Effective Lithography Leveling Improvement was Achieved by Retaining Wafer Back-Surface Nitride During a Novel SMT Nitride Remove Process

The shrinkage of critical dimension requires more delicate lithography leveling; otherwise, it will easily leads to various pattern failures. Worse wafer back-surface flatness caused by repetitive BEOL backside clean leads to worse lithography leveling and defocuses follow. BEOL lithography leveling performance of a 28 nm node wafer is significantly improved by retaining its back-surface nitride during a novel SMT nitride remove process. The thin back-surface nitride layer could resist the corrosion of backside clean chemicals and maintain wafer back-surface flatness in a more preferable level. In this study a conventional one-step batch SMT nitride remove wet process is replaced by two continuous steps. The first one is a single wafer front-side wet process, and the second one is a conventional batch one. The via-5 leveling data demonstrate a 60% to 80% improvement of leveling range from novel two-step SMT nitride remove wafers. These wafers are defocus free, while the baseline wafers still suffer various kinds of defocus defects. Average CP yield improvement is about 0.5%.