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Optimum Phase Condition for Low-Contrast X-Ray Masks

A low-contrast X-ray mask works as an attenuating phase-shift mask. The X-rays which pass through the clear and opaque areas of the mask interfere with each other and therefore enhance the image contrast. In proximity X-ray lithography, the rays which gather on an image plane have a phase difference caused by the optical path length. Thus, the phase shift (PS) on the wafer is determined by the sum of the PS caused by the absorber and the PS caused by the optical path length difference. In line-and-space (L&S) patterns, the phase-shift effect can be simplified by considering two typical rays: the rays passing through the center of the opaque area and those passing through the center of the clear area. With this model, the optimum phase condition is obtained by setting both the absorber PS and the geometric PS as π/2. The 70 nm and 50 nm L&S patterns are resolved under this condition.