Resist underlayer film-forming composition, resist underlayer film, and method of producing semiconductor substrate

A method of producing a semiconductor substrate, the method comprising:

• applying a resist underlayer film-forming composition directly or indirectly on a substrate to form a coating film;
• heating the coating film to form a resist underlayer film;
• applying a resist film-forming composition directly or indirectly on the resist underlayer film to form a resist film;
• exposing the resist film to an electron beam or extreme ultraviolet ray;
• and
• developing the resist film exposed,
• wherein the resist underlayer film-forming composition comprises:
• a polysiloxane compound comprising a first structural unit represented by formula (1);
• and
• a solvent,
• wherein,
• in the formula (1), X represents a group represented by formula (2);
• a is an integer of 1 to 3, wherein in a case in which a is no less than 2, a plurality of Xs are identical or different from each other;
• R1 represents a halogen atom, a hydroxy group, or a monovalent organic group having 1 to 20 carbon atoms;
• and b is an integer of 0 to 2, wherein in a case in which b is 2, two R1s are identical or different from each other, and wherein a sum of a and b is no greater than 3, and
• wherein, in the formula (2), R2 represents a monovalent hydrocarbon group having 1 to 20 carbon atoms;
• n is 1 or 2, wherein in a case in which n is 2, a plurality of R2s are identical or different from each other;
• R3 represents a monovalent organic group having 1 to 20 carbon atoms;
• L represents a single bond or a divalent linking group;
• and * denotes a site bonding to the silicon atom in the formula (1).