Liquid purification or separation – With preliminary chemical manufacture
Reexamination Certificate
1998-11-04
2002-06-25
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
With preliminary chemical manufacture
C210S188000, C210S194000, C210S198100, C210S900000
Reexamination Certificate
active
06409918
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a novel system for providing a relatively constant concentration of ozonated ultrapure water to remote points of use. The present invention may be used in applications requiring a supply of ozonated ultrapure water such as for use in cleaning semiconductor substrates.
In the prior art, silicon substrates for semiconductors and glass substrates for liquid crystals are cleaned using what is called RCA cleaning. RCA cleaning is a high temperature cleaning method that uses concentrated hydrogen peroxide-based chemicals. Examples of these chemicals include a water solution of hydrogen chloride and aqueous hydrogen peroxide, or a water solution of aqueous ammonia and aqueous hydrogen peroxide. This method is expensive due to the costs of purchasing chemicals, obtaining purified water for rinsing and for waste disposal. In addition, air pollution devices are required to decontaminate exhaust and steam produced from the process. This method is also costly in terms of its impact on the environment. The method uses a great deal of water, discharges liquid chemical waste and releases waste gases. In recent years, in order to reduce these drawbacks, wet cleaning processes have been reassessed.
For example, Japanese Laid-Open Patent Number 7-14817 discloses a cleaning method and device for washing silicon wafers. The method reduces chemical concentrations, shortens cleaning times, reduces the number of chemicals, makes recovery of wastes easier and reduces investment in equipment. The method involves placing the object to be washed horizontally inside a cleaning tank. A chemical cleaning step provides a continuous chemical flow from above. The object to be cleaned is rotated under the flow. A purified water cleaning step supplies purified water. The chemical cleaning step and the purified water cleaning step are performed sequentially in the same cleaning tank. Ozonated ultrapure water is proposed as the first chemical solution.
Furthermore, in Japanese Laid-Open Patent Number 8-316187, the present inventors have proposed a method for the efficient removal of metal pollutants and organic pollutants from the top of a semiconductor substrate. The method does not use chemicals such as highly concentrated hydrochloric acid or hydrogen peroxide. Rather, a cleaning method is proposed wherein the cleaning step adds ozone to an acidic aqueous solution containing chloride compounds. This makes processing the waste solution generated by the cleaning process easier.
Ozone dissolved in ultrapure water has a very strong oxidizing effect even at concentrations of only a few ppm. It is highly effective in removing organic substances and metals. It would be desirable to generate a source of ultrapure water containing a high concentration of dissolved ozone. However, the solubility of ozone in water is relatively small. In addition, autolysis of ozone into oxygen also occurs. These limitations prevent the preparation of a high concentration of dissolved ozone in water that can then be diluted to an appropriate concentration.
The realities of the cleaning process are such that the demand for ozonated ultrapure water is not always constant. It is difficult to supply ozonated purified water at a constant concentration that is independent of demand. Because of this problem, the prior art method continually supplies ozonated ultrapure water at a constant flow and constant concentration, even when the demand is low.
The present inventors have previously developed a closed loop system, wherein the makeup water of ultrapure water and the excess ozonated ultrapure water are collected in a tank. Ozone is dissolved in this water to compensate both for the effect of dilution by the makeup water and for the effect of autolysis. Ozone dissolved in ultrapure water is liable to autolyse even at normal temperatures. In this system, if the loop length is too long, the amount of supplemental ozone required to compensate for dilution and autolysis becomes large. In addition, a dissolved ozone concentration gradient develops between the upstream and downstream portions of the pipes. This uneven concentration of dissolved ozone is not desirable. Therefore, when long piping is used, this system is not adequate.
In the water supply system for ozonated ultrapure water previously developed by the present inventors, the ozone dissolving area and the ozonated purified water supply piping system are set up separately. In other words, upon increasing the dissolved ozone concentration to a specified concentration, gas/liquid separation is first conducted in the ozone dissolving area so that no undissolved ozone gas remains in the ultrapure water supply pipe. Alternatively, ozone is dissolved while the gas and liquid are separated using a gas permeable membrane. Ozonated ultrapurified water is then sent to the supply system. This method is adequate if the ozone dissolving area is placed near the point of use. This method is also adequate if ozonated ultrapurified water is transported from the ozone dissolving area to the point of use via a short piping system. However, when using a long piping system, the dissolved ozone concentration is constantly being reduced through autolysis. Thus, it is not possible, using this system, to have points of use remotely located from the ozone dissolving device.
OBJECTS AND SUMMARY OF THE INVENTION
In light of the above, it is an object of the present invention to overcome the limitations of the prior art.
The present inventors have discovered through intensive study, that using the ultrapure water supply pipe as the ozone dissolving area, allows for the dissolved ozone concentration to be maintained at a fairly constant concentration. The fairly constant concentration can be maintained even as autolysis occurs. The present invention was developed based on this finding.
It is an object of the present invention to provide an apparatus for supplying ozonated ultrapure water which can suppress the reduction of dissolved ozone concentration within a supply piping system and which can maintain a relatively constant ozone concentration within the supply system. This apparatus may be applied to ultrapure water supply/circulating systems having any length of piping.
It is another object of the present invention to provide an apparatus for supplying ozonated ultrapure water, including, in sequence, a main ultrapure water supply pipe for supplying ultrapure water, an ozonated gas supply device, a branch pipe off of the main ultrapure water supply pipe and a gas/liquid separation device just before the ultimate point of use.
It is another object of the present invention to provide ozonated ultrapure water to a plurality of points of use while only using a single ozonated gas supply device.
It is another object of the present invention to provide an economical way to provide a source of ozonated ultrapure water to a series of points of use having only a single ozone supply device.
It is another object of the present invention to provide a closed loop system for providing ozonated ultrapure water that conserves unused ozonated ultrapure water.
Briefly stated, the present invention provides an ozonated ultrapure water supply apparatus that prevents reduction of the dissolved ozone concentration in the supply pipe system and maintains the dissolved ozone at points of use along the supply system at desired concentrations. This permits the system to supply ozonated water on long pipes. An apparatus for supplying ozonated ultrapure water includes a sequential arrangement of a main ultrapure water supply pipe for supplying ultrapure water, an ozonated gas supplying device, a branch pipe feeding into a gas/liquid separation device and an ultimate point of use. The flow distance along the main pipe from the ozonated gas supplying device to the branch pipe is selected to achieve a desired level of ozonation.
According to an embodiment of the invention, there is provided an apparatus for supplying ozonated ultrapure water, comprising: a main ultrapure water supply
Ida Junichi
Mizuniwa Tetsuo
Morita Hiroshi
Cecil Terry K.
Drodge Joseph W.
Kurita Water Industries Ltd.
Morrison Law Firm
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