Radiation-sensitive resin composition and pattern formation method

A radiation-sensitive resin composition comprising:

• a resin comprising a structural unit represented by formula (1);
• at least onium salt each comprising an organic acid anion moiety and an onium cation moiety;
• and
• a solvent, wherein
• at least part of the organic acid anion moiety in the at least one onium salt comprises an iodine-substituted aromatic ring structure,
• in the formula (1),
• R is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or a halogenated alkyl group having 1 to 5 carbon atoms, Y1 is a divalent linking group, and X1 is an acid-dissociable group, and
• n is 0 or 1,
• provided that when n is 0, X1 is represented by the following formula (s1) or (s2),
• in the formula (s1),
• Cy is an aliphatic cyclic group formed with the carbon atom,
• Ra01 to Ra03 each independently represent a hydrogen atom, a substituted or unsubstituted monovalent chain saturated hydrocarbon group having 1 to 10 carbon atoms, or a substituted or unsubstituted monovalent aliphatic cyclic saturated hydrocarbon group having 3 to 20 carbon atoms, and optionally at least two of Ra01 to Ra03 taken together represent an aliphatic cyclic structure, provided that the aliphatic cyclic structure does not form a crosslinked structure;
• in the formula (s2),
• Cy is as defined as in the formula (s1), and
• Ra04 is a substituted or unsubstituted aromatic hydrocarbon group;
• in the formula (s1) and formula (s2), each * represents a bond with the oxygen atom in the formula (1).