A method includes receiving a design rule deck including a predetermined set of widths and spacings associated with active regions. The method also includes providing a cell library including cells having respective active regions, wherein widths and spacings of the active regions are selected from the predetermined set of the design rule deck. The method includes placing a first cell and a second cell from the cell library in a design layout. The first cell has a cell height in a first direction, and a first active region having a first width in the first direction. The second cell has the cell height, and a second active region having a second width in the first direction. The second width is different from the first width. The method further includes manufacturing a semiconductor device according to the design layout.