The purpose of the present invention is to provide a method for producing a semiconductor substrate using a composition from which a film having excellent etching resistance, heat resistance, and bending resistance can be formed, and a composition. This method for manufacturing a semiconductor substrate comprises: a step for applying a resist underlayer film-forming composition directly or indirectly to a substrate; a step for forming resist patterns directly or indirectly on the resist underlayer film formed in the application step; and a step for performing etching using the resist patterns as masks, the resist underlayer film-forming composition containing a solvent and a polymer having a repeating unit represented by formula (1). (In formula (1), Ar1 is a divalent group having a 5- to 40-membered aromatic ring. R0 is a monovalent group having a 5- to 40-membered aromatic ring and has at least one group selected from the group consisting of groups represented by formula (2-1) and groups represented by formula (2-2).) (In formulas (2-1) and (2-2), R7 each independently are a C1-20 divalent organic group or a single bond. * is a bond with a carbon atom in an aromatic ring.)