forming first and second conductive structures on a lower structure, the first conductive structures being spaced apart from one another in a first direction, the second conductive structure being formed between the first conductive structures;

forming a first insulating layer on the lower structure, the first insulating layer covering the first and second conductive structures;

transforming the first insulating layer into a preliminary first insulating layer pattern having first holes each exposing the first conductive structures;

forming spacers on inner walls of the first holes;

forming first contacts in the first holes having the inner walls on which the spacers are formed, the first contacts being electrically connected to the first conductive structures;

transforming the preliminary first insulating layer pattern into a first insulating layer pattern having a second hole exposing the second conductive structure by removing a portion of the preliminary first insulating layer pattern located between the spacers; and

forming a second contact in the second hole, the second contact being electrically connected to the second conductive structure,

wherein forming the second contact includes

forming a second conductive layer filling the second hole on the lower structure, the second conductive structure, the first contacts, and the spacers, and

forming the second contact in the second hole by performing a planarization process on the second conductive layer until the first contacts are exposed, and

wherein forming the first contacts includes

forming a first conductive layer filling the first holes on the preliminary first insulating pattern, the spacers and the first conductive structures,

forming mask layer patterns on the first conductive layer, the mask layer patterns extending in the first direction, the mask layer patterns being spaced apart from each other in a second direction substantially perpendicular to the first direction by a width substantially smaller than a width of the first holes such that the spacers are located under a sidewall of the mask layer patterns,

transforming the first conductive layer into first contacts and remaining patterns by etching the first conductive layer by using the mask layer patterns as an etching mask until the spacers are exposed. the first contacts filling the first holes having the inner walls on which the spacers are formed, the remaining patterns being located under the mask layer patterns, and

removing the mask layer patterns from the remaining patterns.