Radiation-sensitive composition, method for forming pattern, and onium salt

A radiation-sensitive composition comprising:

• an onium salt represented by formula (1);
• a polymer comprising a structural unit (I) which comprises an acid-dissociable group;
• and
• a solvent:
• wherein in the formula (1),
• Ar1 is an (a1+b1+1)-valent aromatic ring,
• Ar2 is an (a2+b2+1)-valent aromatic ring,
• X1 and X2 are each independently a monovalent organic group having 1 to 20 carbon atoms, a cyano group, a nitro group, or a halogen atom, when a plurality of X1s and X2s are present, the plurality of X1s and X2s are identical to or different from each other,
• Y1 is a monovalent organic group having 4 to 20 carbon atoms bonded to Ar1 through —O—, —S—, or —SO2—, or a monovalent perfluoroalkyl group having 1 to 20 carbon atoms, when a plurality of Y1s are present, the plurality of Y1s are identical to or different from each other,
• Y2 is a monovalent organic group having 4 to 20 carbon atoms bonded to Ar2 through —O—, —S—, or —SO2—, or a monovalent perfluoroalkyl group having 1 to 20 carbon atoms, when a plurality of Y2s are present, the plurality of Y2s are identical to or different from each other,
• L is a single bond or a divalent linking group having 1 to 5 carbon atoms, and
• a1, a2, b1, and b2 are each independently an integer of 0 to 5, provided that when the aromatic ring of Ar2 is a benzene ring, b1+b2≥1.