field is in proportion to the square of the electric field the intensity of the electric field shown in fig6 is as shown in fig8 and the of electric field strength shown in fig7 is as shown in fig9 this will be further detailed in comparison with the positional relation of first and second phase shift masks fig1 at a shows the relative positional relation between first and second phase shift masks in interference exposure fig1 at b shows the intensity of exposure light along c c in fig1 at a fig1 at c shows the intensity of exposure light along d d fig1 at d shows the intensity of the exposure light along e e referring to fig1 at a and b the intensity of exposure light resulting from interference between exposure light transmitted through the transmission portion tn of first phase shift mask 10a and exposure light transmitted through transmission portion ta of second phase shift mask 10b is largest meanwhile exposure light resulting from interference between exposure light transmitted through the transmission portion ta of first phase shift mask 10a and exposure light transmitted through the transmission portion tn of second phase shift mask 10b is smallest in the intensity of exposure light and almost 0 referring to fig1 at a and c the intensity of exposure light resulting from interference between exposure light transmitted through transmission portion tn.