1. A process monitor mark for monitoring a patterning result of a photoresist layer over a patterned underlayer around the boundary between a peripheral region and a device region of a die, wherein the process monitor mark and a first pattern having a first critical dimension are simultaneously formed in the patterned underlayer at an unused region and the device region of the die respectively, the process monitor mark comprising: a first mark with a second critical dimension, wherein the second critical dimension is as same as the first critical dimension; and a frame mark enclosing the first pattern, wherein for a material layer on the patterned underlayer and below the photoresist layer, a width of the frame mark is about 2.2-3 times of a thickness of the material layer.