plasma nitridation

**Plasma Nitridation of Gate Oxide** is the **incorporation of nitrogen into the Si/SiO₂ interface region via plasma treatment (decoupled plasma nitridation, DPN) — forming oxynitride (SiON) — reducing boron penetration from polysilicon gate and improving reliability and performance by 10-20% compared to pure SiO₂**. Plasma nitridation was a key technology at 65 nm and below before transition to high-k/metal gate. **Decoupled Plasma Nitridation (DPN)** DPN is a two-step process: (1) deposit SiO₂ via LPCVD or PECVD, (2) expose to N₂ plasma (decoupled: ICP source generates plasma, RF substrate bias accelerates N⁺ ions). ICP power is ~500-1500 W, substrate bias ~100-200 W, temperature ~400-500°C. N₂⁺ and N⁺ ions strike the oxide surface, incorporating N atoms into the oxide lattice (Si-O-N and Si-N bonds form). Typical nitridation dose is 5-50 × 10¹⁴ N/cm² (controllable via plasma duration). The process is "decoupled" because ICP and RF are independent, allowing tuning of ion energy and flux separately. **SiON Formation and Nitrogen Profile** Plasma nitridation forms a SiON layer at the Si/SiO₂ interface (or throughout the oxide if dose is high). The nitrogen profile is typically concentrated at the interface (N concentration ~10-30% at interface, decreasing toward surface). The bonding is mixed: some Si-N bonds (Si₃N₄-like), some Si-O bonds (SiO₂-like), and some Si-O-N bridges. The resulting material is intermediate between SiO₂ and Si₃N₄ in properties. **Boron Penetration Reduction** Polysilicon gate (p+ doped with boron for PMOS) is prone to boron diffusion and penetration through the oxide at elevated temperature (>600°C). Boron migrates into the oxide and finally to Si, causing: (1) Vt shift (positive for PMOS due to positive boron charge), (2) leakage increase (boron creates traps in oxide), (3) reliability degradation (boron assists trap-assisted tunneling). Nitrogen incorporation into oxide reduces boron diffusion via: (1) Si-N bonds are stronger than Si-O bonds (B diffusion slowed), (2) nitrogen creates a barrier to boron migration, (3) some boron is trapped by N-containing defects. Boron penetration is reduced ~50-70% with oxynitride vs pure SiO₂. **Reliability Improvement (PBTI/NBTI)** Positive bias temperature instability (PBTI, p-MOSFET under positive gate bias at elevated temperature) is improved by SiON: (1) lower Dit (nitrogen passivates some interface states), (2) slower charge trapping kinetics (N-containing defects have different trap time constants), (3) improved interface stability. NBTI (n-MOSFET, negative bias) is also improved. Typical reliability improvement is 10-20% longer lifetime (1.2-1.5x MTTF increase) with SiON vs SiO₂. **Dielectric Constant Increase** Nitrogen incorporation increases the dielectric constant of the oxide: SiO₂ (k=3.9) → SiON (k=4.5-5.5) depending on N content. Higher k reduces equivalent oxide thickness (EOT) for the same physical thickness, but this benefit is modest (5-15% EOT reduction). The k increase is partially offset by the thickness benefit (thinner equivalent oxide, less effective oxide thickness reduction in practice). **Rapid Thermal Nitridation (RTN)** Rapid thermal nitridation (RTN) uses rapid thermal annealing in N₂ or NH₃ atmosphere: (1) NH₃ RTN (~600-850°C for 10-120 sec in NH₃ atmosphere) — ammonia dissociates (NH₃ → N + 3H) and nitrogen is incorporated into oxide, (2) N₂ RTN — requires higher temperature (>900°C) or extended time for effective incorporation. RTN is simpler than DPN (no plasma required) but less controllable. RTN nitridation is slower than DPN; typically lighter N incorporation. RTN is sometimes used after oxidation (as part of standard RTO — rapid thermal oxidation). **Plasma Nitridation Anneal (PNA)** PNA is a post-nitridation annealing step (thermal anneal in N₂ or inert gas after plasma nitridation) to redistribute nitrogen and improve interface quality. Annealing (500-700°C for 5-30 min) allows: (1) nitrogen migration and stabilization in favorable lattice sites, (2) interface relaxation and defect reduction, (3) Si-N bond strengthening. PNA improves reliability vs non-annealed DPN by ~10%. **N Profile Control (Surface vs Bulk)** Nitrogen incorporation can be tailored to concentrate at the interface or throughout the oxide: (1) short plasma duration → N concentrated at interface (Si-rich oxynitride, SiON where Si:O~1:2), (2) long plasma duration → N throughout oxide (N-rich oxynitride, Si:O:N more balanced). Bulk nitridation (throughout) provides stronger boron barrier but can degrade interface quality. Interface nitridation (concentrated at Si/SiO₂ boundary) improves interface while maintaining oxide quality. **Integration with High-k/Metal Gate** For high-k/metal gate transition (starting ~22 nm), plasma nitridation was sometimes used as an intermediate step: nitrided oxide (SiON) as interfacial layer under HfO₂ high-k. However, nitrogen in oxide near HfO₂ can degrade high-k quality (oxygen scavenging), so modern high-k/metal gate processes avoid SiON in favor of pure SiO₂ IL or no IL. Plasma nitridation is largely obsolete in current high-k processes. **Hydrogen and Depassivation** Nitrogen incorporation can interact with hydrogen passivation (if hydrogen anneal is performed after DPN). N-H bonds are stronger than Si-H bonds in some contexts, potentially affecting interface passivation kinetics. However, combined DPN + hydrogen anneal is typically compatible, with minimal adverse interaction. **Summary** Plasma nitridation was a critical technology for extending gate oxide scaling in pre-high-k CMOS (65 nm to 28 nm), improving boron penetration control and reliability. While largely replaced by high-k/metal gate, plasma nitridation remains relevant for non-volatile memory and select analog/RF applications.