In the manufacture of liquid crystal panel elements and semiconductor elements such as IC and LSI, a chemically vapor-deposited (CVD) electroconductive metal film and an insulating film such as SiO2 film are firstly prepared on a substrate such as silicon wafer or glass. After that, a photoresist is uniformly applied on the electroconductive metal film and the insulating film and then selectively exposed to light and developed to form a resist pattern. Then the substrate is selectively subjected to a dry etching process using the resist pattern as a mask whereupon a fine circuit is formed. After that, photoresist film residue and etching residue which are no longer necessary are washed and removed using a stripping liquid.
Although some stripping effects are achieved when an aqueous solution of sodium hydroxide or a common organic solvent is used solely as the stripping liquid, the resulting stripping effect is not sufficient. Accordingly, various stripping liquids for the photoresist have been proposed already for enhancing the stripping property. One of the common methods among them is a method where a solution of quaternary ammonium hydroxide such as tetramethylammonium hydroxide (TMAH) is used.
In that method, preparation of a stripping liquid is usually carried out in such a manner that a solution prepared from commercially available water-containing crystals of quaternary ammonium hydroxide or a commercially available aqueous solution of quaternary ammonium hydroxide is diluted with an organic solvent such as dimethyl sulfoxide (DMSO) or 3-methoxy-3-methyl-1-butanol (MMB) down to a desired concentration.
On the other hand, it has been known that the stripping property of a stripping liquid is also dependent upon the amount of water contained in the stripping liquid and that the lower the water content, the higher the stripping property. Since a stripping liquid is prepared by diluting a quaternary ammonium hydroxide solution with an organic solvent as mentioned above, it is necessary for lowering the water content in the stripping liquid that a quaternary ammonium hydroxide solution is concentrated so as to make the water content therein low.
With regard to a method for concentrating a quaternary ammonium hydroxide solution to make the water content low, there is a known method whereby a pentahydrate of TMAH (ratio by weight of TMAH to water is 1:1) is dissolved in methanol and concentrated in vacuo according to a batch system and also a method whereby a DMSO solution containing TMAH pentahydrate is dried by adding molecular sieves thereto (please see Patent Documents 1 and 2).
However, in the former method, there are disadvantages that heating for long time is necessary whereby TMAH is apt to be decomposed and that, since the product obtained after the concentration is a solid, its handling is difficult. On the other hand, in the latter method, there are disadvantages that, due to the use of molecular sieves, the cost is high and the operation is complicated. Further, since the presence of fine particles in the concentrated solution and in the stripping liquid is not preferred, there is a disadvantage that the molecular sieves must be removed after drying. Furthermore, the water content in the concentrated solution prepared by such a method is not satisfactorily low. Accordingly, the conventional process for producing a concentrated solution of quaternary ammonium hydroxide still needs to be improved.