Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For halogen or halogen containing compound
Reexamination Certificate
1999-02-03
2001-10-23
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For halogen or halogen containing compound
C205S787500, C205S433000, C205S789000, C204S416000
Reexamination Certificate
active
06306284
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to an apparatus and a method for monitoring fluorine ions in an aqueous solution and more particularly, relates to an apparatus and a method for determining fluorine ion concentration in an aqueous solution wherein the pH value of the aqueous solution is first stabilized within a range between about 4 and about 10 in which the fluorine ion concentration is independent of the pH value of the aqueous solution.
BACKGROUND OF THE INVENTION
Various industrial fabrication processes often employ apparatus for dispensing pre-selected, discrete amounts or doses of a fluid medium for anyone of many different purposes. For example, in connection with semiconductor fabrication operations for producing semiconductor devices from silicon wafers, various processing equipment produces waste water of which the pH value must be controlled. This is usually accomplished by measuring the pH of the waste water and then adding doses of a buffer solution to the waste water from time to time such that the pH value of the waste water is maintained within a pre-selected, allowed range.
For instance, waste water which is produced by an ion implantation equipment usually contains a concentration of fluorine ions. Based on the regulations of environmental protection agency, such concentration can not be higher than 15 ppm when the waste water is released into the environment. It is therefore important that the concentration of fluorine ions in the waste water must be determined accurately. When determining the fluorine ion concentration, it was found that the fluorine ion concentration varies with the pH values of the waste water except in a relatively narrow range. For instance,
FIG. 1
shows the electrical potentials for aqueous solutions that contain different concentrations of fluorine ions plotted against the pH values of the aqueous solutions. As shown in
FIG. 1
, three different fluorine ion concentrations, i.e., 2 mg/l, 5 mg/l and 10 mg/l plotted. For all three concentrations, the electrical potential is not constant at various pH levels except within a relatively narrow range of between about 4 and about 10.
The measurement of fluorine ion concentrations in waste water therefore presents a challenge in that the pH value of the waste water must first be adjusted to within the range of pH where the electrical potentials are constant so that the fluorine ion concentration can be reliably determined.
Conventionally, a buffer solution which is substantially of alkaline nature is added to the aqueous solution to stabilize the chemistry prior to a pH determination. For instance, a 1% alkaline solution of CH
3
COONa has been utilized as the buffer solution. The buffer solution is normally added to the waste water at a constant rate regardless whether it is necessary or whether the waste water is of an acidic nature. The conventional apparatus for dispensing such buffer solutions is therefore inadequate in providing a stabilized aqueous solution for the detection of fluorine ion concentrations. This is particularly the case when a waste water is already alkaline and the addition of the buffer solution further increases its alkalinity.
It is therefore an object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution that is capable of producing accurate results.
It is a further object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution that is capable of making pH adjustment in the solution prior to the determination of the fluorine ion concentration.
It is another further object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution which includes a pH controller, an acid dispenser, a base dispenser and a fluorine ion sensor.
It is still another object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution which is capable of adjusting automatically a pH value of the solution to within a 4~10 range before determining the concentration of fluorine ions.
It is yet another object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution which incorporates an acid dispenser that dispenses H
2
SO
4
and a base dispenser which dispenses NaOH.
It is still another further object of the present invention to provide a method for determining concentration of fluorine ions in an aqueous solution by first adding H
2
SO
4
or NaOH to the solution such that the pH value of the solution falls within the range of 4~10.
It is yet another further object of the present invention to provide a method for determining concentration of fluorine ions in an aqueous solution by adding a buffer solution, an acid or a base into the aqueous solution until a pH value between 4 and 10 is obtained.
SUMMARY OF THE INVENTION
In accordance with the present invention, an apparatus and a method for determining concentration of fluorine ions in an aqueous solution are provided.
In a preferred embodiment, an apparatus for determining concentration of fluorine ions in aqueous solutions is provided which includes a pH adjustment tank for holding an aqueous solution, a pH sensor for sensing a pH value of the aqueous solution in the tank and sending a first signal to a pH controller, a pH controller for receiving the first signal, comparing to a stored pH value, sending out a second signal when the first signal is larger than the stored pH value or sending out a third signal when the first signal is smaller than the stored pH value, an acid dispenser which receives the second signal and delivers an acid to the aqueous solution in the tank, a base dispenser which receives the third signal and delivers a base to the aqueous solution in the tank, and a fluorine ion sensor for sensing a fluorine ion concentration in the aqueous solution which has a modified pH value in the pH adjustment tank.
The apparatus for determining concentration of fluorine ions in aqueous solutions may further include a buffer solution dispenser for delivering a buffer solution to the aqueous solution in the pH adjustment tank. The stored pH value in the pH controller may be in a range between about 4 and about 10. The aqueous solution may be a waste water flow from a semiconductor fabrication facility. A second signal is sent by the pH controller to the acid dispenser to when the first signal is indicative of a pH value larger than about 10. A third signal is sent by the pH controller to the base dispenser when the first signal is indicative of a pH value smaller than about 4. The acid dispenser delivers H
2
SO
4
to the aqueous solution in the tank. The base dispenser delivers NaOH to the aqueous solution in the tank. The modified pH value in the tank is in the range between about 4 and about 10. The apparatus may further include a recording device for recording the fluorine ion concentration detected by the fluorine ion sensor.
The present invention is further directed to a method for determining concentration of fluorine ions in aqueous solutions which can be carried out by the operating steps of first filling a pH adjustment tank with an aqueous solution, sensing a pH value of the aqueous solution in the tank by a pH sensor and sending a first signal to a pH controller, receiving a first signal in the pH controller for comparing to a stored pH value, sending a second signal from the pH controller to an acid dispenser when the first signal is larger than the stored pH value for delivering an acid to the aqueous solution, sending a third signal from the pH controller to a base dispenser when the first signal is smaller than the stored pH value for delivering a base to the aqueous sol
Huang Wei-tien
Yang Feng-Yi
Noguerola Alex
Taiwan Semiconductor Manufacturing Company , Ltd.
Tung T.
Tung & Associates
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