TFT-LCD glass substrate manufacturing method

TFT-LCD glass substrate manufacturing method
TFT-LCD glass substrate manufacturing methods: floating method, orifice down-drawing method, overflow melting method currently used in commercial glass substrates, its main thickness is 0.7 mm and 0.6 mm, and will soon be thinner (such as 0.4 mm) thickness of the process. Basically, one TFT-LCD panel needs to use two glass substrates, which are respectively used as the bottom glass substrate and the bottom plate of the color filter (COLOR FILT ER) (the cross-sectional view of the color filter is shown in Figure 1). Generally, the ratio of the glass substrate manufacturing supplier to the glass panel assembly factory and its color filter processing factory is about 1: 1.1 to 1: 1.3.

The glass substrates used in LCDs can be divided into two categories: alkali glass and alkali-free glass; alkali glass includes soda glass and neutral boron silicate glass, which are mostly used on TN and STN LCD. The main manufacturers are Japanese plate glass ( NHT), Asahi and Central Glass, etc., are mainly produced by the float process; alkali-free glass is made of alkali-free aluminum silicate glass (Alumino Silicate Glass, the main components are SiO2, Al2O3, B2O3 and BaO, etc.), whose total alkali metal content is less than 1%, is mainly used in TFT-LCD. The leading manufacturer is Corning (Corning), which mainly produces by overflow melting process.
The characteristics of ultra-thin flat glass substrates mainly depend on the composition of the glass, and the composition of the glass affects the thermal expansion, viscosity (strain, annealing, transformation, softening and working point), chemical resistance, optical penetration absorption and various properties of the glass The electrical characteristics at various frequencies and temperatures, product quality is not only deeply affected by the material composition, but also depends on the production process.

For glass substrates in TN / STN and TFT-LCD applications, the required characteristics are surface characteristics, heat resistance, chemical resistance, and alkali metal content; the following only describes the main physical characteristics that affect the glass substrates for TFT-LCD as follows :
1. Strain Point: It is an indicator of glass densification, which must be resistant to the high temperature of the production process of liquid crystal displays of optoelectronic products.
2. Specific gravity: For TFT-LCD, notebook computers are currently the largest market, so the lower the density of the glass substrate, the better, so that it is easy to transport and carry.
3. Thermal expansion coefficient: This coefficient will determine the ratio of the expansion or contraction of the appearance size of the glass material due to temperature changes. The lower the coefficient, the better, so as to minimize the thermal expansion and contraction of the large screen.
The remaining physical properties include melting point, softening point, chemical resistance, mechanical strength, optical properties, and electrical properties, etc., which can be regulated according to the specific needs of users.

In the entire glass substrate manufacturing process, the main technologies include feeding, sheet forming and post-processing. The feeding technology is mainly controlled by the quality of the formula. The first is to melt the glass raw materials into a low viscosity and uniformity in a high temperature furnace The glass melt must not only consider the physical and chemical characteristics of the glass, but also need to select the best raw material formula without changing the chemical composition, so as to effectively reduce the glass melting temperature, make the glass clear, and at the same time achieve the specific properties of the glass. Meet the needs of practical applications. The thin plate forming technology is concerned with dimensional accuracy, surface properties and whether further processing and grinding are required to achieve special physical and chemical characteristics. The subsequent processing includes glass division, grinding, washing and heat treatment processes.

So far, there are three main process technologies for the production of glass substrates for flat panel displays, namely Float Technology, Slot Down Draw and Overflow Fusion Technology. "Floating method" due to the relationship of horizontal extension, the surface will produce scratches and irregularities, and then need to be surface polished, so the investment amount is higher, but it can produce a wider glass product (wide width can reach 2.5 Ruler) and large production capacity (approximately 100,000 square meters per month); the "overflow melting method" has the advantages of more controllable surface characteristics, no grinding, and simpler process, etc. It is especially suitable for production thickness less than 2 mm ultra-thin flat glass, but the width of the glass produced is limited to less than 1.5 meters, so the production capacity is small. The floating method can produce glass substrates suitable for various flat panel displays, while the overflow melting method is currently only used to produce TFT-LCD glass substrates. The following only describes the above three process technologies as follows:

(1) Floating method:
As the most well-known flat glass manufacturing technology at present, this method is to transfer the glass paste melted in the furnace to the liquid tin bed. Due to the low viscosity, the thickness of the glass can be controlled by the baffle or tie rod. After the increase, the glass paste will gradually solidify into flat glass, and then use the guide wheel to lead the cured glass plate out, and then through annealing, cutting and other post-processing procedures.

When the ultra-thin flat glass is produced by the float method, the lower glass paste feeding amount should be controlled. The glass ribbon (R ibbon) entering the tin bed is first cooled to about 700 ℃, and the viscosity of the glass ribbon is about 108 poise ( Poise; 1 poise = 1 g / cm • sec), and then use the edge roller to pull the glass paste floating on the liquid tin, and then stretch out, then heat the glass ribbon to 850 ℃, in conjunction with the conveyor belt roller to apply external force It is drawn and drawn, and the ultra-thin flat glass is drawn by the float method as shown in Figure 3.

The float method technology adopts the horizontal lead-out method, so it is easier to use the elongated horizontal production line to meet the annealing requirements. The main reason why the float method technology is not widely used in the production of ultra-thin flat glass with a thickness of less than 2 mm is because it cannot achieve the required economic scale. For example, the daily output of the float method can almost meet the current monthly consumption in the Taiwan market; if the float method is used to produce ultra-thin flat glass, it is generally produced in a discontinuous tank kiln (D a yTank) Therefore, it is very important to optimize the design of the kiln.

(2) Orifice down-drawing method:
As for the special ultra-thin flat glass required for flat panel displays, many manufacturers use the orifice down-draw method, which is an orifice leak plate made of platinum alloy with a low viscosity homogeneous glass paste (Slot Bushing) In the tank, the thickness of the glass is controlled by the force of gravity and pull-down and the size of the opening of the mold. The temperature and the size of the orifice determine the glass output, and the size of the orifice and the speed of the downdraw The thickness of the glass is determined, and the temperature distribution determines the warpage of the glass. The ultra-thin flat glass is drawn by the down hole drawing method as shown in Figure 4.

The flow-through down-draw process can produce 5 to 20 metric tons of ultra-thin flat glass with a thickness of 0.0 3 to 1.1 mm per day. Because platinum metal cannot withstand high mechanical stress, most of the molds made of platinum alloy are generally However, the orifice often deforms when subjected to external forces, resulting in uneven thickness and surface flatness that cannot meet specifications and requirements.

The downdraft method must be annealed in the vertical direction. If it is turned to the horizontal direction, it may increase the contact between the glass surface and the roller and the warpage caused by the horizontal transportation, resulting in a large increase in the defect rate. Such concerns make it necessary to adopt a high design for the construction of the furnace, and at the same time must accurately consider the height required for annealing, which greatly increases the difficulty of the project and is also reflected in the cost of building the factory.

(3) Overflow melting method:
It adopts an elongated fusion pump (Fusion Pump) to transfer the molten glass paste to the center of the molten pump, and then uses the overflow method to transfer the two overflowing glass pastes to the pump. The lower part is combined into an ultra-thin flat glass.

The use of this molding technology also requires the use of heavy molds, so the melt pump mold also faces the problems of deformation due to mechanical stress, maintaining the level of the melt pump, and how to stably melt the glass paste into the melt pump. Because the ultra-flat glass made by the forming technology of the overflow melting method, the thickness and quality of the glass surface depend on the amount of glass paste delivered to the molten pump, stability, levelness, surface properties of the pump and the glass Lead volume.

Melt overflow technology can produce ultra-thin glass substrates with dual primary glass surfaces, compared to the floating method (only a single primary glass surface can be produced) and the orifice down-draw method (cannot produce a primary glass surface), It can eliminate post-processing processes such as grinding or polishing. At the same time, during the manufacture of flat panel displays, there is no need to pay attention to the difference in glass surface properties due to the different glass surfaces that have both original and liquid tin contact, or contact with the grinding media Etc., has become the mainstream of ultra-thin flat glass molding.

Because the alkali-free glass has a special composition formula and has extremely strict standard specifications in terms of thermal stability, mechanical, electrical, optical, chemical and other characteristics, appearance dimensions, surface flatness, etc., its production line adjustment and learning time are longer. The technological threshold for new manufacturers to join the industry is higher.

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