An embodiment of the present invention provides an overlay measuring method which includes the steps of: forming a first grating and a second grating which are located in an overlapping region in different levels, respectively, are arranged with the substantially same pitch, and are inclined to each other so that a bias is changed along the longitudinal direction of the grating; irradiating the first and second gratings with an illumination beam; and obtaining information related to the change of the intensity of a diffraction beam due to the bias from an image pattern of the beam diffracted from the first and second gratings by the illumination beam. Accordingly, the present invention can accurately calculate an overlay error and monitor a process influence.
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