Abstract: An additive for a resist underlayer film-forming composition containing a copolymer having structural units of Formulae (1) to (4), and a resist underlayer film-forming composition containing the additive: (where each R1 is independently a hydrogen atom or methyl group, Ar is arylene group, Pr is a protecting group or a hydrogen atom, X is a direct bond or a —C(?O)O—R2— group, R2 constituting the —C(?O)O—R2— group is a C1-3 alkylene group, the alkylene group is bonded to a sulfur atom, R3 is a hydrogen atom, methyl group, methoxy group, or halogeno group, R4 is a C1-3 alkyl group in which at least one hydrogen atom is substituted with a fluoro group, and Z is an organic group having 4 to 7-membered ring lactone skeleton, adamantane skeleton, or norbornane skeleton).

Abstract: An additive for a resist underlayer film-forming composition containing a copolymer having structural units of Formulae (1) to (4), and a resist underlayer film-forming composition containing the additive: (where each R1 is independently a hydrogen atom or methyl group, Ar is arylene group, Pr is a protecting group or a hydrogen atom, X is a direct bond or a —C(?O)O—R2— group, R2 constituting the —C(?O)O—R2— group is a C1-3 alkylene group, the alkylene group is bonded to a sulfur atom, R3 is a hydrogen atom, methyl group, methoxy group, or halogeno group, R4 is a C1-3 alkyl group in which at least one hydrogen atom is substituted with a fluoro group, and Z is an organic group having 4 to 7-membered ring lactone skeleton, adamantane skeleton, or norbornane skeleton).