Positive resist composition and method of formation of resist patterns

1 2 The invention provides a positive resist composition which has high etching resistance and attains high resolution, and a method of forming patterns by using the positive resist composition. The positive resist composition contains a resin component (A), which has acid dissociable, dissolution inhibiting groups, and exhibits increased alkali solubility under the action of acid, and an acid generator component (B) which generates acid on exposure, wherein the resin component (A) is a polymer comprising structural units (a1) represented by the general formula (I) shown below, and a portion of the hydroxyl groups of the units (a1) are protected by replacing the hydrogen atoms of the hydroxyl groups with acid dissociable, dissolution inhibiting groups represented by the general formula (II) shown below: (wherein, R is a hydrogen atom or methyl group, Ris an alkyl group having 1 to 5 carbon atoms, Ris an alkyl group having 1 to 5 carbon atoms or a hydrogen atom, and X is an aliphatic polycyclic group having 10 to 16 carbon atoms or an aromatic polycyclic hydrocarbon group having 10 to 16 carbon atoms.)