A mask at frequency domain comprises a plurality of amplitude patterns positioned on a first surface of the mask and a plurality of phase patterns positioned on a second surface of the mask. The amplitude patterns have different vertical thicknesses to change the amplitude of an exposing light, and the phase patterns have different vertical thicknesses to change the phase of the exposing light. Preferably, the amplitude patterns are made of inorganic material, such as molybdenum silicide (MoSi), and the phase patterns are made of transparent material, such as quartz. The amplitude patterns and phase patterns are the Fourier transform of a circuit layout, and their numbers and positions are correspondent with each other.