In one embodiment, as shown in FIG. 4E, the fin 116 not covered by the stack structure 120 is recessed through the stop layer 104. The etching of the fins 116 continues until penetrating through the stop layer 104, but the depth D1 of the recess 128 is smaller than the depth D (FIG. 2B) of the fins 116. The etching process for removing the fins 116 to form the recesses 128 includes at least a first etching process and a second etching process. In some embodiments, the first etching process includes a main anisotropic etching process, while the second etching process includes a tuning anisotropic etching process. In some embodiments, the fin 116 is etched through the main anisotropic etching process until the stop layer 104 is penetrated through and then is further etched through the tuning anisotropic etching process for tuning the profile. By adjusting the etching conditions and etching selectivity of the main anisotropic etching process and the tuning anisotropic etching process, the depth and the side profile of the resultant recess 128 are well controlled and the recess 128 has a substantially vertical profile. The top critical dimension CDt, the middle critical dimension CDm and the bottom critical dimension CDb of the resultant recess 128 are substantial equivalent. Later, the strained material portion 130 is formed in the recess 128 and fills up the recess 128. Regarding the base portion 131 of the strained material portion 130 below the top surface 114a of the insulators 114, the top dimension CDt of the base portion 131, the middle critical dimension CDm of the base portion 131 and the bottom critical dimension CDb of the base portion 131 of the strained material portion 130 are substantial equivalent. In some embodiments, the stop layer 104 embedded within the fins 116 is located above the bottom 130b of the strained material portion 130.