US 8,298,748 B2  110  109  virtue of the above combination of the structural units, EL  margin and resist pattern shape can be improved.  In the present invention, as the component (Al’), a copoly› mer represented by general formula (Al ’-11) shown below  which includes three types of structural units is particularly  desirable.  [Chemical Formula 58]  (Al’-11)  In the formulas, Rs, a, R 7 , R1, R 2 , R 3 , Y andx- are the same  as defined above. Further, the plurality ofR, Rs, R  $^{7 }$and a may  be the same or different from each other  The weight average molecular weight (Mw) (the polysty› rene equivalent value determined by gel permeation chroma› tography) of the component (Al’) is not particularly limited,  but is preferably 1,000 to 50,000, more preferably 1,500 to  30,000, and most preferably 2,500 to 25,000. When the  weight average molecular weight is no more than the upper  limit of the above-mentioned range, the resist composition  exhibits a satisfactory solubility in a resist solvent. On the  other hand, when the weight average molecular weight is at  least as large as the lower limit of the above-mentioned range,  dry etching resistance and the cross-sectional shape of the  resist pattern becomes satisfactory.  Further, the dispersity (Mw/Mn) is preferably 1.0 to 5.0,  more preferably 1.0 to 4.0, and most preferably 1.0 to 3.0.  Here, Mn is the number average molecular weight  In the component (A’), as the component (Al’), one type  may be used, or two or more types of compounds may be used  in combination.  In the component (A’), the amount of the component (Al’)  based on the total weight of the component (A’) is preferably 60  10 to 90% by weight, more preferably 20 to 80% by weight,  and still more preferably 30 to 70% by weight.  In the resist composition of the present invention, the com› ponent (A’) may contain "a base component which exhibits  increased solubility in an alkali developing solution under  action of acid" other than the component (Al’) (hereafter,  referred to as "component (A2)").  Further, among R  1 " to R 3 ", at least one group represents an  aryl group. Among R 1 " to R 3 ", two or more groups are pref› erably aryl groups, and it is particularly desirable that all of  R 1 " to are aryl groups.  The aryl group for R  1 " to R 3 " is not particularly limited. For  example, an aryl group having 6 to 20 carbon atoms may be  used in which part or all of the hydrogen atoms of the aryl  In formula (b-1 ), R  1 " to R 3 " each independently represents  an aryl group or an alkyl group. In formula (b-1), two ofR 1 "  to R 3 "  may be bonded to each other to form a ring with the  sulfur atom.  In the formulas above, R  1 "  to R 3 ", Rs" and R  $^{611 }$each indepen› dently represent an aryl group or alkyl group, wherein two of  R  1 " to R 3 " may be bonded to each other to form a ring with the  sulfur atom; and R 4 "  represents an alkyl group, a halogenated  alkyl group, an aryl group or an alkenyl group which may  have a substituent, with the provision that at least one of R  1 "  to R 3 " represents anaryl group, and at least one ofRsi’ andR 6 "  represents an aryl group.  [Chemical Formula 59]  (b-1)  (b-2)  RI’"  R 2 •-!+ R 4 ’"so-$_{3 }$ ~3•  As an onium salt acid generator, a compound represented  by general formula (b-1) or (b-2) shown below can be used.  When the positive resist composition of the present inven› tion includes the component (B), as the component (B), there  is no particular limitation, and any of the known acid genera- tors used in conventional chemically amplified resist compo› sitions can be used. Examples of these acid generators are  numerous, and include onium salt acid generators such as  iodonium salts and sulfonium salts; oxime sulfonate acid  generators; diazomethane acid generators such as bisalkyl or  bisaryl sulfonyl diazomethanes and poly(bis-sulfonyl)diaz› omethaues; nitrobenzylsulfonate acid generators; iminosul› fonate acid generators; and disulfone acid generators.  The positive resist composition of the present invention  may further include an acid-generator component (B) (here- after, referred to as "component (B)") which generates acid  upon irradiation of radial rays.  <Component (B)>  In the resist composition of the present invention, the  amount of the component (A’) can be appropriately adjusted  depending on the thickness of the resist film to be formed, and  the like.  As the component (A2), one type ofresin may be used, or  two or more types of resins may be used in combination.  The component (A2) is not particularly limited, and any of  the multitude of conventional base components used within  chemically amplified resist compositions (e.g., base resins  used within chemically amplified resist compositions for ArF  excimer lasers or KrF excimer lasers, preferably ArF excimer  lasers) can be used. For example, as a base resin for ArF  excimer laser, a base resin having the aforementioned struc› tural unit (al) as an essential component, and optionally the  aforementioned structural units (a2) to (a4) can be used. 