conformality

Conformality is the ability of a deposition process to coat all surfaces with equal thickness regardless of surface orientation and topography. **Quantification**: Ratio of minimum to maximum film thickness across a feature. 100% = perfectly conformal. **Process ranking**: ALD > LPCVD > PECVD > PVD (sputtering) > evaporation, in terms of conformality. **ALD conformality**: Self-limiting surface reactions ensure equal growth on all accessible surfaces. Can achieve >99% conformality in extreme AR features. **LPCVD conformality**: Surface-reaction-limited regime at low pressure allows precursors to diffuse into features before reacting. 90-100% typical. **PECVD**: Moderate conformality. Directional ion component and mass-transport limitations reduce sidewall coverage. **Importance**: Barrier and liner layers must be continuous on all surfaces. Gate dielectrics must be uniform on 3D structures (FinFETs, GAA). **Gap fill**: Conformal deposition fills gaps from bottom-up without voids when combined with proper chemistry. **Challenges**: Extreme AR features (>50:1 in 3D NAND) challenge even ALD due to long diffusion paths. **Measurement**: TEM cross-sections of features at various AR values. **Selectivity alternative**: Area-selective deposition grows film only where needed, complementing conformality.