Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device, the method comprising:

• forming a preliminary source structure including a lower source layer, a first source protective layer, a source sacrificial layer, a second source protective layer and an upper source layer;
• forming a stack structure including first material layers and second material layers, the first material layers and the second material layers alternately stacked over the preliminary source structure,
• forming a hole passing through the stack structure and the preliminary source structure;
• forming a preliminary memory layer on a surface of the hole;
• forming a channel layer on the preliminary memory layer;
• forming a trench passing through the stack structure and the upper source layer;
• forming a first buffer pattern and second buffer patterns by performing a surface treatment on a side portion of the upper source layer and side portions of the second material layers exposed by the trench, wherein a portion of the upper source layer remains between the first buffer pattern and the preliminary memory layer, and wherein portions of the second material layers remain between the second buffer patterns and the preliminary memory layer;
• forming a preliminary buffer layer covering the first buffer pattern, the first material layers and the second buffer patterns;
• removing a portion of the second source protective layer exposed by the trench;
• forming a buffer layer covering the first buffer pattern, the first material layers and the second buffer patterns by performing an oxidation process of the preliminary buffer layer, wherein a dummy pattern is formed by oxidizing a portion of the source sacrificial layer;
• removing a portion of the dummy pattern to expose a remaining portion of the source sacrificial layer;
• forming a cavity exposing a portion of the preliminary memory layer by removing the source sacrificial layer and the first and second source protective layers;
• forming an expanded cavity exposing a portion of the channel layer by removing the portion of the preliminary memory layer;
• and
• forming a source layer in the expanded cavity.