Composition for forming resist underlayer film for electron beam or extreme ultraviolet light lithography, resist underlayer film for electron beam or extreme ultraviolet light lithography, and method for producing semiconductor substrate

The present invention provides: a composition that is used for the formation of a resist underlayer film which is used in electron beam or extreme ultraviolet light lithography, and on which a fine resist pattern is able to be formed, while being capable of forming a resist underlayer film which is easily removed; and the like. A composition for forming a resist underlayer film for electron beam or extreme ultraviolet light lithography, said composition containing a polysiloxane compound having a first structural unit that is represented by formula (1), and a solvent. In formula (1), X is a group represented by formula (2); a is an integer from 1 to 3; R1 is a monovalent organic group having from 1 to 20 carbon atoms, a hydroxy group or a halogen atom; and b is an integer from 0 to 2, provided that (a + b) is 3 or less. In formula (2), R2 is a monovalent hydrocarbon group having from 1 to 20 carbon atoms; n is 1 or 2; R3 is a hydrogen atom or a monovalent organic group having from 1 to 20 carbon atoms; and L