Semiconductor memory device and fabrication method thereof

A method for forming a semiconductor memory device, comprising:

• providing a substrate;
• forming a pair of floating gates on the substrate;
• forming a recessed region in the substrate between the floating gates, wherein an upper surface of the recessed region has a concave profile lower than a surface of the substrate and with a radius between 40 nm and 60 nm in a cross-sectional view perpendicular to the floating gates;
• forming a source line doped region in the recessed region;
• forming an erase gate between the floating gates and on the recessed region, and a word line on the substrate and adjacent to a side of each of the floating gates opposite to the erase gate, wherein a lower surface of the erase gate continuously extends below the surface of the substrate;
• and
• forming a bit line doped region in the substrate and adjacent to the word line.