Optical performance enhancement technique for 45-nm node with binary mask

The optical resolution of Binary mask (BIN) surpasses that of phase shift mask (PSM) when the node size is smaller than 45nm. Therefore, resolution enhancement technology (RET) of the binary mask has become more important in order to realize 45nm node lithography. In this paper, we present a unique way to improve the resolution of conventional binary mask simply by depositing a thin oxide film on the patterned side of the mask. The improvement has been proven by 3D rigorous simulation and real experiment. The simulation result predicts that the binary mask with a thin oxide layer would show increased normalized image log slope (NILS) by more than 10 %, compared to the conventional binary mask. The real experimental evaluation shows even further improved NILS when a thin oxide layer is deposited on the binary mask. The mask structure with a thin oxide layer turns out to have advantages over the conventional binary mask in terms of not only improved NILS but also DOF margin aspects. We further investigated resolution enhancement of the mask structure with a thin oxide layer depending on different duty ratios of the mask pattern.