Semiconductor device and method of manufacturing semiconductor device

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

• forming a stack including alternately stacking sacrificial layers and insulating layers;
• forming a separation sacrificial layer in the stack;
• forming channel structures extending through the separation sacrificial layer and the stack;
• forming a slit extending through the stack;
• forming first openings by removing the sacrificial layers through the slit and forming a second opening by removing the separation sacrificial layer simultaneously;
• forming a conductive layer filling the first openings and extending into the slit and the second opening;
• removing a portion of the conductive layer formed on a lower surface of the second opening and a lower surface of the slit;
• forming a separation structure in the second opening;
• removing a portion of the conductive layer formed on an inner wall of the slit;
• and
• forming a slit structure in the slit.