Semiconductor memory device and manufacturing method of semiconductor memory device

A manufacturing method of a semiconductor memory device, comprising:

• forming a plurality of active areas on a surface of a semiconductor substrate;
• forming an element isolation part between the active areas adjacent to each other;
• forming a tunnel dielectric film on the active areas;
• forming a lower gate part on the tunnel dielectric film, the lower gate part being a part of a floating gate;
• depositing a material of an upper gate part on the lower gate part and on the element isolation part, the upper gate part being a part of the floating gate;
• depositing a masking material on a material of the upper gate part;
• patterning the masking material and the material of the upper gate part so that the upper gate part has larger width than that of the lower gate part on a cross-sectional surface perpendicular to an extension direction of the active area;
• forming a trench within the element isolation part by etching the element isolation part to a deeper position than an interface between the upper gate part and the lower gate part by using the masking material as a mask;
• forming an intermediate dielectric film on an internal wall of the trench;
• and
• depositing a material of a control gate on the intermediate dielectric film, wherein
• a height of a lower end of the control gate from a surface of the semiconductor substrate is lower than a height of the interface between the upper gate part and the lower gate part from the surface of the semiconductor substrate.