For semiconductor production, the process of deposition, lithography, and etching is repeated while sequential patterns are formed on the wafer continuously. In order to make semiconductors work properly, the accurate stacking of patterns in the multiple layers must be carried out in necessary locations to prevent any defects.

In the photo lithography that forms patterns on the wafer by using the stepper, the overlay measurement uses an optical device to measure if the patterns from the previous process and current process are accurately arranged and connected according to the plan.