144  143  US  9,104,101 B2  carbon atoms, and most preferably 1 to 4 carbon atoms. As the  alkyl group, a partially or completely halogenated alkyl group  (hereinafter, sometimes referred to as a "halogenated alkyl  group") is particularly desirable. A "partially halogenated  alkyl group" refers to an alkyl group in which part of the  hydrogen atoms are each substituted with a halogen atom,  whereas a "completely halogenated alkyl group" refers to an  alkyl group in which all of the hydrogen atoms are substituted  with halogen atoms. Examples of the halogen atom include a  fluorine atom, chlorine atom, bromine atom or iodine atom,  and a fluorine atom is particularly desirable. In other words,  the halogenated alkyl group is preferably a fluorinated alkyl  group.  Each of Y" and Z" independently represents a linear or  branched alkyl group in which at least one hydrogen atom has  been substituted with a fluorine atom, wherein the alkyl group  contains 1 to 10 carbon atoms, preferably 1 to 7 carbon atoms,  and more preferably 1 to 3 carbon atoms.  The smaller the number of carbon atoms in the alkylene  group for X" or the alkyl group for Y" and Z" within the  above-mentioned ranges of the number of carbon atoms, the  more the solubility in a resist solvent is improved, and there› fore a smaller number of carbon atoms is preferred.  Further, in the alkylene group for X" or the alkyl group for  Y" and Z", it is preferable that the number of hydrogen atoms  substituted with fluorine atoms is as large as possible, because  the acid strength increases and the transparency to high  energy radiation of 200 nm or less and electron beams is  improved.  The proportion of fluorine atoms within the alkylene group  or alkyl group, namely the fluorination ratio, is preferably  within a range from 70 to 100%, and more preferably from 90  to 100%. A perfluoroalkylene or perfluoroalkyl group in  which all the hydrogen atoms are substituted with fluorine  atoms is the most desirable.  As the onium salt acid generator, onium salts of the above  general formula (b-1) or (b-2) in which the anion moiety  (R$^{4}$"SO$_{3 }$ -) has been substituted with  Ra-coo- (wherein  Ra  represents an alkyl group or a fluorinated alkyl group) (and in  which the cation moiety is the same as that of general formula  (b-1) or (b-2)) may also be used.  Examples of  Ra  in the above formula include the same  groups as those listed above for R 4 ".  Specific examples of  Ra  -Coo- include a trifluoroacetate  ion, an acetate ion, and a 1-adamantanecarboxylate ion.  In the present description, an oxime sulfonate acid genera› tor is a compound having at least one group represented by  general formula (B-1) shown below, and has a feature of  generating acid upon irradiation (exposure). Such oxime sul› fonate acid generators are widely used for chemically ampli› fied resist compositions, and any of these known compounds  may be selected as appropriate.  Preferred examples of the oxime sulfonate acid generator  include compounds represented by general formula (B-2) or  (B-3) shown below.  In formula (B-2), R 33  represents a cyano group, an alkyl  group having no substituent or a halogenated alkyl group, R 34  represents an aryl group, and R$^{35 }$represents an alkyl group  having no substituent or a halogenated alkyl group.  In formula (B-1 ), each of R$^{31 }$and R$^{32 }$independently repre› sents an organic group.  The organic group for R$^{31 }$and R$^{32 }$refers to a group con› taining a carbon atom, and may also include atoms other than  the carbon atom (such as a hydrogen atom, oxygen atom,  nitrogen atom, sulfur atom or halogen atom (such as a fluorine  atom or chlorine atom) or the like).  The organic group for R$^{31 }$is preferably a linear, branched  or cyclic alkyl group, or an aryl group. The alkyl group or aryl  group may have a substituent. There are no particular limita› tions on the substituent, and examples include a fluorine atom  and a linear, branched or cyclic alkyl group having 1 to 6 60  carbon atoms. The expression that the alkyl group or aryl  group "may have a substituent" means that part or all of the  hydrogen atoms of the alkyl group or aryl group may each be  substituted with a substituent.  The alkyl group for R 31  preferably has 1 to 20 carbon 65  atoms, more preferably 1 to 10 carbon atoms, still more  preferably 1 to 8 carbon atoms, still more preferably 1 to 6  In formula (B-3), R 36  represents a cyano group, an alkyl  group having no substituent or a halogenated alkyl group, R 37  represents a divalent or trivalent aromatic hydrocarbon group,  R 38  represents an alkyl group having no substituent or a  halogenated alkyl group, and p" represents 2 or 3.  In general formula (B-2), the alkyl group having no sub› stituent or the halogenated alkyl group for R$^{33 }$preferably has  1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms,  and most preferably 1 to 6 carbon atoms.  [Chemical Formula 75]  [Chemical Formula 73]  (B-3)  (B-1)  -C=N-0-S0$_{2}$ -R 31  I  R32  (B-2)  [Chemical Formula 74]  As the organic group for R 32 , a cyano group, an alkyl group  of 1 to 8 carbon atoms having no substituent, or a fluorinated  alkyl group of 1 to 8 carbon atoms is particularly desirable.  The organic group for R$^{32 }$is preferably a linear, branched  or cyclic alkyl group, an aryl group, or a cyano group.  Examples of the alkyl group and the aryl group for R 32  include the same alkyl groups and aryl groups as those  described above for R 31 .  As the organic group for R 31 , an alkyl group of 1 to 4  carbon atoms which has no substituent, or a fluorinated alkyl  group of 1 to 4 carbon atoms is particularly desirable.  The aryl group for R$^{31 }$preferably has 4 to 20 carbon atoms,  more preferably 4 to 10 carbon atoms, and most preferably 6  to 10 carbon atoms. As the aryl group, a partially or com› pletely halogenated aryl group is particularly desirable. A  "partially halogenated aryl group" refers to an aryl group in  which part of the hydrogen atoms are each substituted with a  halogen atom, whereas a "completely halogenated aryl  group" refers to an aryl group in which all ofhydrogen atoms  are substituted with halogen atoms. 