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

A radiation-sensitive composition comprising:

• an onium salt compound represented by formula (1);
• a polymer comprising a structural unit (I) represented by formula (2);
• and
• a solvent,
• wherein in the formula (1),
• A is a (1+n)-valent organic group having 3 to 40 carbon atoms comprising a cyclic structure, when the group represented by A comprises a linear alkanediyl group, a carbon number of the linear alkanediyl group is 1 or 2, the group represented by A does not contain a cyclohexylcarbonyloxy structure,
• Rf1 and Rf2 are each independently a hydrogen atom, a monovalent organic group having 1 to 20 carbon atoms, a fluorine atom, or a monovalent fluorinated hydrocarbon group, when there are a plurality of Rf1s and Rf2s, the plurality of Rf1s and Rf2s are the same or different from each other, at least one of Rf1 and Rf2 is a fluorine atom or a monovalent fluorinated hydrocarbon group,
• ml is an integer of 1 to 4,
• m2 is 0 or 1,
• n is an integer of 1 to 3,
• Z+ is a monovalent radiation-sensitive onium cation, and the number of fluorine atoms in Z+ is 8 or less,
• wherein in the formula (2),
• RA is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group,
• L is a single bond or a divalent linking group,
• W is a monocyclic lactone structure, a monocyclic carbonate structure, a monocyclic sultone structure, a monocyclic sulfone structure, or a monocyclic ether structure,
• R11 is a monovalent organic group having 1 to 10 carbon atoms, a cyano group, a nitro group, a hydroxy group, or an amino group, when there are a plurality of R11s, the plurality of R11s are each the same or different from each other,
• p is an integer of 0 to 3, and
• q is 0 or 1.