1 Introduction to cleaning method
The main cleaning methods and technologies can be divided into wet cleaning method and dry cleaning method.
Wet cleaning method includes physical cleaning (including brush brushing, high-pressure water column washing, two fluid spraying, ultrasonic (20~50khz) cleaning and chemical cleaning (organic solvent, neutral detergent, chemical cleaning solution and pure water are selected according to pollutants). Dry cleaning method includes ultraviolet ozone, laser cleaning and plasma, among which ultraviolet ozone cleaning method is the most common. The cleaning of TFT display array substrate is a comprehensive application of various cleaning technologies, and different cleaning combinations will be adopted according to the properties of cleaning materials and dirt.
1.1 Wet cleaning
1.1.1 wet chemical cleaning refers to the process of cleaning and drying the substrate surface with a mixture of liquid acid, alkali solvent and deionized water. Chemical cleaning can remove not only organic matters, but also inorganic matters such as metals. The commonly used cleaning agent for removing organic matter is ammonia hydrogen peroxide solution or chromic acid sulfuric acid mixture. Dilute hydrofluoric acid solution can be used to remove inorganic pollutants.
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 liquid impacting the glass surface to remove particles, which has a great relationship with the height of the boundary layer and the velocity of the fluid, and the removal rate of small particles is not high.
1.1.4 two fluid spray cleaning: the principle is similar to high-pressure liquid spray. The second fluid is a high-speed spray device formed by mixing compressed air and liquid, which turns the liquid into a high-speed fluid. Such as SBJ device.
1.1.5 ultrasonic cleaning: the principle is affected by very complex and diverse factors. The decisive theory has not been established yet, but it generally has the following three types of functions:
a. Ultrasonic cavitation
If strong ultrasonic wave is added to the liquid, the cleaning liquid will change one air pressure in the static zone in the center. When the pressure is below 0, the dissolved O2 and other tiny bubbles in the liquid become the core, and countless tiny holes close to the vacuum are generated (cavitation phenomenon). Under the positive pressure of ultrasonic wave, those tiny cavities become adiabatic compression and are finally crushed. Strong shock wave will be generated at the moment of being crushed, which will directly destroy the dirt and disperse it to the liquid for cleaning. According to the cleaning effect of such cavitation on oil contamination, it is mainly used for cleaning mechanical parts. 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. Ultrasonic vibration
When ultrasonic waves are added to the liquid, the liquid molecules vibrate. The acceleration of this vibration is 103 times of the gravitational acceleration at 28kHz and 105 times of the gravitational acceleration at 950 kHz. Under this strong acceleration, dirt can be stripped from the surface of the cleaned object. The 950 kHz ultrasonic wave can not clean the oil stain because it does not produce holes, and can clean the submicron dirt. It is also used for semiconductor related cleaning because it is difficult to erode the metal surface.
c. Reaction promotion of Physical Chemistry
According to the local high temperature and high pressure (1000 air pressure, 5500 ℃) of the hole, the chemical or physical effects due to vibration, stirring and other effects can be emulsified and dispersed. 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 chemistry is to irradiate gas molecules with 185nm and 254nm ultraviolet light generated by low-pressure mercury lamp to break them into high-energy free radicals, and then the generated free radicals react with the treated substances to achieve the removal effect. This method mainly removes 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 organics, and then take the products out of the reaction tank with air flow.
1.2.3 laser assisted system particle removal technology can remove particles of 0.1um or smaller. The principle is that laser heating destroys the low light level environment where particles adhere, so as to remove particles. Generally, the laser energy density is low, causing almost no damage to the substrate.
In addition, the high-speed airflow particle removal technology is also very effective for removing particles on the substrate surface.
1.3 comparison of several common cleaning methods
2.0 source, classification and impact of pollutants
2.1 source of pollutants
Pollutants mainly come from raw material pollution, process pollution and environmental pollution. Among them, raw material pollution includes some impurities brought by the process of glass delivery, packaging, transportation and storage. The process pollution includes the residue of reactants in the process, mechanical wear of the process chamber, chemical reaction on the process surface, etc. Environmental pollution includes human hair, dander, fiber, air dust, mechanical equipment wear, grease, etc.
2.2 classification of pollutants
Pollutants can be divided into inorganic categories such as particles, metals and oxides; Fiber, grease, bacteria and other organic compounds. The particles are mainly dust, impurities in the chamber and etching impurities adhered to the basic surface. Metal pollutants are mainly formed in metal film-forming and graphic processes, and may also be other dust sources. Amorphous silicon atoms are easy to form oxide layer in the environment containing oxygen and water. If n+ amorphous silicon layer is oxidized, the contact resistance of source and drain will increase. Organic impurities exist in many forms, such as human dandruff grease, machine oil, vacuum grease, cleaning solvent and other liquid crystal modules
2.3 impact of various pollution sources
3.0 TFT LCD cleaning equipment
The cleaning equipment for TFT LCD is generally composed of conveyor, UV, plasma cleaning (AP), high-pressure spray cleaning, ultrasonic cleaning, brush (RB), water air spray (SBJ) and air knife drying (AK) or drying. Here are some important units.
3.1 AP unit
Under normal temperature and pressure, ions produced by high-pressure ionization react with and bombard the pollutants on the glass surface, so as to achieve the purpose of cleaning stains and foreign matters. 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 rate of N2 and CDA is conducive to the generation of high density and more free radicals, improving the cleaning effect, but also requires a larger excitation voltage; Reducing the gap between the substrate and the electrode can also improve the cleaning effect, but the gap is too small to prevent the substrate from being damaged; Increasing the excitation voltage can produce more free radicals and improve the cleaning effect, but it will have higher requirements on the anti-static of power supply and equipment, and the increase of high-energy particles will make AP defects more serious. LCD module
3.2 RB unit
Brushing is mainly used to remove relatively large particles (>5um). The effect of brushing is related to the substrate transfer speed, the pressing amount of the brush, and the direction and speed of the brush rotation. The substrate transport speed is fast, and the particle removal rate decreases; When the pressing amount is zero or positive, the brush vibration can also remove some particles. With the increase of pressing amount, the particle removal rate increases. However, too much pressing may damage the film 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 brush zeroing steps are as follows: ① use the newly formed Al film glass (the glass thickness is 0.5mm, and the Al film thickness is 1500-2000 Ω); ② Set the upper and lower pressing amount of the brush to 0.0mm, close the AP and do not use it. Put in the Al film glass for cleaning. After cleaning, take out the Al film glass and take it to the MCR (or mar) to see the appearance; ③ In addition, during the cleaning of Al film glass, you need to go to the side of the brush and listen to whether there is abnormal sound when the brush rotates. Judgment criteria for OK: there is no obvious scratch on the Al film glass after cleaning, or only slight scratch on the front end and / or rear end; 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 excite to generate oxygen free radicals, and act with organic pollutants to cut off the chemical bonds of organic substances, making them volatilize into carbon monoxide, carbon dioxide, water and other gases. The specific reaction principle is as follows:
3.4 ultrasonic cleaning
Several factors affecting ultrasonic cleaning effect:
Relationship with frequency: generally, the lower the frequency is, the more obvious the cavitation effect is, but the noise is relatively high. It is suitable for objects with relatively flat surface. The higher the frequency is, the worse the cavitation effect is, but the noise is relatively low. It is suitable for objects with more microporous blind holes and electronic crystals.
Related to temperature: generally, the medium temperature of 30 ℃ – 50 ℃ has the best cleaning effect.
Related to sound intensity: according to different frequencies, the sound intensity is generally about 1-2w/cm2.
Related to cleaning fluid: Generally speaking, the lower the viscosity of 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 cleaned object.
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 drop is hemispherical on the membrane. If the height is D and the radius is r, 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 washing cleanliness and the better the water droplet infiltration.
Standard: θ ≤ 7 °