Lithography method using photo mask

A method, comprising:

• forming a photoresist layer over a substrate comprising a first region and a second region;
• exposing the photoresist layer to a radiation through a photo mask, wherein the photo mask comprises a plurality of device features in a patterning region of the photo mask corresponding to the first region of the substrate and a plurality of sub-resolution assist features in a non-patterning region of the photo mask corresponding to the second region of the substrate, wherein an area of the photo mask occupied by the plurality of sub-resolution assist features is greater than an area of the photo mask occupied by the plurality of device features, and a portion of the radiation passing through spaces between the plurality of sub-resolution assist features is incident on the second region of the substrate;
• and
• developing the photoresist layer to form a patterned photoresist layer over the first region of the substrate, wherein the patterned photoresist layer is defined by the plurality of device features.

A method, comprising:

• receiving an integrated circuit (IC) design layout comprising a plurality of main features;
• inserting a plurality of first assist features adjacent to the plurality of main features to form a first modified IC design layout, wherein a width of one of the plurality of first assist features is less than a printable dimension of a photolithography process;
• determining a non-patterning region adjacent to the plurality of main features and the plurality of first assist features, wherein a percentage of a total area of the plurality of main features and the plurality of first assist features to an area of an exposure field of a photo mask is lower than about 50 percent;
• inserting a plurality of second assist features in the non-patterning region to form a second modified IC design layout, wherein a width of one of the plurality of second assist features is less than the printable dimension of the photolithography process;
• and
• fabricating the photo mask according to the second modified IC design layout.