forming a mold structure by alternately stacking mold insulating layers and semiconductor patterns in a first direction, the mold structure including a plurality of pre side extension holes defined by the semiconductor patterns and adjacent mold insulating layers, the plurality of pre side extension holes extending in a second direction perpendicular to the first direction;

forming a seed metal film along profiles of each of the pre side extension holes using a reductant having a group IVA or a group IVB element as a central atom;

forming a plurality of silicide patterns, between the mold insulating layers through a silicidation process of the seed metal film and the semiconductor patterns; and

forming a metal conductive film in a remainder of the pre side extension holes, the metal conductive film connected to the plurality of silicide patterns.