AXIAL IMAGE SUPERPOSING (SUPER-FLEX) EFFECT USING THE MASK MODULATION METHOD FOR OPTICAL LITHOGRAPHY

Phase and amplitude modulation in a mask can achieve amplitude superposition for multiple images along the light axis and phase control between them. This improves the depth of focus while keeping high resolution under high coherence illumination in optical lithography. This method explains the mechanism of the depth-of-focus enhancement effect in "peripherally added" or "edge-enhancing" phase-shifting masks and provides the most effective phase-shifter arrangement. Because a high coherency is required in this method, however, the proximity effect becomes a serious problem in practical LSI patterns.