ARDE (Aspect Ratio Dependent Etching) describes how etch rate systematically decreases as feature aspect ratio increases. Root cause: Transport limitations - both reactant delivery and product removal become harder in deeper, narrower features. Knudsen transport: In molecular flow regime (mean free path > feature width), species undergo random reflections off sidewalls. Probability of reaching bottom decreases exponentially with AR. Ion angular distribution: Ions with off-normal angles are shadowed by feature walls, reducing ion flux to bottom of high-AR features. Consequences: Dense features etch slower than isolated features. Small features etch slower than large features on same wafer. Magnitude: Etch rate can drop 30-50% or more at AR > 10:1 compared to open areas. Compensation: Overetch required to clear high-AR features, which stresses selectivity. Mitigation strategies: Lower pressure (more directional ions), higher bias (faster ions), pulsed plasma, cyclic etch-dep processes. Impact on design: Features of different AR need different etch times, causing integration challenges. 3D NAND: Extreme example where ARDE limits practical channel hole depth. Process optimization: Characterize etch rate vs AR curve and adjust target to compensate.