1. An overlay control system, comprising: a metrology tool comprising: a current focus unit configured to determine a current focal length; a previous focus unit configured to determine a previous focal length by using at least two independent variables, wherein one of the independent variables is a thickness of a previous layer, a dielectric layer adjacent to the previous layer, or both of them, and another of the independent variables is a coefficient related to a material of the previous layer, the dielectric layer, or both of them; at least one image capturing unit configured to capture a current layer image of a current overlay mark on a current layer with the current focal length and capture a previous layer image of a previous overlay mark on the previous layer with the previous focal length; and a calculating unit configured to determine an overlay error between the current overlay mark and the previous overlay mark based on the current layer image and the previous layer image; and a controller configured to determine at least one parameter of an operating recipe for performing a photolithography process based on the overlay error.