Mask and method of forming pattern

A mask, comprising:

• a substrate;
• a main pattern disposed on the substrate, wherein the main pattern comprises:
• a first pattern;
• and
• second patterns, wherein the first pattern and the second patterns are arranged in an array configuration, and the second patterns surround the first pattern;
• and
• an assist pattern disposed on the substrate, wherein the assist pattern comprises sub patterns disposed in the first pattern or disposed in the second patterns.

A method of forming a pattern, comprising:

• providing a layout pattern to a computer system, wherein the layout pattern comprises a first pattern and second patterns, and two of the second patterns are disposed at two opposite sides of the first pattern in a first direction;
• forming a first assist pattern in the layout pattern;
• forming a second assist pattern outside the layout pattern;
• and
• outputting the layout pattern, the first assist pattern, and the second assist pattern through the computer system onto a mask, wherein the first assist pattern is located in the layout pattern, and the second assist pattern is located outside the layout pattern.