Semiconductor devices are provided. A semiconductor device includes a substrate. The semiconductor device includes a stack structure on the substrate. The stack structure includes a first insulating material and a second insulating material that is on the first insulating material. The semiconductor device includes a spacer that extends from a sidewall of the first insulating material of the stack structure to a portion of a sidewall of the second insulating material of the stack structure. Moreover, the semiconductor device includes a conductive line that is on the spacer. Methods of forming semiconductor devices are also provided.