Relationship between Normalized Image Log Slope and Exposure Latitude in advanced technodes

It can help evaluate and improve photolithographic process to understand the correlation between metrics, among which NILS and EL are of great importance. As is well known, if both of EL and NILS are evaluated based on aerial image cutline, the relationship between them are fixed, that is, EL is 10 times NILS when ±10% CD is considered. The purpose of this paper is to demonstrate the relationship between EL and NILS with resist effect considered. The study is undertaken with both simulation and experimental data in two litho-processes. One is PTD and the other is NTD while both of them are line-dominated. The results of the experiment provide us many detailed information about this topic. First of all, EL is not always around 10 times NILS when the resist effect participates. Second, a linear relationship between EL and NILS, which was reported by C. Mack, is only proximately true in PTD while the relationship in NTD is much more complicated. Finally, the relationship varies among different pattern type in NTD process.From what we have learnt from above, it can be said that a minimum acceptable NILS can be settled for a PTD process as long as EL value is fixed. While in NTD process, greater attention should be paid since a larger value NILS doesn’t necessarily lead to an improvement of EL for a specific pattern.