A method of manufacturing a semiconductor device includes forming a first sacrificial layer including a nitride over a first source layer, forming a second sacrificial layer including aluminum oxide over the first sacrificial layer, forming a second source layer over the second sacrificial layer, forming a stacked structure over the second source layer, forming a channel layer that passes through the stacked structure, the second source layer, the second sacrificial layer, and the first sacrificial layer, the channel layer being enclosed by a memory layer, forming a slit that passes through the stacked structure and the second source layer, forming a polysilicon spacer in the slit, forming an opening by removing the first sacrificial layer and the second sacrificial layer, exposing the channel layer by etching the memory layer, and forming a third source layer in the opening.