Composition for forming resist underlayer film, patterning process, and resist underlayer film formation process

A composition for forming a resist underlayer film, the composition comprising:

• (A) a polymer compound having a structural unit (a1) represented by the following formula (A-1) and at least one structural unit (a2) selected from structural units represented by the following formulae (A-2) to (A-4);
• (B) a crosslinking agent represented by the following formula (B-1);
• and
• (C) an organic solvent;
• wherein R01 is a hydrogen atom or a methyl group, and R02 is a monovalent organic group containing a group selected from groups represented by the following formulae (R2-1) to (R2-3);
• wherein Ra is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and a dotted line represents a bond;
• wherein, in the formula (A-2), X is an aromatic ring having 6 to 20 carbon atoms or a single bond, R03 is a hydrogen atom or a methyl group, and R04 is a hydrogen atom or a monovalent alkyl group having 1 to 10 carbon atoms and, when R04 is the alkyl group, a hydrogen atom constituting the alkyl group is optionally replaced by a hydroxyl group;
• in the formula (A-3), R05 is a hydrogen atom or a methyl group, R06 is a single bond or a divalent linkage group containing an ester group and having 2 to 10 carbon atoms, and R07 is a tertiary alkyl group having 4 to 20 carbon atoms or a tertiary alkenyl group having 5 to 20 carbon atoms;
• and in the formula (A-4), R05 and R06 have the same meanings as those in the formula (A-3), and R08 is a hydrogen atom or a monovalent primary alkyl group having 1 to 10 carbon atoms;
• wherein R5 represents a methyl group, an ethyl group, a propyl group, an allyl group, a propargyl group or a group represented by the following (B-2), and R1 and R3 each represent a benzoyl group, a toluoyl group, a naphthoyl group or an anthranoyl group;
• and R2 and R4 each represent a hydrogen atom, an acetyl group, an acryloyl group or a methacryloyl group;
• and
• wherein R7 represents a benzoyl group, a toluoyl group, a naphthoyl group or an anthranoyl group;
• and R6 represents a hydrogen atom, an acetyl group, an acryloyl group or a methacryloyl group and, when R2 and R4 are hydrogen atoms, R6 is also a hydrogen atom.