A method for forming a semiconductor structure includes forming a gate stack over a semiconductor substrate; forming a recess in the semiconductor substrate and adjacent the gate stack; and performing a selective epitaxial growth to grow a semiconductor material in the recess to form an epitaxy region. After the step of performing the selective epitaxial growth, a selective etch-back is performed to the epitaxy region. The selective etch-back is performed using process gases comprising a first gas for growing the semiconductor material, and a second gas for etching the epitaxy region.