Gas separation: apparatus – Degasifying means for liquid – With gas contacting means
Patent
1994-12-12
1998-06-16
Smith, Duane S.
Gas separation: apparatus
Degasifying means for liquid
With gas contacting means
95263, 95265, 96203, 210220, B01D 1900
Patent
active
057663211
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a dissolved oxygen reducing apparatus wherein the content of oxygen dissolved in liquids is reduced.
BACKGROUND ART
In semiconductor manufacturing processes, for example, pure water is used in large amounts for the cleaning treatment of semiconductor devices. Pure water is intended to mean water which contains a small amount of impurities. Actually, a small amount of oxygen is contained in pure water as an impurity.
If oxygen is present, even at a very low concentration, in pure water used for cleaning treatment, a native oxide film is formed on the surfaces of the semiconductor device, thereby causing the device characteristics to deteriorate. For pure water used in semiconductor manufacturing processes, or for pure water for semiconductors, there is a demand to reduce the concentration of dissolved oxygen to a level not higher than 10 ppb.
Hitherto, vacuum degassing apparatuses have been frequently employed as a dissolved oxygen reducing apparatus. However, such apparatuses are disadvantageous in that the dissolved oxygen can be reduced to a concentration as high as approximately 100 ppb, that the apparatus is large in size, and that power costs are great, resulting in economically poor situation. In recent years, attention has been directed to a dissolved oxygen reducing apparatus using an inert gas bubbling method, which is free from the above disadvantages.
In the inert gas bubbling method, while untreated water (water to be treated) flows down in a bubbling vessel, an inert gas is jetted in the form of bubbles from the lower portion of the bubbling vessel, so that the water and the inert gas are subjected to gas-liquid contact to substitute the oxygen gas in the water for the inert gas, thereby eliminating oxygen from the water. The bubbling vessel and the pipes through which the liquid is passed are formed of polyvinyl chloride (PVC) which is a kind of plastic. PVC is excellent in its ease of availability, economy, mechanical strength, corrosion resistance and the like, and it has been used for almost all the general-purpose piping for water.
According to the inert gas bubbling method, the concentration of dissolved oxygen in a liquid can be readily reduced to not higher than 100 ppb and may be reduced to approximately 10 ppb, but has never been reduced to not higher than 10 ppb.
In recent semiconductor manufacturing processes, there are some cases where it is desirable to utilize pure water having a concentration of dissolved oxygen of not higher than 10 ppb, in specific steps such as cleaning of the surfaces of a silicon wafer.
In view of the foregoing, the inventors of the present invention made extensive studies and, as a result, found unexpected problems. To describe in detail, where pure water having an amount of dissolved oxygen of not higher than 10 ppb is prepared and supplied, it has been found that oxygen in the air permeates through the walls of the PVC bubbling vessel and the walls of the PVC pipes through which water after the bubbling treatment is fed, resulting in an increased amount of dissolved oxygen.
As stated hereinabove, PVC is readily available and is used in almost all general-purpose piping for water. Moreover, the bubbling vessel is arranged to have a pressure higher than atmospheric pressure, so that it has never been suspected that oxygen permeates from the air which is lower in pressure than within the bubbling vessel. In fact, with pure water having a dissolved oxygen concentration of approximately 100 ppb, oxygen permeating from the air into water has not been investigated. However, since oxygen in the air actually enters water, the concentration of dissolved oxygen in liquids cannot be reduced to a level of not higher than 10 ppb according to the known inert gas bubbling method.
It is therefore an object of the invention to provide a dissolved oxygen reducing apparatus which is able to provide a liquid containing only a very small amount of dissolved oxygen.
SUMMARY OF THE INVENTION
The dissolved oxygen reducing
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Ishihara Yoshio
Matsumoto Koh
Yamane Sumiyo
Yamazaki Hiroshi
Nippon Sanso Corporation
Smith Duane S.
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