S/D), data electrode and channel (Channel) formation
Specific processes include: S/D metal layer sputtering into a film, S/D lithography, S/D wet lithography, channel dry lithography and other processes. After these processes, the source, drain, channel and data lines of the TFT are finally formed on the glass substrate. At this point, the TFT has been produced. The graphics obtained after the process is completed are shown in the following figure.
Step 4: Protective insulation layer (PassiviTIon) and Via formation
The process includes PECVD, photolithography, and dry lithography of vias. After these processes, the final TFT channel protective insulation layer and guide through the hole are formed on the glass substrate. The graphics obtained after the process is completed are shown in the following figure.
Step 5: Formation of transparent pixel electrode ITO
Specifically, the process includes: sputtering of ITO transparent electrode layer, ITO photolithography, ITO wet lithography and other processes. After these processes, the transparent pixel electrode is finally formed on the glass substrate. The graphics obtained after the process is completed are shown in the following figure.
At this point, the whole array process is completed. In short, the array process of 5 times of light is: 5 times of film formation + 5 times of etching
CF process
Color filters can be produced by various methods; photolithography is a typical method. In photolithography, color filters are produced by exposing a glass substrate coated with a photographic color resist through a photomask. The resist is hardened to form the RGB pattern of the LCD.
1.Formation of black substrate (low reflectivity chrome or resin)
The black matrix is formed first to prevent any leakage of the backlight and RGB color mixture.
2.Color resist coating
The colored resist is applied to the entire surface of the glass substrate.
3.Exposure
In order to make the pattern insoluble, it is UV cured by photomask exposure.
4.4Development and bakin
After removing unnecessary portions of the color resist by developing solution, the pattern is cured by baking.
★Repeat 2 to 4 times
The above process from 2 to 4 is repeated three times (for RGB).
5.Indium-tin oxide layer formation
The ITO layer (transparent conductive layer) is formed by the fly sputtering method.
6.Support column spacer (Photo Spacer, PS) formation
PS is formed to more thoroughly control the cell gap between the two glasses (color filter and TFT array) to obtain improved image quality.
Role.
2.High contrast ratio
3.Reduce the scratching of color filters due to the spherical spacer produced by vibration
Cell process
The middle section of the cell is a combination of the front Array glass as the substrate and the color filter glass substrate, and the liquid crystal (LC) is injected between the two glass substrates.
I. PI coating
It provides the required orientation and pre-tilt substrate for the liquid crystal molecules in the cell.
Rubbing (friction)
Rubbing the PI film on top of the glass substrate by using the alignment cloth hairs to create grooves for liquid crystal orientation, so that the liquid crystal is neatly aligned between the upper and lower alignment films in the direction specified.
Like the PI process, Rubbing is especially needed to eliminate ESD.
Pre-Tilt Angle (Pre-TIlt Angle or TBA: Tilt Bias Angle) characteristic refers to the arrangement of liquid crystal molecules on the mating film is not parallel to the film surface, but one end of the molecule in the friction direction relative to the film surface has a certain warp, the stilt angle is called pre-tilt angle. If there is no pre-tilt angle, under the external electric field, the liquid crystal molecules can stand up randomly from two directions, which will cause poor display.
Three, frame glue
Frame glue composition:
(1) Main component : Sealant
(2) Silicon ball : with a certain height, acting as a shim
IV. Liquid crystal drip-fill
The liquid crystal filling method is drop-fill type (One-Drop-Fill).
V. Hot pressing
Pressing and laminating the array glass with the color resist glass.
Sixth, cutting lobes
When making LCD panels it is impossible to produce them one by one, which is too inefficient, so multiple pieces are processed at once and separated by cutting.
Seven, polarizer patch
After that is bonding IC and FPC; assembling backlight module, the whole machine shell and other processes, relative to the front of the high-precision process to be simpler.
Module process
The main process of the module process includes: COG, FPC bonding, assembly, etc.. The following is introduced one by one.
1、COG, FPC bonding
COG (Chip on Glass) and FPC (Flexible Printed Circuit) is a circuit connection method. Due to the many electrodes, one-to-one lining connection is very difficult. Nowadays, the common practice is to make an array of leads on the glass and a corresponding array of leads on the IC/FPC, and to connect the electrodes on the IC/FPC to the electrodes on the glass one-to-one through an anisotropic conductive film (ACF). The schematic diagram of the lead electrode array on the glass is as follows.
The relative positions of IC/FPC, screen, and ACF after bonding are as follows.
2.Assembly
Assembly is the combination of backlight, screen, control circuit board, and touch screen and other components together to form a complete display module. Assembly is generally done by hand, and skilled workers are very important here.
In addition to the above main process, there are some auxiliary processes in the module section, such as: laser cutting, electrical testing after cutting, electrical testing after bonding, electrical testing after assembly, microscopic inspection after cutting, microscopic inspection or automatic optical inspection after binding, shear force peel test after IC bonding, tensile peel test after FPC bonding, electrical aging after assembly, packaging and shipping, etc.
V. Conclusion
Although we are not process processing personnel, but we still need to understand the relevant things, because in the interface with other departments or personnel in order to be more comfortable. Problems can be considered in many aspects, if you do not know the process related knowledge, encounter problems simply will not think of here, so the process of understanding or necessary.
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