ald cycle

An ALD (Atomic Layer Deposition) cycle consists of four sequential steps that deposit exactly one atomic layer of material per cycle. **Step 1 - Precursor pulse**: First precursor gas introduced, chemisorbs onto surface forming a self-limiting monolayer. Excess precursor does not adsorb. **Step 2 - Purge**: Inert gas (N2 or Ar) flushes unreacted precursor and byproducts from chamber. **Step 3 - Reactant pulse**: Second reactant gas introduced, reacts with adsorbed precursor layer to form desired film. Also self-limiting. **Step 4 - Purge**: Inert gas removes unreacted reactant and byproducts. **Self-limiting**: Each half-reaction saturates at one monolayer regardless of exposure time (above minimum dose). This gives atomic-level thickness control. **Growth rate**: Typically 0.5-1.5 angstroms per cycle depending on material. **Cycle time**: 1-30 seconds per cycle. Thicker films require many cycles (slow process). **Temperature window**: ALD has optimal temperature range where self-limiting behavior holds. Too low = condensation. Too high = decomposition. **Conformality**: Near-perfect step coverage (>95%) even in extreme AR features. Key advantage over CVD. **Applications**: High-k gate dielectrics (HfO2), spacers, barrier layers, capacitor dielectrics.