Accordingly, the spaces 610a and 610c in the second-type doped rows 604a-604b may be reused by the logic cell 802 with the cell structure 314 such that the usage rate of the second-type doped rows 604a-604d in the layout system 800 may be increased. Moreover, the resource of the layout system 800 may be increased by placing the rectilinear cell structures 314 into the rectilinear space fragments.

FIG. 9A is a diagram illustrating of a logic cell 902 placed in a mixed-row heights layout system 900 in accordance with some embodiments. The logic cell 902 is configured to be the cell structure 306 and is placed on the second-type doped row 904 in the mixed-row heights layout system 900. During the cell placement operation, the cell structure of the logic cell 902 may be replaced with a logic cell 910 with the cell structure 308 or a logic cell 912 with the cell structure 312 from the cell structure 306 to increase the usage of the first-type doped rows 906 and 908. FIG. 9B is a diagram illustrating of the logic cell 910 placed in the mixed-row heights layout system 900 in accordance with some embodiments. When the logic cell 902 is replaced with the logic cell 910, the logic cell 902 may be re-configured into two portions, i.e. 910a and 910b. The two portions 910a and 910b may be placed in the first-type doped rows 906 and 908 respectively. The logic cell 910 is similar to the above mentioned logic cell 702, thus the detailed description of the logic cell 910 is omitted here for brevity.