Optical Imaging With Phase Shift Masks

Mark D Prouty; Andrew R. Neureuther

doi:10.1117/12.941921

29 June 1984 Optical Imaging With Phase Shift Masks

Mark D Prouty, Andrew R. Neureuther

Proceedings Volume 0470, Optical Microlithography III: Technology for the Next Decade; (1984) https://doi.org/10.1117/12.941921
Event: 1984 Microlithography Conferences, 1984, Santa Clara, United States

Abstract

The imaging characteristics of masks which have arbitrary phase shifts between adjacent regions has been investigated. The process simulation program SAMPLE was extended to compute images of masks with an arbitrary number of regions of varying amplitude and phase transmission. In order to do this, the code was modified to calculate the imaginary, in addition to the real, part of the transmission cross-coefficient and perform complex multiplications within the image intensity summation. The disadvantage of phase shift masks lies in the high printability of defects. Phase shifts of 180° cause defects twice as small to print. This printability is phase shift dependent, and may be considerably reduced with smaller phase shifts. A second disadvantage is that phase shifts occurring in the middle of large clear areas produce such a deep dip in the image intensity that these clear field transitions are impractical. The advantage of phase shift masks lies in reducing proximity effects between adjacent features. The best measure of this reduction is the improvement in image con-trast for periodic arrays of lines. A significant improvement can be obtained, especially at low sigma. This improved contrast enables a smaller linewidth to be used. At a sigma of 0.3 the size of periodic features may be reduced from 1.2 μm to 0.5 μm, still maintaining a contrast of 0.85. Not only is the contrast improved, but its sensitivity to defocus is actually reduced, thereby increasing the useful depth of focus. Phase shifts less than 180° give similar results although the improvement decreases rapidly for phase shifts less than 120°. The image of isolated features may be improved 30 percent by using unprintable phase shifted proximity features. By proper defect control and elimination of clear field transitions, very significant improvements in aerial image quality are possible with phase masks.

Citation Download Citation

Mark D Prouty and Andrew R. Neureuther "Optical Imaging With Phase Shift Masks", Proc. SPIE 0470, Optical Microlithography III: Technology for the Next Decade, (29 June 1984); https://doi.org/10.1117/12.941921

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