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.