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The weight average molecular weights (Mw) ofresins (Ab) are preferably each in the range of 1000 to 200,000. Up to 200,000 is preferred from the viewpoint of the rate of disso› lution of the resin per se in alkali and the sensitivity. The polydispersity index (Mw/Mn) of the resin is preferably in the range of 1.0 to 3.0, more preferably 1.0 to 2.5 and most preferably 1.0 to 2.0. 

With respect to the weight average molecular weight (Mw) of the resin, it is preferably in the range of 1000 to 200,000, more preferably 1000 to 100,000, further more preferably 1000 to 50,000 and most preferably 1000 to 25,000. 

The weight average molecular weight refers to a polysty› rene-equivalent value determined by gel permeation chroma› tography. 

The resin (Ab) of2.0 or below polydispersity index can be synthesized by radical polymerization using an azo polymer› ization initiator. The resin (Ab) exhibiting a further preferred polydispersity index of 1.0 to 1.5 can be synthesized by, for example, a living radical polymerization. 

The resin (Ab) is preferably polymerized by, for example, a generally known anion polymerization method or radical polymerization method. 

In the anion polymerization method, using an alkali metal or organoalkali metal as an initiator, polymerization is gen- erally performed in an organic solvent at -100 to 90° C. in an atmosphere of inert gas, such as nitrogen or argon. In the event of copolymerization, a block copolymer is obtained by performing polymerization while sequentially adding mono› mers to a reaction system, and a random copolymer is obtained by adding a mixture of monomers to a reaction system and carrying out polymerization. 

The alkali metal as the polymerization initiator is, for example, lithium, sodium, potassium or cesium. As the orga› noalkali metal, use can be made of an alkylation, allylation or arylation product of alkali metal mentioned above. For example, there can be mentioned ethyllithium, n-butyl› lithium, sec-butyllithium, tert-butyllithium, ethylsodium, lithium biphenyl, lithium naphthalene, lithium triphenyl, sodium naphthalene, a-methylstyrene sodium dianion, 1,1- dipheny lhexy llithium, 1, 1-dipheny 1-3-methylpenty llithium or the like. 

In the radical polymerization method, in an atmosphere of inert gas, such as nitrogen or argon, polymerization is per› formed in an organic solvent at 50 to 200° C., using any of 45 common radical polymerization initiators comprised of, for example, an azo compound, such as azobisisobutyronitrile or azobisisovaleronitrile, or an organic peroxide, such as ben› zoyl peroxide, methyl ethyl ketone peroxide or cumene hydroperoxide, according to necessity in combination with 50 any of common chain transfer agents, such as 1-dode› canethiol. 

As the organic solvent, there can be mentioned any of those commonly used in the anion polymerization, including an aliphatic hydrocarbon, such as n-hexane or n-heptane; an 55 alicyclic hydrocarbon, such as cyclohexane or cyclopentane; an aromatic hydrocarbon, such as benzene or toluene; a ketone, such as methyl ethyl ketone or cyclohexanone; a polyhydric alcohol derivative, such as propylene glycol monomethyl ether acetate, propylene glycol monomethyl 60 ether, ethylene glycol monobutyl ether acetate, ethylene gly› col mono butyl ether, ethylene glycol monoethyl ether acetate, ethylene glycol monoethyl ether, propylene glycol monoet› hyl ether acetate or propylene glycol monoethyl ether; an ether, such as diethyl ether, tetrahydrofuran or dioxane; ani- 65 sole, hexamethylphosphoramide and the like. These are used as a single solvent or a mixed solvent comprised of two or 

more of these. Preferred solvents are propylene glycol monomethyl ether acetate, propylene glycol monomethyl ether and cyclohexanone. 

When the positive resist of the present invention is exposed to light by means of anArF excimer laser, it is preferred to use a resin containing no aromatic ring as the resin (Ab) from the viewpoint of the transparency to ArF excimer laser light. 

The resin (hereinafter also referred to as resin (A’)) suitable for exposure to ArF excimer laser light will be described below. 

The acid-decomposable group contained in the resin (A’) can be the same as in the above resin (Ab). As a preferred repeating unit containing the acid-decomposable group, there can be mentioned any of the repeating units of general for› mula (A2) above. 

The content of repeating unit containing the acid-decom› posable group, based on all the repeating units of the resin (A’), is preferably in the range of 20 to 50 mo! %, more preferably 25 to 45 mo! %. 

It is preferred for the resin (A’) to further have a repeating unit having at least one group selected from among a lactone group, a hydroxyl group, a cyano group and an alkali soluble group. 

The repeating unit having a lactone group that may be contained in the resin (A’) will now be described. 

Any lactone groups can be employed as long as a lactone structure is possessed therein. However, lactone structures of a 5 to 7-membered ring are preferred, and in particular, those resulting from condensation of lactone structures of a 5 to 7-membered ring with other cyclic structures effected in a fashion to form a bicyclo structure or spiro structure are preferred. The possession of repeating units having a lactone structure represented by any of the following general formu› lae (LCl-1) to (LCl-16) is more preferred. The lactone struc› tures may be directly bonded to the principal chain of the resm. 

Preferred lactone structures are those of the formulae (LCl-1), (LCl-4), (LCl-5), (LCl-6), (LCl-13) and (LCl- 14). The use of these specified lactone structures would ensure improvement in the line edge roughness and develop› ment defect. 

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