Optimum phase condition for low-contrast X-ray masks

In proximity X-ray lithography (PXL), the use of a low-contrast mask strongly improves the resolution. Since a low-contrast mask works as an attenuating phase-shift mask, interference of the X-rays passed through the clear and opaque areas enhances the image contrast. Although previous works showed that the optimum phase shift angle of the X-ray absorber is much smaller than /spl pi/ in PXL, the optimum phase condition has not been clarified yet. In this article, we show the geometric phase shift caused by different optical path lengths plays an important role in image formation. By considering the geometric phase shift along with the absorber phase shift, the phase effect can be clearly understood.