1. A method of inspecting a structure, comprising: preparing a plurality of preliminary spectrums each including a plurality of reference diffraction intensities, the reference diffraction intensities corresponding to critical dimensions of a plurality of reference structures; obtaining a linear spectrum based on the preliminary spectrums in a critical dimension range; radiating first light onto one or more measurement structures; measuring one or more measurement diffraction intensities of the first light diffracted by the measurement structures; and obtaining critical dimensions of the measurement structures based on the measurement diffraction intensities and the linear spectrum.