laser spike anneal
**Laser Spike Anneal (LSA)** is an **ultra-fast thermal anneal technique that heats the wafer surface to 1200–1350°C for microseconds using a laser beam** — enabling maximum dopant activation while minimizing diffusion for ultra-shallow junction formation at advanced CMOS nodes.
**Why LSA Over RTP?**
- RTA (1050°C, 10 sec): Standard activation anneal — causes 5–10nm B diffusion.
- Spike RTA (1100°C, < 1 sec): Reduces diffusion to 2–3nm.
- LSA (1300°C, 200 μs): Maximum activation, < 1nm diffusion — 10x better than spike RTA.
**LSA Operating Principle**
- CW or pulsed laser (808nm, 1070nm, or CO2) scanned across wafer surface.
- Dwell time: 200–500 μs per point (millisecond range — hence "millisecond anneal").
- Temperature: Exceeds 1200°C at surface, decreasing exponentially into bulk.
- Bulk remains cool: Thermal diffusion length $L_{th} = \sqrt{D_{th} \cdot t} \approx 10$–30μm — limits heat penetration.
- Result: Near-melt surface activation, cold bulk — no metal layer damage.
**Activation vs. Diffusion Tradeoff**
- Dopant activation follows Arrhenius — benefits from very high T.
- Diffusion follows Dt — time dominates when T is high but t is very short.
- LSA: T=1300°C, t=200μs → high activation, negligible diffusion.
- B USJ (10keV, 2×10¹⁴ cm⁻²): Xj after LSA < 10nm (vs. > 20nm for RTA).
**Flash Lamp Anneal (FLA)**
- Alternative millisecond technique.
- Xenon arc lamps flash entire wafer simultaneously (1–10ms pulse).
- Less spatially uniform than laser, but higher throughput.
- Applied Materials Vantage Astra (flash); Mattson, Applied Materials Vantage Radiance (LSA).
**Stress Effects**
- LSA creates thermal stress gradients → transient stress during cooling.
- Can cause slip dislocations if temperature gradient too steep.
- Ramp rate and spatial uniformity optimized to avoid slip.
LSA is **the enabling technology for sub-10nm USJ formation** — without it, the shallow junction requirements of 14nm FinFET and below cannot be met while maintaining adequate dopant activation and low contact resistance.