Positive resist composition and process for formation of resist patterns

1. A positive resist composition comprising:

a resin component (A) that exhibits increased alkali solubility under action of acid; and

an acid generator component (B) that generates acid upon irradiation, wherein

said component (A) is a copolymer comprising a structural unit (a1) derived from an (α-lower alkyl)acrylate ester that contains a monocyclic or polycyclic group-containing acid dissociable, dissolution inhibiting group, a structural unit (a2) derived from an (α-lower alkyl)acrylate ester that contains a lactone ring, a structural unit (a3) derived from an (α-lower alkyl)acrylate ester that contains a polar group-containing polycyclic group, and a structural unit (a4) represented by a general formula (1) shown below:

(wherein, R 11 represents a lower alkyl group or a hydrogen atom, and R 12 represents a carboxyl group), and

wherein a proportion of said structural unit (a4) within said component (A) is at least 3 mol % but less than 10 mol %.

2. A positive resist composition according to claim 1 , wherein in said structural unit (a3), said polar group is an alcoholic hydroxyl group.

3. A positive resist composition according to claim 1 , wherein a proportion of said structural unit (a1) within said component (A) is within a range from 20 to 60 mol %.

4. A positive resist composition according to claim 1 , wherein a proportion of said structural unit (a2) within said component (A) is within a range from 20 to 60 mol %.

5. A positive resist composition according to claim 1 , wherein a proportion of said structural unit (a3) within said component (A) is within a range from 10 to 50 mol %.

6. A positive resist composition according to claim 1 , further comprising a nitrogen-containing organic compound.

7. A process for forming a resist pattern, comprising:

applying a positive resist composition according to any one of claims 1 , 2 , 3 , 4 , 5 or 6 to a support,

conducting a prebake,

performing selective exposure,

conducting PEB (post exposure baking), and

performing alkali developing to form a resist pattern.

8. A process for forming a resist pattern according to claim 7 , further comprising:

applying a water-soluble coating formation agent to said resist pattern formed on top of said support, thereby forming a water-soluble coating, and

subjecting a laminate comprising said resist pattern and said water-soluble coating to heat treatment, thereby heat shrinking said water-soluble coating.