Multiple-focus exposure in strong phase-shift lithography: improvement of CD-focus characteristics and CD controllability

To improve the CD controllability of isolated lines, we have developed a new method using multiple-focus exposure in alternating phase-shift lithography. In this paper, the imaging performance such as DOF, exposure latitude, mask linearity, and CD controllability is discussed through both experiments and simulation. Multiple-focus exposure experiments were performed using a KrF scanner by giving a tilt offset between the image focal plane and the wafer leveling plane. For the conventional alternating phase-shift method, the CD-focus curve showed a strong concave shape and thus the DOF was rather small. By applying multiple-focus exposure, the shape of the CD-focus curve changed from concave to flat, and therefore the DOF was much improved. We have also found that the CD controllability considering focus errors can be improved by our method.