Line edge roughness reduction for 7nm metals

Line edge roughness (LER) reduction is critical during the patterning process definition and development, as the critical dimension (CD) and pitch scale in advanced semiconductor technology nodes. In this paper, we will focus on a 7nm self-aligned double patterning (SADP) process for use in back end of line (BEOL). Specifically, we will investigate LER from various lithography options and how LER changes through downstream processes, including mandrel etch, spacer deposition, hard mask open, dielectric etch and wet clean. We characterized LER as a function of several mandrel etch parameters such as O2 flow rate, over etch rate percentage and polymer deposition rates. We also characterized LER response to dielectric etch parameters and found that while some etch processes may smooth high frequency LER, there are additional cases where the final etch and wet-clean increased LER and line wiggling. Overall, we observed that lithography is the primary source of LER and we have the opportunity to reduce LER by both design and process optimization. In this paper we focused on characterization of a standard logic cell with varied CD and pitch. We looked through various designs, retargeting as well as both negative tone developer (NTD) and positive tone developer (PTD) resists for the LER reduction. We also analyzed the image log slope (ILS) of each corresponding edge and the process windows of the resist candidates. We concluded that ILS improvement and resist selection are the primary knobs to reduce LER. With optimization, we can achieve LER close to the process assumption targets for 7nm technology node. Further LER reduction techniques are definitely needed in both 7nm and future nodes even with migration from 193nm to EUV lithography.