In one embodiment, as shown in FIG. 4D, 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 a first etching process and a second etching process, while the first etching process includes a main anisotropic etching process and the second etching process includes an isotropic etching process. In some embodiments, the fin 116 is etched through the main anisotropic etching process until the stop layer is penetrated through and then is over-etched through the isotropic etching process. By adjusting the etching conditions and etching selectivity of the anisotropic etching process and the isotropic etching process, the depth and the side profile of the resultant recess 128 are well controlled and the recess 128 has a narrow-base profile, so that the top critical dimension CDt is substantial equivalent to the middle critical dimension CDm and is larger than the bottom critical dimension CDb of the resultant recess 128. Later, the strained material portion 130 is formed in the recess 128 and fills up the recess 128. For 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 is substantial equivalent to the middle critical dimension CDm of the base portion 131 and the middle critical dimension CDm of the base portion 131 is larger than the bottom critical dimension CDb of the base portion 131 of the strained material portion 130. In some embodiments, the stop layer 104 embedded within the fins 116 is located above the bottom 130b of the strained material portion 130.