Assist Feature Placement Analysis Using Focus Sensitivity Models

Sub-Resolution Assist Features (SRAFs) are placed into patterns to enhance the through process imaging performance of critical features. SRAFs are typically placed using complex rules to achieve optimal configurations for a pattern. However, as manufacturing process nodes are growing increasingly complex, the SRAF placement rules will most likely be unable to produce optimal performance on some critical features. A primary impediment to resolving these problems is identifying poorly performing features in an efficient manner. A new process model form referred to as a Focus Sensitivity Model (FSM) is capable of rapidly analyzing SRAF placement for through process pattern performance. This study will demonstrate that an FSM is capable of finding suboptimal SRAF placements as well as missing SRAFs. In addition, the study suggests that the FSM does not need to comprehend the entire photolithography process to analyze SRAF placement. This results in simpler models that can be generated before a manufacturing process enters its development phase.