RIE lag (Reactive Ion Etching lag), also known as aspect-ratio-dependent etching (ARDE), is a phenomenon in plasma etching where narrow or high-aspect-ratio features etch more slowly than wider or low-aspect-ratio features on the same wafer, even though they are etched simultaneously under identical plasma conditions. The result is that when the widest trench reaches target depth, narrower trenches are shallower, creating a depth differential that depends on feature width and aspect ratio. RIE lag arises from several interrelated transport mechanisms within the features. First, Knudsen transport limitation: as features become narrower and deeper, the probability of reactive neutral species (etchant radicals) reaching the feature bottom decreases because molecules undergo multiple collisions with the sidewalls during their random-walk trajectory through the feature, and many are reflected back out before reaching the etch front. Second, ion angular distribution effects: ions entering narrow features must have near-vertical trajectories to reach the bottom without striking the sidewalls, effectively reducing the ion flux at the bottom of high-aspect-ratio features. Third, etch byproduct redeposition: volatile etch products generated at the feature bottom have a greater probability of redepositing on the sidewalls or bottom surface in narrow features due to the reduced escape solid angle, creating a micro-masking effect. Fourth, charging effects: differential charging of insulating sidewalls and bottom surfaces in narrow features can deflect ions or retard their energy, further reducing etch rate. The severity of RIE lag increases with aspect ratio and is particularly challenging in deep trench etching for DRAM, through-silicon vias (TSVs), and 3D NAND channel holes. Mitigation approaches include increasing process pressure to enhance radical supply, using pulsed plasma to modulate ion energy distribution, optimizing gas chemistry for maximum radical generation, and employing Bosch-type cyclic processes with tuned passivation and etch step durations tailored to combat ARDE.