ALD Reactor Designs
ALD Reactor Designs
ALD reactors must efficiently deliver, react, and purge precursors in rapid cycles:
- Temporal (pulse-purge) ALD: The standard approach. Precursor A is pulsed, then purged with inert gas, then precursor B is pulsed and purged. One cycle = one atomic layer. Cycle time: 1–10 seconds.
- Spatial ALD: Instead of time-sequenced pulses, the wafer physically moves between different gas zones separated by inert gas curtains. Much faster — cycle times under 0.1 seconds. Used for high-throughput applications.
- Batch ALD: Process 100+ wafers simultaneously in a vertical furnace. Lower throughput per cycle but high total throughput. Used for films where absolute thickness control is less critical.
- Plasma-enhanced ALD (PEALD): Uses plasma in the second half-cycle for better reactivity at lower temperatures. Critical for BEOL applications.
Key Concept: ALD Throughput Challenge
ALD's main weakness is speed — depositing 10 nm at 0.1 nm/cycle requires 100 cycles. At ~5 seconds/cycle, that's over 8 minutes per wafer for a single film. Equipment makers address this with multi-station designs (4 wafers simultaneously) and faster purge systems.
ALD Precursors and Common Film Chemistries
ALD Precursors and Common Film Chemistries
Each ALD recipe is a paired metal-precursor + co-reactant chemistry. A few that dominate production:
| Film | Metal precursor | Co-reactant | Application |
|---|---|---|---|
| HfO₂ | HfCl₄, TEMA-Hf, TDMA-Hf | H₂O or O₃ | High-k gate dielectric (45 nm and below) |
| Al₂O₃ | TMA (trimethylaluminum) | H₂O | Capacitor dielectric, encapsulation, blocking oxide |
| TiN | TiCl₄ or TDMAT | NH₃ or H₂ plasma | Barrier, metal gate work-function layer |
| TaN | PDMAT / TBTDET | NH₃ plasma | Cu barrier in advanced damascene |
| W | WF₆ | SiH₄ (nucleation), then H₂ | Plug fill, gap-fill metal |
| Ru | Ru carbonyls, RuO₄ | O₂ or H₂ | Barrierless via fill, capacitor electrodes |
Two practical knobs the engineer tunes per recipe:
- Saturation dose — pulse the precursor long enough that the surface is fully covered, but no longer (extra time wastes precursor and throughput).
- Purge time — must be long enough to remove all precursor and byproducts; otherwise A and B meet in the gas phase and you fall back to CVD-like growth with worse conformality.
Key Concept: Why ALD Pricing Hurts
Many ALD precursors are exotic organometallics costing $10k–$50k per kilogram. With single-digit-percent utilization per cycle, ALD precursor cost can dominate cost-of-ownership for thin high-k and barrier films.
Knowledge Check
Knowledge Check
1 / 2What is spatial ALD?