Method for manufacturing semiconductor structure

A method for manufacturing a semiconductor structure, comprising:

• recessing a substrate from an upper surface of the substrate toward a lower surface of the substrate to form a trench;
• filling a semiconductor material in the trench;
• forming a patterned metal layer on the semiconductor material and forming a patterned hard mask on the patterned metal layer;
• etching a portion of the semiconductor material by using the patterned metal layer and the patterned hard mask as a first etching mask, wherein a remaining portion of the semiconductor material comprises a first portion and a second portion below the first portion, and the second portion is wider than the first portion;
• forming a passivation layer on a sidewall of the first portion;
• and
• removing the passivation layer and etching the second portion of the remaining portion of the semiconductor material by using the patterned metal layer and the patterned hard mask as a second etching mask, thereby forming a semiconductor layer.