Embodiment of the present invention provides a method of forming a semiconductor device in a sidewall image transfer process with multiple critical dimensions. The method includes forming a multi-level dielectric layer over a plurality of mandrels, the multi-level dielectric layer having a plurality of regions covering the plurality of mandrels, the plurality of regions of the multi-level dielectric layer having different thicknesses; etching the plurality of regions of the multi-level dielectric layer into spacers by applying a directional etching process, the spacers being formed next to sidewalls of the plurality of mandrels and having different widths corresponding to the different thicknesses of the plurality of regions of the multi-level dielectric layer; removing the plurality of mandrels in-between the spacers; and transferring bottom images of the spacers into one or more layers underneath the spacers.