Why Annealing Is Needed
Why Annealing Is Needed
After ion implantation, two problems must be fixed through thermal annealing:
- Crystal damage repair: The silicon lattice was disrupted by bombarding ions. Heating allows atoms to return to their proper lattice positions (recrystallization).
- Dopant activation: Implanted dopant atoms sit in random interstitial positions. Annealing moves them into substitutional lattice sites where they become electrically active (donors or acceptors).
The challenge: annealing requires high temperature to activate dopants, but too much heat causes dopant diffusion — spreading the carefully implanted profile. Modern solutions:
- Rapid Thermal Anneal (RTA): Heat to 1000–1100°C for 1–10 seconds using halogen lamps
- Spike anneal: Ramp to ~1050°C with zero hold time at peak temperature
- Laser anneal (millisecond/nanosecond): Heat only the surface to ~1300°C for microseconds — maximum activation with minimal diffusion
Key Concept: The Anneal Trade-off
Higher temperature and longer time = better activation but more diffusion. The industry has progressively moved to shorter, hotter anneals to maximize the activation/diffusion ratio. Laser annealing represents the extreme — near-melting temperatures for mere nanoseconds.
Thermal Budget and Diffusion
Thermal Budget and Diffusion
Every time the wafer sees high temperature, dopants diffuse. The cumulative impact is the thermal budget, often summarised by an effective Dt product:
(Dt)eff = Σi D(Ti) · ti
where D(T) = D₀ · exp(−EA / kT) is Fick's diffusion coefficient and the sum runs over every anneal, deposition, or oxidation step. The resulting dopant spread after a series of steps is roughly √(Dt)eff.
| Dopant in Si | D₀ (cm²/s) | EA (eV) | Practical implication |
|---|---|---|---|
| Boron (B) | ~10.5 | ~3.69 | Fastest diffuser of common dopants — hardest to keep junctions sharp |
| Phosphorus (P) | ~10.5 | ~3.69 | Similar to B; channel-stop and well dopant |
| Arsenic (As) | ~0.32 | ~3.56 | Slow — preferred for shallow N+ source/drain |
| Antimony (Sb) | ~0.21 | ~3.65 | Even slower — buried-layer dopant in BJTs |
Two strategies dominate modern junction engineering:
- Co-implants (C, F, N) trap interstitials and suppress transient-enhanced diffusion (TED) of boron after RTA.
- Millisecond / nanosecond laser anneals push the (Dt) product so low that even boron barely moves, letting the source-drain extension stay under 10 nm deep.
Key Concept: Why As Is the Default for NMOS S/D
Arsenic's diffusion coefficient is ~30× lower than phosphorus at 1000 °C. That's why advanced NMOS source/drain extensions use As (and SiGe:B for PMOS) — they stay where you put them through the rest of the thermal flow.
Knowledge Check
Knowledge Check
1 / 2What are the two main purposes of post-implant annealing?