In one embodiment, as shown in FIG. 4A, the fin 116 not covered by the stack structure 120 is etched and the etching stops at the stop layer 104. 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 an anisotropic etching process and the second etching process includes an isotropic etching process. In some embodiments, the fin 116 is etched through the anisotropic etching process until half or more of the depth of the recess 128 is reached and then is further etched through the isotropic etching process and the stop layer 104 is removed. By adjusting the etching conditions and etching selectivity of the anisotropic etching process and the isotropic etching process, the side profile of the resultant recess 128 is well controlled and the recess 128 has a narrow-base profile (i.e., the bottom of the recess 128 is smaller than the opening of the recess 128), so that the top critical dimension CDt of the resultant recess 128 is substantially equivalent to the middle critical dimension CDm of the resultant recess 128 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. Regarding the strained material portion 130 filled within the recess 128 (i.e., 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 substantially equivalent to the middle critical dimension CDm of the base portion 131 and is larger than the bottom critical dimension CDb of the base portion 131 of the strained material portion 130. In some embodiments, the bottom 130b of the strained material portion 130 is substantially leveled with the stop layer 104 embedded within the fins 116.