TFT-LCD technology is based on semiconductor IC manufacturing and processing. TFT-LCD technology is unique in that it uses a glass substrate instead of traditional silicon wafers. For TFT manufacturing process, film formation, such as CVD, PVD and other processes, are very important links. During the assembly of color filters and TFT substrates, the ODF process has been developed and applied to large-size LCDS.
In the following sections, we will introduce the process of TFT, Cell, and Module one by one
- TFT panel process
First of all, the movement and arrangement of liquid crystal molecules need electrons to drive, so on the liquid crystal carrier – TFT glass, there must be a conductive part to control the movement of the liquid crystal, here will use ITO(Indium Tin Oxide, transparent conductive metal) to do this thing. ITO is transparent, also known as a thin film conductive crystal, so that it does not block the backlight.
The different arrangement of liquid crystal molecules and the rapid movement changes can ensure that each pixel accurately displays the corresponding color, and the image changes accurately and quickly, which requires precision control of liquid crystal molecules. ITO film needs to do a special treatment, just like the printed circuit on the PCB board, drawing a conductive circuit on the entire liquid crystal board.
Conventional LCD screens use the structure of back channel etched (BCE)TFT display pixels.
For the array panel with the etched TFT structure in the back channel, the main process can be divided into 5 steps (5 illuminations) according to the order of the film layers to be made and the relationship between each film layer.
The first layer is the Gate electrode.
The second layer is the Gate insulation layer
The third layer is the Source/Drain electrode
The fourth layer is the Contact Hole
The fifth layer is ITO electrode
The form of the process type is as follows:
Step 1: Gate and scan line formation
Specifically including: Gate layer metal sputtering film, Gate lithography, Gate wet etching and other processes. After these processes, the scanner line and Gate electrode are finally formed on the glass substrate, that is, the gate electrode.
The lithography technique is to copy the Mask graphic structure onto the glass substrate to be etched. The main three processes: photoresist coating, exposure, development
Wet etching: Wet etching uses an appropriate corrosive solution to remove the film to be removed by chemical reaction
Dry engraving: The chemical reaction of process gas and film, as well as the bombardment of the film by plasma, is used to remove the film layer
The second step: gate insulation layer and amorphous silicon Island formation
Specifically including: PECVD three layer continuous film, small island lithography, small island dry lithography and other processes. After these processes, amorphous silicon islands for TFT are finally formed on the glass substrate. The graphics obtained after the completion of the process are shown in the following figure:
PVD(Physical Vapor Deposition) is an advanced surface treatment technology widely used in the world. The working principle is that under vacuum conditions, the gas discharge is used to partially ionize the gas or the evaporated substance, and the evaporation or its reactants are deposited on the substrate while the gas ions or the evaporated substance ions are bombarded. At present, the most used PVD technology on the market is mainly divided into three categories: magnetron sputtering, multi-arc ion plating and evaporation plating.
Step 3: Source, leakage electrode (S/D), data electrode and Channel formation
Specifically including: S/D metal layer sputtering film, S/D lithography, S/D wet engraving, channel dry engraving and other processes. After these processes, the TFT source, leakage electrode, channel and data line are formed on the glass substrate. At this point, the TFT is finished. The graphics obtained after the completion of the process are shown in the following figure:
Step 4: Formation of protective insulation layer (Passivition) and through hole (Via)
Specifically including: PECVD film forming, photolithography, through hole dry engraving and other processes. After these processes, TFT channel protection insulation layer and guide hole are formed on the glass substrate. The graphics obtained after the completion of the process are shown in the following figure:
Step 5: Formation of transparent pixel electrode ITO
Specifically, it includes: ITO transparent electrode layer sputtering film, ITO photolithography, ITO wet engraving and other processes. Through these processes, a transparent pixel electrode is formed on a glass substrate. The graphics obtained after the completion of the process are shown in the following figure:
At this point, the entire array process is completed. In simple terms, 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 a variety of methods; Lithography is a typical method. In lithography, color filters are produced by exposing a glass substrate coated with a photosensitive color resist through a photomask. The resist is hardened to form the RGB pattern of the LCD.
- Form a black matrix (low reflectivity chromium or resin)
A black matrix is formed first to prevent any leakage of the backlight and RGB color mixture.
- Color corrosion resistant coating
The color resist is coated on the entire surface of the glass substrate.
Step 3: Exposure
In order to make the pattern insoluble, it is UV cured by photomask exposure.
- Developing and baking
After removing the unnecessary part of the color resist by a 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).
- Formation of indium tin oxide layer
The ITO layer (transparent conductive layer) is formed by splashing method.
- Photo Spacer (PS) is formed
PS is formed to more thoroughly control the cell gap between the two glasses (color filters and TFT arrays) for improved image quality.
Functions:
- Proper mechanical strength
- High contrast
- Reduce the scratch of the color filter caused by the spherical spacer generated by vibration
- Good uniformity
- Cell process
The middle part of the Cell is based on the Array glass of the front segment as the substrate, combined with the colored filter glass substrate, and the liquid crystal (LC) is injected between the two glass substrates.
- PI coating
Provide the orientation and pretilt substrate required by the liquid crystal molecules in the Cell.
Two, Rubbing(friction)
A groove for liquid crystal orientation is made by rubbing PI film above the glass substrate with matching cloth, so that the liquid crystal is arranged neatly between the upper and lower matching films according to the specified direction.
As with the PI process, Rubbing is especially necessary to eliminate ESD.
Pre-tilt Angle (or TBA: Tilt Bias Angle) means that the arrangement of liquid crystal molecules on the coordinated film is not parallel to the surface of the film, but one end of the molecule has a certain tilt in the friction direction relative to the surface of the film, and the Angle is called the pre-tilt Angle. If there is no pre-tilt Angle, the liquid crystal molecules can stand up randomly from two directions under the external electric field, which will cause poor display.
3, frame glue
Frame glue ingredients:
(1) Main ingredients: sealant
(2) Silicon ball: it has a certain height and plays the role of gasket
Four, liquid crystal drop
The liquid crystal filling method is One-Drop-Fill.
- Hot pressing
The array glass and the color resistance glass are pressed together.
- Cut the segments
It is impossible to produce a piece of liquid crystal panel when making it, so the efficiency is too low, so the one-time processing of multiple pieces is separated by cutting.
Seven, polarizer patch
Then bonding IC and FPC; The process of assembling the backlight module and the whole machine shell is relatively simple compared with the previous high-precision process.
- Module process
The main process of the module includes: COG, FPC bonding, assembly, etc. Here are some of them.
1, COG, FPC bond
COG(Chip on Glass) and FPC (Flexible Printed Circuit) are a type of circuit connection. Because of the number of electrodes, one-to-one wire connections are difficult. It is now common practice to make an array of leads on the glass, and the leads on the IC/FPC are also made into a corresponding array, and the electrodes on the IC/FPC are connected one-on-one with the electrodes on the glass through an anisotropic conductive film (ACF).
- Assembly
Assembly is to combine the backlight, screen, control circuit board, and touch screen and other components together to form a complete display module. Assembly is usually done by hand, and skilled technical workers are very important here. https://www.findhorizon.com/
In addition to the above main process processes, there are some auxiliary process processes, such as: laser tangential, electrical measurement after tangential, electrical measurement after bonding, electrical measurement after assembly, microscopic inspection after tangential, microscopic inspection after binding or automatic optical inspection, IC shear stripping test after bonding, FPC tensile stripping test after assembly, aging after assembly, packaging and shipping
5. Concluding remarks
Although we are not process processing personnel, we still need to understand the relevant things, because we can be more comfortable when interacting with other departments or personnel. Problems can be considered in many ways, if you do not know the relevant knowledge of the process, you will not think of coming here when you encounter problems, so it is necessary to understand the process.