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

A radiation-sensitive composition comprising:

• an onium salt compound represented by formula (1);
• a polymer comprising a structural unit (I) which comprises an acid-dissociable group;
• and
• a solvent,
• wherein,
• W is a cyclic structure having 3 to 40 ring members formed together with the two carbon atoms,
• a formula between the two carbon atoms below represents a single bond or a double bond,
• A is a group represented by formula (A-1), a group represented by formula (A-2), a group represented by formula (A-3), a group represented by formula (A-4), a group represented by formula (A-5), a group represented by formula (A-6), or a group represented by formula (A-7),
• wherein, in the formulas (A-3) and (A-4), RA1 and RA2 are each independently a monovalent hydrocarbon group having 1 to 20 carbon atoms, and * is a bond with a carbon atom,
• R1 is a monovalent organic group having 1 to 20 carbon atoms, a cyano group, a nitro group, a carboxy group, a hydroxy group, an amino group, a halogen atom, or a thiol group, when there are a plurality of R1s, the plurality of R1s are same as or different from each other,
• m1 is an integer of 0 to 4, and
• Z+ is a monovalent radiation-sensitive onium cation.