Patent ID: 8313992

Claim:
A method of making a NAND string, comprising: depositing a photoresist layer over a NAND gate layer located over a substrate; exposing the photoresist layer to radiation though a lithography mask, the lithography mask comprising: a plurality of patterning features formed on a mask substrate; and a first plurality of sub-resolution assist features (SRAFs) formed on the mask substrate; wherein: the plurality of patterning features are dimensioned equal to or greater than a resolution limit of a lithography system used with the lithography mask, the lithography system having a lithography radiation source; each of the plurality of patterning features has a length greater than a width; the plurality of patterning features extend in a first longitudinal direction along their length on the mask substrate; the plurality of patterning features correspond in shape to NAND select gates and NAND control gates; the first plurality of SRAFs are dimensioned less than a resolution limit of the lithography system used with the lithography mask; each of the first plurality of SRAFs has a length greater than a width; the first plurality of SRAFs extend in a second longitudinal direction along their length on the mask substrate, where the second longitudinal direction is substantially perpendicular to the first longitudinal direction; the first plurality of SRAFs are located between two adjacent patterning features; a first end of each of the first plurality of SRAFs connects to a side of a first of the two adjacent patterning features; and a second end of each of the first plurality of SRAFs connects to a side of a second of the two adjacent patterning features; and selectively removing portions of the photoresist layer such that photoresist patterns corresponding to the plurality of patterning features are formed and no photoresist patterns corresponding to the SRAFs are formed; and etching the NAND gate layer using the photoresist patterns as a mask to form a plurality of NAND control gates and select gates.