A method for forming a semiconductor structure includes forming a gate structure over a substrate. The method also includes forming a spacer on a sidewall of the gate structure. The method also includes forming a source/drain recess beside the spacer. The method also includes treating the source/drain recess and partially removing the spacers in a first cleaning process. The method also includes treating the source/drain recess with a plasma process after performing the first cleaning process. The method also includes treating the source/drain recess in a second cleaning process after treating the source/drain recess with the plasma process. The method also includes forming a source/drain structure in the source/drain recess after performing the second cleaning process.