The present invention provides a method for forming a semiconductor structure, comprising: firstly, a substrate is provided, next, a first dry etching process is performed, to form a recess in the substrate. Afterwards, an ion implantation process is performed to a bottom surface of the recess, a wet etching process is then performed, to etch partial sidewalls of the recess, so as to form at least two tips on two sides of the recess respectively, and a second dry etching process is performed, to etch partial bottom surface of the recess, wherein after the second dry etching process is performed, a lower portion of the recess has a U-shaped cross section profile.