Methods for achieving sub-lithographic feature sizes in an integrated circuit (IC) layer are provided that use ion implantation to enhance or reduce the etch rate of a thin masking layer. The etch rates also can be enhanced or reduced at specific locations through multiple implantation steps. The implantation can be performed at tilted angles to achieve sub-lithographic implanted regions that are self-aligned to pre-existing photoresist or hard-mask features over the masking layer on the surface of a substrate. A higher density of features can be achieved in an IC layer than are present in an overlying masking layer with the use of ion implantation.
