A first insulating interlayer is formed on a substrate including first and second regions. The first insulating interlayer has top surface, a height of which is greater in the first region than in the second region. A first planarization stop layer and a second insulating interlayer are formed. The second insulating interlayer is planarized until the first planarization stop layer is exposed. The first planarization stop layer and the first and second insulating interlayers in the second region are removed to expose the substrate. A lower mold structure including first insulation layer patterns, first sacrificial layer patterns and a second planarization stop layer pattern is formed. The first insulation layer patterns and the first sacrificial layer patterns are alternately and repeatedly formed on the substrate, and a second planarization stop layer pattern is formed on the first insulation layer pattern.