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.
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 1DGeneral organic contaminants |
|---|
| Fat on the skin, FluxCosmetic greases, Resin additives, Parting agent,
Wax Machine oil, Lubricant, Solvent and its vapor, Cutting oil, Sulfide etc |
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.
| Table2DVarious 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 3DComparison 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 |
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 |
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 25Ž 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.
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|---|---|---|
| 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. |
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.
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.
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|---|
| Fig 2. Effect of reversal concept of combination of UV/Ozone and wet cleaning |
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