An exemplary method of etching sacrificial layer includes steps of: providing a substrate formed with a sacrificial layer and defined with a first region and a second region, the sacrificial layer disposed in both the first and second regions; forming a hard mask covering the first region while exposing the second region; performing a first etching process on the sacrificial layer to thin the sacrificial layer while forming a byproduct film overlying the thinned sacrificial layer; performing a second etching process on the byproduct film to remove a portion of the byproduct layer for exposing a portion of the thinned sacrificial layer, while another portion of the byproduct film disposed on sidewalls of the thinned sacrificial layer being remained; and performing a third etching process on the thinned sacrificial layer, to remove the portion of the thinned sacrificial layer exposed in the second etching process.