US 8,298,748 B2  12  11  The aliphatic hydrocarbon group may be saturated or  unsaturated. In general, the aliphatic hydrocarbon group is  preferably saturated.  R represents a hydrogen atom, a methyl group or a trifluo› romethyl group,  RA  represents a hydrogen atom or a methyl  group, andR $^{6 }$represents a single bond or an alkylene group of  1 to 3 carbon atoms.  As the structural unit ( a0-1 ), one type of structural unit may  be used alone, or two or more types of structural units may be  used in combination.  In the component (Al’), the amount of the structural unit  (a0-1) based on the combined total of all structural units  constituting the component (Al’) is preferably 1 to 40 mmol  %, more preferably 5 to 35 mo! %, and still more preferably  5 to 30 mmol %. By ensuring the above-mentioned range, the  lithography properties are improved. Further, solubility in an  organic solvent can be increased.  The structural unit (a0-2) is a structural unit that generates  acid upon exposure and contains a group represented by  general formula (a0-2’). The main chain of the structural unit  (a0-2) is not particularly limited. By including the structural  unit (a0-2), outgas can be suppressed. Further, the acid gen› erator can be uniformly distributed within the resist film,  which results in the improvement of the lithography proper› ties. In addition, lowering of Tg can be suppressed. As the  main chain of the monomer for deriving the structural unit  ( a0-2), any one which can be polymerized with the monomers  for deriving the structural units ( a5-l) and ( a0-1) can be used,  e.g., a main chain having a (meth)acryloyl group or a vinyl  group.  Firstly, the cation moiety excluding the anion moiety in  general formula (a0-2’), i.e., the portion excluding x- will be  described.  <<Cation Moiety>>  As specific examples of the aliphatic hydrocarbon group, a  linear or branched aliphatic hydrocarbon group, and an ali› phatic hydrocarbon group confining a ring in the structure  thereof can be given.  The linear, or branched aliphatic hydrocarbon group pref› erably has 1 to 10 carbon atoms, more preferably 1 to 8, still  10  more preferably 1 to 5, and most preferably 1 or 2.  As the linear aliphatic hydrocarbon group, a linear alkylene  group is preferable. Specific examples thereof include a  methylene  group  [----CH 2 -],  an  ethylene  group  [-(CH 2 ) 2 -], a trimethylene group [-(CH 2 )$_{3}$ -],  a tetram› ethylene group [-(CH 2 )$_{4}$ -]  and a pentamethylene group  [-(CH2)s-l- In general formula (a0-2’), R$^{7 }$is the same as defined for R in general formula ( a0-1 ), and is preferably a hydrogen atom  or a methyl group.  Further, R $^{7 }$may be the same or different from R $^{7 }$in general  formula (a0-1).  In general formula (a0-2’), Y represents a divalent linking  group.  Preferable examples ofY include a divalent hydrocarbon  group which may have a substituent, and a divalent linking  group containing a hetero atom.  A hydrocarbon "has a substituent" means that part or all of  the hydrogen atoms within the hydrocarbon group is substi› tuted with groups or atoms other than hydrogen.  The hydrocarbon group may be either an aliphatic hydro› carbon group or an aromatic hydrocarbon group. An "ali- phatic hydrocarbon group" refers to a hydrocarbon group that  has no aromaticity.  As the branched aliphatic hydrocarbon group, branched  alkylene groups are preferable, and specific examples include  various alkylalkylene groups, including alkylmethylene  groups  such  as  ----CH(CH$_{3}$)-,  ----CH(CH 2 CH$_{3}$)-,  ----C(CH$_{3}$ ) 2 -,  -C(CH$_{3}$)(CH 2 CH$_{3}$)-,  -C(CH$_{3}$ )  (CH 2 CH 2 CH$_{3}$)-, and ----C(CH 2 CH$_{3}$ ) 2 -; alkylethylene  groups such as ----CH(CH$_{3}$)CH 2 -,  ----CH(CH$_{3}$)CH(CH$_{3}$)-,  ----C(CH$_{3}$ ) 2 CH 2 -,  -CH(CH 2 CH$_{3}$)CH 2 -,  and  ----C(CH 2 CH$_{3}$ ) 2 ----CH 2 -; alkyltrimethylene groups such as  ----CH(CH$_{3}$)CH 2 CH 2 -, and ----CH 2 CH(CH$_{3}$)CH 2 -; and  alkyltetramethylene  groups  such  as  -CH(CH$_{3}$ )  CH 2 CH 2 CH 2 -, and ----CH 2 CH(CH$_{3}$)CH 2 CH 2 -. As the  alkyl group within the alkylalkylene group, a linear alkyl  group of 1 to 5 carbon atoms is preferable.  The linear or branched aliphatic hydrocarbon group  (chain-like aliphatic hydrocarbon group) may or may not  have a substituent. Examples of substituents include a fluo- rine atom, a fluorinated alkyl group of 1 to 5 carbon atoms,  and an oxygen atom  (=0).  As examples of the hydrocarbon group containing a ring in  the structure thereof, a cyclic aliphatic hydrocarbon group (a  group in which two hydrogen atoms have been removed from  an aliphatic hydrocarbon ring), and a group in which the  cyclic aliphatic hydrocarbon group is bonded to the terminal  of the aforementioned chain-like aliphatic hydrocarbon  group or interposed within the aforementioned chain-like  aliphatic hydrocarbon group, can be given.  The cyclic aliphatic hydrocarbon group preferably has 3 to  20 carbon atoms, and more preferably 3 to 12 carbon atoms.  The cyclic aliphatic hydrocarbon group may be either a  polycyclic group or a monocyclic group. As the monocyclic  group, a group in which two hydrogen atoms have been  removed from a monocycloalkane of 3 to 6 carbon atoms is  preferable. Examples of the monocycloalkane include cyclo› pentane and cyclohexane. As the polycyclic group, a group in  which two hydrogen atoms have been removed from a poly› cycloalkane of7 to 12 carbon atoms is preferable. Examples  of the polycycloalkane include adamantane, norbomane,  isobomane, tricyclodecane and tetracyclododecane.  The cyclic aliphatic hydrocarbon group may or may not  have a substituent. Examples of the substituent include an  alkyl group of 1 to 5 carbon atoms, a fluorine atom, a fluori- nated alkyl group ofl to 5 carbon atoms, and an oxygen atom  (=0).  Examples of aromatic hydrocarbon groups include a diva› lent aromatic hydrocarbon group in which one hydrogen  atom has been removed from a benzene ring of a monovalent  aromatic hydrocarbon group such as a phenyl group, a bi phe› nyl group, a fluorenyl group, a naphthyl group, an anthryl  group or a phenanthryl group;  (Structural Unit (a0-2))  R  -continued 