In the wet cleaning process for electronic materials, the so-called functional water for cleaning which is prepared by dissolving a minute amount of a specific gas or a specific chemical into ultra-pure water has been recognized to be useful and is being used more widely. Apparatus using a module having a built-in gas-permeating membrane in the portion for dissolution or in the portion for degassing and dissolution are widely used. These apparatuses are useful since the entire amount of the gas to be supplied can be dissolved as long as the concentration does not exceed the concentration of saturation when the main stream liquid is preliminarily treated by degassing.
However, an apparatus for dissolving a specific gas such as hydrogen or a specific gas and a specific chemical can provide a functional water containing the gas or the chemical in the prescribed concentration only when the main stream liquid is supplied at a constant flow rate. When the amount of supply of the main stream liquid changes due to some outside factors or due to saving of water, a problem arises in the practical operation that the concentration of the gas or the chemical in the produced functional water fluctuates.
During the period of time when the functional water is not used, occasionally, a small amount of the main stream liquid alone is continuously supplied without supplying the specific gas. This causes another problem in the practical operation in that, at the start of the period of time when the functional water is used, it takes a considerable time until the concentration of the dissolved gas reaches the prescribed value and is stabilized at this value after the start of the supply of the specific gas.
To stabilize the concentration of the dissolved gas, a feed back mechanism in which an output signal is received from the portion for measurement of the concentration placed at the down stream side of the dissolution apparatus and the amount of supply of the gas to be dissolved is controlled, is widely used. However, the desirable results cannot be obtained even when this mechanism is applied to preparation of the functional water. The reason is that a functional water having an undesired concentration is prepared at least during the period of time lag by the feed back and the hunting phenomenon in the concentration cannot be prevented even when the so-called PID control is conducted.
Therefore, an apparatus for continuous dissolution and a process for continuous dissolution which can maintain the quality of the functional water, i.e., the condition in which the concentrations of the gas and the chemical can be kept stable, even when the flow rate of the main stream water changes, has been desired.
The present invention has an object of providing an apparatus for continuous dissolution and a process for continuous dissolution which provides a solution having a constant concentration with stability even when the flow rate of the main stream liquid changes and can supply cleaning water or surface treatment water used for electronic materials which particularly requires a precisely clean surface without loss.
The present invention has another object of providing an apparatus for supplying water containing a dissolved gas which utilizes the apparatus for continuous dissolution providing a solution having a constant concentration with stability.