Resist underlayer composition, and method of forming patterns using the composition

A resist underlayer composition, comprising:

• a polymer comprising a structural unit represented by Chemical Formula 1, a structural unit represented by Chemical Formula 2, or a combination thereof, a photoacid generator having a mass reduction rate of about 0% after 3 minutes from the time of measurement during thermogravimetric analysis (TGA) at 205° C. in an air atmosphere (air gas), and a solvent:
• wherein, in Chemical Formula 1 and Chemical Formula 2,
• A is a heterocyclic group including a nitrogen atom in a ring,
• L1 to L6 are each independently a single bond, a substituted or unsubstituted C1 to C10 alkylene group, a substituted or unsubstituted C1 to C10 heteroalkylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C2 to C20 heterocycloalkylene group, a substituted or unsubstituted C6 to C20 arylene group, a substituted or unsubstituted C3 to C20 heteroarylene group, or a combination thereof,
• X1 to X5 are each independently a single bond, —O—, —S—, —S(═O)—, —S(═O)2—, —C(═O)—, —(CO)O—, —O(CO)O—, —NRa— (wherein Ra is hydrogen, deuterium, or a C1 to C10 alkyl group), or a combination thereof,
• Y1 to Y3 are each independently a hydroxy group, a substituted or unsubstituted C1 to C10 alkyl group, a substituted or unsubstituted C2 to C10 alkenyl group, a substituted or unsubstituted C2 to C10 alkynyl group, a substituted or unsubstituted C1 to C10 heteroalkyl group, a substituted or unsubstituted C2 to C10 heteroalkenyl group, a substituted or unsubstituted C2 to C10 heteroalkynyl group, a substituted or unsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstituted C2 to C20 heterocycloalkyl group, a substituted or unsubstituted C6 to C20 aryl group, a substituted or unsubstituted C3 to C20 heteroaryl group, or a combination thereof, and
• * is a linking point.