Positive resist composition and method of forming resist pattern

A positive resist composition comprising:

• a base material component (A)′ that exhibits increased solubility in an alkali developing solution under action of an acid;
• and
• an acid generator component (B) that generates an acid upon exposure,
• wherein said base material component (A)′ includes a polymeric compound (A1)′ having a structural unit (a10) derived from hydroxystyrene, a structural unit (a11)′ represented by general formula (a11-1)′ shown below, and a structural unit (a13) represented by general formula (a13-1) shown below:
• wherein R represents a hydrogen atom, an alkyl group of 1 to 5 carbon atoms, or a halogenated alkyl group of 1 to 5 carbon atoms;
• R21 represents an alkyl group;
• R22′ represents a group that forms an aliphatic monocyclic group together with the carbon atom to which this R22′ group is bonded;
• Y2 represents a divalent linking group;
• and X2 represents an acid dissociable, dissolution inhibiting group.

A positive resist composition comprising:

• a base material component (A)″ that exhibits increased solubility in an alkali developing solution under action of an acid;
• and
• an acid generator component (B) that generates an acid upon exposure,
• wherein said base material component (A)″ includes a polymeric compound (A1)″ having a structural unit (a10) derived from hydroxystyrene, a structural unit (a11)′ represented by general formula (a11-1)′ shown below, and a structural unit (a12) represented by general formula (a12-1) shown below:
• wherein R represents a hydrogen atom, an alkyl group of 1 to 5 carbon atoms, or a halogenated alkyl group of 1 to 5 carbon atoms;
• R21 represents an alkyl group;
• R22′ represents a group that forms an aliphatic monocyclic group together with the carbon atom to which this R22′ group is bonded;
• R23 represents an alkyl group;
• R24 represents a group that forms an aliphatic polycyclic group together with the carbon atom to which this R24 group is bonded.