two-step anneal (silicide)

**Two-Step Anneal** is the **standard thermal process sequence for forming low-resistivity silicide** — using a first anneal at low temperature to form the metal-rich phase, followed by a selective etch, then a second anneal at higher temperature to convert to the desired low-resistivity phase. **What Are the Two Steps?** - **Step 1 (Low-T Anneal)**: 300-500°C. Forms intermediate phase (CoSi, Ni₂Si, C49-TiSi₂). Silicide forms only on Si, not on dielectric. - **Selective Etch**: Wet chemistry (SPM, piranha) removes unreacted metal from SiO₂/SiN surfaces. Silicide is resistant to this etch. - **Step 2 (High-T Anneal)**: 600-800°C. Converts to final low-$ ho$ phase (CoSi₂, NiSi, C54-TiSi₂). **Why Two Steps?** - **Selectivity**: If the high-T anneal were done first, metal could migrate over the spacer/STI and cause silicide bridging. - **Self-Alignment**: Forming the intermediate phase first and then removing unreacted metal ensures silicide exists only where needed. **Two-Step Anneal** is **the lock-and-key process for selective silicide** — forming the right phase in the right place by carefully separating the reaction into controlled stages.