1. A mark pattern for forming a process monitor mark in a patterned underlayer to monitor a patterning result of a photoresist layer over the patterned underlayer, wherein the patterned underlayer is formed by using a first mask having a first pattern in a main region of the first mask and the mark pattern at an unused region of the first mask and the first pattern possesses a first mask critical dimension, the mark pattern comprising: a second pattern with a second mask critical dimension, wherein the second mask critical dimension is as same as the first mask critical dimension; and a frame pattern enclosing the second pattern.