Pattern Transfer Characteristics of Transparent Phase Shifting Mask

Phase shifting technologies achieve significant improvement in both resolution and depth of focus (DOF). We have proposed a “transparent-type” phase shifting mask which has a simple structure (no Cr layer) that considerably improves the resolution limit of optical lithography.1) In this paper, we describe in detail pattern transfer characteristics of the transparent phase shifting mask with i-line steppers. The effects of numerical aperture (NA) and coherence factor (σ) on the resolution capability and DOF were investigated using simulations and experiments. Patterns are printed using i-line steppers with a numerical aperture (NA) of 0.45 and 0.65. A pattern of 0.23 µm lines and spaces is resolved with a 0.45 NA i-line stepper. DOF for 0.25-µm features is found to be in the 1.5-µm range. Well-resolved 0.18-µm lines and spaces are obtained with a 0.65-NA i-line stepper. Transparent phase shifting mask technologies extend the resolution limit of i-line lithography to the resist features below 0.2 µm.