34  US 7,504,193 B2  33  tion through an ion exchange filter, a step of adding a photo› acid generator and, if desired, an organic basic compound, a  surfactant and the like to the solution to prepare a mixed  solution, and a step of filtering the mixed solution through an  insoluble colloid removing filter.  concentration is, in terms of the solid content concentration of  all resist components, preferably from 2 to 30 mass %, more  preferably from 3 to 25 mass %.  Preferred examples of the dissolution medium used here  include ethylene dichloride, cyclohexanone, cyclopentanone,  2-heptanone, y-butyrolactone, methyl ethyl ketone, ethylene  glycol monomethyl ether, ethylene glycol monoethyl ether,  2-methoxyethyl acetate, ethylene glycol monoethyl ether  acetate, propylene glycol monomethyl ether, propylene gly› col monomethyl ether acetate, toluene, ethyl acetate, methyl  lactate, ethyl lactate, methyl methoxypropionate, ethyl ethox› ypropionate, methyl pyruvate, ethyl pyruvate, propyl pyru› vate, N,N-dimethylformamide, dimethylsulfoxide, N-meth› ylpyrrolidone  and  tetrahydrofuran.  These  dissolution  mediums are used individually or in combination of two or  more thereof. As for the solvent, those containing a propylene  glycol monomethyl ether acetate are preferred, and those  containing a propylene glycol monomethyl ether are more  preferred.  4. Amphoteric Ion Compound  The photosensitive resin composition of the present inven› tion preferably further contains an amphoteric ion compound.  The amphoteric ion compound as used herein indicates a  compound containing both a cation moiety and an anion  moiety within one molecule. Specific examples thereof  include, but are not limited to, amphoteric ions of amino acid,  such as alanine, phenylalanine, asparagine, glycine and  valine.  The amount of the amphoteric ion compound added is  preferably from 3 to 70 mo! %, more preferably from 5 to 50  mo!%, still more preferably from 7 to 40 mo!%, based on the  component (Bl). By adding the component (Y), the sensitiv› ity and contrast are more enhanced.  In the resist composition of the present invention, the gen- eration of particle (particulate aggregate) ascribable to fluc› tuation of sensitivity is preferably reduced as much as pos› sible. Generally, as the metal content is larger, the number of  particles increases because the resin aggregates in the solu› tion by using the metal as the core. Accordingly, in the com› position of the present invention, the content of metal com› ponent such as Na, K, Ca, Fe and Mg contained in the resin as  the component (A) is preferably small. More specifically, the  content of each metal species contained in the resin is pref› erably 300 ppb or less, more preferably 200 ppb or less, still  more preferably 100 ppb or less.  In the case where the metal impurity content in the resin is  large, the number of particles readily increases, because the  resin aggregates in the solvent by using the metal, impurity as  the core along the change in aging. The aggregation of resin  also brings about substantial change in the components con› stituting the resist composition and this is presumed to cause  change in the sensitivity.  Examples of the metal impurity contained in the resist  composition include Na, K. Ca, Fe, Mg, Mn, Pd, Ni, Zn, Pt,  Ag and Cu.  As for the method of decreasing the content of metal impu› rities contained in the resist composition, that is, decreasing  the number of particles, for example, a method of dissolving  the resin in a solvent to prepare a solution and filtering the  solution through an ion exchange filter, and also cleaning by  liquid separation and treatment with acidic ion exchange  resin or chelate resin are known.  The method for decreasing the metal impurity content in  the chemical amplification-type resist composition of the  present invention is preferably a method comprising a step of  filtering a solution containing the resin of the present inven- In the production or the like of a precision integrated circuit  device, the step of forming a pattern on a resist film is per› formed by coating the positive resist composition of the  present invention on a substrate (for example, silicon/silicon  dioxide-coated substrate, glass substrate, ITO substrate or  quartz/chromium oxide-coated substrate), drying it to form a  Also, the organic antireflection film may be a commer› cially available organic antireflection film such as DUV-30  Series, DUV-40 Series (produced by Brewer Science, Inc.),  AR-2, AR-3 andAR-5 (produced by Shipley Co., Ltd.).  The ion exchange filter which can be used is preferably a  cation exchange filter with the ion exchange group being  fixed to a polyethylene-made porous membrane or a polypro› pylene-made porous membrane, and examples thereof  include Ion Clean (produced by Nihon Pall Ltd.) and Ion  Clean AQ (produced by Nihon Pall Ltd.). The filtration rate  through the ion exchange filter is preferably from 500 to  10,000 ml/min/m 2 .  The insoluble colloid removing filter is  preferably a synthetic resin-made filter and examples thereof  include Mykrolis Optimizer DEV-16/40 (polyethylene filter,  produced by Mykrolis Corp.), Microguard Minichem (poly› ethylene filter, produced by Mykrolis Corp.), Enflon (poly› tetrafluoroethylene filter, produced by Nihon Pall Ltd.), Ulti› pore N66 (nylon 66 filter, produced by Nihon Pall Ltd.), Zeta  Plus ( cellulose filter, produced by Cuna) and Electropor II  (nylon 66 filter, produced by Cuna). Also, in the case of  performing the filtration step after adding an ionic compound  such as photoacid generator, the insoluble colloid removing  filter is preferably a filter not having ion exchanging ability.  The ion exchange-type filter and the insoluble colloid remov› ing filter each preferably has a pore size of 0.01 to 0.5 µm,  more preferably from 0.01 to 0.1 µm.  The resist composition of the present invention is coated on  a substrate to form a thin film. The thickness ofthis resist film  is preferably from 0.05 to 4.0 µm.  In the present invention, a commercially available inor› ganic or organic antireflection film may be used, if desired.  Furthermore, an antireflection film may be used by coating it  as a lower layer of the resist.  The antireflection film used as the lower layer of the resist  may be either an inorganic film such as titanium, titanium  dioxide, titanium nitride, chromium oxide, carbon and amor› phous silicon, or an organic film comprising a light absorbent  and a polymer material. The former requires equipment for  the film formation, such as vacuum deposition apparatus,  CVD apparatus and sputtering apparatus. Examples of the  organic antireflection film include a film comprising a diphe› nylamine derivative and formaldehyde-modified melamine  resin condensate, an alkali-soluble resin and a light absorbent  described in JP-B-7-69611 (the term "JP-B" as used herein  means an "examined Japanese patent publication"), a reac› tion product of a maleic anhydride copolymer and a diamine› type light absorbent described in U.S. Pat. No. 5,294,680, a  film comprising a resin binder and a methylolmelamine- based heat crosslinking agent described in JP-A-6-118631,  an acrylic resin-type antireflection film containing a carboxy› lic acid group, an epoxy group and a light absorbing group  within the same molecule described in JP-A-6-118656, a film  comprising methylolmelamine and a benzophenone-based  light absorbent described in JP-A-8-87115, and a film  obtained by adding a low molecular light absorbent to a  polyvinyl alcohol resin described in JP-A-8-179509. 