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.
Knowledge Check
Knowledge Check
1 / 2What are the two main purposes of post-implant annealing?