KrF bilayer resist defects: cause, analysis, and reduction

Dual damascene processing for back end of the line (BEOL) layers can employ bilayer film stack approaches for lithographic patterning. These bilayer resist systems are more prevalent for KrF layers and have many unique characteristics, including silicon-containing photoresists and gap fill underlayer material that must also act as a bottom anti-reflective coating (BARC). Bilayer resists pattern for copper deposition; as such, defect levels are a critical concern, as any post-patterning bridging or residue defects can often times render an entire die inoperable due to electrical shorts or breaks. Here, two such defect types were found: missing resist patterns and resist residue. Through several experiments and with process optimization, the defect origins were elucidated and the defects themselves significantly reduced. This work will detail the examination, root causes and eventual elimination of these significant bilayer resist defects.