ald precursor chemistry
**Atomic Layer Deposition (ALD) Process Chemistry** is the **self-limiting thin-film deposition technique where alternating pulses of two or more chemical precursors react with the substrate surface one atomic layer at a time — providing angstrom-level thickness control, perfect conformality on 3D structures, and composition tunability that makes ALD the indispensable deposition method for gate dielectrics, barrier layers, spacers, and every other film in advanced CMOS where thickness uniformity below 1nm matters**.
**The ALD Cycle**
1. **Precursor A Pulse**: Metal-organic or halide precursor (e.g., TMA — trimethylaluminum for Al₂O₃, or TDMAT — tetrakis-dimethylamido-titanium for TiN) flows into the chamber. Molecules chemisorb onto surface reactive sites (typically -OH groups). Reaction is self-limiting: once all surface sites are occupied, excess precursor does not react.
2. **Purge 1**: Inert gas (N₂ or Ar) flushes unreacted precursor and byproducts from the chamber.
3. **Precursor B Pulse (Co-reactant)**: Oxidizer (H₂O, O₃) or reducer (NH₃, H₂ plasma) reacts with the chemisorbed surface species, completing the desired film chemistry and regenerating surface reactive sites for the next cycle.
4. **Purge 2**: Flushes excess co-reactant and byproducts.
One cycle deposits 0.5-1.2 Å of film. Desired thickness is achieved by repeating the cycle — 100 cycles for 10nm, with thickness precision of ±0.5 Å across a 300mm wafer.
**Self-Limiting Chemistry**
The defining feature of ALD: each half-reaction saturates when all available surface sites have reacted. This provides:
- **Thickness uniformity**: Identical deposition on all surfaces regardless of precursor flux variations (unlike CVD, which is flux-dependent).
- **Conformality**: Inside a 100:1 aspect ratio feature, precursor molecules eventually reach the bottom and saturate all surfaces. 100% step coverage is theoretically achievable (practically >98%).
- **Digital thickness control**: Each cycle adds a fixed amount — thickness is programmed by cycle count.
**Thermal vs. Plasma-Enhanced ALD**
- **Thermal ALD**: Both half-reactions proceed thermally. Temperature window (process window) is 200-400°C for most processes. Lower reactivity limits material choices at low temperature.
- **PEALD (Plasma-Enhanced ALD)**: The co-reactant step uses plasma-generated radicals (O*, N*, H*). Enables lower deposition temperature (50-200°C), higher film density, better electrical properties, and access to materials (metals, nitrides) that are difficult or impossible by thermal ALD alone.
**Key ALD Films in CMOS**
| Film | Precursors | Application | Thickness |
|------|-----------|-------------|----------|
| HfO₂ | HfCl₄/H₂O | High-k gate dielectric | 1.5-2.5 nm |
| Al₂O₃ | TMA/H₂O | Gate cap, passivation | 1-5 nm |
| TiN | TDMAT/NH₃ | Metal gate, barrier | 2-10 nm |
| SiO₂ | BDEAS/O₃ plasma | Spacer, liner | 2-15 nm |
| W | WF₆/Si₂H₆ | Contact fill (nucleation) | 2-5 nm |
ALD Process Chemistry is **the angstrom-precision deposition engine of advanced semiconductor manufacturing** — the only technique that can deposit films with sub-nanometer control on the extreme 3D topographies of FinFET, nanosheet, and CFET architectures.