step coverage

Step coverage measures how well a deposited film coats vertical sidewalls and bottoms of topographic features relative to flat surfaces. **Definition**: Ratio of film thickness on sidewall or bottom to thickness on top flat surface. Expressed as percentage. **Ideal**: 100% step coverage means perfectly conformal coating everywhere. **CVD methods comparison**: ALD ~100% (best). LPCVD 80-100%. PECVD 30-80%. PVD 5-30%. **Mechanism dependence**: Surface-reaction-limited processes (LPCVD, ALD) give best step coverage because precursors reach all surfaces before reacting. **Transport limitation**: In mass-transport-limited regime, reactants deposit preferentially on top surfaces before reaching sidewalls and bottom. **Feature geometry**: Higher aspect ratio features are harder to coat. Step coverage degrades with increasing AR. **Measurement**: Cross-section SEM or TEM of features with deposited film. Measure thickness at top, sidewall, and bottom. **Importance**: Barrier layers, liners, and dielectrics must coat all surfaces to function properly. Void-free gap fill required for ILD. **Bottom coverage**: Ratio of bottom thickness to top. Often worse than sidewall coverage. **Overhang**: Excess deposition at top of feature can close off opening before filling bottom. Leads to voids.