Atomic Layer Deposition (ALD) is the ultra-precise thin film deposition technique that grows materials one atomic layer at a time through sequential, self-limiting surface reactions — achieving angstrom-level thickness control, 100% conformal coverage on 3D structures with aspect ratios >100:1, and composition uniformity across 300 mm wafers, making it the indispensable deposition method for gate dielectrics, barrier layers, and capacitor films at advanced semiconductor nodes where even 1 Å of thickness variation is unacceptable.

The ALD Cycle

Each ALD cycle deposits exactly one atomic layer (~1 Å) through four steps: 1. Precursor A Pulse: Metal-organic or halide precursor (e.g., trimethylaluminum, TMA: Al(CH₃)₃) flows into the chamber. It chemisorbs on the surface, saturating all available reactive sites. 2. Purge: Inert gas (N₂ or Ar) purges excess precursor and byproducts. Only the chemisorbed monolayer remains. 3. Precursor B Pulse: Co-reactant (e.g., H₂O or O₃ for oxides; NH₃ for nitrides) reacts with the chemisorbed layer, forming the desired material (Al₂O₃) and regenerating surface reactive sites. 4. Purge: Remove excess co-reactant and byproducts.

Self-Limiting Growth: Because each precursor saturates the surface, the deposited thickness per cycle is fixed regardless of exposure time or precursor flow rate (once saturation is reached). This self-limiting nature is what gives ALD its extraordinary uniformity and conformality.

Growth Rate: 0.5-2.0 Å/cycle depending on material. A 5 nm film requires 25-100 cycles.

Key ALD Materials in Semiconductor Manufacturing

Thermal vs. Plasma-Enhanced ALD

• Thermal ALD: Both reactions are thermally driven (150-350°C). Truly conformal because reactive species are neutral molecules that diffuse equally into features. Used for DRAM capacitors and gap fill.
• PE-ALD (Plasma-Enhanced): Precursor B is replaced by plasma-generated radicals (O, N, H radicals). Lower deposition temperature (50-200°C) and better film quality for some materials. Conformality slightly reduced in extreme AR due to radical recombination on surfaces. Used for gate dielectrics and low-temperature processing.

ALD Conformality in Extreme Structures

ALD is the only deposition technique that can coat 100:1 AR structures conformally:

• DRAM capacitor holes (6 nm diameter × 600 nm deep): ALD ZrO₂ + TiN coat all surfaces uniformly.
• 3D NAND channel holes (80-100:1 AR): ALD ONO gate stack.
• GAA nanosheet channels: ALD wraps around all sides of suspended nanosheets.

Throughput and Cost

ALD is inherently slow (~1 Å/cycle, 1-10 seconds/cycle). A 5 nm film takes 5-15 minutes. To compensate:

• Batch ALD: Process 50-100 wafers simultaneously in a tube furnace configuration. Used for non-critical films.
• Spatial ALD: Wafer moves over separate precursor zones (no purge needed between zones). Throughput: 10-50× faster than temporal ALD.

ALD is the atomic sculptor of the semiconductor industry — the deposition technique that provides the angstrom-precision film control required for the gate oxides that determine transistor performance and the capacitor dielectrics that define memory density, making it irreplaceable at every advanced node.