Pattern forming method and developer

A pattern forming method comprising:

• applying a photoresist composition onto a substrate to form a resist film;
• exposing the resist film to an ArF excimer laser;
• and
• developing the exposed resist film with a developer comprising an organic solvent, wherein
• the photoresist composition comprises:
• [A] a polymer that has a structural unit (I) having an acid-dissociable group to be dissociated by action of an acid, does not have a phenolic hydroxyl group and exhibits decreased solubility in the developer by dissociation of the acid-dissociable group;
• and
• [B] a radiation-sensitive acid generator,
• the developer comprises a nitrogen-containing compound, and
• the nitrogen-containing compound is at least one of
• (i) a condensed ring compound or bridged cyclic compound comprising at least two nitrogen atoms as ring-forming atoms,
• (ii) a compound having a nitrogen-containing aromatic heterocyclic structure and an acyclic tertiary amine structure in a molecule thereof,
• (iii) an onium salt compound represented by Formula (A-1), or
• (iv) a compound represented by Formula (A-2):
• wherein, in Formula (A-1), R1 is an aliphatic hydrocarbon group having 1 to 12 carbon atoms or a group obtained by substituting a part or all of hydrogen atoms contained in an aliphatic hydrocarbon group having 1 to 12 carbon atoms with a fluorine atom, R2 is an aliphatic hydrocarbon group having 1 to 12 carbon atoms, a branched hydrocarbon group having 3 to 12 carbon atoms, or an alicyclic hydrocarbon group having 3 to 18 carbon atoms, and Z+ is a monovalent onium cation,
• wherein, in Formula (A-2), R3, R4, and R5 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, or any one of R3 to R5 is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms and remaining two of R3 to R5 are combined with each other to be a part of a 3 to 6-membered ring structure that is formed together with the nitrogen atom to which the remaining two of R3 to R5 are bonded, provided that the 3 to 6-membered ring is unsubstituted except the any one of R3 to R5, and that R3 to R5 are not all hydrogen atoms.