Obtaining good post-etching CD uniformity is getting more and more important in advanced processes such as 90 nm, 65 nm, and even 45nm for 300 mm wafers. But process noise greatly impacts the CD uniformity, especially etching bias and metrology noise. To achieve a CD uniformity of below 3 nm for 300 mm post-etch wafers, the metrology noise and process noise must be reduced and compensated for. In this paper, we demonstrate spectroscopic ellipsometry CD with the advantages of high stability and high accuracy to get CD information precisely, and high sensitivity to monitor PEB temperature and exposure energy fine variation in order to compensate for the etching bias.
This study focuses on the feasibility of minimizing the CD uniformity of post-etch wafers by ADI CD compensation for a 300 mm leading-edge fab. Because the CD uniformity of after-development inspection (ADI) wafers from a leading-edge lithographic tool could be in the range of only 3 nm, it is very challenging to reveal the true CD signature of an ADI wafer using a metrology tool. A spectroscopic ellipsometry based metrology tool, SpectraCD, was used in this study. In order to make sure the CD signatures reported by SpectraCD reveal the true behavior of a lithographic tool, the well-published Total Test Repeatability (TTR) test was adopted. In comparison with 3 nm CD uniformity, a 0.2 nm TTR is accurate enough for this study. In addition, from more than 100 wafers produced within a week, the CD signature of ADI wafers is very stable on wafer-to-wafer and lot-to-lot bases. Basically, all the ADI wafers produced from a single post-exposure-bake plate of an exposure tool within a week show very similar CD signatures. The feasibility of reaching a CD uniformity of 3 nm for post-etch wafers will be demonstrated in this study.