Overlay accuracy of EUV1 using compensation method for nonflatness of mask

Two EUVL masks were made using the compensation method for nonflatness of a mask; and the EUV1 was used to evaluate the resulting overlay accuracy. For the same mask, the reproducibility of the intra-field overlay errors was better than 1 nm (3 sigma) without linear components; and that of the flatness was better than 20 nm PV. In contrast, the overlay errors were about 3 nm (3 sigma) for the two masks. The main cause of this degradation in overlay accuracy might be the difference in mask flatness (similar to 260 nm PV). Using overlay patterns corrected by the compensation method reduced the overlay errors to about 2.5 nm (3 sigma). Although the compensation method produced only a small change, it definitely improved the intra-field overlay of the EUV1. Furthermore, the EUV1 was used to evaluate the intra-wafer overlay for 23 shots. The single-machine overlay (SMO) was found to be better than 4.5 nm (Mean + 3 sigma(nonlinear)), and the mix-and-match overlay (MMO) between the EUV1 and an ArF immersion scanner (NSR-S610C) was about 20 nm (Mean + 3 sigma(nonlinear)). The main cause of the MMO errors might be the nonflatness of the mask and wafer chucks of the EUV1. Thus, the chucks must be made flatter to reduce MMO errors. This work was supported in part by NEDO.