Aspect ratio in etching is the ratio of feature depth to width, which fundamentally governs etch difficulty and profile control. Definition: AR = depth / width. A 100nm wide, 1um deep trench has AR = 10:1. Transport limitation: At high AR, reactant species have difficulty reaching feature bottom while etch products struggle to escape. Ion shadowing: Ions at non-normal angles blocked by feature walls at high AR, reducing etch rate at bottom. Neutral transport: Reactive neutrals undergo multiple collisions with sidewalls before reaching bottom, reducing effective concentration. ARDE: Aspect Ratio Dependent Etching - etch rate decreases with increasing AR. Profile effects: Higher AR makes maintaining vertical sidewalls harder. Bowing, tapering, and undercut more likely. Deposition effects: Passivation species also have difficulty reaching bottom at high AR, affecting selectivity and profile. Current challenges: Advanced 3D NAND requires AR > 60:1. FinFET fin etching at tight pitch. TSV etching at AR 10-20:1. Process tuning: Higher ion energy, lower pressure, pulsed plasma help improve high-AR etch performance. Metrology: Measuring CD and profile at high AR requires specialized techniques (TEM, scatterometry).