A method of operating an IC manufacturing system includes determining whether an n-type active region of a cell or a p-type active region of the cell is a first active region based on a timing critical path of the cell, positioning the first active region along a cell height direction in an IC layout diagram of a cell, the first active region having a first total number of fins extending in a direction perpendicular to the cell height direction. The method also includes positioning a second active region in the cell along the cell height direction, the second active region being the n-type or p-type opposite the n-type or p-type of the first active region and having a second total number of fins less than the first total number of fins and extending in the direction, and storing the IC layout diagram of the cell in a cell library.