Polymer with a good heat resistance and storage stability, underlayer film composition containing the polymer and process for forming underlayer film using the composition

An underlayer film composition for semiconductor and display manufacturing processes, the underlayer film composition comprising a polymer including a repeating unit represented by the following Chemical Formula 1:

• in Chemical Formula 1,
• Ar is (C6-C20)arylene;
• R1 to R6 are each independently hydrogen, (C1-C20)alkyl, nitrile, or (C6-C20)arylmethyl, at least one of R1 to R6 being (C6-C20)arylmethyl;
• L is a single bond, (C1-C10)alkylene, or (C6-C20)arylene, alkylene and arylene of L being further substituted with one or more substituents selected from (C1-C20)alkyl, (C6-C20)aryl, and (C1-C20)alkoxy;
• and
• R′ and R″ are each independently hydrogen, (C1-C20)alkyl, (C1-C20)alkoxy, or (C6-C20)aryl, or are linked to each other via (C2-C10)alkenylene to form a fused ring.