forming an etching subject layer on a substrate,

coating a resist underlayer composition on the etching subject layer to form a resist underlayer,

forming a photoresist pattern on the resist underlayer, and

sequentially etching the resist underlayer and the etching subject layer using the photoresist pattern as an etching mask,

wherein the resist underlayer composition includes:

a polymer including at least one of a first moiety represented by Chemical Formula 1-1 and a second moiety represented by Chemical Formula 1-2;

a thermal acid generator including a salt composed of an anion of an acid and a cation of a base, the base having a pKa of greater than or equal to about 7; and

a solvent,

wherein, in Chemical Formula 1-1 and Chemical Formula 1-2,

a and f are each independently an integer of 0 to 3,

when a is 0, R¹ is hydrogen, a C1 to C30 alkyl group substituted with at least one hydroxy group, or a C1 to C30 heteroalkyl group substituted with at least one hydroxy group,

when a is an integer of 1 to 3, R¹ is a hydroxy group and R⁰ is a substituted or unsubstituted C1 to C30 alkylene group, a substituted or unsubstituted C6 to C30 arylene group, a substituted or unsubstituted C1 to C30 heteroalkylene group, a substituted or unsubstituted C1 to C30 heteroalkenylene group, a substituted or unsubstituted C2 to C30 heteroarylene group, a substituted or unsubstituted C2 to C30 alkenylene group, a substituted or unsubstituted C2 to C30 alkynylene group, —(C═O)—O—, —O—, —S—, or a combination thereof,

R² to R⁶ are each independently a substituted or unsubstituted C1 to C30 alkylene group, a substituted or unsubstituted C6 to C30 arylene group, a substituted or unsubstituted C1 to C30 heteroalkylene group, a substituted or unsubstituted C1 to C30 heteroalkenylene group, a substituted or unsubstituted C2 to C30 heteroarylene group, a substituted or unsubstituted C2 to C30 alkenylene group, a substituted or unsubstituted C2 to C30 alkynylene group, —(C═O)—O—, —O—, —S—, or a combination thereof, provided that R⁴ to R⁶ do not comprise —O—, and at least one of R⁴ to R⁶ comprises —S—,

b and c are each independently an integer of 0 to 3,

d and e are each independently an integer of 1 to 3, and

* is a linking point.