Essential Points of UV/Ozone Dry cleaning

The use of UV/Ozone cleaning technique spread in early 1980 with manufacturing of liquid crystal display (LCD), and later development is astonishing. Presently it is used in most highly definition electronic products for cleaning and modification purposes.

1. Cleaning Method and Cleanliness

1-1. Dirt that can not be shifted by water and solvent

There are some stains that cannot be shifted by water and solvent. The Technique to simply remove those kinds of stains is Photo Cleaning Technique (Hereinafter UV/Ozone cleaning).
In the world of nanometer, organic contaminants cannot be seen by eyes, but they surely form a thin layer on the surfaces. When topcoat or a photo-orientation coat are printed, these organic layers damage the definition of the printed patterns and cause some pinhole defect.
As shown in table 1, those contaminants that can be removed by light are organic compounds, namely fatty compounds.

Table 1DGeneral organic contaminants
Fat on the skin, FluxCosmetic greases, Resin additives, Parting agent, Wax
Machine oil, Lubricant, Solvent and its vapor, Cutting oil, Sulfide etc

1-2. Cleanliness and water contact angle

Table 2 shows many cleaning methods. The well-known wet cleaning technique is excellent for removing bulky dirt, but its disadvantage is
that it cannot remove them completely.

Table2DVarious cleaning methods
Dry cleaning UV/Ozone cleaning , Plasma cleaning , Ion bombarding
Sputtering cleaning , Vacuum baking , Dry ice blasting
Wet cleaning Water washing , Alkali cleaning , Acid cleaning
Detergent cleaning , Solvent cleaning , Jet cleaning

Also solvent remain on the surface, which in itself can pollute the surface. Table 3 shows the cleaning performance of different method of cleaning by evaluating the contact angel of a drop of water.
A solid surface has a physical inherent surface tension, which is accompanied with a water contact angle. These surface tension and contact angle vary depending on the formation of organic compounds on the surface, a contaminated surface means a smaller surface tension and a wider contact angle, a more clean surface means a greater surface tension and a smaller contact angle.
Hydro-carbonic solvent are known to have a strong solvency against fat, but after cleaning by Hydro-carbonic solvent, the contact angle seems to be wider than un-cleaned surface as shown in table 3. That is attributed to remaining of solvent on the surface. In wet cleaning, if the surface is not cleaned carefully, contact angle of glass won't go below 10 degree

.

Table 3DComparison of the cleaning powers
among the various cleaning methods
Cleaning conditions Contact angle (degree)
Non-treated glass plate 25
Synthetic detergent ⇒ Water ⇒ Hydrocarbon solvent 39
Synthetic detergent ⇒ Water ⇒ Pure water ⇒ Dry 17
Synthetic detergent ⇒ Water ⇒ IPA 13
Ozone solution <10
Synthetic detergent ⇒ Water ⇒ Plasma 5
Synthetic detergent ⇒ Water ⇒ UV/O3 4

1-3. Monomolecular layers of contaminations

Steam test is one method of testing the cleanliness of a glass. It is the method of studying a thin membrane, when steam condensed on the surface after holding the glass upon the hot water (not boiling), and when the layer vaporized. Table 4 shows the results when a quartz glass was used. According to the table, when the contact angle is more than 10 degree the surface still is polluted by more than a monomolecular layer of fatty membranes. The thickness of the membrane at that time is said to be more than 1 nm. This phenomenon, when topcoat or a photo-orientation coat are printed, won't let the print layer adhere to the board and in some parts peel off and cause various abnormalities.
The UV/Ozone and plasma cleaning methods are the best to demonstrate the high degree of cleaning power. From table 3 & 4 it can be realized that both methods must be employed to have a less than monomolecular pollution layer (<1nm). In another word, it indicates that one part or all the surface of the substrata have been exposed as shown in Fig 1.

Table 4. Result of Steam Test of Fused Quartz and Contact angle of Water Droplet
Result Appearance of Condensation on polished Quartz Contact Angle
(Degree)
Monomolecular
layers of Contamination
Excellent Fringes Uniform Rainbow-like Fringes during both Condensation and Evaporation 4 <0.1
Good Fringes Uniform Fringes during Condensation, Irregular Fringes during Evaporation 4 <0.1
Poor Fringes Irregular Fringes during Condensation 4 <=0.1
Orange Peel No Colors, large Droplets visible, Transparent 5-10 0.1-1
Fog Translucent due to large number of small Droplets >10 >1

2. A hydrophilic and a hydrophobic glass

Wettability of a glass surface

Photo 1 is a glass that was cleaned off by UV/Ozone Cleaning after being washed by wet cleaning method. Photo 2 is a glass that is washed only by wet cleaning method, and photo 3 is an unprocessed glass.
Even the same glass, depending on the surface purity, the wettability can vary that much. Unlike water, ink etc are sticky and don't shrink easily, at out look it seems to be painted, but at hydrophobic condition one part rise, ink with bad adherent is the same.
At 25Ž temperature, the pure water surface tension is 72mN/m. The Wetting Tension Test Mixture with value of 30 to 73 are sold in the market. They can be used to examine the surface wetting tension of a processed surface in details.
Are you aware of the surface tension of the ink that you are using? In order to have ink with good adhesion power, the surface must be cleaned properly. By increasing the cleanliness of the surface, a good adhered printing is possible and there will be no need to add some expensive extra additives and reduce the quality of paint.

Photo 1. Glass that was cleaned off by
UV/Ozone Cleaning after being washed by wet cleaning method. A reagent of a wet ability was applied to the surface of the glass that spread
all over the glass.
Photo 2. The spread water on the glass after being washed by wet cleaning method only.
The water dappled instead of spreading.
Photo 3. The spread water on an unprocessed glass again the water dappled.


3. Caution on re-pollution

A glass surface with less than a single molecular layer of an organic pollution is the most clean surface conditions. Even in laboratories and for example in clean rooms, there are some particles of volatile organic compounds and sulfuric compounds floating in the air. If the clean glass in left neglected in there it will get re-polluted. It is said that the contact angle of highly clean surface will return to 20 degree after 30 to 60 minutes. It also can be said that an ultra clean surface can not be kept for a long time.

4. Particles free cleaning process

UV/Ozone cleaning method can reduce the surface pollution to less than a monomolecular and up to now the process shown in Fig 1 has been used to complete the process.
UV/Ozone cleaning has no effect on particles. In other hand, the wet cleaning is unable to remove the organic compounds entirely and leaves several molecule layers of fatty film on the surface( Fig 1). Those particles sucked into fatty films cannot be rinsed off.
Reversing the cleaning concept of completion of cleaning by UV/Ozone method, by adding the UV/Ozone cleaning prior to wet cleaning was born on the production.
As shown in schematic diagram 2, if after UV/Ozone cleaning that removes almost all the fatty films, the surface is washed by pure water, then all the free particles will be washed away and a particle free, organic compound free surface can be achieved. This method is not just used in Liquid crystal display elements but also in manufacturing the mask and the reticle.
Cleaning the optical board need a more sophisticated process
that will not be discussed in text.

Fig 2. Effect of reversal concept of combination
of UV/Ozone and wet cleaning

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