Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1998-12-01
2003-11-18
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C423S531000, C521S033000, C525S344000
Reexamination Certificate
active
06649070
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for utilizing a sulfuric acid/peroxide liquid mixture. More particularly, it relates to a method for utilizing a sulfuric acid/peroxide liquid mixture that can be used with advantage as a method for re-utilization of a used-up sulfuric acid/peroxide liquid mixture.
2. Description of the Related Art
A sulfuric acid/peroxide liquid mixture is so far used in large quantities as a rinsing liquid for a semiconductor wafer mainly in semiconductor manufacturing processing in the form of a sulfuric acid/aqueous hydrogen peroxide or a sulfuric acid/ozone liquid mixture.
This sulfuric acid/peroxide liquid mixture is a representative rinsing solution used mainly for exfoliating a photoresist on a wafer or removing particles of metal, in particular heavy metal. This liquid mixture is used in significantly larger quantities than in the case of other rinsing solutions, such as ammonia/aqueous hydrogen peroxide, hydrochloric acid/aqueous hydrogen peroxide or dilute fluoric acid.
The result is that a large quantity of a waste liquid is generated after use. The waste solution of this liquid mixture presents the following problems in connection with disposal:
(i) If the waste liquid is mixed with water, significant heat evolution occurs (in connection with the heat of hydration of sulfuric acid);
(ii) peroxo acid (Caro's acid) exhibiting strong oxidating power is left; and
(iii) aqueous hydrogen oxide is left such that vigorous foaming (oxygen yielding) is likely to occur as a result of impacts or mixing of impurities.
Due to the above-described problems of the heat evolution, abrupt rise in pressure caused by foaming, or to possibility of deterioration of the equipment due to oxidation, it has been retained to be difficult, in view of safety or maintenance, to transport the waste solution directly to purify it to sulfuric acid. Thus, according to a customary process, the waste solution is diluted and cooled with a large quantity of water, neutralized with an alkali such as slaked lime, agglomerated with an inorganic flocculent and with a high molecular flocculent, in this order, and finally allowed to flow into an effluent as waste water.
Since the amount of water or chemical agents, such as flocculants, are used in an amount which ultimately is tens to hundreds of that of the original sulfuric acid/peroxide liquid mixture, the processing cost, that is the equipment or maintenance cost, cost of the chemical agents or of disposal of sludge, is increased. If this is to be deemed to be unavoidable, the amount of the waste material is also increased prohibitively.
It has so far been felt that the waste sulfuric acid/peroxide liquid mixture cannot be disposed of except by causing it to flow into an effluent, without the possibility of its re-utilization.
SUMMARY OF THE INVENTION
In view that the disposal of the waste sulfuric acid/peroxide liquid mixture has many disadvantages, as described above, the present inventors have made attempts to develop a technique for efficiently re-utilizing the waste liquid and for suppressing the problem relating to its disposal. The present invention, completed in the course of these researches, is aimed to develop a technique of utilizing the sulfuric acid/peroxide liquid mixture, such as its waste liquid, and to provide a method for utilizing the waste liquid and for suppressing the problem relating to its disposal.
According to the present invention, there is provided a method for utilizing a sulfuric acid/peroxide liquid mixture including decomposing peroxide in the sulfuric acid/peroxide liquid mixture to utilize the liquid mixture as sulfuric acid.
Since the peroxide in the sulfuric acid/peroxide liquid mixture is decomposed to utilize the resulting liquid mixture as sulfuric acid, a waste liquid of the sulfuric acid/peroxide liquid mixture can be effectively re-utilized if the waste liquid is processed by this technique. This enables effective utilization of the waste liquid to suppress the problem raised in connection with disposal. The present invention, which provides an effective technique for the waste liquid of the sulfuric acid/peroxide liquid mixture in which sulfuric acid/peroxide is left as a component, can be applied to a sulfuric acid/peroxide liquid mixture other than a waste sulfuric acid/peroxide liquid mixture as a method for utilizing the sulfuric acid/peroxide liquid mixture if it is desired to utilize it as sulfuric acid.
The present invention is completed on the basis of the following information obtained by the present inventors. That is, the present inventors have conducted perseverant investigations to overcome the aforementioned problem and accordingly have found that, on adding a decomposition accelerator, such as nitric acid, to a sulfuric acid/peroxide liquid mixture, such as its waste liquid material produced from, for example, the semiconductor manufacturing process, the waste liquid material can be utilized as regenerated sulfuric acid. This finding has led to completion of the present invention.
According to the present invention, the novel method for utilizing the sulfuric acid/peroxide liquid mixture is provided whereby it is now possible to utilize the waste liquid material of the sulfuric acid/peroxide liquid mixture as sulfuric acid. The result is that the waste liquid need not be disposed of to render it possible to make effective utilization of resources and to reduce the amount of the waste material, such as effluent water. Moreover, with the present invention, the peroxide can be completely decomposed without lowering the concentration of sulfuric acid in the liquid waste material. The resultant product can be as a modifier, such as a sulfonating agent or a hydrolysis accelerator, for a variety of compounds, such as polymers. The decomposed liquid obtained on decomposing the peroxide can be used as a modifier for the waste resin material, whereby the waste material can be converted to a high added-value material to contribute to maintenance of global environment through reduction of the waste material and effective utilization of resources.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be explained with reference to a preferred embodiment which is merely illustrative and is not intended to limit the scope of the invention.
The present invention exploits the sulfuric acid/peroxide liquid mixture as sulfuric acid by decomposing the peroxide in the sulfuric acid/peroxide liquid mixture and finds most effective application in case the sulfuric acid/peroxide liquid mixture represents a waste liquid resulting from various fields of use of the sulfuric acid/peroxide liquid mixture because the various problems caused by disposal can then be obviated.
According to the present invention, the sulfuric acid contained in the sulfuric acid/peroxide liquid mixture has a concentration not lower than 70 wt %, preferably not lower than 80 wt %. The reason is that, if the sulfuric acid concentration is less than 70 wt %, peroxides are not liable to be decomposed even on addition of a decomposition accelerator as later explained.
There is no particular limitation to the fields of application to which were put the sulfuric acid/peroxide liquid mixtures in use. In general, the sulfuric acid/peroxide liquid mixtures are preferably derived from a variety of rinsing processes. For example, the sulfuric acid/peroxide liquid mixtures used as a rinsing liquid for various parts or products in the electrical field, in particular those used in the production process of semiconductors for resist exfoliation or as a variety of rinsing liquids, are preferred because the sulfuric acid in the waste sulfuric acid/peroxide liquid mixture used in the semiconductor manufacturing process usually has a concentration of 70 wt % or higher.
The peroxides in the sulfuric acid/peroxide liquid mixture may be enumerated by hydrogen peroxide, ozone and peroxo acid (Caro's acid). It is noted that peroxo sulfuric acid is usually generated by injectin
Cintins Ivars
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
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