millisecond anneal
**Millisecond anneal** (also called **ultra-fast anneal**) is a thermal processing technique that heats the wafer to very high temperatures (**1,000–1,400°C**) for extremely short durations (**0.1–10 milliseconds**) using lasers or flash lamps. This activates dopants with **minimal diffusion**, enabling the ultra-shallow junctions needed in advanced transistors.
**Why Millisecond Anneal?**
- In modern transistors, source/drain junctions must be **extremely shallow** (a few nanometers) to prevent short-channel effects.
- Traditional rapid thermal anneal (RTA, ~1–10 seconds) activates dopants but causes significant **thermal diffusion**, deepening the junction beyond acceptable limits.
- Millisecond anneal achieves **high dopant activation** (often >90%) while keeping diffusion to **sub-nanometer** levels — the wafer simply isn't hot long enough for atoms to move far.
**Methods**
- **Flash Lamp Anneal (FLA)**: Uses an array of xenon flash lamps to illuminate the entire wafer surface for **0.5–20 ms**. The wafer surface heats rapidly while the bulk remains cooler, creating a steep thermal gradient.
- **Laser Spike Anneal (LSA)**: A focused laser beam scans across the wafer, heating a narrow stripe for **0.2–1 ms**. The beam dwells briefly on each spot before moving on.
- **Pulsed Laser Anneal**: Uses pulsed excimer or solid-state lasers for even shorter exposures (microseconds to nanoseconds). Can achieve surface melting and rapid recrystallization.
**Temperature-Time Tradeoff**
- **Conventional RTA**: ~1,000°C for 1–10 seconds → good activation, significant diffusion.
- **Spike Anneal**: ~1,050°C for ~50 ms → better control, moderate diffusion.
- **Millisecond Anneal**: ~1,200–1,400°C for 0.1–10 ms → excellent activation, minimal diffusion.
- **Sub-Millisecond**: ~1,300°C+ for microseconds → near-zero diffusion, possible surface melting.
**Challenges**
- **Temperature Non-Uniformity**: At these timescales, achieving uniform temperature across the wafer is difficult. Pattern density variations cause local heating differences.
- **Thermal Stress**: Extreme temperature gradients between the hot surface and cool bulk can cause **wafer warpage** or even cracking.
- **Metrology**: Measuring temperature accurately during millisecond-scale heating is extremely challenging.
- **Integration**: Process windows are very tight — small variations in energy or dwell time significantly affect results.
Millisecond anneal is **essential for nodes below 14nm** — without it, achieving the abrupt, shallow junctions needed for high-performance FinFET and gate-all-around transistors would be impossible.