A method for manufacturing a semiconductor device (10), a semiconductor device (10), and a memory device (1) are provided. The method includes forming (11, 21) shallow trench isolation structures (400) in a substrate (100), where first trenches (B) are defined between adjacent protruding portions of the shallow trench isolation structures (400); forming (12, 22) floating gates (500) in first trenches (B) and performing a first etching on a part of the protruding portions to form second trenches (C) between the adjacent floating gates (500); performing (13, 23) a narrowing process on the floating gates (500); performing (14, 24) a second etching on a remaining part of the protruding portions to widen the second trenches (C); and forming (15, 26) a control gate (600) partially filled in the second trenches (C).