1.0 Introduction of cleaning method

The main cleaning methods and technologies can be divided into: wet cleaning method and dry cleaning method. The wet cleaning method includes physical cleaning (including brush scrubbing, high-pressure water column flushing, two-fluid spraying, ultrasonic (Ultrasonic, 20~50kHz) cleaning, etc.) and chemical cleaning (selecting solvents according to pollutants, divided into organic solvents, Neutral lotion, chemical cleaning solution and pure water, etc.). Dry cleaning methods include ultraviolet ozone, laser cleaning and plasma, among which ultraviolet ozone cleaning is the most common. The cleaning of the TFT array substrate is a comprehensive application of various cleaning technologies, and different cleaning combinations will be adopted according to the nature of the cleaning objects and dirt.

1.1 Wet cleaning

1.1.1 Wet chemical cleaning is a procedure in which the surface of the substrate is cleaned and dried with a mixture of liquid acid, alkali solvent and deionized water. Chemical cleaning not only removes organic substances, but also inorganic substances such as metals. The commonly used cleaning agent for removing organic matter is ammonia water-hydrogen peroxide solution or chromic acid-sulfuric acid mixture. Diluted hydrofluoric acid solutions can be used to remove inorganic contaminants.

1.1.2 Brushing: a mechanical cleaning method that uses a brush to roll on the surface of the substrate to remove particles and organic films. It is very effective to remove particles larger than 5um in diameter.

1.1.3 High-pressure liquid spray: Use the pressure of the liquid to impact the glass surface to remove the particles, which has a great relationship with the height of the boundary layer and the velocity of the fluid, and the removal rate of smaller particles is not high.

1.1.4 Two-fluid spray cleaning: The principle is similar to high-pressure liquid spray. The two-fluid is a high-speed spray device formed by mixing compressed air and liquid to turn the liquid into a high-speed fluid that is atomized. Such as SBJ device. LCD module

1.1.5 Ultrasonic cleaning: The principle is caused by very complex and diverse factors. The decisive theory has not yet been established, but generally there are the following three types of effects:

a. Ultrasonic Cavitation

If strong ultrasonic waves are added to the liquid, the cleaning liquid will change 1 air pressure in the quiet zone in the center. When the pressure is below 0 atmospheric pressure, the minute air bubbles such as O2 dissolved in the liquid become nuclei, and countless tiny holes (cavitation) that are close to a vacuum are generated. Under the positive pressure of ultrasonic waves, those tiny cavities become adiabatic compression and are eventually crushed. At the moment of being crushed, a strong shock wave is generated to directly destroy the dirt and disperse it into the liquid for cleaning. According to the effect of such cavitation cleaning on oil stains, it is mainly used for cleaning mechanical parts and the like. Generally, a cleaning machine with a frequency of about 28kHz to 50kHz and an intensity of 0.5 to 1W/cm2 is used. LCD module

b. Acceleration (Ultrasonic Vibration)

When adding ultrasonic waves to the liquid, the liquid molecules vibrate. The acceleration of this vibration is 103 times the acceleration of gravity at 28 kHz and 105 times the acceleration of gravity at 950 kHz. Under this strong acceleration, the dirt can be peeled off from the surface of the cleaning object. The 950 kHz ultrasonic wave cannot clean oil stains because it does not generate cavities, but can clean sub-micron stains. It is also used for semiconductor-related cleaning due to the difficulty of etching metal surfaces. LCD module

c. Physicochemical reaction promotion

According to the local high temperature and high pressure (1000 atmospheric pressure, 5500 °C) generated by the cavity, chemical or physical effects are generated due to vibration, stirring, etc., and emulsification and dispersion can be carried out. These can promote chemical reactions.

1.2 Dry cleaning

Dry cleaning mainly includes photochemical cleaning (UV), physical cleaning and plasma cleaning.

1.2.1 Ultraviolet photochemistry (ultraviolet) is the use of 185nm and 254nm ultraviolet rays generated by a low-pressure mercury lamp to irradiate gas molecules to crack them into high-energy free radicals, and the generated free radicals react with the treated material to achieve a scavenging effect. The method is mainly to remove organic pollutants.

1.2.2 Plasma removal method (plasma) uses plasma to generate free radicals to react with pollutants. For example, oxygen plasma can remove photoresist and fine organic matter, and then the product is taken out of the reaction tank by air flow. LCD module

1.2.3 The laser-assisted system particle removal technology can remove particles of 0.1um or smaller. The principle is that laser heating destroys the low-light environment where the particles are adhered, thereby removing the particles. Generally, the laser energy density is low and causes little damage to the substrate.

In addition, the high-velocity airflow particle removal technology is also effective for removing particles from the substrate surface. LCD module

1.3 Comparison of several common cleaning methods

2.0 Source, classification and impact of pollutants

2.1 Sources of pollutants

Pollutants mainly come from raw material pollution, process pollution and environmental pollution. Among them, the pollution of raw materials includes some impurities brought by the glass factory, packaging, transportation and storage process. Process contamination includes residues of reactants in the process, mechanical wear of the process chamber, chemical reactions on the process surface, etc. Environmental pollution includes human hair, dander, fibers, air dust, mechanical equipment wear, grease, etc.

2.2 Classification of pollutants

Pollutants can be divided into inorganic types such as particles, metals and oxides; organic types such as fibers, oils and bacteria. Among them, the particles are mainly dust, impurities in the chamber and etching impurities, etc., which adhere to the basic surface. Metal contaminants are mainly formed during metal film formation and patterning, and may also be other sources of dust. Amorphous silicon atoms easily form an oxide layer in an environment containing oxygen and water. If the n+ amorphous silicon layer is oxidized, the contact resistance of the source and drain will increase. Organic impurities exist in various forms, such as human dander grease, mechanical oil, vacuum grease, cleaning solvent and other liquid crystal modules

2.3 Influence of various pollution sources

3.0 TFT LCD cleaning equipment

The cleaning equipment of TFT LCD is generally composed of conveyor, UV, plasma cleaning (AP), high pressure spray cleaning, ultrasonic cleaning, brush (RB), water spray (SBJ) and air knife drying (AK) or drying. Several groups of important units are now introduced.

3.1 AP unit

Under normal temperature and pressure conditions, the ions generated by high-voltage ionization react with and bombard the contaminants on the glass surface to achieve the purpose of cleaning stains and foreign objects. The gate/SD layer is not used in production because it damages the metal layer and produces AP defects.

Process conditions: N2 flow 150lpm, CDA flow 0.3lpm, Gap -5mm, voltage 7KV.

Increasing the flow of N2 and CDA is beneficial to generate high density and more free radicals and improve the cleaning effect, but it also requires a larger excitation voltage; reducing the gap between the substrate and the electrode can also improve the cleaning effect, but If the gap is too small, it is necessary to prevent the substrate from being damaged; increasing the excitation voltage can generate more free radicals and improve the cleaning effect, but the requirements for power supply and equipment anti-static will be higher, and the increase of high-energy particles will make AP defects more serious. . LCD module

3.2 RB unit

Brushing is mainly to remove relatively large particles (>5um). The effect of brushing is related to the conveying speed of the substrate, the pressing amount of the brush and the direction and speed of the rotation of the brush. The substrate conveying speed is fast, and the particle removal rate decreases; the pressing amount is zero or a positive number, and the brush vibration can also remove a part of the particles. As the pressing amount increases, the particle removal rate increases. But pressing too much may damage the membrane surface.

For the zero adjustment of the brush, we define the position when the brush is exactly tangent to the substrate as the zero point of the brush. The position of the zero point is directly related to the actual pressing amount, so the adjustment of the zero point of the brush is very important. The steps of brush zero adjustment are as follows: ① Use newly formed Al film glass (glass thickness 0.5mm, Al film thickness 1500-2000Å); ② Set the upper and lower pressing amount of the brush to 0.0mm, the AP is turned off and does not need to be turned on. The Al-coated glass is cleaned. After cleaning, take out the Al-coated glass and take it to the MCR (or MAR) to see the appearance; ③ In addition, during the cleaning of the Al-coated glass, you need to go to the brush to listen to whether there is any abnormal sound when the brush rotates. Judgment of OK standard: After cleaning, the Al-coated glass has no obvious scratches, or only the front and/or rear parts are slightly scratched; there is no abnormal sound during the rotation of the brush.

3.3 UV unit

The UV unit is equipped with a low-pressure mercury lamp that generates 172/185/254nm ultraviolet rays. Under the action of ultraviolet rays, oxygen molecules are excited to generate oxygen free radicals, and they interact with organic pollutants to cut off the chemical bonds of organic substances, turning them into carbon monoxide, carbon dioxide, water, etc. The gas evaporates. The specific reaction principle is as follows:

3.4 Ultrasonic cleaning

Several factors affect the effect of ultrasonic cleaning:

Relationship with frequency: Generally, the lower the frequency, the more obvious the cavitation effect, but the noise is relatively high, which is suitable for objects with relatively flat surfaces. The higher the frequency, the worse the cavitation effect, but the noise is relatively low. It is suitable for objects with many micro-hole blind holes and electronic crystals.

It is related to temperature: generally, the medium temperature of 30℃-50℃ has the best cleaning effect.

It is related to the sound intensity: according to different frequencies, the sound intensity is generally selected at about 1-2w/cm2.

Related to cleaning fluid: Generally speaking, the lower the viscosity of the cleaning fluid, the higher the air content, and the better the cleaning effect.

It is related to the depth of the cleaning solution and the position of the object to be cleaned.

3.5 Drying device

3.6 Hot plate drying

4.0 Confirmation of cleaning effect

4.1 Particle removal rate

Removal rate (%) = (number of particles before cleaning - number of particles after cleaning) / number of particles before cleaning × 100%

Standard: T≤100EA; above 3um≤40EA

4.2 Contact angle test

The water droplet is hemispherical on the film, and the height is d and the radius is r, then the contact angle θ=2arctand/r.

The size of the contact angle measures the removal of organic matter from the substrate. The smaller the contact angle, the less organic matter, the higher the cleaning degree, and the better the infiltration of water droplets.

Standard: θ≤7°