Abstract: A method of manufacturing a semiconductor device includes forming a first lower overlay key including first and second patterns in a lower layer, forming a first upper overlay key including third and fourth patterns in an upper layer vertically disposed on the lower layer, irradiating a first measurement light to a first region of interest (ROI) over first portions of the first and second patterns to detect a first overlay error and irradiating a second measurement light to a second ROI over second portions of the first and second patterns, the second ROI being different from the first ROI, to detect a second overlay error.