1. A method, comprising: moving a first wafer stage of a first exposure apparatus to a first location so that a first imaging sensor integrated with the first wafer stage is able to image a mask; performing a first exposure process to capture a first physical image of the mask in the first exposure apparatus operating in a first mode using a first exposure source and the first imaging sensor, wherein the first mode is one of a scan mode and a step-and-scan mode; moving the first wafer stage to a second location so that a first wafer secured on the first wafer stage adjacent to the first imaging sensor is able to be exposed by a second exposure process; performing the second exposure process on the first wafer in the first exposure apparatus operating in a second mode using the first exposure source and the mask, wherein the second mode is the other of the scan mode and the step-and-scan mode; moving a second wafer stage of a second exposure apparatus to a third location so that a second imaging sensor integrated with the second wafer stage is able to image the mask; performing a third exposure process to capture a second physical image of the mask in the second exposure apparatus operating in the first mode using a second exposure source and the second imaging sensor, wherein the second exposure apparatus is different from the first exposure apparatus, wherein the second exposure source is different from the first exposure source, and wherein the second imaging sensor is different from the first imaging sensor; moving the second wafer stage to a fourth location so that a second wafer secured on the second wafer stage adjacent to the second imaging sensor is able to be exposed by a fourth exposure process; performing the fourth exposure process to the second wafer in the second exposure apparatus operating in the second mode using the second exposure source and the mask; comparing the first physical image of the mask and the second physical image of the mask for a difference therebetween; and determining an action according to the difference.