notching

Notching is an undercut defect at the bottom of etched features caused by charge buildup on insulating layers during plasma etching. **Mechanism**: When etch reaches an insulating layer (oxide), positive charge accumulates from trapped ions. This deflects subsequent incoming ions sideways into the feature base, causing lateral etching. **Profile**: Characteristic foot-shaped undercut at the interface between conducting and insulating layers. **Charge buildup**: Insulating surfaces cannot dissipate charge. Electric field builds, deflecting ion trajectories. **Feature dependence**: Worse in isolated features than dense arrays due to different charging conditions. **Impact**: Reduces CD control at bottom of features. Can undermine structural integrity. **Mitigation**: Pulsed plasma - off-cycles allow charge dissipation. Low-frequency bias reduces charging. **Electron flooding**: Supplying electrons during etch neutralizes surface charge. **Endpoint control**: Minimize overetch time on insulating surfaces. Precise endpoint detection critical. **Design consideration**: Layout-dependent notching can cause systematic yield loss. **Characterization**: Cross-section SEM to visualize notch profile and quantify lateral extent.